Cosmogenic activation of silicon

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.

21 pages, 16 figures, 10 tables. Accepted version of the article published in Physical Review D