Ionization Yields for Fission Fragments

The ratio of fission fragment-to-alpha particle ionization produced in total stopping was measured in the gases helium, argon, and a helium plus 0.25% argon mixture. A parallel-plate gridded electron collection chamber was used. Gas purity and electron loss were checked by making absolute average energy per ion pair measurements for alpha particles. These agreed with the values reported for total ion collection. The difference in ionization defects between helium and argon is consistent with the value predicted by Knipp and Ling on the basis of the greater importance of elastic atomic collisions in argon. Because adding a small amount of argon to helium enables the metastable helium atoms to produce ionization, the difference in the fission fragment-to-alpha particle ionization ratios between helium and the helium plus argon mixture shows that the ionization- to-excitation ratio in helium is lower for fission fragments than for alpha particles. Arguments are given which indicate that very little "defect" is present for fission fragments in the helium plus argon mixture. (auth)