Journal of Nuclear and Radiochemical Sciences, Vol. 1, No. 1, pp. 1−4, 2000

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Reviews
Superheavy Elements with the Berkeley Gas-Filled Separator
K.E. Gregorich* and V. Ninov
Lawrence Berkeley National Laboratory, MS-88, Berkeley, CA 94720, USA
Received: October 13, 1999
In April and May of 1999 the Berkeley Gas-filled Separator was used to search for the production and decay ofelement 118 from the 86Kr + 208Pb reaction, according to Smolañczuk's predictions of relatively large production
rates. Three decay chains, each consisting of an implanted heavy ion, followed by a rapid (ms) succession ofhigh-energy (> 10 MeV) alpha-particle decays were detected. These chains are consistent with the production anddecay of element 118 with mass number 293. These results a) show experimental evidence for the existence ofshell-stabilized superheavy elements, b) provide experimental values for refinement of nuclear mass models in the
superheavy element region, and, most importantly, c) present a “new” reaction pathway for the production ofsuperheavy elements.

In the early 1970s, the heaviest known isotopes had proton andneutron numbers near Z~106 and N~158. The experimental
trends in nuclear stability indicated that with increasing Z andN, spontaneous fission (SF) would dominate the decay, resulting in ever-decreasing half-lives. However, according to thesuperheavy element (SHE) predictions,1 closed proton andneutron shells near Z=114 and N=184 should result inincreased stability.
Traditionally, the best nuclear reactions for production of
SHE at particle accelerators were thought to involve 48Ca projectiles with actinide targets, and several attempts to produce
SHE were made 2 which ultimately proved unsuccessful.
Recent very exciting results from experiments with 48Ca projectiles and 238U, 242Pu, and 244Pu targets have recently beenreported,3 with indications that element 114 isotopes may havebeen produced in the 48Ca bombardments of Pu targets.
Heavy element production rates in experiments4 to synthesize elements 110−112 indicate that it will be difficult to produce elements beyond Z=113 by the “cold fusion” approach5of bombarding Pb or Bi target nuclei to produce heavy compound nuclei at low excitation energies. Standard extrapolations of these production cross sections indicate that large
increases in beam currents will be needed to approach production rates of one atom per month.
However, the recent predictions of Smolañczuk6 indicatethat the cold fusion of projectiles near 86Kr with 208Pb or 209Bi
targets should lead to relatively large SHE production rates.
According to his calculations, the 208Pb (86Kr, 1n)293118 reaction should proceed with a cross section of 670 pb. In addition, Smolañczuk predicted that these SHE isotopes would
decay by a rapid series of high-energy α-particles. The predicted decay properties of 293118 and the resulting daughter
activities are presented in Table 1.

Based on these predictions, we searched for SHE productionin the interaction of 86Kr projectiles with 208Pb targets using therecently completed Berkeley Gas-filled Separator7 (BGS) at
the 88-Inch Cyclotron of the Lawrence Berkeley National
Laboratory (LBNL). Results from these first experiments havebeen published. 8 A schematic of the BGS is presented in
Figure 1. The 86Kr19+ beam was produced in the Advanced
Electron Cyclotron Resonance Source9 and accelerated by the
88-Inch Cyclotron to an energy of 459 MeV (∆E = 2.3 MeV
FWHM). During the experiments, the beam current averaged
0.3 particle microAmperes (2.0 × 1012 ions/s). At the BGS, thebeam first passed through a 0.1 mg/cm2 carbon vacuum window before entering the 1.0 torr He gas filling the BGS. The
Kr beam then passed through the 208Pb targets mounted at the
periphery of a 14-inch diameter rotating wheel positioned 0.5
cm downstream of the vacuum window. The targets consistedof 300−400 µg/cm 2 208 Pb metal evaporated onto the downstream side of 40 µg/cm2 carbon foils. The Pb layer was covered by an additional 10 µg/cm2 layer of carbon to prevent
sputtering of the 208Pb and to aid in the blackbody radiation
cooling of the targets. The 86Kr energy at the center of the208
Pb target material was 449 MeV.10 The compound nucleus
evaporation residues (EVRs) recoiled out of the target at near
the beam momentum (EEVR = 131 MeV). Upon passing
through the He gas, the EVRs experience many charge-changing collisions, leading to a well-defined average charge state.
This average charge state is nearly proportional to the EVR
velocity, resulting in large a large charge and velocity acceptance in the BGS. The EVRs then passed through a verticallyfocusing quadrupole magnet, a 25° dipole magnet with astrong horizontally focusing field gradient, and a 45° flat-field

TABLE 1: Predicted6 Decay Sequence from 293118
A

ZN
118175289
116173285
114171281
112169277
110167273
108165
269
Sg163
265
Rf161
293

Qα(MeV)
12.23
11.37
11.18
11.00
10.77
9.69
8.35
SF

T1/2
31 µs – 310 µs
960 µs – 9.6 ms
800 µs – 8.0 ms
610 µs – 6.1 ms
620 µs – 6.2 ms
120 ms – 1.2 s
8.0 min – 80 min
41 min*Corresponding author. E-mail: KEGregorich@lbl.gov. FAX: +1510-486-7983.

Figure 1. Schematic of the Berkeley Gas-filled Separator.

© 2000 The Japan Society of Nuclear and Radiochemical Sciences

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Gregorich

J. Nucl. Radiochem. Sci., Vol. 1, No. 1, 2000

dipole magnet before entering the detector chamber. Other
nuclear reaction products, unreacted beam, and scattered beam
particles take on a lower magnet rigidity in the He gas, and are
directed toward the beamstop along the upper edge of the vacuum chamber shown in Figure 1. The efficiency of the BGS
for focusing element 118 EVRs onto the focal plane detector
was estimated by measuring the α-decay of Po EVRs from a86
Kr + 116Cd test reaction, and comparing with the expected Po
production rates.11 This efficiency agrees well with Monte
Carlo simulations of ion trajectories through the separator.
Upon entering the detector chamber, the EVRs passed
through a parallel plate avalanche counter 12 (PPAC) wheretime and position were recorded. The Si-strip focal plane
detector was located 30 cm downstream of the PPAC. This Sistrip detector was an 80 mm wide × 35 mm high and was
divided into 16 vertical strips. Along each of these strips, thevertical position was measured by resistive charge division.
The energy and position resolution of the Si-strip detector were
determined using implanted recoil atoms from a 86Kr + 116Cd
test reaction. The energy resolution for 5−9 MeV α particles
was 30 keV FWHM, and the position resolution in the verticaldirection was ± 0.58 mm. Directly behind the focal plane
detector was a second Si-strip punch-through detector, used toreject particles passing through the focal plane detector.
Events registering signals in both the Si-strip focal plane detector and either the PPAC or punch-through detector were interpreted as reaction products passing through the separator,
while events leaving only a signal in the Si-strip detector wereassumed to be decays of nuclei previously implanted in the Sistrip detector. The focal plane detector had a calculated efficiency of 60% for detection of full-energy 12-MeV α-particles
from nuclei implanted to a depth of 14 µm. The other 40% of
the time, α particles escape out the front of the detector,“escape alphas”, depositing a partial energy (1.5 ≤ E(MeV) ≤4.0). For both the focal plane detector and the punchthroughdetector, the energy, position and time of all events wererecorded. The Si strip detector events were recorded over two
amplifier gain settings: a low-energy setting covering 0.5−15
MeV, and a high-energy setting covering energies up to 200
MeV.
Two separate 86Kr + 208Pb experiments were carried out. In
the first experiment, the overall rate of events registering
greater than 0.5 MeV in the focal plane detector was 50 s−1. In
this experiment, a 5 cm × 5 cm Si-strip detector (which did notcover the entire focal plane detector) was used as thepunchthrough detector. The integrated beam flux for thisexperiment was 0.7 × 10 ions. For the second experiment,with an integrated beam flux of 1.6 × 1018 ions, an 8 cm × 3.5
cm Si-strip detector was used as the punchthrough detector,
providing complete coverage of the focal plane detector.
Improvements to the beamstop within the BGS reduced theoverall count rate in the Si-strip focal plane detector to lessthan 20 s−1. The low-energy spectrum recorded in the focal
plane detector for the entire second experiment is presented in
Figure 2. Figure 2(a) contains the ungated spectrum. Figure2(b) shows the remaining events after applying a veto based on
all events recorded in the PPAC. Figure 2(c) includes the
additional sensitivity of applying a veto for all events registering in the punchthrough detector. Figure 2(d) shows the spectrum of all events from Figure 2(c) with 8.1 ≤ E(MeV) ≤ 13.0
which are correlated in position and time (within 1 s) with an
implanted recoil. Note that the 3 events labeled with arrows
18

Figure 2. The low-energy spectrum in the focal plane detector for theentire second experiment. The gating conditions for the four panels
are explained in the text.

Figure 3. Observed decay chains from the reaction of 449-MeV 86Kr with 208Pb.

Superheavy Elements
are members of a single decay chain. The detection rate for αparticles with 4.0 ≤ E(MeV) ≤ 13.0 was 0.5 s−1.
Three decay chains were observed, each consisting of an
implanted heavy ion recoil (with implantation energies consistent with that expected for an element 118 recoil) correlated inposition and time with chains of 6 to 7 subsequent α-particle
decays. This corresponds to a production cross section of
2.2 +2.6–0.8 pb. The observed correlation chains are shown in
Figure 3, interpreted as the decay of 293 118. The first two
chains were observed in the first experiment, and the third wasobserved in the second experiment. For the third chain, we
have indicated that the α-decay of the 293118 was not detected
because it occurred during the 120 µs dead time of the acquisition system while recording the recoil implantation event.
An analysis of the positions of the members of each chain
was carried out. The FWHM position resolution for EVR-αcorrelations was 0.58 mm. It was assumed that the position
resolution for correlations between pairs of escape alphas is thesame, and for pairs of full energy α-particles it is half as large.
From this, the 1-sigma position resolutions for the event types
were calculated: σrecoil = 0.230 mm, σα = 0.087 mm, and σescape= 0.174 mm. The weighted average and the reduced Χ2 for the
positions in each chain were calculated, and are presented in
Figure 4. The large Χ2 for the second chain results from theerror in determining the positions near the bottom of the strip.
(The positions were determined by comparing the pulseheights from the top of the strips with the total pulse height.
At a 3.5 mm vertical position, the signal for escape alphas
measured at the top of the strip is as small as 1% of the fullscale values for the amplifier and ADC.)
During the entire experiment, one additional chain wasrecorded, which could possibly be interpreted as being due tothe implantation and decay of an atom of 293118. It occurred instrip 14 during the first experiment. A recoil implantation
event was recorded, followed 190 µs later by a 12.0-MeV α
particle, then followed 936 µs later by a 11.4-MeV α particle,
then followed at 1.125 ms, 1.740 ms, 1.503 ms, and 13.879 slater by escape alpha events, with energies of 1.3, 3.1, 2.3, and
1.2 MeV, respectively. The vertical positions (in mm) in strip
14 of the implantation, the two α events, and the four escapes
were: 18.3, 18.6, 18.5, 19.1, 19.4, 19.3, and 19.8. Since strip
14 was not backed by the punchthrough detector in the firstexperiment, we cannot be sure of the validity of the escapealpha events. In fact, the positions of the escape events are
significantly different from those for the recoil and the fullenergy alphas. We have therefore decided not to include thisFigure 4. Analysis of the vertical positions of the three observed
chains from the implantation and decay of 293118. The dashed lines
are the weighted average positions of the chains. The heavy lines
show the measured positions of the chain members, and the thin lines
indicate the 1-σ limits on the measured positions.

J. Nucl. Radiochem. Sci., Vol. 1, No. 1, 2000

3

chain in any analysis of decay properties or cross section.
Based on the sequences shown in Figure 3, the half-lives ofthe decay chain members were calculated.13 These half-lives
are presented together with the calculated α-decay hindrance
factors based on α-decay systematics for unhindered transitions14 in Table 2, showing that the observed α-transitions havehindrance factors in the range expected for odd-N nuclei.
The observation of long decay chains of rapid, high-energy
α-particles indicate the formation of SHE, as there are noknow nuclei that could exhibit a similar decay pattern. Theenergies of the observed α-particles agree well with the predictions of Smolañcuk, supporting the proposed Z and A assignments. Statistical model considerations suggest that the ratio
Γn/Γα is be proportional to exp{-[Sn – (Bα - Qα)]/T}, where Sn is
the neutron separation energy, Bα is the Coulomb barrier for α
emission, Qα is the energy released in removing an α particle
from the nucleus, and T is the nuclear temperature.
Substituting appropriate values for the binding energies6 andbarriers15 gives Γn/Γα ~ 60 and similarly, Γn/Γp ~ 200, indicating that neutron emission is the most probable deexcitation
path. Since the excitation energy of the compound nucleus is~13 MeV,6 emission of two particles (i.e., 2n, αn, or pn, etc.) isenergetically forbidden. In addition, extrapolation of the Qα
values from known isotopes of Hs and Sg to neutron numbers
at the ends of our chains indicate that deexcitation by α emission is extremely unlikely. Therefore, while other assignments
are not absolutely ruled out, we are confident in the assignmentof these chains to the decay of 293118.
TABLE 2: Measured α-Particle Energies, Half-Lives, andHindrance Factors14 for Decay Chain Members
A

293
289

ZN
118175E-α (MeV)
12.39

Measured T1/2
120 +180 µs

H.F.a
8.7 (4.3 – 14)116173

11.63

600 +860 µs

3.6 (1.8 – 8.9)

+870

285

114171

11.34

580

µs

3.0 (1.5 – 7.4)28111216910.68

890 +1300 µs

0.5 (0.2 – 1.2)

+4.7

0.4 (0.2 – 1.0)277110167

10.18

3.0

273

108165

9.78
9.47

1.2 +1.7 s

269

Sg163

8.74

~22 scms

110 (56 – 270)b
15 ( 8 – 37)b
~4.2

a

HF for the most probable half life with HF for lower and upper
half-life limits in parentheses.
bhindrance factors assuming each α-energy has 50% abundance.
chalf-life is approximate because maximum search time is poorly
defined.

In Figure 5, we compare our measured α particle energies
with predictions of several modern nuclear mass models. The
best agreement with our observations is obtained with
Smolañczuk’s prediction. The finite range droplet model16 andthe Thomas-Fermi model17 are macroscopic-microscopic models, which both use Möller’s shell effects. It appears that theirpredicted Z = 114 shell effect near N = 175 is too strong. Qα
values from the SLy4 Skyrme-Hartree-Fock Bogoliubov
method 18 shows extra stability near Z=116 which does notappear in the experimental data. The empirical mass model of
Liran and Zeldes19 may not be appropriate for extrapolation
into this region.
We have presented evidence for the first synthesis of new
superheavy elements ( 293118 and its decay products 289116,
285
114, 281112, 277110, 273Hs, and 269Sg). The measured α-decayenergies can be used directly for refinement of mass and shellmodels. Our results show an unexpected viability of the cold
fusion approach to the synthesis of superheavy nuclei using
projectiles heavier than 70Zn with targets near 208Pb. The 2.2

4Gregorich

J. Nucl. Radiochem. Sci., Vol. 1, No. 1, 2000
References

Figure 5. Comparison of the α-particle energies observed in thiswork with the predictions of various mass models for the N – Z = 57
nuclei.

pb cross section, while it than 300 times smaller than that predicted by Smolañczuk, is orders of magnitude larger than
expected from extrapolations from cold fusion reactions withlighter projectiles.4 This relatively large cross section may be
explained by the idea of “unshielded fusion” 20 where withheavier projectiles, the optimal bombarding energy for the 1ndeexcitatoin channel is above the Coulomb barrier so that the
Coulomb barrier is no longer the first thing stopping the fusion
process.
We gratefully acknowledge the operations staff of the 88Inch Cyclotron for providing intense, steady beams of 86Kr.
We thank B. Lommel and W. Talheimer of Gesellschaft für
Schwerionenforschung for providing the entrance windows
and lead targets. We thank N. Kurz and H. Essel for help insetting up the data acquisition system, and G. Münzenberg andS. Hoffman for their support. We gratefully acknowledge R.
Smolañczuk for helpful discussions and continuous theoreticalsupport. We thank M. Steiner, J. Yurkon, and D.J. Morrissey
at Michigan State University for lending the PPACs. Financial
support was provided by the Office of High Energy andNuclear Physics, Nuclear Physics Division and the Office ofEnergy Research, Office of Basic Energy Sciences, Chemical
Sciences Division of the U.S. Department of Energy underContract No., DE-AC03-76SF0098 and Grant No., DE-FG0688ER40402.

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