Molecular effects in the energy loss of alpha particles in gaseous media

Abstract

Abstract Previous measurements in gaseous hydrocarbon and carbon-oxygen compounds in the range 0.3 ≦ E α ≦ 2.0 MeV revealed that the Bragg additive rule failed to give molecular stopping cross sections e α from the measured atomic solid carbon stopping cross section e(C) and diatomic e expt (H 2 ) or e expt (o 2 ). For example, when hydrogen was present in the compound, the molecular data could be fitted either by using e(C) and an e'(H) which was higher than 1 2 e expt (H 2 ) by as much as 21%, or by using 1 2 e expt (H 2 ) and another e'(C) which was greater than e(C) by as much as 22%. A similar difficulty has also been encountered in interpreting the stopping cross sections recently measured for C-H-F and C-H-O compounds. e(C), e expt (H 2 ) and e expt (O 2 ) can be used to fit (CH 3 ) 2 O, but it is necessary to use e'(C) and e expt (H 2 ) for C 2 H 2 F 2 . Nevertheless, a closer examination of the stopping cross sections for eight hydrocarbon compounds reveals a correlation between the stopping cross sections of atomic hydrogen and carbon which are required to satisfy the Bragg rule, and the type of bond associated with the atomic carbon in the molecule. The stopping cross section of a carbon atom is greater when the atom is bound by a triple bond than a single bond, whereas that of a hydrogen atom is less in a triple-bond compound than in a single-bond compound. Double-bonded compounds yield a carbon stopping cross section between the single-bond and triple-bond values at lower energies and closer to the triple-bond values at higher energies.