ON EXTENDING THE ACCURATE AND USEFUL COUNTING RATE RANGE OF GM COUNTER DETECTOR SYSTEMS

A study is described to extend the accurate and useful counting rate r ange of the GM (Geiger-Muller) counter detector system. The idealized simple one-parameter paralyzable and non-paralyzable models were first investigated with the decaying source method and were found to be ina dequate for all the systems tested. The use of a slightly more complex two-parameter model was investigated next and, in spite of initial po sitive indications, the resulting model yielded inaccuracies larger th an +/-10% for over one-third of the counting rate range. However, use of the decaying source method to provide a wide range of accurate usef ul counting rates was found to be reproducible to within +/-1%. Previo us formulas for variance found in the literature were studied by Monte Carlo simulation and were found to be valid when the respective appro priate assumptions of the idealized models were applicable. Actual exp erimental variances were found to be fairly well predicted by a modifi ed Kosten (1943) model that has a variable dead time. This model has t he advantage that it always gives conservative estimates. At present t he counting rate range of GM counter detector systems can be extended to the +/-1% accuracy level only at the cost of having to use the deca ying source method, which involves the use of a pure short-lived radio isotope such as Mn-56. Use at this accuracy level without having to us e the decaying source method must await further modeling or other expe rimental developments. (C) 1997 Elsevier Science Ltd. All rights reser ved.