PNNL

Abstract

The production of 3H, 7Be, and 22Na by interactions of cosmic-ray particles with silicon can produce radioactive backgrounds in detectors used to search for rare events. Through controlled ir10 radiation 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 +/- 24) atoms/(kg day) for 3H, (8.1 +/- 1.9) atoms/(kg day) for 7Be, and (43.0 +/- 7.1) atoms/(kg day) for 22Na. 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 15 cosmic-ray production rate of (124 +/- 24) atoms/(kg day) for 3H, (9.4 +/- 2.0) atoms/(kg day) for 7Be, and (49.6 +/- 7.3) atoms/(kg day) for 22Na. 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 dark matter searches.