Observation of cold scission of highly excited fissioning nuclei

Abstract

The time scale of fission at initial nuclear temperatures of about 5 MeV is deduced from the number of neutrons evaporated prior to and after scission in the reactions 144 Sm and 154 Sm+ (838 MeV) 32 S. The prescission lifetime for fission with symmetric mass splits is longer than for asymmetric mass splits. For symmetric mass splits the excitation energy at scission is only about 60 MeV and independent of the initial excitation energy which is consistent with a prescission lifetime in the order of 10 ~2 0 s. PACS numbers: 25.70.Jj, 25.70.Gh, 25.85.Ge, 27.70.+q Fifty years after the discovery of nuclear fission the dynamics of the division of the nucleus into two parts of roughly equal size is still one of the most interesting processes of collective flow of nuclear matter and an ideal example of the yet unsolved nuclear many-body problem. The time scale of nuclear fission has recently been studied 1 " 4 very intensively up to nuclear temperatures of about 2.7 MeV by several groups. One method employed 1 " 3 the number of neutrons evaporated prior to scission to deduce the prescission lifetime by calculating the mean evaporation time and thus relating the prescission neutron multiplicity with an absolute time scale. A second method 4 exploited the giant-dipole-resonance y rays emitted from the composite systems produced in heavy-ion-fusion reactions. Both methods led to the conclusion that nuclear fission at temperatures up to 2.7 MeV is very slow, about a few 10 ~2 0 s. In other words, nuclear fission, or rather scission, occurs at the very end of the nuclear deexcitation chain. An exciting question, then, is whether this finding prevails also at higher excitation energies or nuclear temperatures. In the present experiment we have investigated for the first time the fission time scale of nuclei at initial temperatures of about 5 MeV or excitation energies of about 500-600 MeV in nuclei of mass close to 180. Furthermore, we have measured for the first time the time scale of fission with symmetric and asymmetric mass splits. At such high temperatures one might expect that the temperature dependence of the nuclear surface tension 5 and/or nuclear dissipation 6 could drastically influence the time scale of nuclear fission. In the following we will briefly discuss the experiment and then present the results for the number of neutrons emitted prior to and after scission, which we use to establish a relative and, with certain assumptions, absolute time scale.Targets of isotopically enriched 144 Sm and 154 Sm with thicknesses of 185 and 216 ^g/cm 2 , respectively, on carbon backings of 50 //g/cm 2 were bombarded with 838-MeV 32 S ions at the VICKSI accelerator in Berlin. Two heavy fragments were detected in coincidence with two x and y position-sensitive low-pressure multiwire chambers providing also good time resolution of about 0.2 ns. The absolute time-of-flight resolution in reference to the cyclotron frequency was 0.8 ns. The centers of these detectors were positioned to the left and right of the beam at 0i = -4...

Investigation of the velocity distribution of evaporation residues produced in20Ne+197Au reactions atE/A=5.7?11.4 MeV/u

“…Since the velocity shift has its largest value for the heaviest residue, one can conclude that the mean energy of the forward peaked nucleons is higher than that of the thermal nucleons (characterized by an isotropic distribution) 9 The velocity distribution of 2os, 206Rn is similar to that of the Francium isotopes, it peaks, however, at a significant lower velocity. To explain this behavior one has to assume either the emission of two preequilibrium nucleons or of one particle with A = 2, since its momentum transfer to the compound nucleus is roughly double the value of one nucleon.…”

“…Hilscher et al [9] conclude from the observation of an excitation -energy -independent post-scission neutron multiplicity, that fission becomes dominant at Ex--60MeV. Natowitz et al [10] have analysed fission excitation functions for several nuclei leading to ZCN = (67-89).…”

Evaporation residues from heavy (Z=90?93) compound nuclei at excitation energies of Ex ? (75?110) MeV

“…We therefore conclude that the low temperature at scission T~oi~s ~ 1.5 MeV as indicated in [1][2][3][4][5] is not equal to the temperature Tfls ~ beyond which fission cannot compete with particle emission; rather it is Tm~ > T~ci~. Using a value of a = A/8 for the level density parameter, we obtain from our measurements Tm~ ~-(1.8-1.9) MeV.…”

“…On the other hand, experiments, performed at SHIP recently, that used considerably higher bombarding energies of E/A = 14.5 MeV/u [2], resulted in an evaporation residue cross section O-ER--~2.3 mb for the system 4~ + 181Ta, which is more than an order of magnitude higher than the cross sections obtained close to the barrier [I]. Qualitatively this high cross section can be under-* Present address: Instituto Nazionale Fisica Nucleare, Laboratori di Legnaro, 1-35020 Legnaro, Padova (Italy) stood on the basis of recent experimental and theoretical studies I- [3][4][5][6][7][8][9][10][11][12]. It indicates a dominance of a fast particle emission over the slow fission process, especially in the early stage of the compound nucleus deexcitation, i.e.…”

Role of dynamical deformation in pre-scission neutron multiplicity

“…Neutron evaporation is one of the dominant decay mode in case of heavy-ion induced reactions. It is often used as the most effective probe to investigate nuclear properties like nuclear temperature [1,2], level density [3,4], fission lifetime [5][6][7][8][9], nuclear dissipation [10][11][12][13][14][15][16], etc. Moreover, in the contest of nuclear power production, neutron multiplicity plays the pivotal role in the operation of nuclear reactors.…”