Discrepancies exist between the many published measurements of W̄, the average energy to produce an ion pair for helium. Since results for other gases have been relatively consistent, the cause of the discrepancies usually has been attributed to the need to use ultrapure helium in order to avoid Penning ionization. This arises from the deexcitation of highly excited metastable states by collisions with contaminant atoms. A new experimental value of W̄ has been established as 43.5 ± 0.3 eV per ion pair for241 Am alpha particles completely stopped in purified helium. Particular attention has been paid to the establishment and maintenance of pure helium. The present result agrees well with theoretical predictions corrected for molecular ion formation. Gas mixture studies have been made which indicate that nonmetastable excited states of helium are responsible in part for the Penning ionization effect, previously attributed to metastable states alone.

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