Synthesis of a new element with atomic number Z = 117.
ABSTRACT 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 Z ¼ 117
Yu.Ts. Oganessian,1,*F.Sh. Abdullin,1P.D. Bailey,2D.E. Benker,2M.E. Bennett,3S.N. Dmitriev,1J.G. Ezold,2
J.H. Hamilton,4R.A. Henderson,5M.G. Itkis,1Yu.V. Lobanov,1A.N. Mezentsev,1K.J. Moody,5S.L. Nelson,5
A.N. Polyakov,1C.E. Porter,2A.V. Ramayya,4F.D. Riley,2J.B. Roberto,2M.A. Ryabinin,6K.P. Rykaczewski,2
R.N. Sagaidak,1D.A. Shaughnessy,5I.V. Shirokovsky,1M.A. Stoyer,5V.G. Subbotin,1R. Sudowe,3A.M. Sukhov,1
Yu.S. Tsyganov,1V.K. Utyonkov,1A.A. Voinov,1G.K. Vostokin,1and P.A. Wilk5
1Joint Institute for Nuclear Research, RU-141980 Dubna, Russian Federation
2Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
3University of Nevada Las Vegas, Las Vegas, Nevada 89154, USA
4Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USA
5Lawrence Livemore National Laboratory, Livermore, California 94551, USA
6Research 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 Z ¼ 117 is reported. The isotopes293117
and294117 were produced in fusion reactions between48Ca and249Bk. 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 ? 111, 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 than208Pb arises in every advanced model
of nuclear structure. Reactions involving doubly magic
208Pb and singly magic
neutron-rich projectiles as heavy as64Ni or70Zn have
been used for the synthesis of new heavy elements.
These reactions, termed cold fusion, led to the observation
of isotopes with Z ? 113 and N ? 165 [3,4], stabilized by
the Z ¼ 108 and N ¼ 162 shell gaps occurring for de-
formed shapes. The dramatic drop of the production cross
section with increasing Z practically excludes the continu-
ation of such experiments for heavier elements.
A new method of synthesizing superheavy elements,
with Z ? 112 and neutron numbers closer to the predicted
spherical shell closure at N ¼ 184, 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 Z ¼ 112 and 14 new iso-
topes of new elements with Z ¼ 113–116 and 118 were
identified  among the products of heavy-ion fusion
reactions employing doubly magic48Ca projectiles and
actinide radioactive targets of U-Cm and Cf, respectively.
The sequential ?decays of theheaviest even-Znucleiwere
found to be terminated by spontaneous fission (SF) of the
descendant even-even or even-odd nuclei with Z ¼ 114,
112, or 110 (T?? TSF), with total decay times in the range
209Bi target nuclei and stable
of about 0.1 s to 1 min depending on neutron number .
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 Z ¼ 112
and Z ¼ 114 isotopes obtained at Dubna  were recently
confirmed in several independent experiments [5–8].
We present here the experimental evidence for synthesis
of a new chemical element with Z ¼ 117; see Fig. 1. The
identified293117 and294117 isotopes were produced in the
fusion reaction between48Ca 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
the existence of the island of stability for heaviest nuclei.
The249Bkwas 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 of249Bk, only 1.7 ng of252Cf,
and no other detectable impurities. Six arc-shaped targets,
each with an area of 6:0 cm2, 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=cm2of249Bk. 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
and theheavy-ioncyclotronU-400atJINR. Adetailed
description of experiment will be given in a forthcoming
PRL 104, 142502 (2010)
Selected for a Viewpoint in Physics
9 APRIL 2010
? 2010 The American Physical Society
paper ; 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 4 cm ? 12 cm Si-detector array with 12
vertical position-sensitive strips surrounded by eight
4 cm ? 4 cm side 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
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 Z ? 102 was determined
from the sum Etotþ 23 MeV, where Etotis 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
(FWHM) of the strip detector in registering correlated
decay chains of the ER-?1-?2-?3-SF type was ?1:2 mm.
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-
252No measurement. The position resolution
ergy expected for Z ¼ 117 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
the48Ca projectiles at 252 MeV energy in the middle of
the249Bk target, the excitation energy of the compound
nucleus297117 is estimated to be E?¼ 39 MeV, near the
expected maximum for the total ER cross section (sum of
3 n and 4 n evaporation channels ). The intensity of the
48Ca-ion beam was 7 ? 1012ions=s at 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 2:4 ? 1019. 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-MeV48Ca 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. 1 for the
averaged decay properties assigned to293117 isotope). All
FIG. 2 (color).
48Ca þ249Bk run (E?¼ 39 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.
Energy spectra recorded during the 252 MeV
FIG. 1 (color).
ing from the isotopes A ¼ 294 (single event) and A ¼ 293
(average of five events) of the new element Z ¼ 117. The
deduced and predicted  lifetimes (? ¼ T1=2=ln2) and
?-particle energies are shown in black and blue, respectively.
Observed decay chains interpreted as originat-
PRL 104, 142502 (2010)
PHYSICAL REVIEW LETTERS
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 E?1¼ 11:03 ? 0:08 MeV and
T?1¼ 14ðþ11;?4Þ ms. 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 E?2¼ 10:31 ? 0:09 MeV and
T?2¼ 0:22ðþ0:26;?0:08Þ s. The third ? transition was
observed as having E?3¼ 9:74 ? 0:08 MeV and E?3¼
9:48 ? 0:11 MeV, and T?3¼ 5:5ðþ5:0;?1:8Þ s. In all
five cases the decay chains ended with the spontaneous
fission with TSF¼ 26ðþ25;?8Þ s.
At the E?¼ 39 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 isotope293117. This conclusion is supported
by the systematics of the cross sections ?xnðE?Þ measured
previously for production of isotopes of superheavy nuclei
with Z ¼ 108, 112–116, and 118 in48Ca-induced reactions
, by calculations made directly for the evaporation
residues of the reaction249Bk þ48Ca [12–14] as well as
by the result of the249Bk þ48Ca experiment performed at
lower beam energy (see below). In the E?energy 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 10?3=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 1:2 ? 10?4=s
(beam on) and 7 ? 10?5=s (beam off). We have calculated
the total numbers of random sequences  imitating each
of the observed five decay chains, by using extended
intervals of time (?t ? 5T1=2), ?-particle energy and po-
sition (both exceeding 4.7 standard deviations), to be 6 ?
10?6, 10?3, 10?5, 3 ? 10?11, and 3 ? 10?11.
The experiment was continued at a
247 MeV for 70 d with a total beam dose of 2 ? 1019.
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 Z ¼ 111 did
not undergo SF, but instead emitted an ? particle with
E?4¼ 9:00 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 
imitating the observed decay chain amounts to 6 ?
10?11. Therefore, we assign this chain to the decay of the
neighboring odd-odd nucleus294117. Note that this decay
chain was registered when about 30% of249Bk decayed to
249Cf. Attributing it to the decay of294ð118Þ nucleus is
unlikely due to the small production yield and significantly
different decay properties.
48Ca energy of
The decay properties of the neighboring isotopes293117
and294117, their daughters289115 and290115, as well as
granddaughters285113 and286113, 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 of281111 can be
explained by comparing the results of the present experi-
ment with the properties of the neighboring even-Z nuclei.
In the TSFðNÞ systematics, 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
Minimum values of TSFare characteristic of the transition
region N ¼ 168–170 wherethe effect of nuclear shells is at
a minimum. Indeed, the Z ¼ 110 darmstadtium isotopes
with N ¼ 169 and N ¼ 171, as well as the Z ¼ 112, N ¼
170, and N ¼ 172 copernicium isotopes, undergo SF
rather than ? decay . For the odd-Z nuclei produced
in the reactions237Np þ48Ca and243Am þ48Ca, the high
hindrance of SF for nuclei with an odd number of protons
and the relatively low T?for the isotopes of elements 113
and 115 with N ¼ 169–173 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 105s for268Db. In the reaction249Bk þ48Ca, the
daughter nuclei that originate from the evaporation resi-
dues293117 and294117 have one or two extra neutrons
compared with those produced in the lower-Z reactions. A
closer approach to the shell at N ¼ 184 should result in a
decrease in their decay energy Q?and 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 Z ? 111; see Fig. 3. In analogy with the
neighboring even-Z isotopes, all the nuclei in the decay
chains of293117 and294117 with Z > 111 and N ? 172 are
expected to undergo ? decay. The nucleus281111 (N ¼
170) lies in the ‘‘critical’’ region, and may avoid SF only
because of the hindrance resulting from an odd proton.
Despite a hindrance of 3 ? 104with respect to its even-
even neighbor282112 , the isotope281111 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 N ¼ 162 and N ¼ 184. The
presence of an extra and unpaired neutron in the neighbor-
ing isotope282111 further hinders SF relative to the ?
decay of this nucleus. In Figs. 3(a) and 3(b), the experi-
mental values of decay energies Q?and half-lives T?are
presented for isotopes with Z ¼ 111, 113, 115, and 117.
Increasing the neutron number in the heaviest nuclides
results in a decrease of Q?and a considerable increase in
T?. An especially strong growth of T?ðNÞ is observed for
the isotopes of elements 111 and 113. Except for281111, all
the nuclides presented in Fig. 3 are ? emitters; for them T?
N ¼ 184
PRL 104, 142502 (2010)
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  are in a good agreement with our experiment.
The cross sections for producing the nuclei of element 117
in the reaction249Bk þ48Ca are ? ¼ 0:5ðþ1:1;?0:4Þ pb
and ? ¼ 1:3ðþ1:5;?0:6Þ pb at E?¼ 35 MeV and E?¼
39 MeV, respectively. These values are similar to the
results of previous experiments where cross sections for
the reactions of233;238U,237Np,242;244Pu,243Am,245;248Cm,
and249Cf targets with48Ca beams have been measured .
In conclusion, a new chemical element with atomic
number 117 has been synthesized in the fusion of249Bk
and48Ca. The data are consistent with the observation of
two isotopes of element 117, with atomic masses 293 and
294. These isotopes undergo ? decay with E?¼ 11:03ð8Þ
and 10.81(10) MeV and half-lives 14ðþ11;?4Þ and
78ðþ370;?36Þ ms, respectively, giving rise to sequential
?-decay chains ending in spontaneous fission of281Rg
(TSF? 26 s) and270Db (TSF? 1 d), respectively. The de-
cays of 11 identified isotopes substantially expand our
knowledge of the properties of odd-Z nuclei in the region
of the most neutron-rich isotopes of elements 105–117.
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 with48Ca,
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
We are grateful to the JINR Directorate and the U-400
cyclotron and ionsource crewsfortheir 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
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).
neutron number for the isotopes of elements with Z ¼ 111–117
(new results in red). All the nuclides with N > 165 have been
produced in48Ca induced reactions. Our T?(exp) values are
given for the nuclei belonging to the
(5 events). The limit for T?ð281RgÞ was estimated from the
measured half-life and number of observed nuclei.
(a) ?-decay energy and (b) half-lives versus
293117 decay chain
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