Internal Pairs from Aligned Nuclei

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Abstract
The problem of pair emission from light nuclei where the emitting state is oriented is examined. More specifically as long as the polarization of neither of the pair is observed, only nuclear alignment plays a role. Detailed plane wave calculations presented for the pure magnetic multipole show that if the dihedral angle between the planes of the alignment axis and recoil momentum and the plane of the pair is summed over, the distribution of pairs is the product of the unaligned distribution and the angular distribution of spin-one particles (photons). The ratio of pairs with fixed angle between the two members and fixed angle between the alignment and recoil directions to the number of photons with the latter angle also fixed is independent of alignment. For electric transitions the corresponding pair distribution is an incoherent sum of transverse and longitudinal parts. The first part, with nuclear orientation, is again multiplied by the spin-one angular distribution function but the longitudinal part is multiplied by a spin-zero angular distribution function. Hence, alignment will influence the ratio of pair and photon angular distributions because of the presence of the longitudinal field in the electron-nucleus interaction.

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