Cosmogenic activation of silicon
R. Saldanha, R. Thomas, R. Tsang, Á. Chavarría, R. Bunker, Jonathan L. Burnett, S. R. Elliott, Ariel Matalon, P. Mitra, A. Piers, Paolo Privitera, Karthik Ramanathan, R. Smida
Abstract
The production of $^{3}\mathrm{H}$, $^{7}\mathrm{Be}$, and $^{22}\mathrm{Na}$ 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 $(112\ifmmode\pm\else\textpm\fi{}24)\text{ }\mathrm{atoms}/(\mathrm{kg}\text{ }\mathrm{day})$ for $^{3}\mathrm{H}$, $(8.1\ifmmode\pm\else\textpm\fi{}1.9)\text{ }\text{ }\mathrm{atoms}/(\mathrm{kg}\text{ }\mathrm{day})$ for $^{7}\mathrm{Be}$, and $(43.0\ifmmode\pm\else\textpm\fi{}7.2)\text{ }\text{ }\mathrm{atoms}/(\mathrm{kg}\text{ }\mathrm{day})$ for $^{22}\mathrm{Na}$. 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 $(124\ifmmode\pm\else\textpm\fi{}25)\text{ }\text{ }\mathrm{atoms}/(\mathrm{kg}\text{ }\mathrm{day})$ for $^{3}\mathrm{H}$, $(9.4\ifmmode\pm\else\textpm\fi{}2.0)\text{ }\text{ }\mathrm{atoms}/(\mathrm{kg}\text{ }\mathrm{day})$ for $^{7}\mathrm{Be}$, and $(49.6\ifmmode\pm\else\textpm\fi{}7.4)\text{ }\text{ }\mathrm{atoms}/(\mathrm{kg}\text{ }\mathrm{day})$ for $^{22}\mathrm{Na}$. 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.