Cosmogenic activation of silicon

The production of H-3, Be-7, and Na-22 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 +/- 24) atoms/(kg day) for H-3, (8.1 +/- 1.9) atoms/(kg day) for Be-7, and (43.0 +/- 7.2) 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 +/- 25) atoms/(kg day) for H-3, (9.4 +/- 2.0) atoms/(kg day) for Be-7, and (49.6 +/- 7.4) atoms/(kg day) for Na-22. 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.