Pulse height defect due to electron interaction in the dead layers of Ge(li) γ-Ray detectors

Abstract

Gamma rays giving rise to ionizing electrons in the dead layer of Ge(Li) detectors result in slow pulses of reduced amplitude which degrade energy spectra and coincidence resolution. It is shown that when internal conversion electrons from 207 Bi (∼ 1 MeV) impinge on a detector with a thin dead layer of 10 μm, the peaks characteristic of the K and L electrons are well defined; but when they impinge on a detector with a dead layer of 0.3 mm, the electron spectrum shows no features except a continuum which rises at the low energy end. The distribution of risetimes, resulting from the interaction of conversion electrons in the latter detector, exhibits a peak which indicates an abundance of slow pulses. A similar peak is apparent in the risetime distribution from γ-rays in the continuum of the energy spectrum but not from γ-rays in the full energy peak. This suggests that a significant fraction of the pulses in the continuum is due to electron and/or positron interaction in the dead layers. A pulse shape discrimination technique was developed whereby defective pulses are identified and rejected. Gamma-ray spectra from a single Ge(Li) detector and from a Ge(Li)-NaI(TI) 3-crystal pair spectrometer were studied. A comparison of these spectra with and without rejection of slow pulses shows that in an 11 mm thick detector the fraction of the continuum due to defective pulses approaches 50%. The discrimination technique is described and results of several experiments are presented and discussed.