Mathematics – Probability
Scientific paper
Aug 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002acasn..43..272l&link_type=abstract
Acta Astronomica Sinica, vol.43, no.2, p. 272-283
Mathematics
Probability
Two-Photon Annihilation, Absorption Cross Section, Absorption Coefficient
Scientific paper
The two-photon annihilation (?-? annihilation) is an important absorption mechanism in the theory of radiative transfer of gamma ?-ray astronomy. The expression of the annihilation cross section given by QED theory in the frame of momentum center is changed into the laboratory reference frame by the use of the skill of relativistic transformation. The new expression of cross section is dependent on the energies and the angle between the moving directions of two photons before annihilation, i.e. ??, ??' and ?, which is more useful for the applied purposes in ?-ray astronomy. Furthermore, a simplified direction-averaged expression of annihilation cross section ?(?, ?') for the conventional isotropic ambient radiation field in astrophysics is derived; and the direction-averaged threshold condition of annihilation and the match condition of the energies of two photons to achieve the maximum annihilation probability are obtained. It is further confirmed that the ?-? annihilation can be compared with the electron-photon scattering process (the Comptonization) in ?-ray astronomy due to the nearly same cross sections. Finally, the annihilation absorption coefficients k?? (.??)??? in some typical ambient low-frequency radiation field in astrophysics, e.g. the bremsstrahlung, the black body and the non-thermal power law fields are calculated, and the detailed physical discussions on our calculations are given.
Chen Leon L.
Ling Jia-Jie
Liu Dang-Bo
Xu D. You J. H. Y.
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