Evaporation ofαparticles from the31Pnucleus

Abstract

The energy spectra of α-particles have been measured in coincidence with the evaporation residues for the decay of the compound nucleus 31 P, produced in the reaction 19 F (96 MeV) + 12 C. The data have been compared with the predictions of the statistical model code CASCADE. It has been observed that significant deformation effect in the compound nucleus need to be considered in order to explain the shape of the evaporated α-particle energy spectra.One of the main motivations of the low energy heavy ion reactions studies has been to extract informations on the statistical properties of the hot, rotating nuclei. The informations on the main ingredients of the statistical description, i.e., the nuclear level densities and the barrier transmission probabilities, are usually obtained from the study of the evaporated light particle spectra. The validity of the statistical model depends crucially on the successful description of the light particle emission data and the model, so far, has been overwhelmingly successful in explaining a wide variety of nuclear reaction data in low energy regime. In this perspective, recent studies on the evaporated α-particle energy spectra has evoked a lot of interest (see, for example, ref.[1] and references therein). It has been observed that the standard statistical model calculations failed to predict the shape of the evaporated α-particle energy spectra satisfactorily. A large number of experiments have been performed to study this anomaly over a wide range of compound nuclear masses A CN in the range of ∼60-170 [1-6], and in all cases it has been found that the average energies of the measured α-particle energy spectra are much lower than the corresponding theoretical predictions. Several attempts have been made in the past few years to explain this anomaly. Some of the authors [2,3] argued that the discrepancy was due to the lowering of the emission barriers of the hot nuclei as compared to the fusion barriers for the corresponding relatively 'cold' inverse absorption channels as a result of the excitation energy and angular momentum dependent deformation of the emitting system in the former. On the other hand, there is another group of authors who claim that the anomaly may be well explained by incorporating spin dependent level density in the standard statistical model prescription and emission barriers need not be changed [4,5]. Moreover, it has also been observed that the magnitude of the discrepancy has some entrance channel dependence [6], the discrepancy being more for the more symmetric entrance channels. This is indicative of the fact that the magnitude of the phenomena may also be linked with the entrance channel dynamics of the system. Intuitively, the shape of the α-particle spectra would be affected by the deformation of the equilibrating system if it remains deformed over a time scale comparable to the mean life time of α emission. For heavier nuclei (A CN > 60), the theoretical calculations of shape equilibration time, using the code HICOL [1,7] show that these ti...

Light-charged-particle evaporation from hot 31P nucleus at E*∼ 60 MeV

“…This is realized by varying only the radius parameter r • . The optimum value of r • is found to be ∼ 1.56 fm which is consistent with our earlier work [9]. The increased radius parameter lowers the emission barrier and hence enhances the low energy yield by increasing the transmission coefficients.…”

“…But our previous studies on the intermediate mass fragment emission did not reflect the existence of any such longlived dinuclear configuration [13,14] of the 31 P compound nucleus. Earlier, we have measured also α-particle spectra in coincidence with the evaporation residues in the 96 MeV 19 F on 12 C reaction [9] and have observed that the measured spectra deviate from the statistical model predictions both at low as well as at high energy sides. Assuming a deformed configuration of the compound nucleus, the full energy spectra could be explained satisfactorily.…”

“…The region is bounded by the yrast line defined by the lowest energy state that occurs for a given angular momentum. It has been observed in the previous experiments [5,9] that the yrast line has to be modified to explain the experimental data. Different attempts have been pursued using different tools to modify the yrast line, such as, radius parameter r • , deformation parameters δ 1 , δ 2 and the angular momentum j.…”

“…However, recent studies reveal that, at high excitation energy and angular momentum, the energy spectra of evaporated light charged particles, are no longer consistent with the standard statistical model predictions. A large number of experiments have been performed in last few years to study this anomaly over a wide range of compound nuclear masses (A CN ) in the range ∼60-170, for example [1][2][3][4][5][6][7][8][9][10][11][12], and in most cases (with the exception of [1]), it has been found that the statistical model fails to explain the observed light charged particle energy spectra. Choudhury et al [2] have measured the α-particle energy spectra in the reaction 214 MeV 32 S on 27 Al and observed that the measured spectra deviate considerably from the standard statistical model predictions.…”

Deformation in28Si*produced via the16O+Kundu1, Bhattacharya2, Bhattacharya3 et al. 2013Phys. Rev. CSelf Cite

“…An orbiting dinuclear complex is assumed to be more deformed than a fully shape equilibrated CN; this motivated us to look for such enhanced deformation in a 16 O + 12 C dinuclear system at same excitation energies using the LCP spectroscopy technique. In this technique, LCPs, evaporated from the excited composites, are used to study the properties of hot rotating nuclear systems as a function of excitation energy, angular momentum, and deformation [11,[13][14][15][16][17][18][19][20][21][22][23][24][25] using statistical model codes, e.g., CASCADE [26], GANES [27], CACARIZO [28].…”