Isomer yields in nuclear fission

A Al-Adili,M Österlund,S Pomp,Z Gao,M Lantz,A Solders

doi:10.1051/epjconf/202125600002

Abstract

The generation of angular momentum in the fission process is still an open question. To shed light on this topic, we started a series of measurements at the IGISOL-JYFLTRAP facility in Finland. Highprecision measurements of isomeric yield ratios (IYR) are performed with a Penning trap, partly with the aim to extract average root-mean-square (rms) quantities of fragment spin distributions. The newly installed Phase-Imaging Ion-Cyclotron Resonance (PI-ICR) technique allows the separation of masses down to tens of keV, which is suffcient to disentangle many isomers. In this paper, we first summarize the previous measurements on the neutron and proton-induced fission of uranium and thorium, e.g. the odd cadmium and indium isotopes (119 ≤ A ≤ 127). The measurements revealed systematic trends as function of mass number, which stimulated further exploration. A recent measurement was performed at IGISIOL and several new IYR data will soon be published, for the first time. Secondly, we employ the TALYS nuclear-reaction code to model one of the newly measured isomer yields. Detailed GEF and TALYS calculations are discussed for the fragment angular momentum distribution in 134I.

Highlights

While isomeric fission yields are an important part of a full fission-process description, the data libraries are incomplete and lack important systematic investigations of the isomer production cross section
E.g. the conflicting experimental results concerning a possible saw-tooth trend in the average J(A) [5]. Another important question is how angular momentum is shared among individual and collective degrees of freedom and how that eventually depends on the compound nucleus and excitation energy
We provide TALYS with input files parameterising the excitation energy with Gaussian distributions, which is a good first order approximation for each neutron emission channel

Summary

Introduction

While isomeric fission yields are an important part of a full fission-process description, the data libraries are incomplete and lack important systematic investigations of the isomer production cross section. Often isomeric ratio data are originating from γ-ray measurements, and there are wide discrepancies in the reported yields, most likely due to lack of nuclear structure data. Novel methods that reduce the dependence on level structures are important to improve the isomeric yield data situation. E.g. the conflicting experimental results concerning a possible saw-tooth trend in the average J(A) [5]. Another important question is how angular momentum is shared among individual and collective degrees of freedom and how that eventually depends on the compound nucleus and excitation energy. With the aid of nuclear de-excitation codes, one can model the angular momentum population of the primary fission fragments [6].

Summary of IYR experiments at IGISOL

Conclusions and outlook