Relative determination of W-values for alpha particles in tissue equivalent and other gases. [5. 4 MeV alpha particles]

W (the average energy to form an ion pair) for 5.4 MeV /sup 241/Am alpha particles in a Rossi-type tissue equivalent (T.E.) gas, argon and methane was determined to an accuracy better than 0.2% using a new automated data handling system. A vibrating reed electrometer and current digitizer were used to measure the current produced by completely stopping the alpha particles in a large cylindrical ionization chamber. A multichannel analyzer, operating in a slow multiscalar mode, was used to store pulses from the current digitizer. The dwell time, on the order of 60 minutes per channel, was selected with an external timer gate. Current measurements were made at reduced pressures (approximately 200 torr) to reduce ion-recombination. The average current, over many repeated measurements, was compared to the current produced in nitrogen and its previously published W-value of 36.39 +- 0.04 eV/ion pair. The resulting W-values were (in eV/ion pair): 26.29 +- 0.05 for argon, 29.08 +- 0.03 for methane and 30.72 +- 0.04 for T.E. gas, which had an analyzed composition of 64.6% methane, 32.4% CO/sub 2/, and 2.7% nitrogen. Although the methane and argon values agree within 0.1% with previously published values, the value for T.E. is 1.2% lower than the single previously reported value