Instrumental Requirements for High-Resolution Gamma-Ray Spectrometry Using Lithium-Drifted Germanium Detectors

Recent development of pre-amplifiers employing field-effect transistors have made it possible to achieve energy resolutions less than one kilovolt (FWHM) with lithium-drifted germanium gamma-ray detectors. This coupled with the availability of 4096-channel ADC's makes it possible to measure gamma-ray energies to better than 1 part in 104 over a reasonable energy range. To realize this degree of precision it has been necessary to re-examine all elements of a laboratory pulse-height analysis system. This includes linear amplifiers, ADC's, test pulse generators, and data storage and analysis techniques. To establish the limits of present state-of-the-art equipment for precision gamma-ray spectrometry, an extensive evaluation has been made of amplifiers, threshold amplifiers, gain stabilizers, and 4096-channel ADC's. Several noise sources exist that are amplitude and rate dependent, thus affecting energy resolution as a function of gamma-ray energy. These effects make it difficult to obtain accurate experimental measurements of the Fano factor for germanium. Recent measurements of the Fano factor obtained with a 4096-channel ADC, are presented. Techniques are described for the measurement of system linearity, stability, and dynamic performance of spectrometers under changing experimental conditions. The evaluation of analogue-to-digital converters has included their use both with conventional "hard-wired" pulse analyzers and programmed data processors. Precision pulse generators, methods for measurement of system linearity, and computer programs for data analysis developed to obtain precision values for energies and intensities of gamma rays in complex pulse-height spectra are described. Data are presented to demonstrate the precision which can be obtained with equipment presently available.