A High Stability Gamma-Ray Spectrometer for Use at High Counting Rates

A spectrometer has been developed for rapid quantitative determination of nitrogen content in bulk media using neutron capture gamma-ray analysis. Four NaI(Tl) scintillation detectors, each operated at counting rates of the order of 105, per second, are utilized to permit rapid measurement. Resolution loss due to pile-up is minimized by the use of a 40-millimicrosecond pulse width at the photomultiplier anode followed directly by a low level biased silicon diode preselector. Those portions of pulses exceeding the diode bias are stretched and the four stretcher outputs are fed to a common amplifier of the nonoverloading type which has been modified to improve stability. The preselector reduces by almost two orders of magnitude the effective pulse rate which must be handled by the amplifier. Since pulse-height stability requirements are stringent, temperature regulation of the scintillation detectors and preselector units is used. Detailed measurements indicate that the NaI crystal-plus-photomultiplier combination contributes more instability than all the other spectrometer components combined.