Improvements in the absolute standardization of large-area reference sources

Using a simple gas-gain model, the efficiency sensitivity of gas-flow proportional counters to changes in the influence quantities (applied anode voltage, gas purity, gas-flow rate, ambient pressure and temperature) was analyzed and applied for the absolute standardization of large-area reference sources. Thus, the uncertainty of measurement was evaluated by taking into account the fluctuations of these quantities in normal conditions of counter operation. It was shown that the uncertainty is small in these conditions but accidental and large changes in the influence quantities may give rise to large errors of measurement. In addition, a cleaning procedure and a simple way of stabilizing the gain of gas-flow proportional counters were used to improve the absolute standardization of large-area reference sources. As a result, measurements were performed with uncertainties smaller than 0.55% and the reproducibility better than 0.42%.