Needle beam studies of HPGe detectors for photon efficiency calibration from 6 to 25 keV

Narrow beams of radiation of appropriate energy were used to investigate the uniformity of charge collection efficiency of a planar HPGe detector of top-hat construction. The front surface of the crystal was examined to a depth f ∝0.5 mm, and deviation from uniformity was shown to be less than 1% for all of this surface layer to the outer edge of the crystal. The crystal entrance window was also shown to be uniform. The spectrometer response function remained unchanged across the crystal, except at the outer edge, where an increase in low-energy tailing was discernible. Thus, for the purpose of modelling low-energy photon interactions in the surface layer, the uniformly sensitive volume was precisely defined by the external dimensions of the crystal, which were measured by radiography. A grooved HPGe detector was also studied and the performance compared with that of the top-hat detector. This work confirmed the validity of assumptions that had been made in determining the efficiency of the top-hat detector as a function of energy from 6 to 25 keV. A combination of computer modelling and coincidence measurements had given an uncertainty of less than 1% (1σ). (orig.)