Speaker
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Abstract:
The pairing correlation is essential for understanding low-energy nuclear structure, particularly under varying conditions of temperature and angular momentum.At low temperatures and angular momentum, nucleons near the Fermi surface tend to pair up, leading to strong pairing correlations.However, as temperature (T) or angular momentum (J) increases, these pairing of nucleons is broken down and thus decrease in pairing correlations. But under specific conditions of temperature and angular momentum pairing can re-occur for specific neutron and proton number & this phenomenon is known as "pairing re-entrance phenomenon". This phenomenon was first predicted by Kammuri[1] and further explained by Moretto[2] using the Bardeen-Cooper-Schrieffer theory at finite temperatures (FTBCS) and angular momentum.This re-entrance of pairing correlations significantly influences nuclear level density and can be probed by studying level densities as functions of temperature and angular momentum. There is no experimental observations on this important phenomenon to our knowledge . In the present work , we therefore investigated the influence of T and J dependent nuclear level density on neutron evaporation spectrum and looking for pairing effect in Ge isotopes.
The experiment was conducted at the BARC-TIFR Pelletron Linac Facility using a specialized setup for measuring gamma-ray multiplicity gated neutron spectra. The pulsed beam of 9Be with an energy of 28 MeV was used to bombard on self-supporting 64Ni targets, producing the compound nuclei 73Ge. The neutrons from 73Ge were measured in coincidence with a low energy gamma rays. An array of 15 Liquid Scintillation (LS) detectors, placed at a flight path of 72.5 cm were used to detect fast neutrons by time-of-flight (TOF) technique and the gamma rays were detected using an array of 38 BGO detectors arranged in two close-packed groups of 19 detectors each, for the angular momentum information populated in the 73Ge. The BGO groups were positioned approximately 7 cm on either side of the target. The energy and time signals from each BGO detector, gamma-ray multiplicity, LS detector energy, pulse shape discrimination (PSD), and TOF with respect to the filtered radiofrequency (RF) signal were recorded in an event-by-event mode using a VME-based data acquisition system. Calibration of the detector systems was meticulously performed. The energy of BGO and LS detectors was carried out using standard gamma sources and time calibration was done using precision time calibrator.
The TOF spectra obtained from the LS detectors were converted into neutron energy spectra using appropriate Jacobian transformations & efficiency correction. The experimental data reveal no significant deviation in the inclusive spectra. However,a small deviations is observed starting from a neutron energy of 10.5 MeV when compared to predictions from the conventional statistical model calculations in the fold-gated (related with J) neutron evaporation spectra for folds 6, 7, and 8. This deviation occurs at a temperature T∼1.55±0.05 MeV and high angular momenta J∼14±2ℏ estimated from the excitation energy and fold distribution respectively. These deviations reasonably explained invoking the combined effect of J and T dependent nuclear level density and enhancement due to pairing correlation as reported in [3]. These results indicate a weak signature of pairing re-entrance phenomenon in 72Ge at such high T and J which aligns with theoretical predictions from the FTBCS with quasi-particle fluctuations[4].This understanding will throw light on the intricate interplay of temperature and angular momentum on pairing correlations for many body finite quantum system.
References:
1. Kammuri T 1964 Prog. Theor. Phys. 31 595–608
2. Moretto L G 1971 Phys. Lett. B 35 379–82
3. Mitra A, et.al., J. Phys. G: Nucl. Part. Phys. 36 095103, (2009)
4. N. Quang Hung et. al., PHYSICAL REVIEW C 84, 054324 (2011)
