Ionization defect for alpha particles in various gases

The ionization produced by the Li7* and o<-particles from the reaction BlO(njo<)Li7* as they were stopped in argonj argon-nitrogen~ and argoncarbon dioxide gas mixtures was measured. A gridded parallel-plate ionization chamber employing electron collection was used. Plutonium alpha particles were used for calibration and the ionization electron pulses were amplified and sorted-with an electronic pulse height analyzer. The neutron source was a 300 kev linear positive ion accele= rator utilizing the D-D reaction. The neutrons were thermalized by blocks of paraffin stacked around the chamber. The thin boron film was made by evaporating boron enriched to 95 percent with B10 onto an aluminum backing. The gas mixtures were purified by continuous circulation over hot calcium~magnesium alloy. The energies for the alpha particles and lithium recoils as calcu= lated from the measured ionization yields were L468 ± 0.002 and 0.782 ± 0.004 Mev in argon3 1.460 t 0.002 and 0.784 t 0.002 Mev in argon plus 2 percent nitrogen~ and 1.444 t 0.002 and 0.774 t 0.002 Mev in argon plus 5 percent carbon dioxide. The uncertainties quoted are the standard deviations for the series of measurements. These results should be compared to the energies 1.473 ± 0.002 Mev for the alpha particles and 0.841 ± 0.002 Mev for the lithium recoils determined from conservation of momentum. The 5 kev ionization defect measured for the alpha particles in argon was assumed to be due to source thickness. This leads to a 6 kev correction to be applied to all values obtained for the lithium recoils. The results of this experiment along with those of other investigators demonstrate the constancy of the average energy required to produce an ion pairj W, for alpha particles in argon above an energy of 1.4 Mev. FurtherJ *This report is based on a Ph.D. thesis by David W. Curtis submitted August~ 1955 to Iowa State CollegeJ Ames 9 Iowa. This work was done \mder contract with the Atomic Energy Commission. ..