Evaporation and fission decay of 132Ce compound nuclei at Ex=122 MeV: some limitations of the statistical model

Abstract

Abstract. Light charged particle (LCP) emission in the evaporation residue (ER) and fusion fission (FF) channels have been studied for the 200 MeV32 S+ 100 Mo reaction, leading to 132 Ce composite nuclei at Ex=122 MeV. The main goal was to study the decay of 132 Ce on the basis of an extended set of observables, to get insights on the fission dynamics. The proton and alpha particle energy spectra, their multiplicities, ER-LCP angular correlations, ER and FF angular distributions, and ER and FF crosssections were measured. The measured observables were compared with the Statistical Model (SM). Using standard parameters, the model was able to reproduce only the pre-scission multiplicities and the FF and ER cross-sections. The calculation was observed to strongly overestimate the proton and alpha particle multiplicities in the ER channel. Disagreements were also observed for the ER-LCP correlations, the LCP energy spectra and the ER angular distribution. By varying the SM input parameters over a wide range of values, it is shown that it is not possible to reproduce all the observables simultaneously with a unique set of parameters. The inadequacy of the model in reproducing the ER particle multiplicities is also observed analysing data from the literature for other systems in the A ≈ 150 and E x ≈ 100-200 MeV region. These results indicate serious limitations about the use of the SM in extracting information on fission dynamics.

Time delays in heavy-ion-induced fission of medium-Znuclei, measured by crystal blocking

“…However, there is a large discrepancy for the two measurements with 32 S. We note that our estimated experimental values in Table I are in reasonable accord with the 130 mb measured for 200 MeV 32 S on 100 Mo [31]. The discrepancy of the calculated cross sections from the measured ones is difficult to remove by adjustment of parameters in the statistical model.…”

“…This may be justified by the fact that the maximum value of the spin is otherwise much larger than the critical value, I c , corresponding to B f = 0. This is not the case for 32 S bombardment and the discrepancy between measured and calculated cross sections must for this case be found elsewhere, perhaps in the competition from emission of light, charged particles [29][30][31]. The omission of these channels in the simulations is a deficiency but it appears difficult to include them in a consistent manner [30,31].…”

“…This is not the case for 32 S bombardment and the discrepancy between measured and calculated cross sections must for this case be found elsewhere, perhaps in the competition from emission of light, charged particles [29][30][31]. The omission of these channels in the simulations is a deficiency but it appears difficult to include them in a consistent manner [30,31]. Furthermore, the measured α and proton multiplicities are small (a few percent) in coincidence with fission and larger by an order of magnitude for evaporation residues, so inclusion of these channels appears mainly to reduce the fission cross section without much change of the distribution of fission over the evaporation cascade.…”

“…The fill-in of the blocking-dip minimum is due to fission after emission of several neutrons (here two to four). For simplicity, we ignore emission of light, charged particles 044609-6 (protons and α particles), although it is known that this can be a significant channel for the fused nuclei [29][30][31]. Our aim is mainly to investigate whether the blocking results are consistent with the standard picture of multichance fission and a very broad lifetime distribution.…”

“…[29][30][31]. The sensitivity of the long-lifetime component to η is smaller for 48 Ti and 58 Ni bombardment because there is a high-spin region with very low fission barrier and predominantly first-chance fission.…”