Speaker
Description
Measurement of lifetime of nuclear excited states and extraction of electromagnetic transition strengths from that provides direct insight into nuclear structure. Gamma ray coincidence spectroscopy with new-age fast scintillator detectors, such as, LaBr$_{3}$(Ce) and CeBr$_{3}$ serves as a useful tool for lifetime measurements in sub-nanosecond ranges. Although the energy resolution of CeBr$_{3}$ is slightly poorer than that of LaBr$_{3}$(Ce), but with comparable time resolution and without any internal activity, CeBr$_{3}$ scintillator detectors emerge as a potential alternative to LaBr$_{3}$(Ce).
At Variable Energy Cyclotron Centre (VECC), Kolkata, 1.5′′ x 1.5′′ CeBr$_{3}$detectors coupled with a new Photo-Multiplier tube Hamamatsu R13089-100 has been characterized. An energy resolution of 4.1% has been obtained at 662 keV of $^{137}$Cs source. Absolute photo-peak efficiencies at different source-to-detector distances have been measured [1]. GEANT4 simulation has been carried out which reproduces pulse height spectra and absolute efficiencies reasonably well. The best time resolution (TAC FWHM) of 199(2) ps between two CeBr$_{3}$detectors for 1173-1332 energy cascade of $^{60}$Co source and 327(3) ps for 511-511 keV of $^{22}$Na source have been obtained after optimizing various parameters. With the knowledge of basic characteristics of two detector set-up, time-walk response for this set-up was determined using Mirror Symmetric Centroid Difference (MSCD) method [2]. $^{152}$Eu source has been used to calibrate Prompt Response Difference[PRD(E$_{\gamma}$)] curve for different high voltages at various CFD delays [3]. For each set-up, known lifetimes of two states of $^{133}$Cs - (3/2)$^{+}$state at 384 keV and (5/2)$^{+}$state at 161 keV, populated via electron capture decay of $^{133}$Ba, have been reproduced.
Nuclei near $^{208}$Pb region are expected to have spherical structure at lower spin and collective structure at higher spin and excitation energies. For even-even Po (Z=84) isotopes in this region, the variation of R$_{4/2}$ ratio approaches towards vibrational limit as neutron holes increase whereas, E2 transition strength increase from $^{210}$Po to $^{206}$Po [4]. The low-lying states of neighboring odd-A nuclei in this region are mainly described by the coupling of one neutron hole with the nearest even-even core. The lifetime measurement of low-lying states of Po isotopes will be of great importance to understand the interplay between single particle and collective structure. Lifetime of (11/2)$^{-}$state at 1521.85 keV of $^{209}$Po has been determined. The excited states of $^{209}$Po were populated via electron capture decay of $^{209}$At which was produced using the reaction $^{209}$Bi ($\alpha$, 4n) $^{209}$At at 52 MeV beam energy at VECC, kolkata. The value obtained has been found to be in good agreement with the previously reported value [5].
References:
[1] S.Das, et al., Proc. of DAE-BRNS Symp. on Nucl. Phys.70 (2019) 986
[2] J.-M.Regis, et al., NIM A 622 (2010) 83-92
[3] J.-M.Regis, et al., NIM A 684 (2012) 36-45
[4] M. Stoyanova, et al., PRC 100, 064304 (2019)
[5] V. Karayonchev, et al., PRC 103, 044309 (2021)
