Independent Normalization for $γ$-ray Strength Functions: The Shape Method

Abstract

The Shape method, a novel approach to obtain the functional form of the γ-ray strength function (γSF) in the absence of neutron resonance spacing data, is introduced. When used in connection with the Oslo method the slope of the Nuclear Level Density (NLD) is obtained simultaneously. The foundation of the Shape method lies in the primary γ-ray transitions which preserve information on the functional form of the γSF. The Shape method has been applied to 56 Fe, 92 Zr, 164 Dy, and 240 Pu, which are representative cases for the variety of situations encountered in typical NLD and γSF studies. The comparisons of results from the Shape method to those from the Oslo method demonstrate that the functional form of the γSF is retained regardless of nuclear structure details or J π values of the states fed by the primary transitions.

Comprehensive test of the Brink-Axel hypothesis in the energy region of the pygmy dipole resonance

“…An analog analysis of the final-state energy dependence shows comparable agreement. Finally, we test the spin independence of the present results by applying a novel approach to extract the energy dependence of the GSF in a largely model-independent way (the so-called Shape method [57]). Here, we use the capability of the OSCAR array to resolve the decay to the ground state and to the first excited 2 + state studying again the case of the 120 Sn isotope (regions 1 and 2 in Fig.…”

“…, one can extract information on the shape of the γ strength from the intensities N D proportional to the average, partial radiative width Γ γ (E i , E f , E γ , J π i ) in the diagonals [57]. The GSF deduced from the Shape method is shown in Fig.…”

“…Further tests of the BA hypothesis include a demonstration of the independence of the GSFs from the energies and spins of initial and final states. The latter utilizes the novel Shape method [57] which allows a largely modelindependent extraction of the energy dependence of the GSF from the selective decay to specific final states.…”