Accurate Determination of the Stability of β-Proportional Gas Counters Against Variations in the Energy Threshold

This paper expresses the stability of the response probability of a counting channel against small variations in the energy threshold in terms of a sensitivity factor, defined as the ratio between a relative change in the response probability and the relative variation in the energy threshold producing this change. According to this definition the sensitivity factor is equal to the intensity at the energy threshold of the pulse-height spectrum obtained from the radiation counter, divided by the area under this spectrum, the pulse height being expressed with the energy threshold as unity. In a β-counting channel supplied with a proportional gas counter, a minimum exists in the lower end of the pulse-height spectrum as a result of the superposition of a primary spectrum due to the ionization caused by the β-particles in the counting gas, and a secondary spectrum which is attributed to 'after pulses'. The latter spectrum is correlated with the former; this, in connection with the fact that the primary spectrum depends on the energy spectrum of the β-particles, indicates the existence of a relation between the smallest obtainable sensitivity factor and the end-point energy of the nuclide. This relation was investigated in the case of a windowless 2π gas-flow counter for β-emitting nuclides with end-point energies in the range from 0.167 to 1.70 MeV. It was found that the smallest obtainable sensitivity factor increased from 0.50% to 4% when the end-point energy was reduced from the upper to the lower of the above-mentioned limits. For endpoint energies higher than about 0. 25 MeV, the relation could be described by a simple analytical expression, regardless of the shapes of the β-spectra. Such a description of the counting channel is more satisfactory than the usually specified plateau slopes of the high-voltage characteristics, since it allows a direct comparison with regard to stability between counting channels of different designs and modes of operation. (author)