Large Germanium Lithium-Drift p-i-n Diodes for Gamma-Ray Spectroscopy

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Abstract
Germanium lithium-ion drift p-i-n diodes with depletion depths up to 11 mm and active volume 6 cm3 have been fabricated from 10 ohm-cm, gallium-doped material using a high-power drift method. Diodes were immersed in chloroform and drifted at ohmic dissipation power levels in the range 50-100 W at 30-50 V D. C. drift bias. Diodes were electrothermally stabilized during drift by the boiling action of the chloroform (B. P. = 61.3°C). Peak power levels of up to 250 W at 200 V bias were obtained at the commencement of the chloroform drift, but were reduced to within the range given above following a rapid increase in leakage current during the first few hours of drift. Diode temperatures were estimated at 10-20°C above the chloroform boiling point. Drift times were typically 500 to 700 hours for depletion depths of 8 and 10 mm respectively. Seven diodes with depletion depths 6-11 mm have been operated as γ-ray spectrometers in vacuum (10-6 mm Hg) at liquid N2 temperatures (77°K) with bias up to 1200 V and leakage currents ~10-9A. Some typical spectral response characteristics are given for diodes used as photoelectric γ-ray spectrometers and the effect of carrier trapping on energy resolution is demonstrated. Resolutions of 3.5 and 8.0 keV on the full-energy peaks of 122 and 1353 keV γ-rays were obtained from a device of average performance (5 cm2 x 7 mm depletion depth). The intrinsic full-energy peak efficiency of a 4.