Recoil of Fission Products in Homogeneous Carbon Structures

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Abstract
An experimental investigation of the average range of fission fragments in pyrolytic graphite was performed by exposing coupons of this material bearing a thin electromagnetically deposited layer of 235U to thermal neutrons. For the experiments, reliable and reproducible sectioning techniques were developed for the determination of ranges in thick targets with variable structure. The procedures used to obtain the range average in terms of the experimental data are described in detail, and an empirical formula for the calculation of ranges in pyrocarbons is given. Analytical expressions were generated for describing the penetration of the recoil‐fission fragments in the target material and evaluating various theoretical models of electronic and nuclear‐stopping mechanisms in a reference structure. It was found that for carbons of homogeneous structure and with void sizes smaller than Bohr's straggling parameter, the penetration curves could be obtained as convolutions of a kernel of the form of a normal‐distribution curve. For the thin extended sources used in this investigation, the various integrations performed mitigate straggling effects so that the only important parameter for data correlations is the average range, which was obtained for four mass chains in six different experiments. The values so obtained compared favorably with Lindhard's and Frank's microscopic models for the calculation of the average range in carbon.