Interference parameters inγ-ray optics

Calculations are given for the interference (or "screening") parameters $\xi \left(L\lambda \right)$ arising in low-energy $\gamma$-ray optics. In $\gamma$-ray emission, these parameters arise from the interaction of the emitted photon with the surrounding electron cloud of the atom, giving a phase shift to the emitted photon which depends on the multipolarity of the nuclear transition. In $\gamma$-ray absorption experiments, the parameters specify the effects of the interference between photoelectron and internal-conversion electron absorption, and the interference between nonresonant scattering from the atomic electrons and resonant scattering from the nucleus. These interference effects give rise to a dispersion term in the total cross section, thereby producing a small asymmety in the absorption spectrum and slightly shifting the center of the absorption line. These effects can be very important in determining precise isomer shifts and in analyzing Mössbauer tests of time-reversal invariance. We present calculated screening parameters for $E1\mathrm{}$, $M1\mathrm{}$, and $E2\mathrm{}$ $\gamma$-ray transitions, and we examine their dependences on the nuclear charge and the $\gamma$-ray energy. We compare the results with the available experimental determinations.