It's not so much that the gamma radiation doesn't kill coronavirus - it does - but that it degrades the way the masks work after they're zapped, according to a report by MIT News.
The testing involved putting some masks in a container for irradiation from some Cobalt-60 in an MIT lab.
Michael Short, an MIT professor of nuclear science and engineering, then bicycled the masks over to Brigham and Women's Hospital for further testing - He sent three hours being sprayed with a mist of saccharin. He never got a taste of the stuff, so the masks seemed to be OK, but then further testing back at MIT showed that the irradiation had damaged the masks' ability to repel or capture even smaller salt molecules shot at it. And that meant they would be useless as the sort of viral shield medical workers need.
Researchers uploaded their results to an online site on March 28. Since then, a Chicago non-profit has delivered a mask sterilizing system - which uses concentrated hydrogen peroxide vapor - to an abandoned Kmart in Somerville, where it will sterilize up to 80,000 masks a day.