Timing Characteristics of Large Coaxial Ge(Li) Detectors for Coincidence Experiments

The high efficiencies of large volume ´coaxial´ Ge(Li) detectors make them attractive for coincidence experiments. However the electric field in this configuration is not uniform and hence the charge collection times vary widely as a function of position in the detector. The resultant variations in output pulse shape limit coincidence resolving times. In our studies one 511 keV annihilation quantum (22Na source) was detected in a scintillation counter to establish the zero (start) time and its partner in the Ge(Li) detector (at 1500 V) was accepted only when a full energy pulse was registered. The stop pulse to the timesorter was generated by the leading edge of the Ge(Li) preamplifier pulse at ~ 10% (50 keV) of maximum amplitude. Scanning the detector with a 1 mm wide collimated ¿-beam resulted in time distributions whose full widths at half-maximum (fwhm) were ~ 8 nsec in the coaxial region of the detector broadening to ~ 35 nsec at the closed end. The distributions are skew in shape with long approximately exponential tails having half value slopes of ~ 5 to ~ 20 nsec. Similar studies with stop pulses generated at ~ 50% of maximum amplitude show much broader time distributions. The position variation in pulse shapes due to charge collection time was confirmed from oscilloscope photographs by triggering with the scintillation counter ´start´ pulse.