Observation of Superheavy Nuclei Produced in the Reaction of 86Kr with 208Pb

Following a prediction by Smolan ́czuk [Phys. Rev. C59, 2634 (1999)], we searched for superheavy element formation in the bombardment of 208Pb with 449-MeV86Kr ions. We have observed three decay chains, each consisting of an implanted heavy atom and six subsequent a d cays, correlated in time and position. In these decay chains, a rapid (ms) sequence of high energy a particles (Ea $ 10 MeV) indicates the decay of a new highZ element. The observed chains are consistent with the formation of293118 and its decay by sequential-particle emission to289116, 285114, 281112, 277110, 273Hs (Z 108) and269Sg (Z 106). The production cross section is 2.2 20.8 pb.

The synthesis of new heavy nuclei has fundamental interest for nuclear physics and chemistry. The heaviest nuclei provide a laboratory to test our ideas of nuclear structure at the limits of large numbers of protons in the nucleus. For over 25 years, scientists have sought to find or synthesize superheavy nuclei at or near the region Z 114 and N 184 [1], although some calculations suggest that the region of maximum stability may be near Z 120 or Z 126 [2,3].
The synthesis of elements 110-112 [4][5][6][7] and element 114 [8] has invigorated this quest. However, it has proven difficult to proceed beyond element 112 [9] using the so-called "cold fusion" approach [10] of bombarding Pb or Bi target nuclei to produce heavy compound nuclei at low excitation energies. The usual extrapolations of existing data on the synthesis of elements 110-112 indicate that to reach still heavier elements will require orders of magnitude increases in accelerator beam currents and new target technologies.
However, the recent prediction of Smolańczuk [11] indicates that the cold fusion reaction of 86 Kr with 208 Pb should produce superheavy nuclei ( 293 118 and its decay products) with an evaporation residue (EVR) cross section of 670 pb. This would represent a dramatic increase in cross section. His predicted decay sequence [12] for the products of the 208 Pb͑ 86 Kr, n͒ 293 118 reaction is shown in Table I.
We have studied this reaction at the 88-Inch Cyclotron of the Lawrence Berkeley National Laboratory, using the Berkeley gas-filled separator [13]. A schematic diagram of the separator is shown in Fig. 1. A 86 Kr 191 beam produced with the Advanced Electron Cyclotron Resonance source [14] was accelerated to 459 MeV[DE͑FWHM͒ 2.
3 MeV] at an average current of ϳ300 particle nanoamperes (1.9 3 10 12 ions͞s). It went through the 0.1 mg͞cm 2 carbon entrance window of the separator and struck a 208 Pb target placed 0.5 cm downstream from the window. The targets were 300 450 mg͞cm 2 thick (sandwiched between 40 mg͞cm 2 C on the upstream side and 10 mg͞cm 2 C on the downstream side) [15]. Nine of them were mounted on a wheel that was rotated at 400 rpm. The beam energy at the center of the target was 449 MeV [16]. The beam intensity was monitored by two silicon detectors (mounted at 630 deg with respect to the incident beam) that detected elastically scattered beam particles from the target. During the first experiment (8)(9)(10)(11)(12) April 1999), a dose of 0.7 3 10 18 ions was delivered to the target and two correlated EVR-a-particle decay chains were observed. During the second experiment (30 April-05 May 1999), a dose of 1.6 3 10 18 ions was delivered and one correlated EVR-a-particle decay chain was observed.
The EVRs (E ϳ 131 MeV) were separated spatially in flight from beam particles and transfer reaction products by their differing magnetic rigidities in the gas-filled separator. The separator consists of three magnets, a vertically focusing quadrupole magnet followed by a strong horizontally focusing gradient dipole magnet and a flat field dipole magnet. The separator is filled with helium gas at a pressure of 1 torr. We have estimated the magnetic rigidity (Br) to be 2.11 Tm [17]. The optimal magnetic field setting was obtained by scaling the values from the measured focal plane EVR distributions for the The efficiency of the separator for transport and implantation of EVRs was estimated by studying the 86 Kr 1 116 Cd reaction to make a-particle emitting 194 198 Po isotopes. By comparing the measured Po implantation rates with predicted EVR production cross sections [18], we estimate a separator efficiency of ϳ75%. This efficiency agrees with Monte Carlo simulations of ion trajectories through the separator.
In the focal plane region of the separator, the EVRs passed through a 10 cm 3 10 cm parallel plate avalanche counter (PPAC) [19] that recorded the time, DE, and x, y positions of the particles. In the first experiment, the PPAC was placed ϳ3 cm from the focal plane detector while in the second experiment, the PPAC was ϳ29 cm from the focal plane detector. In the second experiment, the time of flight of the EVRs between the PPAC and the focal plane detector was measured. In both experiments, the PPAC was used to distinguish (99.1% efficiency) between particles hitting the focal plane detector that were beam related and events due to the decay of implanted atoms.
After passing through the PPAC, the recoils were implanted in a 16-strip, 300-mm thick passivated ion implanted silicon detector at the focal plane that had an active area of 80 mm 3 35 mm. The strips were position sensitive in the vertical (35 mm) direction. The position resolution along each strip was measured to be 0.58 mm for recoil-a correlations in the 86 Kr 1 116 Cd reaction. The energy response of each strip of the focal plane detector was calibrated using implanted recoils. An average energy resolution of 30 keV for 5-9 MeV a particles was measured for this detector. The focal plane detector had an estimated efficiency of 60% for the detection of full energy 12 MeV a particles following implantation of a 293 118 nucleus to a calculated depth of 14 mm. A second silicon strip "punch-through" detector was installed behind this detector to reject particles passing through the primary detector. In the first experiment, a 50 mm 3 50 mm de-tector was used that did not back the entire focal plane detector, while in the second experiment a detector was used that backed the full focal plane detector.
In the first experiment, with a beam current of ϳ300 particle nanoamperes of 86 Kr striking a 208 Pb target, the average total counting rate (E $ 0.5 MeV) in the focal plane detector was ϳ50 s 21 . A modification of the beam stop reduced this rate to ϳ15 20 s 21 in the second experiment. The number of particles with energies, 4 # E # 13 MeV, was 0.5 s 21 . In Fig. 2, the low energy spectrum recorded in the focal plane detector during the entire second experiment is displayed under several conditions. In Fig. 2(a), we show the ungated spectrum. In Fig. 2(b), the spectrum after applying the PPAC veto is shown. In Fig. 2(c), we display the effect of adding the veto of the "punch-through" detector to the total veto. Finally, in Fig. 2(d), we show the spectrum of all events with 8.1 # E # 13.0 MeV satisfying both requirements, which were correlated in position and time (within 1 s) with an implanted recoil. Note that 3 of the 16 counts shown in Fig. 2(d) are part of a single decay chain.
We have observed three decay chains consisting of an implanted heavy atom correlated in position and time with six subsequent a decays for the reaction of 449-MeV 86 Kr with 208 Pb. This corresponds to a production cross section of 2.2 12. 6 20.8 pb. The observed correlations are shown in Fig. 3 in terms of the predicted decay sequences for 293 118. For the third observed chain, we have chosen to indicate the presence of a "missing" a particle. This first a-particle decay could have been missed because it occurred within the 120-ms dead time (after recoil implantation) of the data acquisition system. Based upon the sequences shown in Fig. 3, the half-lives [20] Fig. 2(d), the probability of a chance correlation causing these decay chains is negligible. Chance correlations do limit our ability to unambigiously assign correlations involving a decay and fissionlike decays with lifetimes greater than 20 min, i.e., decays at the end of the observed chains.
The energies of the observed a particles and their lifetimes agree remarkably well with the predictions of Smolańczuk [12]. The overall agreement supports the proposed assignments, and there are no known nuclei that exhibit the observed decay pattern. Thus this observation must be taken as evidence for the formation of new nuclei with very high Z. We considered the possibility that the completely fused system deexcited by emitting an a particle or proton instead of a neutron. Statistical model considerations suggest that the ratio of G n ͞G a would be proportional to exp͕2͓S n 2 ͑B a 2 Q a ͔͒͞T ͖, where S n is the neutron separation energy, B a is the Coulomb barrier for a emission, Q a is the energy released in removing an a particle from the nucleus, and T is the nuclear temperature. Substituting in these relationships appropriate values of the binding energies [12] and barriers [21] gives G n ͞G a ϳ 60 and G n ͞G p ϳ 2000, indicating that neutron emission is the most probable deexcitation path. Since the excitation energy of the completely fused system is 13 MeV [11], emission of two neutrons is energetically forbidden.
In Fig. 4, we compare our measured values of the a-particle energies with the predictions of several modern mass models. The best agreement with our observations is obtained with Smolańczuk's prediction. The finite range droplet model [22] and the Thomas-Fermi model [23] predict appropriate values of the decay energies for the decay of 293 118, 289 116, and 273 Hs (Z 108), but fail for Z 106, and especially, Z 114. The empirical mass model of Liran and Zeldes [24] is not suitable for extrapolation into this region.
We have presented evidence for the first synthesis of new superheavy elements [ 293 118 and its decay products 289 116, 285 114, 281 112, 277 110, 273 Hs (Z 108) and 269 Sg (Z 106)]. Our results show the unexpected viability of the cold fusion approach to the synthesis of superheavy nuclei using projectiles heavier than 70 Zn [9]. The production cross section may be explained by the idea of "unshielded fusion" where, with heavier projectiles, the optimal bombarding energy for the 1n deexcitation channel is above the Coulomb barrier.
We gratefully acknowledge the operations staff of the 88-Inch Cyclotron for providing intense, steady beams