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Register a new userDependence of spin induced structural transitions on level density and Neutron emission spectra
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Mamta Aggarwal
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The impact of spin induced deformation and shape phase transitions on nuclear level density and consequently on neutron emission spectra of the decay of compound nuclear systems 112^Ru to 123^Cs (N = 68 isotones) is investigated in a microscopic framework of Statistical theory of superfluid nuclei. Our calculations are in good accord with experimental data for evaporation residue of 119^Sb^* and 185^Re^* and show a strong correlation between spin induced structural transitions and NLD. We find that the inverse level density parameter K increases with increasing spin for all the systems, but it decreases with a deformation or a shape change that results in the enhancement of level density and emission probability. A sharp shape phase transition from oblate to uncommon prolate non-collective in well deformed nuclei leads to band crossing and enhancement of level density which fades away while approaching sphericity at or near shell closure manifesting shell effects.
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V.A. Plujko
, A.N. Gorbachenko (Taras Shevchenko National University;n Institute for Nuclear Research
, Kiev
2002
The response function approach is proposed to include vibrational state in calculation of level density. The calculations show rather strong dependence of level density on the relaxation times of collective state damping.
Nuclear reactions of interest for astrophysics and applications often rely on statistical model calculations for nuclear reaction rates, particularly for nuclei far from $beta$-stability. However, statistical model parameters are often poorly constrained, where experimental constraints are particularly sparse for exotic nuclides. For example, our understanding of the breakout from the NiCu cycle in the astrophysical rp-process is currently limited by uncertainties in the statistical properties of the proton-rich nucleus $^{60}$Zn. We have determined the nuclear level density of $^{60}$Zn using neutron evaporation spectra from $^{58}$Ni($^3$He, n) measured at the Edwards Accelerator Laboratory. We compare our results to a number of theoretical predictions, including phenomenological, microscopic, and shell model based approaches. Notably, we find the $^{60}$Zn level density is somewhat lower than expected for excitation energies populated in the $^{59}$Cu(p,$gamma$)$^{60}$Zn reaction under rp-process conditions. This includes a level density plateau from roughly 5-6 MeV excitation energy, which is counter to the usual expectation of exponential growth and all theoretical predictions that we explore. A determination of the spin-distribution at the relevant excitation energies in $^{60}$Zn is needed to confirm that the Hauser-Feshbach formalism is appropriate for the $^{59}$Cu(p,$gamma$)$^{60}$Zn reaction rate at X-ray burst temperatures.
Starting from a Skyrme interaction with tensor terms, the $beta$-decay rates of $^{52}$Ca have been studied within a microscopic model including the $2p-2h$ configuration effects. We observe a redistribution of the strength of Gamow-Teller transitions due to the $2p-2h$ fragmentation. Taking into account this effect results in a satisfactory description of the neutron emission probability of the $beta$-decay in $^{52}$Ca.
According to the driving potential of a fissile system, we propose a phenomenological fission potential for a description of the pre-neutron emission mass distributions of neutron-induced actinide fission. Based on the nucleus-nucleus potential with the Skyrme energy-density functional, the driving potential of the fissile system is studied considering the deformations of nuclei. The energy dependence of the potential parameters is investigated based on the experimental data for the heights of the peak and valley of the mass distributions. The pre-neutron emission mass distributions for reactions 238U(n, f), 237Np(n, f), 235U(n, f), 232Th(n, f) and 239Pu(n, f) can be reasonably well reproduced. Some predictions for these reactions at unmeasured incident energies are also presented.
Incident neutron energy dependence of delayed neutron yields of uranium and plutonium isotopes is investigated. A summation calculation of decay and fission yield data is employed, and the energy dependence of the latter part is considered in a phenomenological way. Our calculation systematically reproduces the energy dependence of delayed neutron yields by introducing an energy dependence of the most probable charge and the odd-even effect. The calculated fission yields are assessed by comparison with JENDL/FPY-2011, delayed neutron activities, and decay heats. Although the fission yields in this work are optimized to delayed neutron yields, the calculated decay heats are in good agreement with the experimental data. Comparison of the fission yields calculated in this work and JENDL/FPY-2011 gave an important insight for the evaluation of the next JENDL nuclear data.
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