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Synthesis of a New Element with Atomic Number Z=117



The discovery of a new chemical element with atomic number Z=117 is reported. The isotopes (293)117 and (294)117 were produced in fusion reactions between (48)Ca and (249)Bk. Decay chains involving 11 new nuclei were identified by means of the Dubna gas-filled recoil separator. The measured decay properties show a strong rise of stability for heavier isotopes with Z > or = 111, validating the concept of the long sought island of enhanced stability for superheavy nuclei.
Synthesis of a New Element with Atomic Number Z¼117
Yu. Ts. Oganessian,
*F. Sh. Abdullin,
P. D. Bailey,
D. E. Benker,
M. E. Bennett,
S. N. Dmitriev,
J. G. Ezold,
J. H. Hamilton,
R. A. Henderson,
M. G. Itkis,
Yu. V. Lobanov,
A. N. Mezentsev,
K. J. Moody,
S. L. Nelson,
A. N. Polyakov,
C. E. Porter,
A. V. Ramayya,
F. D. Riley,
J. B. Roberto,
M. A. Ryabinin,
K. P. Rykaczewski,
R. N. Sagaidak,
D. A. Shaughnessy,
I. V. Shirokovsky,
M. A. Stoyer,
V. G. Subbotin,
R. Sudowe,
A. M. Sukhov,
Yu. S. Tsyganov,
V. K. Utyonkov,
A. A. Voinov,
G. K. Vostokin,
and P. A. Wilk
Joint Institute for Nuclear Research, RU-141980 Dubna, Russian Federation
Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
University of Nevada Las Vegas, Las Vegas, Nevada 89154, USA
Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USA
Lawrence Livemore National Laboratory, Livermore, California 94551, USA
Research Institute of Atomic Reactors, RU-433510 Dimitrovgrad, Russian Federation
(Received 15 March 2010; published 9 April 2010)
The discovery of a new chemical element with atomic number Z¼117 is reported. The isotopes 293117
and 294117 were produced in fusion reactions between 48 Ca and 249Bk. Decay chains involving 11 new
nuclei were identified by means of the Dubna gas-filled recoil separator. The measured decay properties
show a strong rise of stability for heavier isotopes with Z111, validating the concept of the long sought
island of enhanced stability for superheavy nuclei.
DOI: 10.1103/PhysRevLett.104.142502 PACS numbers: 27.90.+b, 23.60.+e, 24.60.Dr, 25.70.Jj
The existence of the heaviest atomic nuclei bound
against immediate disintegration depends on the detailed
properties of proton and neutron quantum states; see, e.g.,
[1,2] and references therein. Therefore, studies aimed at
the identification of new superheavy elements contribute to
the fundamental knowledge of nuclear potentials and the
resulting nuclear structure. The concept of an ‘‘island of
stability’’ existing near the next spherical doubly magic
nucleus heavier than 208Pb arises in every advanced model
of nuclear structure. Reactions involving doubly magic
208Pb and singly magic 209 Bi target nuclei and stable
neutron-rich projectiles as heavy as 64Ni or 70 Zn have
been used for the synthesis of new heavy elements.
These reactions, termed cold fusion, led to the observation
of isotopes with Z113 and N165 [3,4], stabilized by
the Z¼108 and N¼162 shell gaps occurring for de-
formed shapes. The dramatic drop of the production cross
section with increasing Zpractically excludes the continu-
ation of such experiments for heavier elements.
A new method of synthesizing superheavy elements,
with Z112 and neutron numbers closer to the predicted
spherical shell closure at N¼184, was pioneered at the
Flerov Laboratory of Nuclear Reactions (FLNR) of Joint
Institute for Nuclear Research (JINR) about a decade ago.
Four new isotopes of element Z¼112 and 14 new iso-
topes of new elements with Z¼113116 and 118 were
identified [1] among the products of heavy-ion fusion
reactions employing doubly magic 48Ca projectiles and
actinide radioactive targets of U-Cm and Cf, respectively.
The sequential decays of the heaviest even-Znuclei were
found to be terminated by spontaneous fission (SF) of the
descendant even-even or even-odd nuclei with Z¼114,
112, or 110 (TTSF), with total decay times in the range
of about 0.1 s to 1 min depending on neutron number [1].
The probabilities of formation and the decay properties of
these 18 new nuclei provide evidence of a considerable
increase in nuclear stability with increasing neutron num-
ber in the nucleus. The production cross sections, the iden-
tification, as well as the decay properties of the Z¼112
and Z¼114 isotopes obtained at Dubna [1] were recently
confirmed in several independent experiments [58].
We present here the experimental evidence for synthesis
of a new chemical element with Z¼117; see Fig. 1. The
identified 293117 and 294 117 isotopes were produced in the
fusion reaction between 48Ca projectiles and radioactive
249Bk target nuclei followed by the emission of four and
three neutrons, respectively. The decay properties of the
resulting 11 new neutron-rich nuclides offer additional
experimental support for the nuclear shell model predicting
the existence of the island of stability for heaviest nuclei.
The 249Bk was produced at Oak Ridge National Laboratory
(ORNL) through intense neutron irradiation of Cm and Am
targets for approximately 250 d in the High Flux Isotope
Reactor. The Bk chemical fraction, separated and purified
at the Radiochemical Engineering Development Center at
ORNL, contained 22.2 mg of 249Bk, only 1.7 ng of 252Cf,
and no other detectable impurities. Six arc-shaped targets,
each with an area of 6:0cm
2, were made at the Research
Institute of Atomic Reactors (Dimitrovgrad, RF) by depos-
iting BkO2onto 0:74 mg=cm2Ti foils to a thickness of
0:31 mg=cm2of 249Bk. The targets were mounted on the
perimeter of a disk that was rotated at 1700 rpm perpen-
dicular to the beam direction. The experiments were per-
formed employing the Dubna gas-filled recoil separator
[10] and the heavy-ion cyclotron U-400 at JINR. A detailed
description of experiment will be given in a forthcoming
PRL 104, 142502 (2010)
Selected for a Viewpoint in Physics
9 APRIL 2010
0031-9007=10=104(14)=142502(4) 142502-1 Ó2010 The American Physical Society
paper [11]; here we present the basic features. Evaporation
residues (ER) passing through the separator with an overall
transmission about 35% were registered by a time-of-flight
system with a detection efficiency of 99.9%, and were
implanted in a 4cm12 cm Si-detector array with 12
vertical position-sensitive strips surrounded by eight
4cm4cmside detectors. The position-averaged detec-
tion efficiency for particles emitted from implanted
nuclei was 87% of 4. The energy resolution for par-
ticles implanted in the focal-plane detector measured as a
full width at half maximum (FWHM) was 60–140 keV,
depending on the strip and the position within the strip.
Alpha-escape signals detected in the side detectors had an
energy resolution of 160–230 keV. If an particle was
detected only by a side detector (its position was lost), the
total energy was estimated as a sum of the energy measured
by the side detector and half of the threshold energy
(0:5 MeV), with the uncertainty in the total energy in-
creased to 0:4 MeV. The total kinetic energy (TKE)
released in the SF of nuclei with Z102 was determined
from the sum Etot þ23 MeV, where Etot is the observed
energy signal (with a systematic uncertainty of about
5 MeV when both fission fragments were detected) and
23 MeV is the correction related to the pulse height effect
and energy loss in the dead layer detector as determined
from a 252No measurement. The position resolution
(FWHM) of the strip detector in registering correlated
decay chains of the ER-1-2-3-SF type was 1:2mm.
In order to reduce the background rate in the detector, the
beam was switched off for at least 3 min after a recoil
signal was detected with parameters of implantation en-
ergy expected for Z¼117 ERs, followed by an -like
signal with an energy between 10.7 and 11.4 MeV, in the
same strip, within a 2.2 mm wide position window. For
the 48Ca projectiles at 252 MeV energy in the middle of
the 249Bk target, the excitation energy of the compound
nucleus 297117 is estimated to be E¼39 MeV, near the
expected maximum for the total ER cross section (sum of
3 n and 4 n evaporation channels [1]). The intensity of the
48Ca-ion beam was 71012 ions=sat the target.
Irradiation at this beam energy was performed for 70 d
between July 27 and October 23, 2009, with a total beam
dose of 2:41019. The beam was interrupted for a total
beam-off time of 79 h. The energy spectra of the -like
signals registered by the front detector during all 1680 h of
the irradiation and those registered only in the beam-off
intervals are presented in Fig. 2(a). The background in the
beam-off spectrum is due to the decay of Po isotopes that
are daughters of the heavier nuclei produced in transfer
reactions. We observed five position-correlated decay
chains in the 252-MeV 48Ca irradiation; in each case,
two or three decays were observed between the time of
arrival of the ER and the detection of SF (see Fig. 1for the
averaged decay properties assigned to 293117 isotope). All
FIG. 2 (color). Energy spectra recorded during the 252 MeV
48Ca þ249 Bk run (E¼39 MeV). (a) Total energy spectra of
beam-on -like signals and beam-off particles. (b) Total
fission-fragment energy spectra, both beam on and beam off.
The arrows show the energies of events observed in the corre-
lated decay chains; see Fig. 1.
FIG. 1 (color). Observed decay chains interpreted as originat-
ing from the isotopes A¼294 (single event) and A¼293
(average of five events) of the new element Z¼117. The
deduced and predicted [9] lifetimes (¼T1=2=ln2) and
-particle energies are shown in black and blue, respectively.
PRL 104, 142502 (2010) PHYSICAL REVIEW LETTERS week ending
9 APRIL 2010
five of the first particles emitted after the implantation of
the recoils have the same energies (within the energy
resolution of the focal-plane detector), yielding average
values for the energy E1¼11:03 0:08 MeV and
T1¼14ðþ11;4Þms. The energies of the particles
emitted by the daughter nuclei and detected in three out of
five chains were the same within the accuracy of the
measurements, resulting in E2¼10:31 0:09 MeV and
T2¼0:22ðþ0:26;0:08Þs. The third transition was
observed as having E3¼9:74 0:08 MeV and E3¼
9:48 0:11 MeV, and T3¼5:5ðþ5:0;1:8Þs. In all
five cases the decay chains ended with the spontaneous
fission with TSF ¼26ðþ25;8Þs.
At the E¼39 MeV excitation energy, the maximum
cross section is expected for the 4 n evaporation channel;
therefore we assign the observed decay chains as originat-
ing from the isotope 293117. This conclusion is supported
by the systematics of the cross sections xnðEÞmeasured
previously for production of isotopes of superheavy nuclei
with Z¼108, 112–116, and 118 in 48Ca-induced reactions
[1], by calculations made directly for the evaporation
residues of the reaction 249Bk þ48 Ca [1214] as well as
by the result of the 249Bk þ48 Ca experiment performed at
lower beam energy (see below). In the Eenergy range
between 8.8 and 11.3 MeV, where we expect particles of
the first five transitions 117 !115 !113 !111 !
109 !107, the counting rate was 0:17=s(with beam on)
and 103=s(beam off) for the whole area of the front
detector. Similar spectra of fission fragmentlike sig-
nals measured under the same conditions are shown in
Fig. 2(b). In the energy range ESF 135 MeV, the SF
counting rate in the front detector was 1:2104=s
(beam on) and 7105=s(beam off). We have calculated
the total numbers of random sequences [15] imitating each
of the observed five decay chains, by using extended
intervals of time (t5T1=2), -particle energy and po-
sition (both exceeding 4.7 standard deviations), to be 6
106,103,105,31011, and 31011 .
The experiment was continued at a 48Ca energy of
247 MeV for 70 d with a total beam dose of 21019.
The resulting excitation energy of the compound nucleus
297117 was about 35 MeV, favoring the 3 n reaction chan-
nel. A new decay chain was detected involving six con-
secutive decays and ending in SF; see Fig. 1. In this
chain, the great-granddaughter nucleus with Z¼111 did
not undergo SF, but instead emitted an particle with
E4¼9:00 MeV. It was followed by at least two more
transitions and then, after about 33 h, the fission event was
recorded. The total number for random sequences [15]
imitating the observed decay chain amounts to 6
1011. Therefore, we assign this chain to the decay of the
neighboring odd-odd nucleus 294117. Note that this decay
chain was registered when about 30% of 249Bk decayed to
249Cf. Attributing it to the decay of 294ð118Þnucleus is
unlikely due to the small production yield and significantly
different decay properties.
The decay properties of the neighboring isotopes 293117
and 294117, their daughters 289 115 and 290115, as well as
granddaughters 285113 and 286 113, do not display substan-
tial differences. These decay properties change signifi-
cantly for the great-granddaughter nuclei. Despite the
strong hindrance resulting in the relatively long half-life,
SF is a principal decay mode of the odd-even nucleus
281111 (see Fig. 1). On the other hand, the heavier isotope
282111 undergoes decay. The SF decay of 281111 can be
explained by comparing the results of the present experi-
ment with the properties of the neighboring even-Znuclei.
In the TSFðNÞsystematics, the decrease in the half-life with
increasing neutron number in the region of nuclei with
N>162 changes to a strong increase in stability as N
approaches the spherical shell at N¼184 [16].
Minimum values of TSF are characteristic of the transition
region N¼168170 where the effect of nuclear shells is at
a minimum. Indeed, the Z¼110 darmstadtium isotopes
with N¼169 and N¼171, as well as the Z¼112,N¼
170, and N¼172 copernicium isotopes, undergo SF
rather than decay [1]. For the odd-Znuclei produced
in the reactions 237Np þ48 Ca and 243Am þ48Ca, the high
hindrance of SF for nuclei with an odd number of protons
and the relatively low Tfor the isotopes of elements 113
and 115 with N¼169173 result in a preference for
decay [17,18]. Spontaneous fission is observed only in the
isotopes of element 105, where the decay half-life ex-
ceeds 105sfor 268Db. In the reaction 249 Bk þ48Ca, the
daughter nuclei that originate from the evaporation resi-
dues 293117 and 294 117 have one or two extra neutrons
compared with those produced in the lower-Zreactions. A
closer approach to the shell at N¼184 should result in a
decrease in their decay energy Qand an increase in T
with respect to the neighboring lighter isotopes at the same
Z. This regularity is clearly observed experimentally for all
the isotopes with Z111; see Fig. 3. In analogy with the
neighboring even-Zisotopes, all the nuclei in the decay
chains of 293117 and 294 117 with Z>111 and N172 are
expected to undergo decay. The nucleus 281 111 (N¼
170) lies in the ‘‘critical’’ region, and may avoid SF only
because of the hindrance resulting from an odd proton.
Despite a hindrance of 3104with respect to its even-
even neighbor 282112 [1], the isotope 281111 undergoes SF
with a probability bSF 83%. Accordingly, even the high
hindrance caused by the odd proton does not ‘‘save’’ the
nucleus from SF because of the weakening of the stabiliz-
ing effect of neutron shells N¼162 and N¼184. The
presence of an extra and unpaired neutron in the neighbor-
ing isotope 282111 further hinders SF relative to the
decay of this nucleus. In Figs. 3(a) and 3(b), the experi-
mental values of decay energies Qand half-lives Tare
presented for isotopes with Z¼111, 113, 115, and 117.
Increasing the neutron number in the heaviest nuclides
results in a decrease of Qand a considerable increase in
T. An especially strong growth of TðNÞis observed for
the isotopes of elements 111 and 113. Except for 281111, all
the nuclides presented in Fig. 3are emitters; for them T
PRL 104, 142502 (2010) PHYSICAL REVIEW LETTERS week ending
9 APRIL 2010
is smaller than TSF. This is another indication of the high
stability of the superheavy nuclei with respect to SF. From
the experimental and theoretical -particle energies given
in Fig. 1, it is obvious that for all the nuclei in the decay
chains of the isotopes of element 117, the macroscopic-
microscopic calculations of the masses of the superheavy
nuclei [9] are in a good agreement with our experiment.
The cross sections for producing the nuclei of element 117
in the reaction 249Bk þ48 Ca are ¼0:5ðþ1:1;0:4Þpb
and ¼1:3ðþ1:5;0:6Þpb at E¼35 MeV and E¼
39 MeV, respectively. These values are similar to the
results of previous experiments where cross sections for
the reactions of 233;238U,237 Np,242;244Pu,243Am,245;248 Cm,
and 249Cf targets with 48 Ca beams have been measured [1].
In conclusion, a new chemical element with atomic
number 117 has been synthesized in the fusion of 249Bk
and 48Ca. The data are consistent with the observation of
two isotopes of element 117, with atomic masses 293 and
294. These isotopes undergo decay with E¼11:03ð8Þ
and 10.81(10) MeV and half-lives 14ðþ11;4Þand
78ðþ370;36Þms, respectively, giving rise to sequential
-decay chains ending in spontaneous fission of 281Rg
(TSF 26 s) and 270Db (TSF 1d), respectively. The de-
cays of 11 identified isotopes substantially expand our
knowledge of the properties of odd-Znuclei in the region
of the most neutron-rich isotopes of elements 105–117.
These nuclei generally display a trend of increased stability
with larger neutron number N. The longer half-lives offer
the potential for investigation of the chemistry of super-
heavy elements and establishing their location in the peri-
odic table. The new isotopes, together with superheavy
nuclides previously synthesized in reactions with 48Ca,
present a consistent picture of nuclear properties in the
area of heaviest nuclei. They demonstrate the critical role
of nuclear shells and represent an experimental verification
for the existence of the predicted island of stability for
superheavy elements.
We are grateful to the JINR Directorate and the U-400
cyclotron and ion source crews for their continuous support
of the experiment. We acknowledge the support of the
Russian Federal Agency of Atomic Energy, RFBR Grants
No. 07-02-00029, No. 09-02-12060, and No. 09-03-12214,
and of the U.S. Department of Energy through Contracts
No. DE-AC05-00OR2272 (ORNL) and No. DE-AC52-
07NA27344 (LLNL), and Grants No. DE-FG-05-
88ER40407 (Vanderbilt University) and No. DE-FG07-
01AL67358 (UNLV). 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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FIG. 3 (color). (a) -decay energy and (b) half-lives versus
neutron number for the isotopes of elements with Z¼111117
(new results in red). All the nuclides with N>165 have been
produced in 48Ca induced reactions. Our T(exp) values are
given for the nuclei belonging to the 293 117 decay chain
(5 events). The limit for Tð281RgÞwas estimated from the
measured half-life and number of observed nuclei.
PRL 104, 142502 (2010) PHYSICAL REVIEW LETTERS week ending
9 APRIL 2010
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The fusion reaction of 48Ca projectiles with 238U target nuclei was studied at the velocity filter SHIP of GSI in Darmstadt. Two decay chains were measured, which fully confirm data that were previously assigned to the isotope 283112 in experiments at the Flerov Laboratory in Dubna. Two other events are consistent with a 50% spontaneous-fission (SF) branch of this isotope. The mean value obtained for the half-life of 283112 is (6.9 +6.9 -2.3 s, the α energy is (9.520±0.015) MeV, and the total kinetic energy (TKE) of SF is (238±14) MeV. The half-life of the α decay daughter nucleus 279Ds is (0.18 +0.32 -0.07 s, and the TKE of SF is (210 +32 -11 MeV. The cross-section deduced from all four events is (0.72 +0.58 -0.35 pb , measured at an excitation energy of 34.6MeV of the compound nucleus 286112.
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The nuclear shell model predicts that the next doubly magic shell closure beyond 208Pb is at a proton number between Z=114 and 126 and at a neutron number N=184. The outstanding aim of experimental investigations is the exploration of this region of spherical superheavy elements (SHE’s). This article describes the experimental methods that led to the identification of elements 107 to 112 at GSI, Darmstadt. Excitation functions were measured for the one-neutron evaporation channel of cold-fusion reactions using lead and bismuth targets. The maximum cross section was measured at beam energies well below a fusion barrier estimated in one dimension. These studies indicate that the transfer of nucleons is an important process for the initiation of fusion. The recent efforts at JINR, Dubna, to investigate the hot-fusion reaction for the production of SHE’s using actinide targets are also presented. First results were obtained on the synthesis of neutron-rich isotopes of elements 112 and 114. However, the most surprising result was achieved in 1999 at LBNL, Berkeley. In a study of the reaction 86Kr+208Pb→294118*, three decay chains were measured and assigned to the superheavy nucleus 293118. The decay data reveal that, for the heaviest elements, the dominant decay mode is alpha emission, not fission. The results are discussed in the framework of theoretical models. This article also presents plans for the further development of the experimental setup and the application of new techniques. At a higher sensitivity, the exploration of the region of spherical SHE’s now seems to be feasible, more than 30 years after its prediction.
Fusion reactions leading to the formation of superheavy element 117 are systematically analyzed. Among the reactions considered, the 250Bk(48Ca,4n)294117 reaction has the largest evaporation residue (ER) cross section of about 2 pb. However, this reaction is hard to realize experimentally because it is difficult to accumulate sufficient amount of target material due to the short lifetime of 250Bk nucleus. For the reaction 48Ca+249Bk, our estimation shows that the ER cross sections in 3n and 4n channels may be expected to be greater than 1 pb. Therefore, 48Ca and 249Bk should be the optimal projectile-target combination for synthesis of superheavy element 117 in practice. In addition, as a main result of systematic analysis, we find that the ER cross section exponentially depends on the mass difference (in unit of temperature) of fission and neutron emission saddle points. Therefore, it is of essential importance for the successful synthesis of superheavy nuclei to select the isotopic composition of projectile and/or target so as the mass difference of fission and neutron emission saddle points as large as possible. Entrance channel effects are examined by means of a comparison of the reactions 48Ca+245Bk, 50Ti+243Am, and 55Mn+238U leading to the same compound nucleus 293117. The ER cross sections of the reactions 50Ti+243Am and 55Mn+238U are much smaller than that of 48Ca+245Bk.
Nuclear reactions leading to the formation of new superheavy (SH) elements and isotopes are discussed in the paper. “Cold” and “hot” synthesis, fusion of fission fragments, transfer reactions, and reactions with radioactive ion beams are analyzed along with their abilities and limitations. If the possibility of increasing the beam intensity and the detection efficiency (by a total of one order of magnitude) is found, then several isotopes of new elements with Z=120-124 could be synthesized in fusion reactions of titanium, chromium, and iron beams with actinide targets. The use of light- and medium-mass neutron-rich radioactive beams may help us fill the gap between the SH nuclei produced in the hot fusion reactions and the mainland. In these reactions, we may really approach the “island of stability.” Such a possibility is also provided by the multinucleon transfer processes in low-energy damped collisions of heavy actinide nuclei. The production of SH elements in fusion reactions with accelerated fission fragments looks less encouraging.
After a brief introduction of the role of shell effects in determining the limiting nuclear masses, the experimental investigation of the decay properties of the heaviest nuclei is presented. For the production of superheavy nuclides fusion, reactions of heavy actinide nuclei with 48Ca-projectiles have been used. The properties of the new nuclei, the isotopes of elements 112–118, as well as of their decay products, together with the known data for the light isotopes with Z ≤ 113, give evidence of the significant increase of the stability with the neutron number of the heavy nucleus. The obtained results are discussed in the context of the theoretical predictions about the 'island of stability' of the hypothetical superheavy elements.
A prescription for the error analysis of experimental data in the case of stochastic background is formulated. Several relations are given which allow to establish the significance of mother-daughter relationships obtained from delayed coincidences. Both, the probability that a cascade is produced randomly and the probability that the parameters of an observed event chain are incompatible with known properties of a given species are formulated. The expressions given are applicable also in cases of poor statistics down even to single events.