Evolution of giant dipole resonance width at low temperatures – New perspectives

Abstract

High energy photons from the decay of giant dipole resonances (GDR) built on excited states provide an excellent probe in the study of nuclear structure properties, damping mechanisms etc., at finite temperatures. The dependence of GDR width on temperature (T) and angular momentum (J) has been the prime focus of many experimental and theoretical studies for the last few decades. The measured GDR widths for a wide range of nuclei at temperatures (1.5 < T < 2.5 MeV) and spins (upto fission limit) were well described by the thermal shape fluctuation model (TSFM). But, at low temperatures (T < 1.5 MeV) there are large discrepancies between the existing theoretical models. The problem is compounded as there are very few experimental data in this region. At Variable Energy Cyclotron Centre, Kolkata, a programme for the systematic measurement of GDR width at very low temperatures has been initiated with precise experimental techniques. Several experiments have been performed by bombarding 7–12 MeV/nucleon alpha beam on various targets (⁶³Cu, ¹¹⁵In and ¹⁹⁷Au) and new datasets have been obtained at low temperatures (T < 1.5 MeV) and at very low spins ($J < 20 \hbar$). The TSFM completely fails to represent the experimental data at these low temperatures in the entire mass range. In fact, the GDR width appears to be constant at its ground state value until a critical temperature is reached and subsequently increases thereafter, whereas the TSFM predicts a gradual increase of GDR width from its ground state value for T > 0 MeV. In order to explain this discrepancy at low T, a new formalism has been put forward by including GDR induced quadrupole moment in the TSFM.