Superheavy compound nuclei investigated with in-flight mass spectroscopy

Volume 42B, number 2

PHYSICS LETTERS

27 November 1972

SUPERHEAVY COMPOUND NUCLEI INVESTIGATED WITH IN-FLIGHT MASS SPECTROSCOPY P. COLOMBANI, B. GATTY, J.C. JACMART, M. LEFORT, J. PETER, M. RIOU, C. STI~PHAN and X. TARRAGO lnstitut de Physique Nucldaire, 91 406 Orsay, France

Received 12 October 1972 Targets of 2°sPb, 2a2Th and 2aSU have been bombarded with 500 MeV S4Kr ions in an attempt to observe superheavy nuclei which could be formed by fusion. The mass identification was done in flight on recoiling nuclei. No superheavy compound nuclei were observed from any of the targets. Upper limits of formation cross sections were found of ,~ 1 tzb for Pb, 5 ub for Th and 80 ~b for U targets. One of the experiments done at Orsay to look for the possible formation of superheavy nuclei by a process of fusion between heavy nuclei and krypton ions as projectiles, will be described here. Although the resuits were negative within the experimental conditions, they are interesting by the cross section limits they give for the investigation of what could be responsible for the non-observation of these nuclei. It has been reasonably assumed that the island of stability predicted by a number of theoreticians in the region Z = 114, N = 184 contains nuclei that, if formed in their ground states, have half-lives which are long enough to be observed [1]. In these conditions, the choice of the proper formation reaction depends on two main conditions. 1) The superheavy compound nucleus has to be as near as possible to the predicted island of stability. In fact, all the possible combinations obtainable with available targets and beams give very neutron deficient nuclei. Because of its curvature, it is not possible to shoot directly at the/3 stability line which passes right through the calculated doubly-closed shell nucleus ?198X 184: Consequently, the compound nuclei which can be synthesized are predicted to have, due to spontaneous fission or a-particle decay, much shorter half-lives than those predicted for the nuclei in the center of the island of stability. This means that the highest efficiency will be obtained for a system with the shortest detection time, that is, with an in-flight system. One can notice, however, that orparticle or proton emission (which could compete 208

with neutron emission) leads to nuclei that are closer to the stability line and so have longer half-lives. 2) The second condition is that the compound nucleus should be formed with a not too high excitation energy such that it has a reasonable chance to decay to the ground state by particle emission instead of by fission. These reactions are very endoenergetic, and, using calculated masses [3], one can notice that the sum B + Q of the Coulomb barrier plus the negative Q value decreases slowly to zero for incident particles from Zn to Xe. Therefore, the excitation energy of the compound nuclei at the Coulomb barrier would be lower for heavier projectiles. On the other hand, heavier projectiles will introduce higher angular momenta in the nuclear reaction. The fact that there is a limitation of the angular momentum for the formation of a compound nucleus [4] and also that a higher angular momentum enhances the fission probability, are arguments against too heavy particles. In these conditions, it appears that the krypton beam produced by the accelerating system ALICE from Orsay [2] is very suitable for this type of experiment. The energy can vary up to the maximum value of 454 MeV for krypton ions with charge state 23 +, and 505 MeV for charge state 24 +. The intensity on the target was about 2 X 108 particles/s. The experimental procedure is described elsewhere [5] and only the principle of the measurement will be presented.

Volume 42B, n u m b e r 2

PHYSICS LETTERS

The compound nucleus of about 300 u should recoil in the direction of the beam with a kinetic energy of about 125 MeV. With such a mass and energy, it is not affected much by multiple scattering in the target or by emission of a few particles (0.6 ° for perpendicular emission of a 10 MeV neutron and 2 ° for a 5 MeV a-particle). An electrostatic deflector placed behind the target separates the main beam (3 ° deviation) from the compound nuclei (12 ° deviation). The identification method consists of three simultaneous measurements relative to the compound nucleus: energy, time of flight and magnetic rigidity; although the first two measurements are sufficient to give the mass, one can obtain a better precision (A m/rn~ 1.5%) if combined with a magnetic rigidity measurement implying an integer value of the charge state of the detected nucleus. An array of 16 solid-state detectors was set in the focal plane of'the spectrometer, covering 13% in energy.The total aperture of the spe~rometer is about 3 °. The overall efficiency of the system is between 5 and 10% for nuclei living longer than 6 × 10 -7 s, which is the time of flight between the target and detectors for superheavy nuclei. The efficiency has been tested with the reaction l16Cd(84Kr, 3n)197po the cross section of which is known to be about 3 mb [6]. ;l'he system proved to be very selective, without random coincidences and allowed a good identification, with only a few hundred atoms formed. Targets of 2°spb (separated isotope), 232Th and 238UF 4 were bombarded by 84Kr ions at different energies between 440 and 500 MeV. No events were observed: the corresponding cross section upper limits deduced from this experiment are given in fig. 1. The lower values obtained for Th ( ~ 5 #b) and for Pb ( ~ 1/ab) than those for U ( ~ 80/ab) are only due to improvements in the beam intensity. The target thickness was ~ 1 mg/cm 2. The energy loss of the incident beam in these targets was then about 13 MeV. It means that an excitation energy range of 10 MeV was covered for each run. Incident energies, varied stepwise, were chosen to obtain a continuous range of excitation energy, the limits of which are indicated in fig. 1. These limits were evaluated from the Q-values of NIX [1] also indicated in the figure. From the position of the classical Coulomb barrier, represented in fig. 1 and corresponding approximately to the threshold for transfer reactions

27 November 1972 [

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Fig. 1. Upper limits of the cross section for compound nudeus formation as a function of the bombarding energy of 3a4Kron targets of 2°sPb, 232Th and 238U. The Q values have been calculated from the masses given by Nix [ 1]. Numbers with stars indicate the excitation energies of the compound nucleus for the corresponding incident energies. The symbol B (1, 33) has been used for B = Z tZ2e2/re(A~/3A~/3) (expressed in MeV) with re= 1:33 fm observed to correspond to the transfer reaction threshold on Th [7] and to elastic scattering measurements [ 8]. [7] and to elastic scattering measurements [8], it can be seen that for Pb and Th targets the measurements almost began at this threshold. For the lead target, the compound nucleus is 2118"'174" 92y It would have been formed in this experiment with an excitation energy between 17 and 34 MeV. Because the neutron separation energy is about 8 MeV, two or three neutron emissions should be sufficient for decay to the ground state. According to the calculation of Nix [ 1] ground states would have in this region a fission barrier of about 8 MeV (with about 1 h lifetime) and a-particle half-lives of the order of ms which is longer than our detection time (6 X 10 -7 s). In the case of thorium, the compound nucleus is 316 X 190" The neutron separation energy is about 126 6 MeV. This nucleus, excited to energies between 209

Volume 42B, number 2

PHYSICS LETTERS

14 MeV and 50 MeV would have to evaporate between two and six neutrons, or one a-particle and three neutrons, without fissioning to decay to the ground state. But the ground state of the compound nucleus has only a fission barrier of 4.7 MeV (halflife of 10 -10 s) which gives less chance to form a compound nucleus and a greater fission probability along the deexcitation chain. The a-particle half-life is also very short (10 -10 s); three a-particle emissions are necessary before encountering a nucleus with longer half-life than our detection time. One must, however, notice that in this "overshooting" reaction, the successive a-particle, or eventually proton emissions would lead to nuclei nearer the predicted region of stabili!~. In the case of uranium, the compound nucleus ~282X is further away from the predicted island of stability. The fact of having not observed superheavy nuclei in this experiment could be due to wrong evaluations of the lifetimes of the nuclei in the predicted island of stability. Another reason could be the very low formation cross section which is the point to be discussed now. The cross section for the formation of a superheavy nucleus in its ground state can be written oSH = orPCFPD,where o r is the usual reaction cross section, PCF is the complete fusion probability, PD is the probability of decaying to the ground state by light particle or photon emissions without fissioning. In the case of this work, o r is about 250 mb for an incident energy 5% above the Coulomb barrier. The two other factors have not yet been properly calculated for superheavy nuclei. Our cross section limits of about 1/ab probably indicate that the product PCF X PD must be smaller than 10 -5. Although PD must be very small, it seems, as has been pointed out recently [9, 10], that there is also a very strong limitation on the angular momentum and on the incident energy allowed for the formation of a complete fusion compound nucleus. The non-observation of binary fission products in the bombardment of U by 500 MeV krypton [11] seems to confirm this assertion and leads to the idea that PCF is also very small.

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27 November 1972

Further calculations are needed in order to know which incident energy range would give the best chance to form a compound nucleus and then, what are the expected cross sections. The authors are much indebted to the members of the cyclotron crew who have expended maximum effort to improve and run the machine. The authors are also grateful to all their collaborators from the laboratory who, by different means have helped them and had to concede some priorities. It is a pleasure for them to acknowledge fruitful discussions with S.G. Nilsson and J.R. Nix.

References [1] C.F. Tsang and S.G. Nilsson, Nucl. Phys. A140 (1970) 289; D. Vautherin, M. Veneroni and D.M. Brink, Phys. Lett. 33B (1970) 381; J.R. Nix, Los Alamos Preprints LA-DC-72-335 (1972) and LA-DC-72-769(1972). [2] M. Lefort and A. Cabrespine, Internat. Conference on nuclear reactions induced by heavy ions, Heidelberg (1969) 557. [3] W.D. Myers and W.J. Swiatecki, Nucl. Phys. 81 (1966) 1 and Berkeley Report UCRL 11 980 (1965). [4] S. Cohen, F. Plasil and W.J. Swiatecki, Third Internat. Conference on reactions between complex nuclei, Asilomar (1963) 325; J.B. Natowitz, PR-CI (1970) 623 and PR-CI (1970) 2~I$7~ [5] P. Colombani et al.,Leysin Conf. 1970 C E R N Vol. 2, 726; Intern. Conf. on Heavy ion physics, Dubna (1971); Third Internat.Transplutonian element symposiumArgonne 1 9 7 1 - Nucl. Instr.(to be published). [6] H. Gauvin, Y.Le Beyec, M. Lefort and C. Deprun, Phys. Rev. Lett. 28 (1972) 697. [7] R. Bimbot et al.,Nucl. Phys. A189 (1972) 539. [8] P. Colombani et al.,European Conf. on Nucl. Phys. Aix-~n-Provence 1972. [9] F. Plasfl,European Conf. on Nucl. Phys. Aix-en-Provence 1972. [10] W.J. Swiatecki, European Conf. on Nucl. Phys. Aix-enProvence 1972. [ll] B. Tamain, M. Lefort,C. Ngo and J. Peter,European Conf. on Nucl. Phys. Aix-~n-l~ovenc~ 1972 and Nucl.

Phys., to be published.