A Comparison of the Primary Processes of the Radiolyses Induced by α-, β-, and γ-Rays

Abstract

By using the cross sections derived by the binary encounter collision theory and the assumption of a continuous slowing down, the G-values for the ionization and excitation of helium gas irradiated by α-, β-, and γ-rays have been calculated. In the radiolysis by high-energy electrons, the G-values started to decrease at an incident electron energy of about 103 eV with a decrease in the energy. Calculation showed that, if the radiation energy is extremely high, the G-values are independent of the kind of radiation. In the radiolysis induced by α-particles, the ratio of the yield resulting from the direct interaction with α-particles to that due to the secondary electrons was calculated as a function of the energy of the incident α-particles. When the energy was 8 MeV, the ratio for ionization was calculated to be 1.31.

Scintillation of liquid helium for low-energy nuclear recoils

“…This rise in the W-value at low projectile energy is the result of the decrease in probability of ionization compared to excitation of helium atoms in this low energy range. The trend of increasing W-value for helium recoils with decreasing energy is what is expected theoretically [51], and has been observed to occur in a number of pure gases and mixtures [60,61]. However, we can find no report of a measurement of this phenomenon in helium.…”

“…7. This ratio, obtained from a summation of the contributions from the various microscopic processes, has a magnitude and energy dependence that is somewhat different from that obtained theoretically by Sato et al [51], who did not consider the variation of charge state of the projectile.…”

“…It also depends on the number and type of excited state atoms that are generated. The only estimates of the number of excitations produced for particles with energies above 100 keV, of which we are aware, are those of Sato et al [51,52] who calculate that 0.45 atoms are promoted to excited states for every ion produced. Of the excited atoms 85% are predicted to be in spin singlet states and 15% in triplet states.…”

Concept for a dark matter detector using liquid helium-4

“…The ionization and excitation yields due to a recoil helium nuclei moving in liquid helium are important premise parameters needed for the design of a heliumbased dark matter detector. Sato et al [53] have studied the ionization and excitation yields of an alpha particle (He 2+ ) in liquid helium using the collision cross sections derived with the binary encounter theory [54]. In their analysis, the charge exchange collisions are ignored and the fraction of the alpha particle energy that is lost to elastic collisions with surrounding He atoms (nuclear stopping) is not included.…”

“…3. The ratio of N Sec el to N el decreases with decreasing E and is only a few percent when E ∼ 100 keV [53]. We shall neglect N Sec el in the following analysis for simplicity.…”