Low Energy Cross-Sections for Positron Interactions with Cyclic Hydrocarbons

Abstract

We report total cross-section measurements for positron scattering on cyclic hydrocarbons: benzene, aniline, and cyclohexane. Measurements were done by an absolute transition method in the energy range 1.5-20 eV (0.4-20 eV for cyclohexane). High cross-sections for all measured molecules were observed at low energies. In the case of cyclohexane the cross-section in the zero energy limit tends to a constant value. In aniline a weakly accented peak slightly above the positronium formation threshold is observed. Similar but less visible bump was observed in the case of benzene. Measurement of total cross-sections in nitrogen and argon used for calibration of the apparatus are also presented.

Positronium Formation in Organic Liquids

“…In degassed cyclohexane at 20 • C the three lifetimes (τ 3 , τ 2 , τ 1 ) amount to 3.22 ns, 445 ps (and 125 ps), with intensities 35%, 44% and 21%, respectively (and the variance 1.05). Generally, results for benzene (reported in detail previously [20]) and cyclohexane are quite similar; this probably reflects similar cross-sections in the gas phase [22].…”

Positron Scattering at Thermal Energies

“…Present MERT extrapolation towards zero--energy of positron low-energy total cross-sections H2, Ar, N2, C6H6, and C6H12. Experimental data are from Karwasz et al [21,22].…”

“…Cross-sections used here for benzene and cyclohexane have been reported in [21] and for N 2 , Ar and H 2 in [22]. Unfortunately, the two earlier experiments, from Detroit [15] and Tokyo [16] used slightly modied experimental settings for dierent gases so the comparison between dierent targets is not straightforward.…”

Positron Scattering on Atoms and Molecules in the Limit of Low Energy

“…In the case of the present apparatus, only at energies below 0.06 eV the angular resolution error becomes significant (above 30%). Therefore, from the experimental point of view, the rise of total cross-sections in the limit of zero energy seems to be proved, at least for targets like N 2 , Ar, H 2 , benzene, aniline, cyclohexane, see experimental data for all these targets in [3,4].…”

“…without arbitrary normalizations) in the range of 0.4-20 eV. The preliminary results for Ar, N 2 , benzene, cyclohexane, and aniline have been presented in [3]. The cross-section is determined using de-Beer-Lambert attenuation law:…”