Metastable Densities in Noble-Gas Plasmas Created by Nuclear Radiations

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Abstract
Optical absorption measurements of metastable densities in plasmas of He, Ne, and He–Ne are reported. The plasmas were created through bombardment of the gases by the products from the boron−10 (n, α) lithium−7 nuclear reaction during irradiation by neutrons from a pulsed nuclear reactor. The experiments covered gas pressures up to 900 Torr, with neutron fluxes from 1012 to 1016 n/cm2sec. The He(21S), He(23S), Ne (1s5), and Ne(1s3) metastable densities were studied in the single gases and in (9/1) He–Ne mixtures. In general, the observed densities increase with pressure up to about 100–200 Torr after which collisional losses, combined with a decrease in the source rate, eventually cause the metastable populations to fall off. The variation with neutron flux typically changes from a linear to a square‐root dependence whenever the destruction by electron superelastic collisions is the dominant loss mechanism. Maximum He(23S) and He(21S) densities of the order of 10−2 to 10−3 and 10−4 to 10−5 atom/cm3 unit flux, respectively, were measured; peak Ne(1s5) and Ne(1s3) densities were roughly 10−3 to 10−5 and 10−4 to 10−6 atom/cm3 unit flux, respectively, depending on the flux level. For pressures below 20 Torr, the Ne‐metastable densities in the (9/1) He–Ne mixture were as much as ten times the densities in neon (at a pressure corresponding to the Ne partial pressure in the mixture). A theoretical model is presented which predicts the behavior of the metastable densities quite well, the largest discrepancy being for He(23S) in helium.