Deterioration of large Ge(Li) diodes caused by fast neutrons

Abstract

The large Ge(Li) gamma-ray detector has become a powerful tool for the investigation of nuclear reactions. Unfortunately these detectors are quite susceptible to fast neutron damage and this makes it difficult to decide whether or not to risk using a detector to study reactions at accelerators where fast neutrons are inevitably present. After ruining several detectors we decided to study the problem in a controlled way. A 30 cm 3 true coaxial diode was systematically irradiated by neutrons from a plutonium-beryllium source. An increase in the width of the 2.614 MeV gamma-ray from 208Tl was first detected after an irradiation of 5 × 10 7 n/cm 2. When the total irradiation had reached 6 × 10 8 n/cm 2, the peak width had increased by more than 50 percent. The irradiated detector was then reprocessed to remove the damage. The diode was again subjected to neutron irradiation. The second curve of resolution deterioration as a function of neutron flux was quite similar to the first one. The procedure was repeated a third time with similar results. Thus, reprocessing a detector effectively removes the neutron damage at a cost of only about 10 to 15 percent of the original price. A method was also developed for using the gamma-ray spectrum to evaluate the amount of neutron flux incident on the detector. The number of counts in the 693 keV peak [from (n,n′) reaction with the 72Ge in the detector] can be multiplied by 20 to get a rough measure of the neutron flux in n/cm 2.