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Study of self-supporting deuterated polyethylene targets Makosky, Lyle M.

Abstract

Thin self-supporting deuterated polyethylene targets for use with the reaction 2ʜ(3ʜe,p)⁴He have been investigated. Both pouring and evaporating techniques for the construction of the polyethylene film were tested. Stable targets, capable of withstanding 690 keV ³He beam current densities of more than 10 μA/cm², were constructed by evaporating 10 μg/cm² thick carbon layers onto both sides of the (C₂D₄)n film ("sandwich" targets). Targets prepared by the pouring technique were found to be mechanically stronger than the "sandwich" targets prepared by evaporation of (C₂D₄)n. The targets exhibit an initial rapid decrease in the deuterium content during the first 150 μc of ³He beam on the target. This effect is attributed to breakdown of the C₂D₄ molecules into their atomic constituents and subsequent diffusion of the deuterium out of the target. After this initial effect, the targets deteriorate by less than 0.05% per μc. The targets were developed for use for total proton reaction cross section measurements by the associated particle technique. The beam profiles of the 15.8 MeV protons from the reaction ²H (³He,p)⁴He, which are relevant to this technique, were measured. The amount of elastic scattering suffered by the protons in the present targets was smaller than that observed with the copper backed heavy ice targets used in previous cross section measurements (Ho 68). The deuterium content of the target was found to be a linear function of the thickness of the carbon layers comprising the target "sandwich". The alpha yield from the reaction ²H (³He,p)⁴He increased by a factor of 1.83 ± 0.35 per 10 μg/cm² carbon layer on the "back" of the target, and by a factor of 1.38 ± 0.26 per 10 μg/cm2 carbon layer on the "front" of the target. As a possible explanation of this effect it is proposed that deuterium, repleased from the polyethylene by the beam, diffuses throughout the target and is trapped by the carbon to a concentration of 0.11 deuterium atoms per carbon atom.

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