Semiconductor Lithium-Ion Drift Diodes as High-Resolution Gamma-Ray Pair Spectrometers

Germanium and silicon p-i-n diodes with active volumes up to 2 cm3 have been fabricated using the lithium-ion drift technique. p-type germanium doped with either gallium or zinc (5-10 ohm-cm) and boron-doped silicon (1100 ohm-cm) were the base materials. Diodes were operated at 77°K in conjunction with a low noise amplifier system (270 RMS electron-holes). They have been used as gamma-ray pair spectrometers, germanium diodes giving a resolution of 7 keV (FWHM) at 2.75 MeV gamma-ray energy increasing to 10 keV at 7.64 MeV; resolution from a silicon device was slightly inferior. A germanium diode has been used as the central detector in a triple-crystal pair spectrometer incorporating two NaI scintillators. This arrangement improved the ratio of the height of the pair peak to the background below the peak from 7:1 to 150:1 for 2.75 MeV gamma-rays from a Na24 source.