Improvements in Large Volume Coaxial Germanium Spectrometers

Large volume (16-54 cm3) germanium lithium drift diodes coaxially drifted with one open end have shown excellent performance as γ-ray spectrometers. However limitations have been found in the coincidence resolving times possible with short coaxial diodes due to large variations in charge collection times which result from the non-uniformity of the electric field - especially in the closed end portion. Leading edge time distributions as a function of axial position were made on a longer (6.6 cm) coaxial diode with one open end using 511 keV γ-rays. In an attempt to improve the electric field uniformity, the closed end was cut off to produce a double open-ended detector 4.2 cm long, and a single open ended detector 2.1 cm long. Both diodes were excellent spectrometers. In the 4.2 cm detector (C = 55 pF) the full width at tenth maximum W0.1 of the leading edge time distributions varied from 34 nsec to 19 nsec due to a large variation in core size along the axis. In the 2.1 cm detector (C = 34 pF) W0.1 varied from 17 nsec in the coaxial region to 30 nsec at the closed end. A 6.1 cm long coaxial diode was made with two open ends, a circular crosssection, and a circular core cross-section nearly uniform along the length. W0.1 of the leading edge time distribution for this improved diode was 19 nsec for the whole detector, and varied by less than 2 nsec from end to end.