Cosmogenic activation in detector materials at shallow depths

TL;DR

This paper analyzes cosmogenic activation in detector materials at shallow depths, focusing on production rates of isotopes like tritium and 60Co, and evaluates suppression factors at various sites to inform low-background experiments.

Contribution

It provides the first detailed calculations of cosmogenic activation in detector materials at shallow depths, including suppression factors and isotope production estimates.

Findings

Calculated tritium production in Ge and Si at shallow depths.

Estimated 60Co production in Cu at shallow depths.

Provided activation suppression factors for several shallow-depth sites.

Abstract

The radioactive decay from long-lived radioactive isotopes produced by cosmogenic activation can be an important background in direct-detection dark matter and neutrino experiments. In general, activation of materials located above ground is dominated by nuclear spallation due to energetic neutrons produced as secondary particles from primary cosmic ray interactions in the atmosphere. As experiments become larger and strive for greater sensitivity to rare events, it is increasingly important to store, assemble, and even fabricate the detector materials underground to mitigate cosmogenic activation. There has been no study of cosmogenic activation in detector materials at shallow depths (< 100 meter-water-equivalent). Unlike at aboveground or at deep depths, where neutrons are the major contributors to activation in materials, there are multiple competing physical processes that contribute to the activation in materials at shallow depths. We present a detailed calculation of the production of tritium in Ge and Si, as well as the production of 60Co in Cu, at shallow depths. We also obtain cosmogenic activation suppression factors and tritium production at several shallow-depth sites including the Stanford Underground Facility (SUF), where the SuperCDMS collaboration stored Ge, Si, and Cu detector materials for a substantial period of time.