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

Molecular stopping cross sections ${\epsilon }_{\alpha }$ of $\alpha$ 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 $\epsilon \left(\mathrm{C}\right)$ calculated from gaseous fluorocarbons agrees with previous measurements by Chu and Powers of $\epsilon \left(\mathrm{C}\right)$ for carbon in solid form but disagrees with $\epsilon \left(\mathrm{C}\right)$ calculated from gaseous hydrocarbons by Bourland, Chu, and Powers. Consistent results are found by using either of two alternatives: (i) Use hydrocarbon gas $\epsilon \left(\mathrm{C}\right)$ and experimental ${\epsilon }_{\mathrm{expt}}$ (${\mathrm{H}}_2$) in hydrogen-enriched C-H-Br or C-H-I compounds, and use solid carbon $\epsilon \left(\mathrm{C}\right)$ everywhere else; or (ii) use solid carbon $\epsilon \left(\mathrm{C}\right)$ in all compounds, but use an ${\overline{\epsilon }}^{\prime }$ (H) which disagrees by as much as 21% from $\frac{1}{2}{\epsilon }_{\mathrm{expt}}$ (${\mathrm{H}}_2$). A plot of ${\epsilon }_{\alpha }$ 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.