The Efficiency Instability Problem of Coaxial Germanium Detectors

The generally accepted belief in constant fullenergy peak efficiency for coaxial germanium gamma ray detectors maintained at liquid nitrogen temperature has been found to be invalid in many cases. A majority of the available raw germanium ingots possess a shortcoming which causes detectors to decline in efficiency with time, if the standard techniques described in the literature have been employed for their production. A strong correlation of efficiency instability with ingot has been established. The time rate of change varies markedly from ingot to ingot. Over 60% of the ingots studied resulted in detectors whose efficiencies declined at rates greater than 8% per year. A substantial fraction of the cases studied displayed relative decreases of 10-20% in a period of six months. Declines of as much as 10-25% in several weeks were also observed.