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 range 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 inadequate for all the systems tested. The use of a slightly more complex two-parameter model was investigated next and, in spite of initial positive indications, the resulting model yielded inaccuracies larger than ± 10% for over one-third of the counting rate range. However, use of the decaying source method to provide a wide range of accurate useful counting rates was found to be reproducible to within ± 1%. Previous formulas for variance found in the literature were studied by Monte Carlo simulation and were found to be valid when the respective appropriate assumptions of the idealized models were applicable. Actual experimental variances were found to be fairly well predicted by a modified Kosten (1943) model that has a variable dead time. This model has the advantage that it always gives conservative estimates. At present the 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 decaying source method, which involves the use of a pure short-lived radioisotope such as 56Mn. Use at this accuracy level without having to use the decaying source method must await further modeling or other experimental developments.