The concept of nuclear photon strength functions: A model-independent approach via ([formula omitted]) reactions

J Isaak,D Savran,B Löher,T Beck,M Bhike,U Gayer,Krishichayan Krishichayan,N Pietralla,M Scheck,W Tornow,V Werner,A Zilges,M Zweidinger

doi:10.1016/j.physletb.2018.11.038

Abstract

Most theoretical approaches used in nuclear astrophysics to model the nucleosynthesis of heavy elements incorporate the so-called statistical model in order to describe the excitation and decay properties of atomic nuclei. One of the basic assumptions of this model is the validity of the Brink–Axel hypothesis and the related concept of so-called photon strength functions to describe γ-ray transition probabilities. We present a novel experimental approach that allows for the first time to experimentally determine the photon strength function simultaneously in two independent ways by a unique combination of quasi-monochromatic photon beams and a newly implemented γ–γ coincidence setup. This technique does not assume a priori the validity of the Brink–Axel hypothesis and sets a benchmark in terms of the detection sensitivity for measuring decay properties of photo-excited states below the neutron separation energy. The data for the spherical off-shell nucleus 128Te were obtained for γ-ray beam-energy settings between 3 MeV and 9 MeV in steps of 130 keV for the lower beam energies and in steps of up to 280 keV for the highest beam settings. We present a quantitative analysis on the consistency of the derived photon strength function with the Brink–Axel hypothesis. The data clearly demonstrate a discrepancy of up to a factor of two between the photon strength functions extracted from the photoabsorption and photon emission process, respectively. In addition, we observe that the photon strength functions are not independent of the excitation energy, as usually assumed. Thus, we conclude, that the Brink–Axel hypothesis is not strictly fulfilled in the excitation-energy region below the neutron separation threshold (Sn=8.78MeV) for the studied case of 128Te.

Highlights

The modeling of a significant part of the synthesis of the elements in the universe is built on basic assumptions in the description of nuclear properties, such as the so-called Brink–Axel (BA) hypothesis [1,2]
The analysis presented in the previous section was performed for all photon-beam settings
It is evident that the experimental data sets for different beam energies are not consistent with each other, i.e., they do not provide a unique shape for the derived photon strength functions (PSF)

Summary

Introduction

The modeling of a significant part of the synthesis of the elements in the universe is built on basic assumptions in the description of nuclear properties, such as the so-called Brink–Axel (BA) hypothesis [1,2]. Solute excitation energy of the nucleus or the specific properties of the nuclear states involved This hypothesis is used as the basis of the treatment of γ -ray transition probabilities in the concept of photon strength functions (PSF). [33] does not allow to draw final conclusions and that statement remains qualitative In this manuscript we present a new experimental technique that allows for the first time to derive the PSF from excitations of the ground state to excited states as well as from the subsequent decay of these excited states in a single experiment in two model-independent ways. This unique combination allows for the experimental extraction of the PSF in two independent ways as outlined below

Experimental approach

Experimental details

Results and discussion

Conclusion