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Super-heavy element research


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A review of the discovery and investigation of the 'island of stability' of super-heavy nuclei at the separator DGFRS (FLNR, JINR) in the fusion reactions of (48)Ca projectiles with target nuclei (238)U-(249)Cf is presented. The synthesis of the heaviest nuclei, their decay properties, and methods of identification are discussed. The role of shell effects in the stability of super-heavy nuclei is demonstrated by comparison of the experimental data and results of theoretical calculations. The radioactive properties of the new nuclei, the isotopes of elements 112-118 as well as of their decay products, give evidence of the significant increase of the stability of the heavy nuclei with rise of their neutron number and approaching magic number N = 184.
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1 © 2015 IOP Publishing Ltd Printed in the UK
Reports on Progress in Physics
Y T Oganessian and V K Utyonkov
Printed in the UK
© 2015 IOP Publishing Ltd
Rep. Prog. Phys.
Reports on Progress in Physics
1. Introduction
The limits of nuclear stability, as it is known, are deter-
mined by interaction of nucleons in the nucleus. The neutron
excess in nuclei leads to a decrease in the neutron separa-
tion energy. The limit comes at Bn = 0 MeV (the neutron drip
line). Similarly, Bp = 0 MeV (the proton drip line) determines
the limit of existence of proton-rich nuclei. Another limita-
tion arises for the extreme nuclear mass, which is dened by
the probability of spontaneous ssion (SF). In fact, when the
ssion barrier vanishes (Bf = 0 MeV), the nucleus loses its
stability against ssion (TSF ~ 10−19 s). In the macroscopic
approach (e.g. the liquid-drop model) the limit of existence
of nuclei appears practically immediately after Z≈100 [1].
At the same time, it has been observed that in the binding
energy of nuclei in their ground and highly deformed states
there are variations depending on the proton or neutron num-
ber (nuclear shells). Also, the macroscopic models could
not explain the variations of the ssion barriers of the heavy
nuclei: two times higher barrier in 208Pb (Bf≈27 MeV), prac-
tically unchanged ssion barrier heights in the isotopes of
the actinides from U to Fm and the nuclear shape isomerism
[2], which manifests itself in heavy nuclei as 35 SF isomers
[3] and many other effects, which have been seen in vari-
ous experiments (gure 1). It had become obvious that the
macroscopic approaches needed some corrections caused by
structure of individual nucleus. The main achievements of the
microscopic theory are connected with the development of
the method of calculating such corrections for the ground and
highly deformed states [9] based on the Nilsson or Woods–
Saxon single particle potential to the smooth, macroscopic
part of the energy.
The main structure corrections are governed by the nuclear
shell effect. The concept of nuclear shells is here dened as
a large-scale non-uniformity in the energy distribution of the
individual particle states near the Fermi energy [10] directly
connected to the nuclear binding energy. In many publications
(e.g. see the reviews [1013] and references therein), a number
of the existing disagreements with experiment were explained
by taking into account the shell effect when calculating the
nuclear energy. One important consequence of these calcula-
tions was the disclosure of a signicant gap in the spectrum of
low lying levels in the region of the hypothetical super-heavy
nuclei (SHN), viz. of a new (following N = 126) closed spheri-
cal neutron shell at N = 184 [1417].
It was also shown that the considerable variations in the
binding energy of spherical nuclei were due to the nuclear
shells and that shell effects might be present also in deformed
‘magic nuclei’ (deformed shells) [9, 10, 18]. And nally,
at further and quite signicant increase of the deformation
Super-heavy element research
YuTsOganessian and VKUtyonkov
Joint Institute for Nuclear Research (JINR), Joliot-Curie 6, RU-141980 Dubna, Russian Federation
Received 24 July 2014, revised 10 November 2014
Accepted for publication 27 November 2014
Published 9 March 2015
Invited by Robert Tribble
A review of the discovery and investigation of the ‘island of stability’ of super-heavy nuclei
at the separator DGFRS (FLNR, JINR) in the fusion reactions of 48Ca projectiles with target
nuclei 238U-249Cf is presented. The synthesis of the heaviest nuclei, their decay properties,
and methods of identication are discussed. The role of shell effects in the stability of super-
heavy nuclei is demonstrated by comparison of the experimental data and results of theoretical
calculations. The radioactive properties of the new nuclei, the isotopes of elements 112–118 as
well as of their decay products, give evidence of the signicant increase of the stability of the
heavy nuclei with rise of their neutron number and approaching magic number N = 184.
Keywords: super-heavy elements, alpha decay, spontaneous ssion
Review Article
Rep. Prog. Phys. 78 (2015) 036301 (22pp)
Review Article
arising in ssion, the shell effects continued to play an impor-
tant role in dening the potential energy and the nuclear iner-
tial masses [7, 10].
The predictions of the nuclear properties change signi-
cantly depending on the effect of the new shells. The sum of
the smooth part of the deformation energy Ed(LD) and the
shell correction ΔEd(Shell) bring forth the appearance of a s-
sion barrier.
For the heaviest nuclei with Z = 112–114 and N = 180–
184 (see gure2), the ssion barrier height may amount to
Bf>6 MeV (higher than for 238U). Therefore, the partial SF
half-lives, as shown in the calculations of [7, 8], increase up to
~105 year (gure1(b)). Then, TSF exceeds the estimates of the
macroscopic models by a factor of 1030! Less striking, but also
quite strong, is the effect expected for deformed nuclei with
Z = 106–108 and N≈162, the effect of the deformed shells
suppresses the probability of SF by a factor of>1020. Here, an
interesting situation arises.
Because of the high stability with respect to SF, the heavi-
est nuclei will undergo α- or β-decay (Tα, Tβ<< TSF). From
the calculated partial half-lives Tα, TEC, β and TSF, shown in
gure3, we may get an impression of the scenarios and the
decay properties of the heaviest nuclei depending on Z and
N. The consecutive α decays will follow until the shell effect
weakens and SF becomes the main decay mode. In the region
of N < 162 this is observed for even–even isotopes [20]. In
the case of odd nuclei, due to the large hindrances to SF, α
decay may occur down to long-living nuclei without competi-
tion from SF. In fact, this is observed in the experiments with
Pb-target based reactions [20, 21]. For the heavier neutron-
rich nuclei, the decay sequences of both even and odd isotopes
will end by SF. The total decay time will be then determined
to a great extent by the neutron number of the parent nucleus.
When approaching the N = 184 shell, we may expect a strong
increase in the decay time.
Above we gave an example of the predictions of the macro-
scopic–microscopic models (MM). Other, purely microscopic
self-consistent approaches as the Hartree–Fock–Bogoliubov
(HFB) model and the relativistic mean eld (RMF) theory
also predict signicant increase of the binding energy of
heavy nuclei at N = 162 and N = 184. In spite of differences
in these models, the maximum shell effect corresponds to N
= 184 (like in MM calculations) but the proton shell closure
with a higher number of protons Z = 120, 122, 124 or even
126 is expected (see review [22] and references therein).
Meantime, uncertainties in the quantitative estimations of the
Figure 1. (a) Fission barrier heights as a function of the ssility parameter x = (Z2/A)/(Z2/A)crit at (Z2/A)crit = 50.883 [4]. Black points show
experimental data, solid line—calculations in the liquid drop model [5, 6], crosses—calculated ssion barrier heights in the macroscopic–
microscopic model [7, 8] for the isotopes of elements 114 and 116, open squares—the same for the nuclei produced in the 242, 244Pu,
245,248Cm + 48Ca reactions. (b) Half-lives with respect to SF as a function of the ssility parameter x. Black points and open circles denote
experimental values TSF(exp) for SF of even–even nuclei from the ground and isomeric states. Dashed line is drawn to guide the eye though
the maximum TSF(exp) values and is extrapolated into the transactinide region according to macroscopic concept. Crosses show calculated
values TSF(th) in the macroscopic–microscopic model [7, 8] for even–even isotopes of elements 112, 114 and 116, open squares—the same
for the nuclei produced in the 242, 244Pu, 245, 248Cm + 48Ca reactions, black squares—the same for the isotopes for which the SF half-lives
have been measured.
0.75 0.80 0.85 0.90
LogT / s
a) b)
6.0 MeV
Fissility Parameter x Fissility Parameter x
neutron capture
fusion with heavy ions
Fission Barrier Height B / MeV
Figure 2. The map of the shell corrections ΔEshell to the nuclear
macroscopic potential energy [18, 19]. The numbers at the contour
curves correspond to the amplitude of the shell correction (in
MeV). Crosses denote nuclei with Z≥104, obtained in cold-fusion
reactions, open circles—in actinides (Act) + 48Ca reactions.
130 140 150 160170 18
Neutron number
Proton number
-2 -3
-6 -6
Rep. Prog. Phys. 78 (2 015) 036301
Review Article
nuclear shell effects do not change the general conclusion of
the theory that in the large interval of masses from 250 to 320
‘islands of stability’ may arise, considerably changing the
limits of existence of atomic nuclei.
2. Reactions of synthesis of super-heavy nuclei
As it was shown in various experiments, the methods of consec-
utive neutron capture previously used for production of nuclei
with Z≤100 or transfer reactions of nuclei, even so heavy as U
+ U, cannot be applied for synthesizing super-heavy elements1.
The only way for producing the heaviest nuclei remains mak-
ing use of complete-fusion reactions which were applied for
the synthesis of elements of the second hundred.
In theory, the process of formation of the evaporation
residue (ER) consists of three consecutive stages. At the rst
stage, colliding nuclei overcome the Coulomb barrier and
approach the point of contact. Quasi-elastic and deep-inelastic
reaction channels dominate at this stage, leading to formation
of projectile-like and target-like fragments in the exit chan-
nel. Then, the composite system can evolve into the congu-
ration of an almost spherical compound nucleus (CN). After
dynamical deformation and exchange by nucleons, two touch-
ing nuclei can re-separate into fragments similar to colliding
nuclei or can go directly to ssion channels without formation
of spherical compound nucleus which is called quasi-ssion.
Finally, the compound nucleus cools down by the emission
of neutrons and γ rays surviving ssion and forming ER in its
ground state. This process takes place in strong competition
with ssion of excited nucleus.
For each angular momentum l the partial ER cross sec-
tionσxn(E*, l) for production of the nal nucleus in its ground
state at projectile energy E and corresponding excitation
energy E* of CN is factorized as the product of the partial
capture cross sectionσcapt(E, l), the fusion probability Pfus(E,l)
and the survival probability Psurv(E*, l),
σσ*= ··*El El PElP El(,)(,) (,)(,).
xn captfus surv (1)
The difference in mass of the initial nuclei AP, AT and the nal
nucleus ACN = AP + AT denes the excitation energy of the
CN E*. The minimum value of E* at the reaction Coulomb
barrier BC is:
*= −=−+EBQ QM MMwith ().
minC CN PT
Energy E*min depends on the masses (in eV) of the interacting
nuclei. When the mass ratio AP/AT increases (from AP = 4 to
20–30), with xed ZCN, the excitation energy increases also
(rise of the Coulomb barrier), but then becomes lower due to
increasing Q. The advance to the nuclei with closed proton or
neutron shells leads to an additional reduction of E*min [23].
Since 1974, the cold fusion reactions of 208Pb, 209Bi with
massive projectiles (AP≥50) have been used in the synthesis
of the heaviest elements. With practically xed mass of the
target (208Pb or 209Bi), the rise of the atomic and mass num-
bers of the evaporation products is entirely connected with
the increase in the mass (and charge) of the projectile. When
the projectile becomes more and more heavy, the excitation
energy of the CN decreases down to E*≈ 15–10 MeV (cold
fusion). Transition to the ground state takes place by the emis-
sion of only one neutron [20, 21]. As a result, the survivability
of the CN Pxn(E*) signicantly increases, this being the main
advantage of cold-fusion reactions.
In cold-fusion reactions, ERs are some 10–15 mass units
shifted from the β-stability line which leads to a consider-
able decrease in their half-lives. Furthermore, the cross sec-
tionof ERs produced in cold-fusion reactions exponentially
decreases with the increase of ZCN because of rise of the
Coulomb repulsion forces. When ZCN changes from 102 to
113, the cross sectiondecreases by a factor of 108. Because of
small neutron excess in the evaporation products and further
considerable decrease of the cross sectionwith increase of the
projectile mass (AP>70), it is impossible to reach the region
Figure 3. Contour map of the calculated half-lives as Log(T1/2) (in seconds) and decay modes of the nuclei with Z≥104 and N≥150 (taken
from [20]). The left graph refers to even–even nuclei, the right one—to odd-A nuclei. The regions corresponding to different decay modes
are shown in different color. The consecutive decay of nuclei, produced in cold fusion and in actinides + 48Ca reactions are also shown (see
text for details).
150 150
155 155
160 160
Proton number
170 170
175 175
207 64
Pb+ Ni
249 48
248 48
Cm+ Ca
185 185
Neutron number Neutron number
248 48
245 48
208 64
Pb+ Ni
208 70
Pb+ Zn
1In this paper, we will use term ‘super-heavy elements’ for the heaviest
nuclei (atoms) which stability is governed by existence of the new spheri-
cal shells at Z = 114 and N = 184 predicted for the rst time within the
macroscopic–microscopic nuclear theory.
Rep. Prog. Phys. 78 (2 015) 036301
Review Article
of SHN. Obviously, for the synthesis of these nuclei (Z≥114)
with high neutron excess, it was necessary to look for other
In order to decrease the factors hindering fusion, it is desir-
able to make use of more asymmetric reactions and to obtain
an increase in the neutron number of the ERs by using both
target and projectile nuclei with maximum neutron excess.
As a target material, it is reasonable to use neutron-rich iso-
topes of the actinides produced in high-ux reactors and
thus have the largest neutron excess. Among the projectiles,
the undoubted advantage is possessed by the doubly magic
nucleus of the rare isotope 48Ca. The CN 292114, produced,
for example, in the fusion of 244Pu and 48Ca (ZPZT = 1880),
acquires 8 additional neutrons compared to the problematic
208Pb + 76Ge (ZPZT = 2624) reaction. Coulomb repulsion in
the reaction 244Pu + 48Ca decreases by almost 40%, which in
turn should lead to a strong decrease in the factors hindering
the formation of the CN.
The last stage—the survival of the CN—is the decisive
one in the given method of synthesis of the heaviest nuclei.
The estimations of E*min and experiments aimed towards
measuring the excitation functions for evaporation prod-
ucts, have shown that the CN with ZCN = 112–118, when
formed in actinides + 48Ca reactions, may attain excitation
energy from 30 to 55 MeV (see section4). This energy will
be released by a cascade emission of 2 to 5 neutrons (the
evaporation of charged particles is signicantly less proba-
ble) and γ-rays. At each step, neutron evaporation and ssion
compete strongly. The survival probability can be expressed
simply as:
PE BBT(*)~ (/)~ exp[
where Bf and Bn are the ssion barrier height and the binding
energy of the neutron, respectively, Т is the temperature of the
CN and х—the number of emitted neutrons.
According to calculations, the ssion-barrier height of
nuclei with Z>104 is entirely determined by the amplitude of
shell correction. The large decrease of production cross sec-
tionsfor increasingly heavy nuclei, observed in asymmetrical
hot-fusion reactions, is connected with their lower surviv-
ability caused by reduce of ssion barriers for CN with an
increase in Z and N. However, if the predictions of the theo-
retical models about the existence of the next closed shell N =
184 are justied, the ssion barrier height will again increase
when advancing the region where NCN≥174 and ZCN≥112. In
turn, the nuclear survivability will increase too and as a result,
one can expect even a rise in the cross section for heavy nuclei
with higher Z and large neutron excess.
3. Experiments
The half-lives of nuclei, products of the complete-fusion reac-
tions of neutron-rich isotopes of heavy actinide elements with
48Ca projectiles, were expected to vary in a wide range: from
microseconds (for the even–even isotopes of the heavy ele-
ments, see, e.g. [7, 22]) up to days (for the products of sequen-
tial decay of the odd nuclei). Their calculated predominant
decay modes were α decay and SF. Production cross sec-
tionsof the ERs could be at the level of a picobarn (10−36 cm2)
or even lower. The recoiling nuclei, formed with full momen-
tum transfer from the projectile to the CN, leave the target
layer in the beam direction. Therefore, experimental equip-
ment should separate them from the beam particles, scattered
nuclei and transfer-reaction products.
The Dubna gas-lled recoil separator (DGFRS) is shown
schematically in gure4. The DGFRS has a DQhQv magnetic
conguration: a at-eld dipole magnet with inclined poles
for horizontal focusing (D) followed by horizontally (Qh) and
vertically (Qv) focusing quadrupole magnets.
The ERs recoil out of a thin target with the momentum
of the beam particle and enter the dipole-magnet chamber
lled by gas at the pressure of about 1 Torr [24]. For keeping
gas inside the DGFRS at the beam intensities of 48Ca up to
1013/s delivered by the U400 cyclotron, a rotating 1.6 μm Ti
window is mounted at its entrance as well as a 0.5–1.5 μm
Mylar xed window separating the detection system. After
emerging from the target layer, the heavy atoms have a large
ion charge (q≈20+) with broad distribution. Due to charge
exchange in consecutive collisions with the gas atoms, the
distribution of the ERs charges rapidly becomes narrower
and the mean charge decreases to the equilibrium value
+) [25] whereas average ion charge of projectiles is
q ≈ 18+ due to their high velocity. In sequential collisions
with the atoms of the medium, the heavy atoms slow down
and move along some average trajectory. Ions with mass (m),
average charge (q) and velocity (v), will be deected in a
magnetic eld strength (B) following a trajectory with cur-
vature radius (ρ). The ER trajectory in the eld of a gas-lled
Figure 4. Layout of the DGFRS and detection system. Lmax
and Bρmax are maximum eld gradient and magnetic rigidity,
Side detectors
“veto” detectors
=3.1 TmBρmax
Lmax=13 T/m
Faraday cup
Rep. Prog. Phys. 78 (2 015) 036301
Review Article
dipole magnet is determined by momentum and charge as
ρ= = 
BmvZ AZe0.0227 [T m] ,
01/31 1/3(4)
where A is the mass number and v0 is the velocity of the elec-
tron (e) in Bohr’s hydrogen atom (2.19 × 106 m s−1).
Among all the reaction products, the complete-fusion reac-
tion products have the largest possible mass. Accordingly, the
other products—target-like or projectile-like ions—will be
spatially separated in ight from the ERs following the tra-
jectories with lower curvature radii. Thus, in experiments on
the synthesis of SHN, the DGFRS suppresses the full-energy
48Ca projectiles, projectile-like ions and target-like nuclei
by factors of about 3 × 1015–1017, 3  × 1013–6  × 1014 and
104–106, respectively [26, 27]. The transmission efciency
of the separator for Z = 112–118 nuclei was estimated to be
about 35–40% [24] for the size of the focal-plane detector of
120 × 40 mm2.
A specic characteristic of the DGFRS is the hydrogen
gas used in the separator, which enables better suppression
of projectile- and target-like recoils at the focal plane than the
helium gas [28, 29] which is used in other separators (e.g.
BGS [29, 30], TASCA [31, 32]). Due to the approximately
linear dependence of the ion charge on velocity, transmission
of ERs produced in the same reaction at different projectile
energies varies weakly. Three isotopes of Fl (Z = 114) were
observed in this reaction with masses 287, 288 and 289 which
differ by only about 0.3%; the change of the DGFRS setting
was not needed. After separation in the DGFRS dipole magnet
from the beam particles and products of unwanted reactions,
the ERs are focused by the quadrupole doublet (see gure4)
onto the separator focal plane located about 4 m down-
stream. The heaviest nuclei, leaving the target with energy
of 35–40 MeV, pass over this distance in about 1 μs. Due to
kinematics of 252No close to that of the SHN produced in
the 48Ca-induced reactions, the 206Pb(48Ca,2n)252No reaction
can be used for testing and calibration of the separator and
detection system. This reaction was used for determining the
optimal thickness of the target also. The total yield of 252No
increases with growth of the target thickness up to approxi-
mately 0.5 mg cm−2. For thicker targets, the yield remains the
same because of the lower transmission caused by multiple
scattering of the recoils [33].
In the experiments on the synthesis of the heaviest nuclei
performed at the DGFRS, targets of actinide oxides were
used with thickness of about 0.4 mg cm−2 electrodeposited on
a 1.6 μm Ti foil. Enriched isotopes of 238U, 237Np, 242,244Pu,
243Am, 245,248Cm, 249Bk and 249Cf were used as target mate-
rial. Each individual target had an area of 5.4–6.0 cm2 in the
shape of an arc segment with an angular extension of 60° and
an average radius of 60 mm. The six segments were mounted
on a disc that was rotated at 2000 rpm in plane perpendicu-
lar to the beam direction. The total area of the target was
32–36 cm2, the total weight of the target material was about
10–15 mg. Before implantation into the detector, the separated
ERs passed through a time-of-ight (TOF) measuring system
that consists of two (start and stop) multiwire proportional
chambers placed within 6.5 cm from each other and lled with
pentane at ≈1.5 Torr. The TOF system allows distinguishing
recoils coming from the separator and passing through the
TOF system from signals, arising from α decay or SF of the
implanted nuclei (without a TOF signal). In order to eliminate
the background from the fast light charged particles (protons,
α’s, etc produced from direct reactions of projectiles with the
DGFRS media) with signal amplitudes lower than the regis-
tration threshold of the TOF detector, a ‘veto’ silicon detector
was placed behind the front detector (gure 4).
ERs are nally implanted in the focal-plane detector con-
sisting of 12 vertical position-sensitive strips (three 4 × 4 cm2
0.3 mm-thick chips, each with four strips) providing hori-
zontal resolution. The vertical position is determined by the
resistive charge division within each strip. The implantation
depth of ERs in the Si detector is lower by several times than
α-particle range of SHN; thus, α particles can escape the
focal-plane detector. To detect these particles as well as ssion
fragments, eight similar detectors without position sensitivity
were located upstream and perpendicular to the focal-plane
detector forming a ve-sided box conguration. This results
in increase of detection efciency for full-energy α particles
from about 52% for focal-plane detector only to approxi-
mately 87% after reconstruction of their energies deposited
in the focal-plane and side detectors. The detection efciency
for SF of the implanted nuclei is close to 100%. Since 2012,
to increase the position granularity of the detectors, which
reduces the probability of observing sequences of random
events that could imitate decay chains of synthesized nuclei,
the new focal-plane detectors have been used. These consiste
of two 6 × 6 cm2 detectors each having 16 strips with position
sensitivity and six similar side detectors without position sen-
sitivity. The detection system of the DGFRS was calibrated
by registering the recoil nuclei and decays (α or SF) of known
isotopes of No and Th and their descendants produced in the
reactions 206Pb(48Ca,2n) and natYb(48Ca,3-5n), respectively.
Using known energies of 215Ra and 217Th the energy resolu-
tions (full width at half maximum (FWHM)) were determined
separately for α particles completely absorbed in the focal-
plane detector, for α particles that escaped this detector with
a low energy release and were registered by a side detector
and for α particles detected only by a side detector (without a
focal-plane position signal). For instance, in a recent experi-
ment with 249Bk target [34] these values were 34–73 keV, 83–
120 keV and 0.73–0.98 MeV, respectively. Fission fragments
from the decay of 252No implants produced in the 206Pb + 48Ca
reaction were used for the total kinetic energy (TKE) calibra-
tion. The typical position resolutions of correlated ER-α and
ER-SF signals were 1.1–1.8 and 0.5–1.2 mm [34], respec-
tively. For α particles detected by both the focal-plane and
side detectors, the ER-α position resolution depends on the
energy deposited in the focal-plane detector and is generally
inferior to that obtained for the full-energy signal.
From theoretical calculations and the available experimen-
tal data, one can estimate the expected α-particle energies of
the ERs and their descendant nuclei that could be produced in
a specic reaction of synthesis. For α particles emitted by the
parent or daughter nuclei, it is possible to choose wide enough
energy and time gates δEα1, δtα1, δEα2, δtα2, etc (accounting for
Rep. Prog. Phys. 78 (2 015) 036301
Review Article
all the uncertainties in the estimation of the expected energies
and half-lives of the synthesized nuclei) and employ a special
low-background detection scheme [35]. For instance, in [34]
during the irradiation of the target, the beam was switched off
after a recoil signal was detected with parameters of implanta-
tion energy and TOF expected for ERs, followed by an α-like
signal within energy interval δEα1 = 10.7–11.3 MeV in the same
strip within some position window (3.2 mm) and time interval
δtα1 (0.4 s). If the rst α particle escaped the focal-plane detec-
tor and a position signal was not detected, then switching off
the beam could be done when the second α particle in the cor-
responding δEα2 (9.6–10.7 MeV) and δtα2 (2 s) intervals was
detected. If, during the rst preset beam-off time interval, an
α particle with energy expected for daughter nuclei was reg-
istered in any position of the same strip, the beam-off interval
could be automatically extended to any time. This operating
mode of the DGFRS is illustrated by detection of long decay
chains of the odd–odd isotope of element 117 produced in the
249Bk(48Ca, 3n)294117 reaction [36]. Figure5 shows the spec-
trum of α-like signals (all events detected by the focal-plane
detector or both the focal-plane and side detectors without a
registered TOF signal) in all strips accumulated during the
247 MeV 249Bk + 48Ca experiment. The α-particle spectrum
detected during beam-off time intervals is also shown. In the
high-energy part of the α-particle spectrum, where the decays
of daughter nuclei 274Bh to 290115 (Eα = 8.5–10.5 MeV) are
expected, 16 events were detected with average counting rate
of about 2/h. This demonstrates very low random probability
for detection of 7 α particles (shown by arrows) which belong
to the decays of the daughter isotopes of 294117 and occur
within about a 3 min time interval after the decay of the parent
nucleus. The calculated numbers of random sequences imitat-
ing each of the observed decay chains ranged from 2 × 10−4
to 3 × 10−20 depending on number of registered nuclei in the
chain and counting rate of random events in the focal-plane
and side detectors [34].
One of the crucial questions in the synthesis of SHN lies in
identication of the new isotopes or experimental determina-
tion of their atomic (Z) and mass (Z + N) numbers. Note, all
the decay chains of SHNs were terminated by SF of previously
unknown nuclei. Moreover, these descendant neutron-rich
nuclides with lower Z cannot be synthesized and identied
in direct reactions because of lack of reactions with stable
projectiles leading to these nuclei. That is why the method
of genetic α-particle correlations between rst observed and
well-known nuclei, widely used during last decades (see, e.g.
[20, 21]), can be applied after independent identication of
one of the members of the decay chain.
At the same time, the super-heavy nuclei and their daugh-
ters can be identied by other methods [37]. One of these is
connected with mechanism of the complete-fusion reaction
which essentially differs from other reactions of 48Ca projec-
tiles with heavy target nuclei. The de-excitation of CN occurs
via evaporation of neutrons; their number depends on the
excitation energy. The shape of excitation functions σxn(E*)
looks like assimetric pseudo–Gaussian curve with FWHM of
about 10 MeV and with maximum located at the energy cor-
responding to the number of evaporated neutrons (see, e.g.
gures 7 and 9 below). The probability of evaporation of
charged particles (protons or α particles) is suppressed due
to high Coulomb barrier in heavy CN. The DGFRS strongly
separates forward-peaked ERs with huge suppression of the
scattered beam particles and the products of incomplete fusion
(like αxn) or transfer reactions.
The measurement of excitation function of the reaction
allows determining the number of evaporated neutrons and
thus, the masses of ERs when two or even three different iso-
topes are registered and difference of their decay properties is
in agreement with expectations for the neighbouring isotopes
of the same element. Practically all the investigated reactions
which resulted in observation of SHN with Z = 112–118 at
the DGFRS were studied at several bombarding energies
[4,26, 27, 34, 36].
An additional approach to the identication of nuclei is
connected with variation of mass and charge of target in the
cross bombardments with 48Ca projectiles. In this method,
one and the same nucleus can be observed either as the prod-
uct of evaporation of a different number of neutrons from
CN produced in the reactions with different target isotopes
or as α-decay product of parent nuclide synthesized in the
reaction with a heavier target nucleus (with higher Z). As an
example, gure6 shows cross reactions used in experiments
on the synthesis of Fl and Lv. Finally, the α decay properties
and SF half-lives of synthesized nuclei can evidently demon-
strate their origin. For α decay of the even–even nuclei, one
may expect with large probability transitions through ground
states of the parent and daughter nuclei. In this case, meas-
ured α-particle energies Eα and partial half-lives Tα should
reect energy (Qα) and probability of α transitions, which can
be compared with predictions of different theoretical models
of α decay and systematics of Qα values for numerous known
Comparison of α-particle energies of even-Z nuclei with
those observed for neighbouring odd-Z nuclei can provide
Figure 5. Total energy spectra of beam-on α-like signals (all events
detected by the focal-plane detector or both the focal-plane and side
detectors without registering TOF signal) and beam-off α particles
recorded during the 247-MeV 249Bk+48Ca run. Durations of the beam-
on and beam-off intervals are given. The arrows show the energies of
events observed in two correlated decay chains of 294117 [36].
213Po (+e-)α
292 h
8.3 h
Counts / 20 keV
678910 11 12
Energy (MeV)
Rep. Prog. Phys. 78 (2 015) 036301
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valuable information for identication. For unhindered α
decays, from Geiger–Nuttall relation Tα(Z, Qα), one can esti-
mate atomic numbers of all nuclei in the α-decay chains of the
new elements. For SF nuclides, the half-lives of nuclei with an
odd number of neutrons and/or protons exceed those for even–
even nuclei by several orders of magnitude [38].
In approaching neutron shell N = 184, a considerable
increase in the stability of SHN is expected. Super-heavy
nuclei—neutron-rich isotopes of elements 112, 113 and
114 produced in the Act+48Ca reactions—have half-lives
from tenths to tens of seconds (gures 6 and 8 below). In
the course of consecutive α decays, the partial half-life Tα
increases. Half-lives of the isotopes of Rf and Db at the end
of decay chains of SHN for which SF was observed reach
hours and days. For identication of atomic numbers of
SHN the methods of fast chemistry may be applied. In con-
trast to short-lived products of the cold-fusion reactions, the
new region of chemistry of the heaviest elements becomes
Most of these methods were applied in experiments on
the synthesis of SHN and will be discussed in the following
section. Even use of combination of a few methods of iden-
tication for the full set of results obtained in the series of
experiments aimed at the investigation of SHN allows us to
determine unambiguously atomic and mass numbers of syn-
thesized nuclei.
4. Results
4.1. Even-Z nuclei
4.1.1. Synthesis and identication of element 114 erovium. The
largest neutron excess in the CN with 114 protons can be achieved
in the 244Pu + 48Ca complete-fusion reaction. The rst super-
heavy nucleus was discovered on June 25, 1999, in experiments
performed at the DGFRS by Dubna (FLNR)-Livermore (LLNL)
collaboration. Two identical decay chains were observed. Each
consisted of two consecutive α decays terminated by SF of the
third nucleus (see gure6). In this experiment, carried out at the
lowest projectile energy only, the parent nucleus was assigned to
288Fl [39]. Later on, the cross-section measurement in combina-
tion with decay properties of produced nuclei gave correct identi-
cation of the mass number of 289 for this isotope of element 114.
In 2003 the study of the 244Pu + 48Ca reaction was continued
at higher 48Ca energies [42] (see gure7). The same isotope
was observed at two higher excitation energies. At increased
energies, another isotope, 288Fl, with different decay proper-
ties was produced also. Its decay chain consisted of α decay
Figure 6. Summary decay properties of the isotopes of even-Z elements observed among the products of 48Ca beam and 238U, 242,244Pu,
245,248Cm and 249Cf target reactions. The numbers of the decay chains of the given isotopes, the products of corresponding xn-evaporation
channels, observed in experiments with use of the DGFRS (in red) [3945], IVO setup with COLD detector (in blue) [4649], SHIP (in
green) [50, 51], BGS (in magenta) [52, 53] and TASCA (in grey) [32, 54] are shown. The average energies of α particles and half-lives are
given for all α emitters observed in these experiments (yellow squares). The energies of rare α lines are given by smaller font. The energy
uncertainties given in brackets correspond to the data with the best energy resolution. For spontaneously ssioning nuclei marked by green
squares the half-lives are listed.
Rep. Prog. Phys. 78 (2 015) 036301
Review Article
of parent nucleus and SF of daughter nuclide which half-life is
lower by factor of 300 than that for daughter isotope in the rst
decay chain. Finally, at the highest energy the third isotope,
287Fl, was observed. The consideration of decay properties of
daughters in the three chains evidently indicated that the sec-
ond chain should originate from even-N nucleus and neigh-
bouring isotopes should have odd number of neutrons. This is
explained by the hindrance factor increasing SF half-lives of
odd-N and odd-Z nuclei by a few orders of magnitude that was
established for many nuclei (see, e.g. [38]). Such assignment
is in agreement with ne structure in α decays of synthesized
nuclei, which will be discussed below (see gures6 and 11).
The measured production cross sectionsof all the three iso-
topes agree well with expectations for complete-fusion reactions
with evaporation of three, four and ve neutrons from CN 292Fl
[5659, 61, 62]. The decay properties of 287Fl were investigated
in more detail and the new lighter isotope 286Fl was synthe-
sized in the cross bombardment of 242Pu by 48Ca [44] in 2003.
The isotope 287Fl, product of the 5n-evaporation channel of the
244Pu + 48Ca reaction, was observed also in the 242Pu(48Ca,3n)
reaction at the three lowest excitation energies of the CN 290Fl
[44] (see gures6 and 7). Further increase of projectile energy
allowed synthesis of the new isotope 286Fl. Its decay chain was
similar to that of its heavier even–even neighbour 288Fl, an α
decay of 288Fl and SF of daughter nuclide 284Cn. In agreement
with theoretical expectations, the stability of nuclei, especially
against SF, decreases with receding from the neutron magic
number N = 184: even–even isotope 286Fl with approximately
equal probabilities undergoes α decay and SF.
In the 238U + 48Ca reaction, investigated in 2003–2004
[44], one event of SF was attributed to the decay of 282Cn and
seven α-decay chains of 283Cn were detected at three projec-
tile energies (see gures6 and 7). In one case 279Ds underwent
α decay, instead of the more probable SF mode, then two more
α decays of 275Hs and 271Sg and SF of 267Rf were registered.
A similar long chain was observed in the 242Pu(48Ca, 3n)287Fl
reaction also [44]. Once again this long decay chain has
been registered in the third cross bombardment 245Cm(48Ca,
2n)291Lv [42, 45] (see below).
Experiments on the synthesis of Fl isotopes were repeated in
other laboratories. A study of the 242Pu + 48Ca fusion reaction
at the BGS was published in 2009–2010. One and two decay
chains of 287Fl and 286Fl, respectively, were observed [52, 53]
with decay modes, half-lives and decay energies in agreement
with results published by the DGFRS group [4245]. Besides,
one more decay chain of the lightest isotope 285Fl, product
of the 5n-evaporation channel of the 242Pu + 48Ca reaction,
was found to consist of ve consecutive α decays terminated
by SF of 265Rf (see gures6 and 7). In addition, the 238U +
48Ca reaction was studied at one 48Ca energy at the SHIP [50]
in 2005–2007. Here two decay chains were measured, which
fully conrm data that were previously assigned to the isotope
283Cn in experiments at the DGFRS. Two ER-SF chains were
assigned to a 50% SF branch of this isotope; this, however,
was not evident from the data where 283Cn was observed as
daughter nucleus after α decay of 287Fl [4245, 52] and the
upper limit of 0.1 was set for a SF branch of 283Cn [44]. And
nally in 2009, the decay properties of 288,289Fl were also
conrmed in experiments [32, 54] performed with use of the
TASCA (see gures6 and 7). In addition, in this work a rare
α-decay branch for 281Ds and SF of 277Hs were detected in one
decay chain of 289Fl.
Figure 7. Excitation functions for the 2n (green triangle up), 3n (red square), 4n (blue circle) and 5n (cyan triangle down) evaporation
channels from the complete-fusion reactions 238U, 242,244Pu, 245,248Cm, 249Cf + 48Ca measured at the DGFRS [3945] (solid symbols) and
the SHIP [50, 51], BGS [52, 53] and TASCA [32, 54] (open symbols). For reference purposes, the Bass barrier [55] is shown by a black
arrow in each the left bottom panel it is labeled with BBass. Lines show the results of calculations [5659]. Vertical error bars
correspond to statistical uncertainties [60] for the DGFRS experiments and available data for other setups. Horizontal error bars represent
the range of excitation energies populated at given beam energy. Position of some symbols were shifted from centers of energy intervals for
avoidance of their mixing. Upper cross-section values are shown by colored arrows.
Excitation energy (MeV)
Cross section (pb)
25 30 35 40 45 50 55 25 30 35 40 45 50 55
Rep. Prog. Phys. 78 (2 015) 036301
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In 2011 a joint IUPAC/IUPAP Working Party (JWP) rec-
ommended that the Dubna–Livermore collaboration be cred-
ited with discovery of the new element 114 produced in the
242Pu(48Ca, 3n)287114 reaction [63]. The element with atomic
number 114 was named erovium (Fl) to honour the Flerov
Laboratory of Nuclear Reactions which was founded by Flerov
and where super-heavy elements were synthesized [64].
The relatively high stability of 283Cn (T1/2 ≈4 s) allowed
researchers to investigate for the rst time, the chemical
properties of element 112. In 2006–2007, the IVO setup was
applied for collecting the 242Pu + 48Ca reaction products in the
chamber lled by a He/Ar carrier gas which delivered volatile
reaction products (including short-lived isotopes of Hg and
Rn) to detection system COLD. Here, atoms were depos-
ited according to their interaction with the detector surfaces.
The COLD consisted of an array of 32 pairs silicon detec-
tors, with the active surfaces facing each other. The surface
of one detector in each of 32 pairs was covered by gold layer.
The temperature gradient was established along detectors by
a thermostat heating at the inlet and a liquid–nitrogen cryostat
cooling near the outlet. The primary fusion-evaporation reac-
tion product 287Fl has a half-life of about 0.5 s which was too
short compared with the average transport time from the reac-
tion chamber to the detector. Thus, only the daughter nucleus
283Cn could reach the detector. In these experiments, ve decay
chains of 283Cn were registered [46, 47]. By directly compar-
ing the adsorption characteristics of 283Cn to that of mercury
and the noble gas radon, it was found that the element Cn was
more volatile than Hg and unlike radon, reveals a metallic
interaction with the gold surface. These adsorption character-
istics establish element Cn as a typical element of group 12.
The results of the study of the chemical properties of Fl are
less denite. In the rst experiment [49] performed in 2007,
three decay chains of 287,288Fl were observed in the reactions
of 48Ca with 242,244Pu. Their deposition on the detectors with
temperature−90, −88 and −4 C indicates that element Fl is
at least as volatile as element 112 (noble-gas-like behaviour).
In the second work [65] published in 2014, two decay chains
were attributed to the 244Pu + 48Ca reaction products 288,289Fl.
But both events were observed at room temperature (+21 C)
indicating volatile-metalic behaviour of Fl.
4.1.2. Synthesis of element 116 livermorium. The rst nucleus
of element 116 was discovered at the DGFRS on July 19, 2000
[40, 41]. Like in 1999 experiment 244Pu + 48Ca, the rst study
of the 248Cm + 48Ca reaction in 2000–2001 was performed
at low-energy side of the excitation function [43, 44]. In this
series of experiments, three similar decay chains of parent
nucleus were observed which were followed by a sequence
of two α decays and SF in each case. The decay properties of
these descendants were in full agreement with those measured
in 1999 in the 244Pu + 48Ca reaction [39]. After measuring
the excitation function of the 244Pu + 48Ca reaction [42], ERs
observed in the 248Cm + 48Ca reaction were identied as the
product of the 3n-evaporation channel, 293Lv.
Investigation of this reaction was continued in 2004 at
higher 48Ca energy [43, 44]. Here, like in the case of the 244Pu
+ 48Ca reaction, increase of the excitation energy allowed us to
observe simultaneously two isotopes: 293Lv and a new lighter
isotope 292Lv. If the same type of nuclear reaction occurred in
both cases, then after α decay of the parent nuclei in the reac-
tion with 248Cm the descendant isotopes should be the same as
those observed in primary reaction with 244Pu. Indeed, decay
properties of the daughter nuclei in the 248Cm + 48Ca reaction
were identical to those registered directly in the reaction with
244Pu [39, 42] (see gures6 and 7).
Experiments were continued with a 245Cm target. In 2003,
at the rst (lowest) bombarding energy two new isotopes of
element 116 were synthesized [42]. The decay properties of
daughter nuclei in one of these were in agreement with those
observed in the 244Pu(48Ca, 5n)287Fl and 242Pu(48Ca, 3n)287Fl
[4244] as well as in the 238U(48Ca, 3n)283Cn [44] reactions.
The α decay of the second isotope lead to decay chains seen
in the 242Pu(48Ca, 4n)286Fl [44] and 238U(48Ca,4n)282Cn [44]
reactions. This indicates observation of the 2n- and 3n-evap-
oration channels of the 245Cm + 48Ca reaction, i.e. 291Lv and
290Lv, respectively (see gures 6 and 7). In 2005, further
increase of 48Ca energy resulted in a reduction in the yield of
291Lv and increase of that of 290Lv [45]. Finally, at the highest
energy, only 3n-evaporation channel was observed. Such vari-
ation of production cross sectionsof two isotopes is in agree-
ment with what it should be expected for the behaviour of the
xn channels of the complete-fusion reactions. The decrease
of the neutron number in the target nuclei, e.g. from 244Pu to
242Pu or from 248Cm to 245Cm, results in rise of the neutron
binding energy and somewhat lower excitation energy of CN
at the fusion barrier (gure 7). This leads to a relative increase
in the yields of lower xn channels. Note, the 2n-evaporation
channel was also observed in two other reactions with some-
what lower-N target nuclei. A single decay chain was assigned
to the 2n-reaction product in the experiment with 242Pu [44]
(gure 7) and four chains were observed in the 243Am(48Ca,
2n)289115 reaction [66] (see below).
Owing to strong correlations in three α-decay chains of
291Lv with following 287Fl and lighter descendants (even up
to 267Rf in one chain), the IUPAC/IUPAP JWP recommended
that the Dubna–Livermore collaboration be credited with dis-
covery of element 116 [63]. The element with atomic number
116 was named livermorium (Lv) in honour of the Lawrence
Livermore National Laboratory because of experiments on
the synthesis of super-heavy elements, including element 116,
were performed in collaboration with group of researchers
from this laboratory [64].
The independent conrmation of the results obtained at the
DGFRS in 2000, 2001 and 2004 in the 248Cm + 48Ca reaction
[40, 41, 43, 44], where element 116 was observed for the rst
time, followed in 2007 in the SHIP experiments [51]. Four and
one decay chains of 292Lv and 293Lv, respectively, were regis-
tered at the excitation energy of 40.9 MeV (see gures 6 and
7). The decay properties of the parent and all the descendant
nuclei are in agreement with those measured at the DGFRS.
One more chain was not denitely assigned. The energy and
lifetime of the rst α decay agree with data determined for 293Lv
but energies of the next three α particles are larger than those
measured for 289Fl, 285Cn [3942] and 281Ds [32, 54] as well
as much longer decay time was observed for the terminating
Rep. Prog. Phys. 78 (2 015) 036301
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ssion event than that seen at the TASCA experiment [32, 54].
The decay properties of detected nuclei are in good agree-
ment with those observed in long decay chains originating from
291Lv which could be produced in the 5n-evaporation channel
of the reaction with 248Cm or in the reaction with lighter Cm
isotope, e.g. 246Cm (percentage of 3.1%) (private communica-
tion by Hofmann). Indeed, α-particle energies and lifetimes of
the second to fourth detected nuclei coincide well with data for
287Fl, 279Ds and 275Hs as well as decay time of SF nucleus is
in agreement with half-life for 271Sg (see gure6). Somewhat
lower α-particle energy for the parent even-odd 291Lv and miss-
ing of one of ve decays (283Cn) are quite possible.
4.1.3. Synthesis of element 118. For the rst time, the decay
chain of the element 118 was synthesized on March 19, 2002,
in the reaction 249Cf(48Ca, 3n)294118 studied at the DGFRS
[45]. Two more decay chains of 294118 were observed in 2005
at higher projectile energy [45]. Finally, the fourth decay
chain of the same isotope was registered during two experi-
ments aimed at the synthesis of element 117 in the 249Bk +
48Ca reaction performed in 2009–2010 and 2012 [36] 2. Iden-
tication of the isotope 294118 was based on the results of
the four cross reactions: 249Cf(48Ca, 3n)294118...282Cn
[45], 245Cm(48Ca, 3n)290Lv...282Cn [42, 45], 242Pu(48Ca,
4n)286Fl 282Cn [44] and 238U(48Ca, 4n)282Cn [44] (see
gure 6). Among the daughter nuclei in the decay chain of
294118, decay properties of two isotopes 286Fl and 282Cn were
conrmed in independent experiments at the BGS [52, 53]
(see subsection 4.1.1. above).
4.2. Odd-Z nuclei
4.2.1. Synthesis and identication of elements 113 and
115 . The discoveries of elements 113 and 115 as the prod-
ucts of the complete-fusion reaction 243Am + 48Ca that led
to the synthesis of isotopes of element 115 and their α-
decay product, element 113 that was also unknown at that
time, were reported in 2004 [67]. The rst decay chain was
observed at the DGFRS on 24 July, 2003. This involved
two new elements at once, 288115 and 284113, followed by α
decays of three new neutron-rich isotopes of the known ele-
ments 280Rg, 276Mt and 272Bh and SF of 268Db (see gure8).
The electron-capture (EC) of 268Db leading to presumably
rapid SF of 268Rf (TSF ~ 1 s [7]) could not be excluded as well.
The run was performed at two projectile energies, which
resulted in observation of two isotopes 288115 and 287115 as
well as their descendant nuclides down to 268Db and 267Db
(gure 8). In the 243Am + 48Ca reaction, the energies of the
bombarding particles and reaction cross sectionswere com-
parable with results of experiments where excitation func-
tions for the reactions 242,244Pu, 245,248Cm + 48Ca have been
measured (see gures7 and 9). Two neighbouring isotopes of
the new element were detected at different 48Ca energies, in
agreement with expectations for the fusion-evaporation reac-
tions. After observing decays of the three nuclei 288115 at the
excitation energy E*≈ 40 MeV, with the increase of energy
to E*≈45 MeV a new decay chain originating from differ-
ent isotope was registered [26, 67] (see, e.g. the difference in
Figure 8. Summary decay properties of the isotopes of odd-Z elements observed among the products of 48Ca beam and 237Np, 243Am and
249Bk target reactions. The numbers of the decay chains of the given isotopes, the products of corresponding xn-evaporation channels,
observed in experiments with use of the DGFRS [26, 27, 34, 36, 6670] (in red), chemical setup (in blue) [26, 71, 72] and TASCA (in
grey) [73, 74] are shown. The average energies or energy intervals of α particles and half-lives are given for all α emitters observed in
these experiments (yellow squares). The energy uncertainties given in brackets correspond to the data with the best energy resolution. For
spontaneously ssioning nuclei marked by green squares the half-lives are listed.
10.63(8) MeV
10.69(8) MeV
10.0(1.1) MeV
8.93(8) MeV
73 ms
4.2 ms
0.44 s
61 s
22 min
10.61(5) MeV
37 ms
10.23(1) MeV
75 ms
10.38(16) MeV
0.09 s
10.33(1) MeV
20 ms
9.28(7) MeV
1.5 s
1.3 h
10.29-10.58 MeV
164 ms
9.10-10.11 MeV
0.91 s
9.09-9.92 MeV
4.6 s
9.17-10.01 MeV
0.45 s / 6 s
8.55-9.15 MeV
10.9 s
26 h
9.76(10) MeV
10.60-11.20 MeV 10.81-11.07 MeV
22 ms 51 ms
9.78-10.31 MeV
650 ms
9.61-9.75 MeV
9.5 s
8.86-9.05 MeV
100 s
9.38-9.55 MeV
4.5 s
8.73-8.84 MeV
44 s
15 h
10.15-10.54 MeV
330 ms
9.47-10.18 MeV
4.2 s
5 ms
9.28(5) MeV
17 s
249 48
Bk +Ca
3n ()
4n ()
243 48
Am +Ca
2n ()
3n ()
31++20 22
4n ()
237 48
Np +Ca
3n ()
2In the course of the long-term work, 249Cf—the product of β decay of
249Bk (330 d)— is being accumulated in the target. The gradual ingrowth of
249Cf in the 249Bk target material during both series of experiments resulted in
production of 294118 with cross sectioncomparable with values measured for
the 249Cf(48Ca, 3n) reaction at close excitation energies (see gures6 and 7).
Rep. Prog. Phys. 78 (2 015) 036301
Review Article
half-lives of the neighbouring 280,Rg and 279Rg, 276Mt – 275Mt
and other descendant nuclei in gure8). The decay properties
of SHN will be discussed in the following sectionbut here we
emphasize that the decay properties of nuclei observed in the
243Am + 48Ca reaction evidently differ from those produced
in the reactions with even-Z target nuclei. One can compare
gures6 and 8 where isotopes of odd-Z elements show decay
properties intermediate between those of neighboring even-
Z nuclei.Their decay chains are longer which is caused by
unpaired protons increasing stability of nuclei against SF by
orders of magnitude, etc.
In addition to the above-mentioned measurements, a
chemical experiment was performed for identication of
the long-living SF isotope 268Db that was observed after ve
sequential α decays of 288115 nuclei [71]. The 243Am target
was bombarded by 48Ca ions with an energy corresponding to
247 MeV in the middle of the target that means E* = 39 MeV
for the CN 291115 (compare with the DGFRS data in gure9).
On leaving the target, the recoiling nuclei passed through a
collimator suppressing the yield of transfer-reaction prod-
ucts (capture angle of±12.5°) and were stopped in a copper
catcher. Each one–two days the front layer of catcher with a
thickness exceeding range of ERs was mechanically cut from
the surface. This part of the catcher could contain 268Db (TSF
~ 1 d) atoms. Then elements of group 4 and 5 were isolated
from actinides [71].
The test experiments [75] have shown that the factor of
separation of transactinides from actinides was more than 104.
On the other hand, from two possible transactinides with Z =
104 and 105, the element of group 4 (Rf) preceded by ve α
decays could be produced in the 243Am + 48Ca reaction only
in the pxn channel or as a result of EC/ β+ decay of one of ele-
ments 115-Bh. However, the pxn channel for x = 1–5 or EC/
β+ decay of 288115 or 284113 would lead to the now known iso-
topes of Fl or Cn whose decay chains strongly differ from the
observed ones (see gures6 and 8). The EC/β+ decay of lighter
descendants 280Rg–272Bh leads to even–even isotopes 280Ds–
272Sg for which SF is expected with half-lives less than 1 s (see
[7] and gure13 below). Therefore, the ~1-d SF, if observed
in transactinides fraction, could originate from the element of
group 5 (Db) only (direct SF or SF with short half-life fol-
lowing EC). In 2004, in the chemical experiment [71] 15 SF
events of the nuclei of a transactinide element were detected.
These showed a half-life of 32
11 h, high total kinetic energy
of SF fragments (~235 MeV) and average neutron multiplicity
per ssion act (4.2) and were produced with a cross sectionof
about 4.2 pb. All the values, within experimental errors, are in
agreement with those (T1/2, TKE, σ3n) measured for 268Db at
the DGFRS in 2003 [67] and later experiments [66, 70] (g-
ures 8 and 9). These results were later (in 2005 [72]) corrobo-
rated in another chemical experiment that attempted studying
more delicate chemical properties of Db within group 5.
In chemistry experiments [71, 72] the SF activity was
produced in the same 243Am + 48Ca reaction (I) at the same
projectile energy (II) with the same decay properties (namely,
decay mode (III), half-life (IV), total kinetic energy (V)) and
the same cross section(VI) as it was observed in the experi-
ment performed at the DGFRS [67]. All the factors allowed
one to conclude that one and the same isotope has been
observed in both the physical and chemical experiments [26,
67, 71, 72]. Simultaneously, all the precursors (Z = 107, 109,
111, 113 and 115) discovered in [67] were identied by the
method of genetic relation between ancestor and descendant
[37]. In our further investigation of the region of odd-Z SHN
we studied neutron-decient isotopes and continued, as well,
the 243Am + 48Ca experiment in a more extended range of pro-
jectile energies for observation of more nuclei and detailed
measurement of the excitation function of this reaction. In
2006 in the 237Np(48Ca, 3n) reaction, we observed two decay
chains originating from the lighter odd–odd isotope 282113
[68] (gure 8). Decay properties of 282113 and its descend-
ant nuclei 278Rg, 274Mt, 270Bh and 266Db were in full agree-
ment with those following from decay properties of heavier
isotopes 283,284113 and other descendants previously produced
in the 243Am + 48Ca reaction.
In contrast to even-Z target nuclei, application of the cross-
bombardment method for odd-Z nuclei is limited by the num-
ber of target nuclei available for experiments, that is 237Np,
241,243Am and 249Bk. The 249Bk(48Ca, 3-4n) reactions lead to
heavier isotopes of element 115 and their descendants; there-
fore only the isotope 289115, the product of the 2n-evaporation
channel of the reaction 243Am + 48Ca, seems to be useful for
cross-bombardment experiments. In 2010–2012, with aim
of synthesizing this isotope as well as measuring excitation
Figure 9. Excitation functions for the 2n (green triangle up),
3n (red square), 4n (blue circle) and 5n (cyan triangle down)
evaporation channels from the complete-fusion reactions 237Np,
243Am, 249Bk + 48Ca measured at the DGFRS [26, 27, 34, 36,
6670] (solid symbols) and TASCA [74] (open symbols). For
reference purposes, the Bass barrier [55] is shown by black arrow
in each panel; in the bottom panel it is labeled with BBass. Vertical
error bars correspond to statistical uncertainties [60] for the DGFRS
experiments and available data from TASCA. Horizontal error bars
represent the range of excitation energies populated at given beam
energy. Symbols with arrows show upper cross-section limits. The
results of theoretical calculations are shown by solid [5659] and
dashed [61] lines.
25 30 35 40 45 50 55
Excitation energy (MeV)
25 30 35 40 45 50 55
Cross section (pb)
10 243Am
Rep. Prog. Phys. 78 (2 015) 036301
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function in a wider energy range, we performed a new series
of experiments with 243Am [66, 70]. The results of all these
experiments are presented in gures8 and 9. Indeed, at the
two lowest 48Ca energies, we detected the product of the
2n-reaction channel, 289115, undergoing two consecutive
α decays and terminated mainly by SF of 281Rg, as it was
observed for the same nuclei synthesized in the reaction 249Bk
+ 48Ca (see below). These chains were not detected at higher
48Ca energies.
In sum with results of 2003, at four energies, 31 decay
chains of 288115, product of the evaporation of three neutrons,
with maximum yield at the excitation energy of about 36 MeV,
were registered. At the energy E*≈45 MeV, we detected two
decay chains of 287115 that were not found at lower 48Ca
energies. Such a behaviour of σxn(E*) is expected for cool-
ing process of exited CN 291115 and is in full agreement with
numerous similar observations for fusion-evaporation reac-
tions3. The radioactive decay properties of 288115, 287115 and
their daughter nuclei discovered in 2003 [26, 67] were com-
pletely conrmed by registration of 28 new decay chains in
the new series of experiments [66, 70].
In four decay chains of 288115, α decay of 268Db was
searched for within beam-off intervals from 2.7 h up to 1–3 d
following decay of 272Bh (see discussion in [34]). No events
were found with Eα = 7.7–8.2 MeV which could be expected
for 268Db. This result is in agreement with chemistry experi-
ments [71, 72] where SF activity was found in the fraction of
transactinide elements. Thus, an upper limit of 7% can be set
for α-decay branch bα for 268Db.
In 2012, the same 243Am + 48Ca reaction was studied at
two close beam energies of 242.1 and 245.0 MeV (E* = 35.1
and 37.5 MeV) at the TASCA [73] applying a high-resolution
α, x-ray and γ-ray coincidence spectroscopy technique. In
total, thirty correlated α-decay chains were detected which
contain ve ER-α-α-SF and two ER-α-SF events ‘compatible
with decay chains proposed to originate from either 289115 or
288115 [73]’. The excitation function of the reaction was not
measured in this experiment; short decay chains and reasons
for doubts about their assignment are not given in [73]. One
long chain of ve α decays was clearly compatible with the
characteristics of the decay chain attributed to start from the
isotope 287115 [66, 67]. The remaining 22 chains are com-
patible with the 31 chains previously assigned to the decay
of 288115 [26, 66, 67, 70]. Here α decay of 268Db was not
observed as well. An upper limit bα≤4% follows from results
of all these experiments. The cross-section values are not
given in [73]. The summary decay properties of 287,288115 are
shown in gure8.
In both experiments at the DGFRS [26, 66, 67, 70] and
TASCA [73], complex spectra of the α-particle energy were
measured (see gure12 below). In decay of 276Mt, that shows
the most complex α-particle spectrum, we could not exclude
observation of two states with different lifetimes [66]. Despite
the single half-life for 276Mt proposed in [73], its decay time
in one chain was t = 8.95 s whereas the average lifetime for
other 15 events was τ = 0.55 s (probability to decay with
t≥ 8.95 s for 16 events with τ = 0.55 s is only 1.5 × 10−6).
Thus, this new long decay time just could conrm the assump-
tion of [66]. In gure8 we present result of a two-exponential
t [76] of all the available data for 276Mt which suggests two
half-lives. However, more statistics is still needed for denite
In addition to studying the formation and radioactive prop-
erties of the products of the 243Am + 48Ca reaction, the TASCA
experiment [73] was aimed at the measuring x- and γ-rays in
coincidence with α particles of nuclei starting from element
115. In one decay chain the escape event with E = 0.825 MeV
was ‘rmly attributed’ to 276Mt (probability of its random origin
is not given in [73]) and was coincident with two Ge-detector
entries at 136 and 167 keV, which are consistent with Z = 107
Kα2 and Kβ energies, respectively [77]. Because of 10–15%
probability of random α-photon coincidences, authors noted
that either one or both of events can also represent γ-rays or
Compton scattered or background events. Besides, several αγ
coincidences were observed for 280Rg and 276Mt.
In spite of the fact that registration of x-rays in coincidence
with α particles of 276Mt was not denitely established, the
experiment demonstrated the prospects for investigation of
nuclear structure of SHN. However, these studies call for con-
siderable increase of production rate of nuclei which could be
reached at the new experimental facilities which are planned
to be put into operation in a few years (see section5).
4.2.2. Synthesis of element 117. The synthesis of heavier
element 117 became feasible when 22.2 mg of the 249Bk tar-
get material was produced at the High Flux Isotope Reac-
tor (HFIR) at the Oak Ridge National Laboratory (ORNL)
[27] and sent to Dubna. For the rst time, element 117 was
observed at the DGFRS on August 20, 2009, in the complete-
fusion reaction 249Bk(48Ca, 4n)293117 [69].
In 2009–2010, at two projectile energies corresponding to
the excitation energies of the CN 297117 of 39 and 35 MeV, we
synthesized two isotopes of element 117 [27, 69] (see gures8
and 9). At the excitation energy of 39 MeV that corresponds to
the expected maximum for production yield of the 4n-evap-
oration channel we have registered ve decay chains of the
odd-even isotope 293117. From the well established behavior
of the excitation functions measured for numerous reactions,
it followed that a reduction of the projectile energy should
result in a decrease of the cross sectionfor the 4n channel and
increase of the cross sectionof a heavier odd–odd isotope with
lower α-particle energy and longer lifetime. Indeed, at 35 MeV
excitation energy, we produced one longer decay chain of the
isotope 294117. As was expected for an odd–odd nucleus, s-
sion of 282Rg and 278Mt is suppressed because of the unpaired
neutrons compared to α emission and gives a longer α chain.
Identication of isotopes of the new element 117 was made
similarly to that of the isotopes with Z = 115 produced in the
243Am(48Ca, 2-4n)287-289115 reaction. Neutron-rich 290115 and
289115 that are daughters of α decay of the isotopes of element
117, produced in the 249Bk(48Ca,3-4n)294,293117 reaction should
have lower α-particle energies and respectively, longer lifetimes
3The products of the 2n-evaporation channel of the 242Pu, 245Cm + 48Ca
reactions, isotopes 288Fl and 291Lv, were observed in our experiments with
comparable cross sectionsat E* = 32–38 MeV [4245] (gure 7).
Rep. Prog. Phys. 78 (2 015) 036301
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compared with the isotopes 288115 and 287115 produced in
the reaction with 243Am. Indeed, α-decay energies of all the
descendant nuclei 289,290115, 285,286113, 282Rg, 278Mt and 274Bh
(products of the 249Bk + 48Ca reaction) have smaller Eα and
longer Tα when compared with neighboring isotopes 287,288115,
282−284113, 278−280Rg, 274−276Mt and 270−272Bh observed in the
reactions with 243Am and 237Np (see gures8 and 12 below).
Moreover, analogously to lighter isotopes of odd-Z nuclei with
Z≤111 and N≤163, the α-decay energies of all the products
of the reaction with 249Bk including parents 293117 and 294117
have intermediate values between those measured for neighbor-
ing even-Z nuclei (see gures6, 8, 11 and 12 below).
The number of consecutive α decays originating from
289115 and 288115 differ. Decay chains of 289115 are termi-
nated mainly by SF of 281Rg but all 31 [26, 66, 67, 70] and
22 [73] observed decay chains of 288115 ends in SF of 268Db
(gure 8). Furthermore, in spite of complex α-particle spectra
of odd-Z nuclei, decay properties of 289115 and 288115 and
descendant nuclei are different (compare Tα values for nuclei
in gure8 and shape of α-particle spectra in gure12 below,
especially for isotopes of element 113). At the same time, α-
particle energies, decay times and decay modes of isotopes
289115, 285113, and 281Rg observed in the reactions with 243Am
and 249Bk agree. Therefore, one can conclude that isotope
289115 was produced in cross reactions with two target nuclei
to provide cross bombardment evidence for the discovery of
the new elements 113,115 and 117.
These conclusions were conrmed also by the following
experiments aimed at the measurement of excitation functions
for isotopes of element 117 [34, 36]. In 2012 the 249Bk + 48Ca
reaction was studied at ve excitation energies within inter-
val E* = 30.4–48.3 MeV. In two campaigns with 249Bk target
four decay chains of 294117 were produced at two laboratory-
frame beam energies Elab = 244 and 247 MeV (E* = 32.6 and
35.1 MeV) and 16 chains of 293117 were observed at three
higher energies Elab = 252, 256 and 260 MeV (E* = 39.3, 42.6
and 46.0 MeV) (see gures8 and 9) providing additional evi-
dence of the identication of the nuclei of element 117.
The same reaction was studied in 2012 at the TASCA at 48Ca
energies of 252, 254 and 258 MeV [74]. In total, four decay
chains, two long and two shorter ones (not published yet), all
terminated by SF, were observed. The decay properties of the
nuclei in the long chains registered at the two largest 48Ca ener-
gies and assigned to 294117 are in good agreement with results
reported from the DGFRS group. In addition, in both chains
events with Eα = 7.89 and 7.90 MeV were found 1.3 h and 1.6 h
after α decay of 274Bh and assigned to 270Db (Tα = 1 h) fol-
lowed by SF of 266Lr (TSF = 11 h). This was not observed in
experiments at the DGFRS and needs a short comment.
Consecutive α decays of nuclei 288115–272Bh or 294117–
274Bh in all the observed decay chains occurred within 2 and
7 min, respectively. The probability of random observation
of any of daughter nuclei with Eα = 8.5–10.5 MeV (see g-
ures5 and 12 below) within beam-off interval of 10 min at the
DGFRS was about 2.5 × 10−3 (at Elab = 243 MeV in [66, 70])
and 2.8 × 10−4 (at Elab = 247 MeV in [34, 36]). But the situa-
tion differs at the end of decay chains where the time interval
between the last α particle and SF increases up to hours and
tens of hours. The probability of the appearance of a random α
particle within this interval increases accordingly.
However, the probability of the random origin Perr of events
within some energy range (e.g. Eα = 7.7–8.2 MeV in [34])
and relatively long time interval, say 2 h, was not presented
in [74]. The given total numbers of beam-off α particles with
Eα = 6–12 MeV in the detector pixels where decay chains
were observed [74] results in a rather large Perr value of 0.5.
Furthermore, one could expect similar decay properties for
270Db (N = 165) and 268Db (N = 163) which follows from close
predicted Qα values (8.31 and 8.26 MeV, respectively [78]) as
well as from comparable α-decay half-lives of other N = 163,
165 isotopes 269,271Sg and 270,272Bh (see gures 6 and 8 and
table1 below). But, as mentioned above, α decay of 268Db was
not seen in the DGFRS [26, 66, 67, 70], TASCA [73] and chem-
istry experiments [71, 72]. Thus, the partial Tα value for 268Db
exceeding that for 270Db, suggested in [74], by more than two
orders of magnitude (≥300 h) seems to be very unlikely. That is
why we do not include results for 270Db in gure8 and below
until nishing detailed analysis of the data ([36] in [74]).
4.3. Attempts to produce Z = 119 and 120 nuclei
By now, ten reactions of actinide target nuclei 233,238U, 237Np,
242,244Pu, 243Am, 245,248Cm, 249Bk and 249Cf with 48Ca were
used for investigation of the region of SHN. In the 233U + 48Ca
reaction no decay chains were observed with an upper cross
sectionlimit of 0.6 pb [44]. Nine other reactions resulted in
synthesis of 25 even-Z and 29 odd-Z nuclei.
The decay properties of these nuclei revealed a signicant
increase in their stability as they approached the predicted
neutron shell N = 184. The nuclides with the largest neu-
tron and proton numbers that were synthesized in reactions
with the heaviest target nuclei 248Cm, 249Bk and 249Cf that
are 293Lv (N = 177), 294117 (N = 177) and 294118 (N = 176),
respectively, still possess 7–8 fewer neutrons than the pre-
dicted magic number N = 184. One can expect that increasing
the number of neutrons in these nuclei would result in fur-
ther increase of their stability. Unfortunately, nuclides with
Z> 98 do not exist in quantities sufcient for producing tar-
gets for these types of experiments. Therefore, isotopes with
larger neutron excess can be reached only if they are formed
as heavier ERs. For this purpose, one needs to use complete-
fusion reactions with projectiles heavier than 48Ca. One
should also note that increasing the atomic number brings us
closer to the closed proton shell at Z = 120–126 predicted by
some microscopic models, which could also increase shell
effects. However, most calculations predict much lower cross
sectionsfor complete-fusion reactions with projectiles heav-
ier than 48Ca [59, 61, 62, 7983].
In 2007 the 244Pu(58Fe, xn)302−x120 reaction was studied at
the DGFRS [84]. No decay chains consistent with fusion-evap-
oration reaction products were observed during an irradiation
with a beam dose of 0.7 × 1019 330 MeV 58Fe projectiles. The
sensitivity of the experiment corresponds to a cross sectionof
0.4 pb for the detecting a single decay; the upper cross-section
limit was set at 1.1 pb. In 2007–2008 a more symmetric reaction
238U + 64Ni leading to the same CN was used at the SHIP [85].
Rep. Prog. Phys. 78 (2 015) 036301
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Table 1. Decay properties of nuclei.
Z N A No. observed a Decay mode, branch b Half-life c E α (MeV) Q αexp (MeV) Qαth (MeV)
118 176 294 4 (4/4) α0.69
0.64ms 11.66 ± 0.06 11.82 ± 0.06 12.11
117 177 294 5 (5/5) α51
38ms 10.81–11.07 11.18 ± 0.04 11.43
176 293 15 (15/15) α22
ms 10.60–11.20 11.32 ± 0.05 11.53
116 177 293 5 (5/5) α57
ms 10.56 ± 0.02 10.71 ± 0.02 11.09
176 292 9 (8/7) α13
ms 10.63 ± 0.02 10.78 ± 0.02 11.06
175 291 4 (4/4) α19
ms 10.74 ± 0.07 10.89 ± 0.07 10.91
10.50 ± 0.02
174 290 11 (11/11) α8.3
ms 10.85 ± 0.07 11.00 ± 0.07 11.08
115 175 290 6 (5/6) α650
ms 9.78–10.31 10.41 ± 0.04 10.65
174 289 16 (15/16) α330
120ms 10.15–10.54 10.49 ± 0.05 10.74
173 288 46 (41/46) α164
30ms 10.29–10.58 10.63 ± 0.01 10.95
172 287 3 (3/3) α37
44ms 10.61 ± 0.05 10.76 ± 0.05 11.21
114 175 289 16 (14/16) α1.9
s9.84 ± 0.02 9.98 ± 0.02 10.04
9.48 ± 0.08
174 288 35 (31/30) α0.66
0.14s9.93 ± 0.03 10.07 ± 0.03 10.32
173 287 19 (18/17) α0.48
s10.03 ± 0.02 10.17 ± 0.02 10.56
172 286 27 (22/14) α:0.6 SF:0.4 0.12
s10.21 ± 0.04 10.35 ± 0.04 10.86
171 285 1 (1/0) α0.13
113 173 286 6 (5/6) α9.5
s9.61–9.75 9.79 ± 0.05 9.98
172 285 17 (17/17) α4.2
s9.47–10.18 10.01 ± 0.04 10.21
171 284 47 (39/47) α0.91
s9.10–10.11 10.12 ± 0.01 10.68
170 283 2 (2/2) α75
ms 10.23 ± 0.01 10.38 ± 0.01 11.12
169 282 2(2/2) α73
ms 10.63 ± 0.08 10.78 ± 0.08 11.47
112 173 285 17 (16/16) α28
9s9.19 ± 0.02 9.32 ± 0.02 9.49
172 284 37 (34/-) SF 98
ms 9.76
171 283 33 (23/31) α:1. SF:0.1 4.2
s9.53 ± 0.02 9.66 ± 0.02 10.16
9.33 ± 0.06
8.94 ± 0.07
170 282 14 (14/-) SF 0.91
0.33ms 10.68
169 281 1 (1/1) α0.10
s10.31 ± 0.04 10.46 ± 0.04 11.21
111 171 282 6 (6/6) α100
s8.86–9.05 9.16 ± 0.03 9.85
170 281 20 (17/2) α:0.1 SF:0.9 17
s9.28 ± 0.05 9.41 ± 0.05 10.48
169 280 45 (41/42) α4.6
s9.09–9.92 9.91 ± 0.01 10.77
168 279 3 (2/2) α90
ms 10.38 ± 0.16 10.53 ± 0.16 11.08
167 278 2 (2/2) α4.2
7.5ms 10.69 ± 0.08 10.85 ± 0.08 11.30
110 171 281 17 (17/1) α:0.07 SF:0.93 12.7
4.0s8.73 ± 0.03 8.85 ± 0.03 9.30
169 279 36 (31/4) α:0.1 SF:0.9 0.21
0.04s9.71 ± 0.02 9.85 ± 0.02 10.24
167 277 1 (1/1) α0.006
s10.57 ± 0.04 10.72 ± 0.04 10.79
109 169 278 5 (5/5) α4.5
s9.38–9.55 9.58 ± 0.03 9.55
168 277 2 (2/-) SF 5
ms 9.84
167 276 43 (43/41) α0.45
s9.17–10.01 10.03 ± 0.01 10.09
166 275 3 (3/3) α20
ms 10.33 ± 0.01 10.48 ± 0.01 10.34
165 274 2 (2/2) α440
ms 10.0 ± 1.1 10.2 ± 1.1 10.63
9.76 ± 0.10
108 169 277 1 (1/1) SF 3
ms 9.03
167 275 4 (4/4) α0.20
s9.31 ± 0.02 9.45 ± 0.02 9.41
165 273 1 (1/1) α0.2
1.2s9.59 ± 0.04 9.73 ± 0.04 9.78
(Continued )
Rep. Prog. Phys. 78 (2 015) 036301
Review Article
No events originating from isotopes of element 120 were
observed. Measured upper cross-section limit of 0.09 pb was
set at a mean excitation energy of 36.4 MeV and beam dose
of 2.1  × 1019. A cross-section limit of 0.56 pb was set in the
248Cm + 54Cr reaction at the SHIP in 2011 [86].
Two experiments with 50Ti beam and target nuclei 249Cf
and 249Bk were performed at the TASCA in 2011 and 2012.
These were aimed at the synthesis of elements 120 and 119,
respectively [87] (The results have not yet been published).
5. Discussion
5.1. Production cross sections
The excitation functions σxn(E*) of the reactions leading to
production of isotopes of elements 112–118 shown in g-
ures7 and 9 are typical for the process of de-excitation of
CN via evaporation of neutrons. The maximum cross-sec-
tion values correspond to evaporation of three to four neu-
trons depending on neutron excess in the CN. In comparison
with these reaction channels, the evaporation of two neu-
trons observed in the reactions with 242Pu, 243Am and 245Cm
is suppressed by shift of maximum of cross section σ2n
below the Coulomb barrier (E* < Emin). One should note
that different orientations of the deformed target nucleus at
the touching point with spherical nucleus 48Ca change loca-
tion and shape of the Coulomb barrier with regard to the
Bass barrier calculated for spherical nuclei. As it follows
from measured cross sections σxn(E*) at the lowest exci-
tation energies E*, in the reactions Act + 48Ca the CN is
formed mainly in ‘equatorial’ collisions, characterized by
minimum distances between the centers of the interacting
nuclei, which leads to shift of the cross-section maxima by
several MeV to higher values than the corresponding Bass
barriers (see gures7, 9 [5659]).
With movement of the Coulomb barrier to higher energies
E*>50 MeV, one can expect a decrease of σxn(E*) because
of ascending competition from ssion of CN. Indeed, in the
reactions of 48Ca with 244Pu and 242Pu at E* = 53 and 50 MeV,
respectively (see gure 7), the only single decay chains of
287Fl and 285Fl, products of the 5n-evaporation channel, could
be observed. Thus, all the ERs in the Act + 48Ca reactions
were registered within relatively narrow excitation-energy
interval 29≤E*≤53 MeV.
In this energy range, the total fusion-evaporation cross
sectionsσtot =
σxn(E*) reach maxima at E*≈ 40 MeV (hot
fusion); transition of nucleus to the ground state occurs via
evaporation of 3–4 neutrons. This energy is about three times
larger than excitation energy of nuclei at the cross-section
maxima in the cold-fusion reactions used for the synthe-
sis of elements 110–113. The maximum total ER cross sec-
tionsmeasured in the experiments on the synthesis of elements
with Z≥102 in cold fusion and in 48Ca-induced reactions are
shown in gure10.
In cold-fusion reactions, as it follows from the gure10(a),
the production cross sectiondrops by about 8 orders of magni-
tude when ZCN increases from 102 to 113. Such an effect, as a
result of growth of the Coulomb factor k = Z1·Z2 / (A11/3 + A21/3)
by 44% (gure 10(c)), is associated with potential energy sur-
face of the colliding system which causes the hindrance of the
formation of CN with stronger Coulomb interaction.
One can note that in theory, the dynamics of the CN forma-
tion in competition with the quasi-ssion process is the most
vague reaction stage. For calculation of the probability Pfus,
different models are used which suppose different mecha-
nisms of CN formation. In the multidimensional Langevin-
type dynamical approach [5659, 79], it is assumed that two
touching nuclei lose their ‘individualities’ at the touching
point and one strongly deformed heavy nucleus with summary
mass evaluates in the multidimensional space of deformations
107 167 274 6 (5/6) α44
s8.73–8.84 8.94 ± 0.03 8.83
165 272 44 (39/44) α10.9
s8.55–9.15 9.18 ± 0.01 9.08
164 271 2 (2/2) α1.5
s9.28 ± 0.07 9.42 ± 0.07 9.07
163 270 1 (1/1) α61
s8.93 ± 0.08 9.06 ± 0.08 8.63
106 165 271 4 (4/2) α:0.6 SF:0.4 1.6
min 8.54 ± 0.08 8.67 ± 0.08 8.71
163 269 1 (1/1) α2
10min 8.57 ± 0.10 8.70 ± 0.10 8.32
105 165 270 6 (6/-) SF e) 15
163 268 66 (66/-) SF e) 26
4h d) 7.80
162 267 3 (3/-) SF e) 1.3
161 266 1 (1/-) SF e) 22
min 7.52
104 163 267 2 (2/-) SF 1.3
161 265 1 (1/-) SF 2
min 7.27
a Number of observed decays and number of events used for calculations of half-lives / α-particle energies, respectively.
b Branching ratio is not shown if only one decay mode was observed.
c Error bars correspond to 68%-condence level.
d The value obtained combining the results of physical and chemical experiments.
e The SF mode was observed but EC/β+ or α decay is not excluded.
Table 1. (Continued)
Z N A No. observed a Decay mode, branch b Half-life c E α (MeV) Q αexp (MeV) Qαth (MeV)
Rep. Prog. Phys. 78 (2 015) 036301
Review Article
into a spherical CN or goes into ssion channels. A somewhat
different scenario of this process is assumed in the ‘dinuclear
system model’ [81, 82]. The dinuclear system stays in con-
tact conguration and undergoes successive transfer of all
nucleons from the lighter nucleus to the heavier partner in
competition with the quasi-ssion processes. Here two touch-
ing nuclei keep their relative distance and their ‘individual-
ity’. In still another approach, the extended versions of the
‘fusion-by-diffusion’ model [61, 83], the stochastic process of
shape uctuations leads to the overcoming of the saddle point.
Several analytical formulas for description of the fusion prob-
ability are proposed [59, 62, 80]. This reaction stage plays a
decisive role in cold-fusion reaction.
On the contrary, in more asymmetric reactions of 48Ca with
actinide nuclei the Coulomb repulsion is less. For super-heavy
nuclei with Z = 112–118 the Coulomb factor changes by 6.5%
only. Because the initial states of the CN ZCN = 112–118 are
similar, this allows a uniform description of their transition to
ground state via emission of neutrons and γ-rays. The calcu-
lated survivability of the CN, which depends on the thermo-
dynamic characteristics of the heated nuclei in the course of
their cooling down via emission of neutron(s) and on ssion
barriers, should correlate with ER cross sectionsas obtained
in the experiment.
In gure 10(b), the total cross section σtot =
measured in the experiments in all the reactions of fusion
of 48Ca with the target nuclei of Pb, Ra and actinide targets
U-Cf are shown. The calculated values of (Bf–Bn) are shown
in gure10(d).
Comparing these, one can see that the relatively high cross
sections for production of ERs in hot fusion reactions with
48Ca are connected with high survivability of the heated CN.
This provides direct evidence of the presence of the high s-
sion barriers in nuclei with Z < 120 which appears due to
nuclear shell effects.
5.2. Decay properties
5.2.1. Alpha decay. The energies of α particles for even-Z
nuclei 281Cn, 294118, 291Lv, 292Lv, 293Lv and their descendants
registered by the focal-plane detector only or together with
the side one at the DGFRS [3945] are shown in gure11.
Here we also show results of experiments carried out with the
use of other facilities (see gure 6): chemical setup IVO +
COLD [4649] and separators SHIP [50, 51], BGS [52, 53]
and TASCA [32, 54].
In agreement with expectations, the α decays of the even–
even nuclei 294118, 290,292Lv and 286,288Fl are terminated by SF
Figure 10. Maximum cross sectionsof the production of the isotopes of the heavy elements in (a) cold fusion reactions: 208Pb, 209Bi
+ 48Ca, 50Ti, 54Cr, ...70Zn (E* = 12–20 MeV) and (b) hot fusion reactions: 208Pb, 226Ra, 233,238U, 242,244Pu, 243Am, 245,248Cm, 249Bk and
249Cf + 48Ca (E* = 35–40 MeV) (data from DGFRS and other separators are in blue and red, respectively). In plot (c), Coulomb factors
Z1·Z2 / (A11/3 + A21/3) for nuclei in the cold (open circle) and 48Ca-induced (closed circle) reactions are shown. (d) Difference of ssion
barrier heights (involving non-axial shapes) and neutron binding energies of the CN in 48Ca-induced reactions calculated in macroscopic–
microscopic nuclear model [8890] and corrected for the odd-even effect are shown. Arrows show number of neutrons in the CN with the
given atomic number.
Rep. Prog. Phys. 78 (2 015) 036301
Review Article
of short-lived isotopes 282,284Cn (see gures6, 11). The isotope
286Fl has an SF branch of 40
% (68% condence level).
The α-particle energy spectra of these nuclei are characterized
mainly by one α line, which corresponds to the ground-state
to ground-state transitions as the most probable for even–even
nuclei. In gure11 the spectra of events registered solely by
the focal-plane detectors with energy resolution better than
0.1 MeV are shown by green histograms.
The decay chains originated from the even–odd nuclei
291,293Lv are terminated mainly at later stage, by SF of
279,281Ds which have small α-decay branch of 11
6 % and
%, respectively. Two SF events of four observed in the
238U + 48Ca reaction at the SHIP [50] were assigned to 50%
SF branch of 283Cn that seemingly does not contradict results
observed at the DGFRS in the same reaction [44]. Here, the
three decay chains of seven were detected as ER-SF sequences
with presumably missing α particles of 283Cn. However, when
283Cn is produced after α decay of the parent nucleus 287Fl
[4245, 52], only the upper limit of 7% for the SF branch of
283Cn can be derived. It seems that the population of isomeric
and ground states, even with comparable lifetimes, in direct
reaction and after α decay cannot be excluded (compare with
261Rf ( [91] and references therein). Further α decay of the
even–odd nuclei leads to SF of 265,267Rf and 277Hs (SF branch
for 271Sg is not excluded with probability of 40%).
The α-particle energy spectra of the even–odd isotopes are
broader. The complex spectra are clearly seen for 283Cn, 289Fl
and 291Lv. In addition to the main α line at Eα = 9.53 MeV, two
lines with lower energies of 9.33 and 8.94 MeV were regis-
tered for 283Cn [4247] whereas a spectrum of 287Fl is consist-
ent with a single α transition. The hindrance factors (HF) of
, 0.3
and 2.9
, respectively, for three energies
of 283Cn can be estimated as a ratio of partial half-lives which
are determined by numbers of observed events and half-lives
expected for Z = 112 nucleus with measured α-particle ener-
gies from, e.g. semi-empirical systematics (Viola–Seaborg
formula [92]). Such data could be decisive for predictions of
the low-lying quasi-particle states of nuclei. Predominantly
single-line spectrum is seen for 293Lv but two different ener-
gies were observed for 289Fl with Eα = 9.48 and 9.84 MeV.
Both these α decays are unhindered (HF = 1.1
and 1.1
The energy spectra of α particles for odd-Z nuclei 282113,
287,288115, 293,294117 and their descendants registered at the
DGFRS [26, 27, 34, 36, 6670] and TASCA [73, 74] are shown
in gure12. Even in cases with relatively low statistics, one can
see wider energy distributions for these nuclei than those shown
in gure11. The α-decay chains of four parent nuclei end by SF
of isotopes 266,267,268,270Db. Only odd–even 281Rg undergoes SF
with small branch for α decay (bα = 12
%) which is followed
by SF of 277Mt (see gure8).
The decay properties of nuclei produced in the fusion-
evaporation reactions with 48Ca are shown in gures 6 and
8 and table1. The α-decay energies of nuclei Qαth calculated
within macroscopic–microscopic model [88, 89] are given in
column 9. For even–even nuclei with Z = 114–118 and N =
172–176 the values ΔQα = QαexpQαth amount to from−0.5
to−0.1 MeV. Measured in experiments α-decay energies Qαexp
are lower and correspondingly, the half-lives Tαexp of super-
heavy nuclei are larger than theoretical values. For even-Z
isotopes of elements 106–116 with odd number of neutrons
N = 165–177 the difference ΔQα varies within−0.8–0.4 MeV.
Finally, for odd-Z nuclei of elements 107–117 the discrepan-
cies are ΔQα = −1.1–0.4 MeV in assumption that the meas-
ured maximum α-particle energy corresponds to transitions
through the ground states of odd-Z nuclei which is not always
Figure 11. α-particle energy spectra for even-Z nuclei registered by the focal-plane detector only or together with the side one at the
DGFRS [3945], IVO + COLD [4649], SHIP [50, 51], BGS [52, 53] and TASCA [32, 54]. Note, the energy resolution of α particles ΔEα
detected simultaneously by the focal-plane and side detectors was up to 0.2 MeV (spectra for the events with energy resolution better than
0.1 MeV shown in green). The data from the IVO + COLD are included if ΔEα are published.
Rep. Prog. Phys. 78 (2 015) 036301
Review Article
valid. From comparison of decay properties of 43 α-decaying
nuclei produced in the Act + 48Ca reactions, it follows that
their agreement with theoretical predictions is not only quali-
tative but, to some extent, quantitative. Here one should bear
in mind that accuracy of predictions of masses of the known
nuclei reaches 0.3–0.4 MeV [89] and 0.4–0.7 MeV for some
other approaches (see, e.g. review [22] and references therein).
Note, results of Qαth calculations within other MM models and
purely microscopic approaches (HFB, RMF) also agree with
experimental data ([22] and references therein). Therefore, the
hypothesis of existence of super-heavy nuclei received experi-
mental conrmation.
5.2.2. Spontaneous ssion. For 16 of the 54 synthesized
nuclei SF was observed. For seven even-Z nuclei SF is the
predominant mode of decay (see gure6 and table1). In ve
nuclei, SF competes with α decay. As to the odd-Z nuclei, SF
was registered for six nuclides (gure 8 and table1). For the
remaining nuclides SF was not observed. The partial SF half-
lives of even–even isotopes with N≥162, produced in fusion
reactions with 48Ca, together with the half-lives of SF nuclides
with N<162, are shown in gure13. Two isotopes of element
112 with N = 170 and 172 are located in a region, where a
steep rise of TSF(N) is expected. Indeed, in the even–even iso-
topes 282Cn and 284Cn the difference in two neutrons increases
the partial half-life TSF by two orders of magnitude4. A similar
effect is also observed for the even–even isotopes of element
114. Addition of two neutrons to the nucleus 286Fl (TSF≈0.3 s)
leads to increase of the stability of 288Fl with respect to SF by
at least a factor of 15. For even–even isotopes 288Fl, 290Lv,
292Lv and 294118 SF was not observed.
It is signicant that the rise of stability with respect to SF is
observed for the heavy nuclei with Z≥110, which are 10–12
neutrons farther from the closed neutron shell N = 184. On
moving to the nuclei with Z<110 and N<170 the probability
of SF decreases again when the closed deformed shell N =
162 is approached. The stabilizing effect of the N = 162 shell
manifests itself in the properties of the even–even isotopes of
Rf, Sg and Hs.
Because of the high hindrance of SF in the nuclei with odd
number of protons (and neutrons) and relatively low Tα, the
isotopes of elements 113 and 115 produced in the reactions
237Np + 48Ca and 243Am + 48Ca with N = 169–173 undergo
α decay [26, 27, 34, 36, 6670, 73]. Spontaneous ssion is
observed only at the end of chain for isotopes of element 105
(or their EC/α-decay products). For 268Db α decay seems to be
less probable (see above).
In case of the reaction 249Bk(48Ca,3-4n)294,293117, the
daughter nuclei have one or two extra neutrons compared to
the reaction with 243Am. In analogy with the neighbouring
even-Z isotopes all the nuclei in the decay chains of 293117
Figure 13. Common logarithm of partial spontaneous ssion half-
life versus. neutron number for even–even isotopes of elements
with Z = 98–114 [32, 4245, 49, 5154, 93]. Dashed lines show the
theoretical TSF values [7, 8] for even–even Z = 108 (in magenta),
110 (in violet) and 112 (in blue) isotopes.
145 150 155 160 165 170 175 180 185
Log T(s)
Neutron number
N=1 26
N=1 25 N=184
Z=1 21
4From measured SF half-lives of the even–even isotopes 282Cn and 284Cn
it follows that the odd neutron in the 283Cn nucleus imposes a hindrance to
spontaneous ssion exceeding 5000.
Figure 12. α-particle energy spectra for odd-Z nuclei registered by the focal-plane detector only or together with the side one at the
DGFRS [26, 27, 34, 36, 6670] and TASCA [73, 74].
Counts / 20 keV
Energy (MeV)
28211328311 3
910119 10 11
Rep. Prog. Phys. 78 (2 015) 036301
Review Article
and 294117 with Z > 111 and N ≥ 172 will undergo α decay.
The nucleus 281Rg (N = 170) belonging to the ‘critical’ region
between neutron shells N = 162 and 184 might avoid SF due to
the hindrance resulting from an odd proton. The hindrance of
the SF in 281Rg with respect to its even–even neighbuor 282Cn
[4245, 52, 53] is ~ 2 × 104. Despite this, the isotope 281Rg
undergoes SF with a probability of about 90%. Accordingly,
even the high hindrance governed by oddness does not ‘save’
the odd nucleus (281Rg, 277Mt) from SF which is caused by the
weakening of the stabilizing effect of the neutron shells at N
= 162 and N = 184. However an extra neutron and the double
effect of oddness favor the α decay in the neighboring isotopes
280Rg and 282Rg.
Practically for none of the odd-Z nuclei the branch for
EC/β+ was observed. For example, isotope 288Fl, EC prod-
uct of 288115 and its descendants were not seen in the 243Am
+ 48Ca reaction, which indicates that for nuclei in the decay
chain of 288115 the probability for EC/β+ is less than 2%.
Meanwhile, nuclei at the end of decay chains, neutron-rich
odd–odd isotopes 266,268,270Db, could undergo EC decay lead-
ing to SF of even–even Rf isotopes for which TSF = 23 s, 1.4 s
and 20 ms are predicted [7]. For odd–even isotopes 267Db as
well as 281Rg values of the observed half-lives are somewhat
lower than the bulk of data in the systematics of Tβ versus
Qβ for known isotopes of Np-Db (see, e.g. gure7 in [27]).
This could indicate a larger probability of undergoing SF than
EC/β+ decay leading to 1 h 267Rf and 13 s 281Ds.
Spontaneously ssioning nuclei with Z = 108–112 and N =
168–170 located between N = 162 and N = 184 neutron shells
have the lowest TSF values. The α-decay chains of the heaviest
nuclei both with even and odd Z, N numbers are terminated
here by SF. The decay chains of nuclei outside this region end
by SF of neutron-rich isotopes of Rf and Db located closer
to the neutron shell at N = 162. Thus, the decay chains of
the heaviest nuclei synthesized in the Act + 48Ca reactions are
not connected with a region of known nuclei. They form an
isolated region of the heaviest nuclei—some ‘island of super-
heavy nuclei’. The existence of this island and relatively high
stability of SHN are determined in full by the new closed
shells at N = 184 and Z = 114 (possibly 120–126).
Investigation of SF of SHN is of independent interest.
Extensive information about the SF process, obtained for
nuclei in the region of actinides (especially for 252Cf), can be
essentially extended for heavier and super-heavy nuclei whose
ssion barrier is entirely determined by the inuence of the
new nuclear shells.
6. Perspectives
For the synthesis of super-heavy nuclei with the largest neu-
tron excess N = 175–177, the heavy isotopes 244Pu, 248Cm,
249Bk and 249Cf produced at high ux reactors were used as
target nuclei. Experiments aimed at the synthesis of the new
isotopes of element 118 in the 249–251Cf + 48Ca reactions are
being considered at FLNR. As a target material, there will
be used the long-lived isotopes of Cf extracted from ‘old’
252Cf neutron sources which were made 30–40 years ago at
the HFIR (ORNL). With such a target of mixed isotopes of
californium, in the 3n- and 4n-evaporation channels, the three
new isotopes - 293118, 295118 and 296118—could be produced.
Note that 251Cf is the heaviest nuclide that can be produced at
the HFIR in required amounts. For the synthesis of elements
with Z>118, projectiles of heavier than 48Ca have to be used.
Another direction is to recede from the shell at N = 184
and investigate SHN with lower neutron excess that could
result in connection of region of known nuclei and ‘island of
SHN’. For these purposes, light isotopes of actinides, such as
233–236U, 239,240Pu or 241Am, can be used in the reactions with
48Ca which leads to isotopes of Z = 112–115 elements with N
= 165–172. However, decit of neutrons—shift to the edge
of island of stability and withdrawal from the shell at N =
184—can result in reduce of production cross sectionscaused
by decrease of ssion barriers and correspondingly, surviv-
ability of CN. This effect was already revealed in the experi-
ments on the synthesis of light isotopes of elements 112–114
in the 233U, 237Np and 239Pu + 48Ca reactions [44, 68]. Further
attempts in this direction, with the existing sensitivity of
experiment, does not seem a trivial task. At the same time,
discovery of SHN raises many questions concerning the valid-
ity of the application of the nuclear shell model and the limits
of existence of the heaviest nuclei, about the sizes and nuclear
density of the SHN, atomic structure and chemical proper-
ties of super-heavy elements. Still an open question is: can the
SHN be produced in nucleosynthesis?
Obviously, further investigations of the heaviest nuclei and
synthesis of the new elements call for considerable increase
in the sensitivity of equipment used in experiments. Our
knowledge of the decay properties of SHN and their produc-
tion cross sections in the 48Ca-induced reactions as well as
the latest achievements in accelerator and plasma physics,
reactor technologies and experimental techniques form the
basis for creation of the new laboratory—type of a factory for
continuous production of super-heavy nuclei (SHE Factory).
Expected yield of SHN can be increased by almost two orders
of magnitude compared with the existing level. Since the
beginning of 2013, at FLNR (JINR) the construction of the
SHE Factory is under way.
It is our pleasure to convey gratitude to all 60 co-authors of
the original publications for their hard work and active par-
ticipation in the experimental programme on the synthesis and
study of the properties of super-heavy elements. Presented
results from the DGFRS were obtained owing to common
efforts of many our colleagues over fteen years. Not all of
them are co-authors of the papers. We are grateful to them for
their important contributions.
The investigations reported in this review involve numerous
experiments performed in Dubna in 1999–2014. This was pre-
ceded by the 5 year work aimed at increase of the 48Ca beam,
further development of methods of preparation of actinide tar-
gets, modernization of the DGFRS and its detection system
for registration of rare events of production and decay of SHN,
Rep. Prog. Phys. 78 (2 015) 036301
Review Article
etc. The experiments were carried out in collaboration with
groups from US National Laboratories in Livermore and Oak
Ridge, RIAR in Dimitrovgrad (Russia), Vanderbilt University
in Nashville and University of Tennessee in Knoxville (USA),
PSI in Villigen (Switzerland), RIEPh in Sarov (Russia). The
investigation of SHN is one of the high-priority directions in
the scientic programme of the JINR. During all stages of
the experiments, we received support from the Committee of
Plenipotentiaries of the Governments of the JINR Member
States, JINR Directorate and Scientic Council.
These studies were performed with the support of the Russian
Ministry of Atomic Energy, of the Governor of the Moscow
region and permanently of the Russian Foundation for Basic
Research, including recent Grants No. 13-02-12052 and No.
13-03-12205. Research at ORNL was supported by the U
S DOE Ofce of Nuclear Physics under DOE Contract No.
DE-AC05-00OR22725 with UTBattelle, LLC. Research at
LLNL was supported by LDRD Program Project No. 08-ERD-
030, under DOE Contract No. DE-AC52-07NA27344 with
Lawrence Livermore National Security, LLC. This work was
also supported by the U S DOE through Grant No. DE-FG-
05-88ER40407 (Vanderbilt University). These studies were
performed in the framework of the Russian Federation/U S
Joint Coordinating Committee for Research on Fundamental
Properties of Matter.
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