High-Resolution Beta- and Gamma-Ray Spectrometer

A high-resolution semiconductor beta- and gamma-ray spectrometer is described. The spectrometer consists of a silicon or germanium detector, low-noise field-effect transistor preamplifier, and a linear amplifier. The requirements for the different sections for high-resolution performance are outlined. The detectors used are low-capacitance, low-leakage devices cooled to low temperatures. The preamplifier utilizes the low-noise characteristics of a cooled junction-field-effect transistor. These characteristics are described and several types of FET's are compared. A single FET, incorporated in a voltage-sensitive input stage, provides an optimum signal-to-noise ratio. Pulse generator resolution of the preamplifier for zero external capacitance is 0.4 keV FWHM (Ge) with a slope of 0.038 keV/pF. Typical resolution obtained in the spectrometer in the low-energy region is 0.7 keV. Recent measurements show that the cooling system is one of the main noise sources of the spectrometer. More than 20% improvement in resolution was observed in preliminary experiments when this noise source was suppressed.