Measurement of Molecular Stopping Cross Sections of Halogen-Carbon Compounds and Calculation of Atomic Stopping Cross Sections of Halogens

Abstract

Molecular stopping cross sections ${\ensuremath{\epsilon}}_{\ensuremath{\alpha}}$ of $\ensuremath{\alpha}$ particles of energy 0.3-2.0 MeV have been measured (probable error 0.9-2.1%) in gaseous (or vapor) C${\mathrm{F}}_{4}$, ${\mathrm{C}}_{2}$${\mathrm{F}}_{6}$, ${\mathrm{C}}_{3}$${\mathrm{F}}_{8}$, ${\mathrm{C}}_{4}$${\mathrm{F}}_{8}$, C${\mathrm{Cl}}_{4}$, CCl${\mathrm{F}}_{3}$, C${\mathrm{Cl}}_{2}$${\mathrm{F}}_{2}$, CH${\mathrm{Cl}}_{2}$F, CBr${\mathrm{F}}_{3}$, ${\mathrm{C}}_{2}$${\mathrm{Br}}_{2}$${\mathrm{F}}_{4}$, ${\mathrm{C}}_{2}$${\mathrm{H}}_{3}$Br, ${\mathrm{C}}_{2}$${\mathrm{H}}_{5}$Br, and ${\mathrm{C}}_{2}$${\mathrm{H}}_{5}$I. Atomic stopping cross sections for F, Cl, Br, and I have been calculated (probable error 2.3-4.4%) by application of the Bragg additive rule. It is found that carbon $\ensuremath{\epsilon}(\mathrm{C})$ calculated from gaseous fluorocarbons agrees with previous measurements by Chu and Powers of $\ensuremath{\epsilon}(\mathrm{C})$ for carbon in solid form but disagrees with $\ensuremath{\epsilon}(\mathrm{C})$ calculated from gaseous hydrocarbons by Bourland, Chu, and Powers. Consistent results are found by using either of two alternatives: (i) Use hydrocarbon gas $\ensuremath{\epsilon}(\mathrm{C})$ and experimental ${\ensuremath{\epsilon}}_{\mathrm{expt}}$ (${\mathrm{H}}_{2}$) in hydrogen-enriched C-H-Br or C-H-I compounds, and use solid carbon $\ensuremath{\epsilon}(\mathrm{C})$ everywhere else; or (ii) use solid carbon $\ensuremath{\epsilon}(\mathrm{C})$ in all compounds, but use an ${\overline{\ensuremath{\epsilon}}}^{\ensuremath{'}}$ (H) which disagrees by as much as 21% from $\frac{1}{2}{\ensuremath{\epsilon}}_{\mathrm{expt}}$ (${\mathrm{H}}_{2}$). A plot of ${\ensuremath{\epsilon}}_{\ensuremath{\alpha}}$ vs the atomic number ${Z}_{2}$ of the stopping medium shows that the halogen measurements at ${Z}_{2}=9, 17, 35, \mathrm{and} 53$ agree well with the existing theory and other experimental measurements for neighboring ${Z}_{2}$ values.