Pulse Height Dependence on Source Position in a Closed-End Coaxial Hyper-Pure Germanium Gamma-Ray Detector

The source position effect on peak center pulse height has been investigated on a 67cm3 closed-end coaxial hyper-pure germanium gamma-ray spectrometer. When a Na-22 source was moved on the axis of the detector, the peak center energy of 1274keV gamma-rays started to decrease at 20cm and a 44eV lower pulse height was observed at 1.5cm in front of the detector. The peak center energy did not change for moving the Na-22 source on a line(B) drawn through the detector center and perpendicular to the axis. The peak center energies observed for the source positions on the axis were lower than those observed on the line B, and the difference at 100cm distance was 15eV. This dependence is similar to that observed in a 73.2cm3 closed-end coaxial Ge(Li) detector reported by K. Shizuma, et al(JAERI-M 6497 (March 1976)). The shaping time of the main amplifier was 2¿sec. The amount of the decrease in the pulse height at 1.5cm source-to-detector distance became smaller as the shaping time of the amplifier increased; the decrease was 178, 44, 24, and 10eV for the shaping time of 1, 2, 4, and 8¿sec, respectively. The risetimes of the pulses were found to depend on the source position; the average risetime increased from 108nsec to 126nsec when the source moved from 30cm to 1.5cm in front of the detector. It is concluded that the risetime dependence on the source position is one of the origins which result in the pulse height dependence on the source position.