Calibration of Radionuclides with a Coincidence System Using a Pressurized Proportional 4π β-Detector

Radionuclide calibrations with a 4π β-γ coincidence system are described In which a pressurized gas-flow counter is used in the β-channel. This detector is operated in the proportional region and is of the small pillbox type, slide operated and modified for use at pressures up to 75 atm. Flow operation with purified gas is necessary to prevent gain instability resulting from gas deterioration effects. This entails the use of a precision density regulator to maintain stable gas multiplication. The pressurized β-detector permits electronic variation of the β-channel detection efficiency through low energy pulse-height discrimination. It is desirable, if not essential, to incorporate a low noise linear amplifier of the double differentiating type. A discriminator with cross-over pick off timing may then be used to avoid delay mismatching of coincident events as the threshold level is varied. Variation in β-channel efficiency is required in most calibration procedures to determine experimentally the functional relations expressed in the general coincidence formulae Nβ = N0F (Nc/Nγ) or NβNγ/Nc = N0 G(Nγ/Nc) Any one of several basic methods may be used to vary the efficiency provided they all yield equivalent efficiency functions. Variation of source self-absorption and foil absorption have already been shown, in this respect, to be equivalent within the experimental errors. Similar comparisons have now been made to show that, within small limits of uncertainty, calibrations using low energy discrimination are identical with those obtained by absorption methods. The capability of using discrimination methods is of obvious practical advantage in simplifying procedures, particularly in efficiency tracing and calibrations of complex decay. Repeated measurements over an extended range of efficiency values can be made with a single source. Thus the internal consistency of these data, used to determine the efficiency functions, is determined primarily by counting statistics. Relatively unambiguous data fitting can then be achieved, by an iterative least-squares method, to a polynomial of order consistent with a well defined precision of measurement. The energy resolution of the small pressurized β-detector is adequate to permit absolute calibrations of some nuclides difficult to standardize with counters operated at atmospheric pressure or by other means. In particular, the calibration of the electron capture species 197Hg is described in which coincidences with γ-X-ray sum pulses in the γ-channel are used as a measure of the β-detector efficiency for low energy Auger-photon cascades. (author)