Cosmogenic activation of silicon

TL;DR

This study measures the production rates of cosmogenic isotopes in silicon caused by cosmic-ray neutrons, providing essential data to improve background estimates in rare-event detection experiments.

Contribution

It provides the first direct measurements of cosmogenic isotope production rates in silicon due to cosmic-ray neutrons at sea level.

Findings

Measured production rates for $^{3}$H, $^{7}$Be, and $^{22}$Na in silicon.

Quantified total cosmic-ray induced isotope production rates at sea level.

Results help optimize timing for silicon detector fabrication to minimize backgrounds.

Abstract

The production of $^{3}$H, $^{7}$Be, and $^{22}$Na by interactions of cosmic-ray particles with silicon can produce radioactive backgrounds in detectors used to search for rare events. Through controlled irradiation of silicon CCDs and wafers with a neutron beam that mimics the cosmic-ray neutron spectrum, followed by direct counting, we determined that the production rate from cosmic-ray neutrons at sea level is ($112 \pm 24$) atoms/(kg day) for $^{3}$H, ($8.1 \pm 1.9 $) atoms/(kg day) for $^{7}$Be, and ($43.0 \pm 7.1 $) atoms/(kg day) for $^{22}$Na. Complementing these results with the current best estimates of activation cross sections for cosmic-ray particles other than neutrons, we obtain a total sea-level cosmic-ray production rate of ($124 \pm 24$) atoms/(kg day) for $^{3}$H, ($9.4 \pm 2.0 $) atoms/(kg day) for $^{7}$Be, and ($49.6 \pm 7.3 $) atoms/(kg day) for $^{22}$Na. These measurements will help constrain background estimates and determine the maximum time that silicon-based detectors can remain unshielded during detector fabrication before cosmogenic backgrounds impact the sensitivity of next-generation rare-event searches.