Improvements in Encapsulated Silicon Junction Alpha Detectors

Encapsulated silicon junction alpha detectors made with material of resistivity 1000 ohm-cm and with front layer depth of 2 microns have been available for some time now in window areas of 5, 20, and 200 sq. mm. Recent measurements on the best of these units have given resolutions of 0.6%, 0.9% and 4% for 5.5 Mev alpha particles. For wider application, similar units have been made with front layer depth of 0.2 micron and using material of resistivity up to 30000 ohm-cm. Excitation of carriers by radiation of optical wavelengths shows that the effective dead layer is less than 0.03 micron. The necessity of making a solder contact and seal to the extremely thin front layer raises problems in the achievement of high stability, low leakage, and low noise at high voltages. Thus the depletion layer depths obtainable in encapsulated units with high resistivity material do not yet approach those obtainable in unencapsulated units in vacuum. A representative measurement obtained recently gave a resolution of 0.6% with a window area of 5 sq. mm. (junction area 20 sq. mm.) and depletion layer depth 180 microns.