Practical, accurate methods for measuring the m-values of ionizing gases

For accurate measurements of processing-level dose rates with ionization chambers, it is essential to estimate the ion collection factor accurately. This is considerably affected by volume ion recombination in the ionizing gases. If the well-known formulae of Boag are considered, the m-value of the gases and the theoretical equivalent gap length between the chamber electrodes have to be estimated accurately. However, with the commonly used ionizing gases, with the exception of air, or with the usual shapes of practical chamber electrodes, it is difficult to obtain accurate values for m and the gap length. The reason for the difficulties are reviewed briefly from theoretical and experimental viewpoints. The principles of new, practical and reliable methods of obtaining the two kinds of quantity are then presented. Insofar as Boag's theory is valid, the methods presented are expected to be useful for obtaining the m-values of any kind of gas under any conditions of interest, as well as the equivalent gap length for ionization chambers with any shape of electrodes, however complex. Using several kinds of ionization chamber, both of common and special design, the m-value of atmospheric air was determined to be quite stable over a range of ionization rates from about 10-2 to about 103 esu/cm3.s (3.3 pC/cm3.s to 0.33 μC/cm3.s). The m-values of some ionizing gases, namely He, Ne, Ar, Kr, Xe, CO2 and C2H4, are also reported. Using the normalized saturation curve of air, a practical and reliable method of obtaining the equivalent gap length was established. From the above results, it seems to be possible to measure higher dose rates, up to 105 Gy/h in air (approximately 107 R/h), even under poor saturation conditions, with an error of less than a few per cent. (author)