THE NIST RAPID IRRADIATION AND COUNTING SYSTEM

Apparatus is being developed for short irradiation and rapid counting at the NIST Reactor, optimized for accurate neutron activation analysi s via activation products ranging in half-life from about 500 ms to 50 0 s. This facility is designed to irradiate a sample either in a well- thermalized neutron flux at 3.10(13)n.cm(-2).s(-1), or in a higher flu x with a larger fast and epithermal component. The design transfer tim e for a l-ml rabbit is 500 ms, measured to 10 ms precision. Timing inf ormation for both irradiation and counting will be transferred automat ically to the activation analysis workstation computer. The gamma-ray spectrometer system is selected and tuned for accurate measurement at high and varying counting rates, using loss-free counting (LFC) techno logy. A detailed calibration and characterization of this system has b een performed. Accurate measurement requires that attention be paid to the systematic and random errors to which LFC is subject, but this re quirement is minor compared to the advantages of undistorted spectral shape, the ability to solve the decay equations exactly, and the wider useful range of counting rates in the spectrometer system.