RAINIER: A Simulation Tool for Distributions of Excited Nuclear States and Cascade Fluctuations

TL;DR

RAINIER is a new simulation tool that models gamma-ray decay in nuclear states, incorporating realistic fluctuations and enabling comparison with experimental data, filling gaps left by previous methods.

Contribution

RAINIER introduces a comprehensive simulation approach for gamma-ray decay that includes cascade fluctuations across a wide range of initial states, surpassing existing tools in scope and realism.

Findings

Cascade fluctuations significantly affect gamma-ray spectra.

RAINIER effectively models decay from diverse initial states.

Comparison with experimental data validates RAINIER's accuracy.

Abstract

A new code has been developed named RAINIER that simulates the $\gamma$-ray decay of discrete and quasi-continuum nuclear levels for a user-specified range of energy, angular momentum, and parity including a realistic treatment of level spacing and transition width fluctuations. A similar program, DICEBOX, uses the Monte Carlo method to simulate level and width fluctuations but is restricted to $\gamma$-ray decay from no more than two initial states such as de-excitation following thermal neutron capture. On the other hand, modern reaction codes such as TALYS and EMPIRE populate a wide range of states in the residual nucleus prior to $\gamma$-ray decay, but do not go beyond the use of deterministic functions and therefore neglect cascade fluctuations. This combination of capabilities allows RAINIER to be used to determine quasi-continuum properties through comparison with experimental data. Several examples are given that demonstrate how cascade fluctuations influence experimental high-resolution $\gamma$-ray spectra from reactions that populate a wide range of initial states.