Efficient Monte Carlo simulation of coincidence effects in radioisotope decays including γ-γ angular correlations

Abstract

Many nuclear decays lead to the emission of multiple gamma-rays or X-rays in close temporal coincidence. Nuclear polarization effects lead to angular correlations between these successive gamma-ray emissions that depend on the spins of the states involved and the multipolarities of the transitions in question. To accurately simulate the transport and detection of such coincident gamma decays, these polarization effects should be taken into account. However, most standard Monte Carlo codes either ignore coincidence effects completely, or treat the direction of each gamma-ray emitted in a decay cascade as uncorrelated and isotropic. We have developed tools to facilitate the accurate simulation of decays or arbitrary radioisotopes based on level-scheme and decay data available in the Evaluated Nuclear Structure and Decay Files (ENSDF) library. Analogue simulation of coincidence effects is inefficient in many practical cases, necessitating the use of variance reduction techniques. Implementing biased sampling in situations where coincidence effects are important is recognized as a complicated problem. We have developed a modified approach that simplifies the coding of variance reduction techniques for such cases. We demonstrate that a combination of deterministic transport and forced collision biasing allows for efficient simulation of coincidence effects in gamma-ray detection.