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.

Highlights in gravitational lensing

“…With the detailed observational information available for QSO 0957+561 a where ugUu is the velocity dispersion of the lens galaxy in units of 300 km/s and At,,, is the time delay in years. QSO 0957+561 has now been monitored since its detection in 1979 in the optical (hnderriest et a1 1989, Schild 1990, Florentin-Nielsen 1984 and in the radio (Roberts et a1 1991). It is diflicult to extract reliable information on the time delay from the data, and various different values have been published.…”