Energy per Ion Pair Measurements in Pure Helium and Helium Mixtures

Abstract

A value for the average energy dissipated by an ionizing particle per ion pair produced for 241Am alpha particles completely stopped in purified helium has been established as 43.5 +/- 0.5 eV on a relative basis. Although this value differs from those previously reported by other researchers, indirect experimental evidence points to the conclusion that the purity of the gas was higher than that of other workers. Direct measurement of the water vapour impurity level has been carried out, together with indirect estimates of other impurity concentrations. The accuracy of the absolute measurements of the energy per ion pair was primarily limited by the accuracy of the source emission rate measurements. These were carried out by 2pi alpha particle spectrometry. Sources emitting reduced energy alpha particles have been produced. The difficulties of calibration over 2pi geometry have been investigated and have led to a study of energy straggling in absorbers. Ionization experiments using reduced energy sources at low gas pressure have proved unsatisfactory. Gas mixture studies have been used to show that non-metastable excited states of helium must be responsible in part for the Penning ionisation effect observed in contaminated helium, previously attributed to metastable states alone.