Speaker
Description
The investigation of collective motion in nuclei, especially through vibrational modes, presents a valuable approach to examining various aspects of the structure of quantal many-body systems. One example of such a collective nuclear vibration is isovector giant dipole resonance (IVGDR) which is described as an out-of-phase oscillation between neutrons and protons. Generally, the width ($\Gamma_G$) of IVGDR is related to the various damping mechanism of this collective vibration and is an important observable to understand the structural details of excited nuclei. Over the years, significant number of experiments have been performed to understand the variation of $\Gamma_G$ with temperature (T) and spin (J) in different mass regions. Generally, $\Gamma_G$ increases with T within the range of 1 MeV $\lesssim$ T $\lesssim$ 3 MeV and further increase in T beyond this range may lead to a saturation in $\Gamma_G$ [1]. For temperature below 1 MeV, $\Gamma_G$ is found to be almost constant to its ground state value [2,3,4,5] and this suppression may results from various microscopic properties such as pairing fluctuation, shell effect etc. [6]. The Thermal Shape Fluctuation Model (TSFM) [7] is the most widely accepted model to explain the variation of the GDR width at mid-T region. The suppression of $\Gamma_G$ at low temperatures can be successfully explained by the Phonon Damping Model (PDM) [8] and the Critical Temperature Fluctuation Model (CTFM) [5]. This suppression of $\Gamma_G$ seems to reflect a widespread trend (for A ~ 30-208); however, an exception has been identified on $^{114}$Sn [9]. In a recent experiment, measurement of high-energy γ-rays have been performed for $^{124,136}$Ba at temperature around 1.1 MeV to study the properties of IVGDR over a wide N/Z range [10]. It has been observed that for $^{124}$Ba, $\Gamma_G$ shows little sensitivity to temperature, whereas for $^{136}$Ba, it increases significantly. A more comprehensive study across a broader mass range is needed to examine the impact of N/Z asymmetry and other microscopic properties on the temperature dependence of $\Gamma_G$.
With this motivation a systematic study of $\Gamma_G$ with T is conducted in an isotopic chain of Te, where shell effect is almost negligible. $^{116,120,128}$Te are populated at low temperature (0.8 MeV – 1.2 MeV) region via α-induced fusion reaction using K-130 cyclotron at VECC, India. The high energy γ-rays (E$_\gamma$ > 4 MeV) emitted from the decay of Te-isotopes are detected by using the Large Area Modular BaF$_2$ Detector Array (LAMBDA) [11]. A multiplicity filter [12] consisting of 50 BaF$_2$ (each having dimensions 3.5 × 3.5 × 5 cm$^3$ ) elements are used to measure the angular momentum (J) populated by the compound nucleus in event by event mode. A detailed offline analysis has been performed under CERN ROOT framework to extract the meaningful GDR spectra from raw spectra after incorporating different cuts. The measured spectra is analyzed by using statistical model code TALYS. The result shows that for all isotopes the $\Gamma_G$ remains almost constant upto T ~ 1 MeV and then increases with temperature. Details will be presented in the conference.
References :
[1] D. R. Chakrabarty, et al., Eur. Phys. J A 52 (2016) 143.
[2] S. Mukhopadhyay, et al., Phys. Lett. B 709 (2012) 9.
[3] Balaram Dey, et al., Phys. Lett. B 731 (2014) 92.
[4] Debasish Mondal, et al., Phys. Lett. B 784 (2018) 423.
[5] Deepak Pandit et al., Phys. Lett. B 713 (2012) 434.
[6] A. K. Rhine Kumar, et al., Phys. Rev. C 91 (2015) 044305.
[7] Y.Alhassid, et al., Phys. Rev. Lett. 61 (1998) 1926.
[8] N. D. Dang, et al., Phys. Rev. C 87 (2013) 054313.
[9] A. Stolk et al., Nucl. Phys. A 505 (1989) 241.
[10] C. Ghosh, et al., Phys. Rev. C 96 (2017) 014309.
[11] S. Mukhopadhyay et al., NIM A 582 (2007) 603.
[12] Deepak Pandit, et al., Nucl. Instr. Meth. A 624 (2010) 148.
