Highly sensitive nuclear spectrometer apparatus and method

Abstract

A nuclear spectrometer employs asymmetrical weighting functions to optimize energy resolution and throughput. Photons striking a semi-conductor detector generate current which is amplified, converted to a voltage step, and fed to a fast analog-to-digital converter (ADC) for pile-up rejection and a slow ADC whose output is examined for low-energy pile-up, slope corrected, and buffered for photon energy measurement. Pile-up is detected with a unique pair of leading and trailing weighting functions whose outputs have sharp rising and falling edges respectively, nearly independent of photon energy. Digital triangular shaping is used to locate the step in the buffer. Asymmetry of the triangular response is used to reject very low-energy pile-up. The step is also rejected if the average noise nearby exceeds a threshold. When a valid step is located, the buffered data stream surrounding the step is multiplied by an asymmetrical pair of leading and trailing weighting functions whose integrals are equal, though their widths and shapes vary depending on the time intervals to the preceding and following steps. The leading weighting function has a cusp shape that comes to a peak at the gap surrounding the step. The trailing function has its peak shifted slightly away from the gap by multiplying the cusp shape by a rising exponential K(1-e.sup.-nt). A multi-channel analyzer attached to the weighting function circuit bins the measured photon energies to accumulate a spectrum.