Characteristics of Some Large, Coaxial Lithium-Drift Semiconductor Gamma-Ray Spectrometers

Large-volume, lithium drift germanium and silicon diodes have been fabricated using the "coaxial" configuration and their diode and γ-ray spectrometer characteristics investigated at liquid nitrogen temperatures (770K). Techniques are described which have been used to drift volumes up to 40 cm3 from 10 ohm-cm, Ga-doped germanium and 22 cm3 from 1100 ohm-cm, B-doped silicon. Drift data, diode d.c. conditions for best spectrometer operation, and experimental resolutions for Co-57,60 source γ-rays are tabulated for Ge diodes, 14.5 - 40 cm3 active volume, and other sections taken from these. γ-ray spectra up to 17 MeV γ-ray energy are presented for typical diodes Resolutions (FWHM) of 2.0, 3.4 and 7.3 keV on the full-energy peak of 122, 1173 and 2754 keV γ-rays have been obtained from a 14 cm3 Ge(Li) diode using a cooled, FET input stage preamplifier. The performance of a coaxial Ge(Li) diode with a guard-ring construction for high bias voltage operation is described. Coaxial Si(Li) diodes primarily intended for application as γ-ray pair spectrometers have been drifted to 22 cm3 active volume. The spectral responses to 122 and 2754 keV γ-rays from a smaller, 11.5 cm3 Si(Li) diode are given. Problems in drifting "single-ended" coaxial Si(Li) diode structures are discussed.