Forward angle scattering effects in the measurement of total cross sections for positron scattering

Abstract

Measurements of total scattering by positron impact have typically excluded a significant portion of the forward scattering angles of the differential cross section. This paper demonstrates the effect that this can have on measurements of the total cross section. We show that much of the apparent disagreement between experimental measurements of positron scattering from atoms and molecules may be explained by this excluded angular range. It is shown that this same effect may also lead to an anomalous energy dependence of some cross sections.

Low-energy positron interactions with krypton

“…they are ∼50% higher between 1 and 7 eV and ∼20% higher at 10 eV, with the difference decreasing with impact energy down to about 8% at 50 eV. In general, the present results are higher than all others at most energies below the Ps threshold, and we have demonstrated in a previous publication [20] that this is due, in large part, to differences in the degree of discrimination in the various experiments against forward elastic scattering. Poor angular discrimination in the forward direction results in a smaller total scattering cross section due to the exclusion of regions of the angular scattering intensity.…”

“…The extent of the problem depends on the angular discrimination of the apparatus and the nature of the differential elastic scattering cross section in this forward angle region. It has been discussed in detail in a recent publication [20].…”

Experimental and theoretical cross sections for positron collisions with 3-hydroxy-tetrahydrofuran

“…28 As we have extensively covered this issue in a dedicated paper 28 and we have revisited those details in our most recent study, 9 we only recall here that this effect causes our measured TCS to be underestimated with respect to their "true" value. This effect can be corrected for, provided that the angular discrimination of the spectrometer and the elastic DCS for the target species of interest are known at any given energy.…”

“…This effect can be corrected for, provided that the angular discrimination of the spectrometer and the elastic DCS for the target species of interest are known at any given energy. 28 This can be achieved, for instance, by following the procedure outlined by Hamada and Sueoka. 29 The missing angular ranges 0 • ± θ min and 180 • ± θ min , where θ min is the angular discrimination, can be calculated, for instance, using Eq.…”

Positron collisions with ethene

“…This effect results in the scattered positron count rate being somewhat overestimated, and therefore, the measured TCSs are somewhat smaller in magnitude than their "true values." At any given energy, the extent of the forward angle scattering effect depends on the angular discrimination of the apparatus and on the nature of the elastic differential cross sections (DCSs) for the target in question in this forward angle region [25]. From the geometry of the scattering and detection regions, the angular acceptance of the Trento apparatus is estimated to be θ ∼ 4 • [3].…”