双光子湮灭吸收截面和吸收系数

双光子湮灭造成的γ射线吸收是γ射线天文学辐射转移理论的一个重要内容。通过相对论变换，将目前QED理论给出的仅在动量中心参考系中表示的双光子湮灭截面推广到实验室系（观测者系），并用湮灭前两光子的能量hw,hw‘及夹角θ表示此截面，便于γ射线在天文学中的实际应用。进一步，在各向同性周边辐射场的一般前提下，求出方向平均的简化截面公式σ（w,w‘），得出方向平均的湮灭阈值条件和湮灭概率最大时两个光子能量的匹配条件。进一步确认在γ射线天文学中，双光子湮灭截面和γ光子－电子的康普顿散射截面确有可比性。最后完成了天体物理中几种常见低频辐射场如热韧致辐射场、黑体辐射场以及非热的幂律场中双光子湮灭吸收系数krr(hw)-hw的计算曲线，并给出较详细的物理讨论。

Absorption Cross Section and Absorption Coefficient of the γ-γ Annihilation

The two-photon annihilation (annihilation) is an important absorption mechanism in the theory of radiative transfer of 7-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 relativistie 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(Tiu>) ~ fiuj in some typical ambient low-frequency radiation field in astrophysics, e.g. the bremsstrahlung, the black body and the nonthermal power law fields are calculated, and the detailed physical discussions on our calculations are given.