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We calculate the contribution to elastic photon scattering from an atom due to scattering off the bound atomic electrons (R'ayleigh scattering). We compare predictions resulting from our numerical evaluation of the relativistic secon&mder S matrix in a ammed central potential mth other theories, particularly the form-factor approximation. We give a prescription for accurate 0(1/o) evaluation of total-atom Rayleigh amplitudes (summed over electrons) and present sample tabulations for lead (Z = 82) for energies of experimental interest in the range from 22.1 to 2750 AU. Based on our prescription ee compare elastic-scattering cross sections using Rayleigh amplitudes with seled~el experiments and are able to remove the large factorwf-bvo discrepancies previously reported.

The validity of form-factor, modified-form-factor and anomalous-scattering-factor approximations in predictions of elastic photon-atom scattering is assessed with the aid of the state-of-the-art numerical calculation of Rayleigh scattering obtained using the second-order Smatrix theory, in the photon energy range from 100 eV to 1 MeV. A comparison is made with predictions from Smatrix theory in the same atomic model for representative low-Z (carbon, Z = 6) and high-Z (lead, Z = 82) elements to get a general idea of the validity of these simpler more approximate methods. The importance of bound-bound contributions and the angle dependence of the anomalous scattering factors is discussed. A prescription is suggested, with the assumption of angle independence, that uses simpler approaches to obtain the elastic scattering cross sections in the soft-X-ray regime at the level of accuracy of the S-matrix calculation, failing at large momentum transfers for high-Z elements. Predictions from this prescription are compared with experiment. With starting point the many-body elastic scattering amplitude, a detailed discussion is presented of the partition of the elastic scattering amplitude into Rayleigh and Delbrtick scattering components. This partition of the optical theorem reveals contributions from bound-bound atomic transitions, bound pair annihilation and bound pair production that are not usually associated with elastic scattering. In the parti-* Present address: 395 Henry Street, Apartment IN, Brooklyn, New York 11201, USA. tioned optical theorem for Rayleigh scattering, as in the many-body optical theorem for scattering from excited states, subtracted cross sections naturally appear. These terms are needed, in addition to the familiar terms for photoionization, to relate the real and imaginary parts of the scattering amplitude.

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Calculated values of the stopping power of ions in superheated liquid have been used in predicting the minimum neutron energy that will nucleate vapor bubbles in a superheated liquid. Such predictions are useful in understanding the -behavior of a new class of neutron detectors based on superheated halocarbon or hydrocarbon drops suspended in aqueous gel.