Cosmic Ray Events and Natural Radioactivity in CCD Cryostats

Abstract

We have found that many materials that are most commonly used in CCD cryostats are weakly radioactive and therefore contribute to what is rather liberally labelled as Cosmic Ray Events. Some standard optical glasses that are extensively used for lenses and optically flat windows such as UBK-7 contain large amounts of potassium 40, which renders them useless as windows in CCD dewars. Cobalt 60 is sometimes found in excessive amounts in some steel alloys, even in Covar used as a thermal match to silicon. The choice of materials that emit a minimum of ionizing radiation is discussed.