Dependence of spin induced structural transitions on level density and neutron emission spectra

Abstract

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 dnesity 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.

Excitation energy and angular momentum dependence of the nuclear level density parameter around A$\approx $110

“…In contrast to the strong dependence of the LD parameter on the angular momentum reported in the above mentioned works, experimental data for the heavier systems showed less sensitivity of k on J [35,36]. Some of the experimental data on the angular dependence of k could be successfully explained by the theoretical calculations by considering spin induced deformation and shape phase transitions under the framework of a statistical theory of hot rotating nuclei [41,42]. In view of the observed variation of the level density parameter with J in the earlier works, it will be interesting to extend the study for similar systems especially to higher spin regions.…”

“…It is observed that the experimental k-values increases as a function of the mean angular momentum for both the 16 O + 93 Nb and 20 Ne + 93 Nb reactions in the measured angular momentum range of <J>≈ 20 -36 . The experimental results have been compared with the microscopic calculations performed using the statistical model of hot rotating nuclei [41,42,62], described briefly in the following section (Sec. III C).…”

“…To investigate the observed energy and angular momentum dependence of the level density parameter theoretically, a microscopic calculation has been performed within the theoretical framework that involves the statistical theory [41,42,42,63], and the Macroscopic-Microscopic approach using the triaxially deformed Nilsson-Strutinsky model [64,65]. In this model, the excited compound nuclei are described as the thermodynamical system of fermions incorporating their collective and non-collective rotational degrees of freedom.…”

“…In the level density formulations based on the simplistic FGM, the level density parameter does not explicitly depend on the excitation energy or angular momentum. However, a number of earlier studies have shown that the parameter k (and thus a) depends on excitation energy (temperature) [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34], and angular momentum [35][36][37][38][39][40][41][42] both, in an intricate manner. Such departures from the FGM may not be surprising as the actual single-particle spectrum of a nucleus is considerably complicated than the simple Fermi gas picture.…”

Impact of the quenching of shell effects with excitation energy on nuclear level density

“…The experimental techniques such as the measurement of primary γ−ray spectra [17], direct counting of nuclear levels [18][19][20][21], analysis of neutron resonance spacings [22], and SM analysis of particle-evaporation spectra in Heavy-Ion fusion reactions [23][24][25][26][27][28][29][30][31][32][33][34][35][36] to compute the nuclear level density provide useful information on various aspects of statistical behaviour of compound nuclei and level densities. Comparison of the spacings of nuclear levels in the neutron resonance data for closed shell and mid-shell nuclei at a similar excitation energy reveal [37][38][39][40] that the larger shell gap in magic nuclei results in much smaller density of resonances pointing towards a strong dependence of level density on nuclear shell structure [41][42][43][44][45]. The nuclear shell effects [46][47][48], that arise from one-body interactions, are known to provide extra stability to the closed shell magic nuclei that seem to play a major role in the nuclear phenomena like production of super heavy elements (SHE) [49,50], fission isomers [51], superdeformed nuclei [52] and changing magicity in exotic nuclei [53][54][55].…”