Giant dipole resonance width in nuclei near Sn at low temperature and high angular momentum

Abstract

High energy γ-rays in coincidence with low energy yrast γ-rays have been measured from 113 Sb, at excitation energies of 109 and 122 MeV, formed by bombarding 20 Ne on 93 Nb at projectile energies of 145 and 160 MeV respectively to study the role of angular momentum (J) and temperature (T) over Giant Dipole Resonance (GDR) width (Γ). The maximum populated angular momenta for fusion were 67h and 73h respectively for the above-mentioned beam energies. The high energy photons were detected using a Large Area Modular BaF 2 Detector Array (LAMBDA) along with a 24-element multiplicity filter. After pre-equilibrium corrections, the excitation energy E * was averaged over the decay steps of the compound nucleus (CN). The average values of temperature, angular momentum, CN mass etc. have been calculated by the statistical model code CASCADE.Using those average values, results show the systematic increase of GDR width with T which is consistent with Kusnezov parametrization and the Thermal Shape Fluctuation Model. The rise of GDR width with temperature also supports the assumptions of adiabatic coupling in the Thermal Shape Fluctuation Model. But the GDR widths and corresponding reduced plots with J are not consistent with the theoretical model at high spins.

The hot GDR revisited

“…Fig. 1 Schematic view of the experimental setup for the LAMBDA spectrometer in a 7 × 7 matrix arrangment along with the low energy γ -ray multiplicity filter [21] In the early experiments NaI(Tl) detectors were used, mainly because they were readily available. One of the main sources of background in these experiments is due to neutrons which can be separated from γ -rays by a time of flight measurement and NaI(Tl) exhibits mediocre timing properties.…”

“…Figure 1 shows a schematic view of a typical set up for such low energy experiments taken from Ref. [21].…”

“…Then the weighted average of the nuclear temperature is calculated only from the initial value down to T eff . Such approach, followed by different authors [21,50], leads to temperatures which are slightly higher than the one obtained from the average of the whole decay sequence, the difference becoming increasingly important with the excitation energy of the hot decaying system. In some cases, at low excitation energies, the temperature was extracted from the excitation energy of the compound system populated in the reactions corrected for the average rotational energy and for the GDR resonance energy [52,53].…”

Decoupling the effect of temperature on GDR widths in excited compound nucleus144Sm

“…The average temperature is defined as 〈 〉 T =Σ Σ w T w i i i , where w i is the weight for the ith step in decay chain and was calculated as the γ-ray yield in the energy range of 12-25 MeV. A few authors have reported an 'effective' temperature (T eff ) [12,17,50], which is defined as the temperature at which there is 50% of the high energy γ-ray yield (12)(13)(14)(15)(16)(17)(18)(19)(20) which is emitted between the threshold temperature (below which there is no significant change in GDR width) and the temperature of the initial compound nucleus. The effective temperature, in turn, considers only GDR contributing events in the decay cascade and thus, the average temperature 〈 〉 T in present context is similar to T eff .…”

“…In the past few years, a large number of experiments have been dedicated to study the evolution of GDR widths with spin and temperature. Several authors have reported the measurement of GDR widths at finite temperatures in different mass regions, and these measurements could be categorized as low temperature measurements (below 1.5 MeV) [6][7][8][9][10][11][12][13], moderate temperature measurements (1.5-2.5 MeV) [14,15], and high temperature measurements (above 2.5 MeV) [16][17][18]. The general experimental method adopted in these experiments is to detect high energy GDR γ-rays in coincidence with low energy γ-ray multiplicity, which could be related to the total angular momentum of the nucleus.…”