Large-Volume Coaxial Germanium Gamma-Ray Spectrometers

Germanium lithium-drift p-i-n diodes for high resolution 7-ray spectroscopy have been made with sensitive volumes in the range 16 to 54 cm3 (1 to 3.3 in. 3) using the coaxial method of drift. Details of the construction, mounting and preparation as spectrometers are given. The shape of the undrifted p-type core has been determined by scanning with a collimated 7-ray beam and by copper plating the surface of a sectioned diode. The characteristics as spectrometers, using 7-rays of energies in the range 0.1 to 7.5 MeV, are shown. At 122 keV energy, resolutions of 5.0 and 4.4 keV (fwhm) were obtained with diodes of sensitive volumes 23 and 54 cm3 respectively. A 16 cm3 volume diode had a resolution of 4.8 keV at 1533 keV 7-ray energy, and a 45 cm3 diode had resolutions of 6.5 and 19 keV at energies of 1.1 and 5.1 MeV respectively. The diodes were operated at bias voltages in the range 600 to 1500 V. Intrinsic full energy peak efficiencies are shown for different diode sizes over a range of 7-ray energies. For the 54 cm3 volume diode, these efficiencies are 21% and 8% respectively for 7-ray energies of 0.5 and 1.0 MeV with a source to diode distance of 30 cm. The larger diodes show a greatly enhanced peak/total ratio, and the higher efficiencies make these spectrometers extremely useful in reaction y-ray, y-7 coincidence, and n, y studies.