IONIZATION YIELD FORMATION IN ARGON-ISOBUTANE MIXTURES AS MEASURED BYA PROPORTIONAL-COUNTER METHOD

By using the proportional-counter method, mean ionization yields produ ced by 5.9 keV photons in argon-isobutane mixtures were measured as a function of the mixture composition, total gas pressure, and gas gains between about 10(3) and 2 x 10(4). It was found that the yield for a given mixture is gain dependent: an initial increase of the yield at l ow gas gains is followed by almost constant yield values for gains bet ween about 2 X 10(3) and 8 x 10(3); at still higher gains the yield de creases. Moreover it was observed that the ionization yield at constan t gas gain depends on the total pressure of the gas mixture. This depe ndence leads to a rather clear correlation between yields and isobutan e partial pressure, To explain the measurements a simple model of the mean energy W per ion-pair formed in Penning gas mixtures is also give n. A direct comparison of calculated W values and measured data showed that the results of our proportional-counter experiments performed at high gas gains were strongly influenced by secondary effects induced during the formation of avalanches in the counter when using low isobu tane partial pressures. Therefore, Penning mixtures with incomplete qu enching should not be used as a counting gas in proportional counters if both high gas gain and good energy resolution are required, as for instance, in high-resolution low-energy (sub-keV) X-ray spectroscopy.