Finite Slit-Length Correction in Positron Annihilation Experiments

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Abstract
The effect of the detector slit length in measurements of the angular correlation of annihilation radiation is treated theoretically. A general formula is derived, which expresses the coincidence rate from a single crystal in terms of the anisotropic momentum distribution, and its behavior is investigated in the limits of very long and very short slits. The result for the special case of an isotropic momentum distribution is compared with an earlier result of Arifov, Gol'danskii, and Sayasov. It is further concluded that line slits are to be preferred to point slits for the measurement of strongly anisotropic momentum distributions, about which no a priori information is available.