The Opacity at High Temperatures due to Compton Scattering.

The contribution to the radiative opacity of matter arising from Compton scattering is obtained for the entire region in which degeneracy in the electron distribution may be neglected. This corresponds to temperatures up to about 125 Kev (1 Kev = 11605800 K) and densities up to roughly 10 gm at the highest temperatures considered. It is found that, at temperatures for which the Klein-Nishina cross- section should be used, parts of the scattering into and out of a beam of radiation do not cancel, as previously assumed. The result of this is to lower the opacity by about 13 per cent at kT = 20 Kev and 37 per cent at kT = 125 Kev. Although absorption resulting in pair-creation may be neglected at these temperatures, we find that pair-production is responsible in another way for a sharp rise in the opacity by increasing the density (N) of electrons and positrons significantly at the highest temperatures we consider. In fact, at kT = 125 Kev, the value of N corresponding to thermal equilibrium is large and almost independent of the matter density (p), where p does not include the mass of the electrons and positrons resulting from pair-creation. Hence we conclude that for kT > 125 Kev, the average mean free path A = (pK ) i is small and independent of p. On the other hand, for kT 30 Kev, we have the familiar situation that the Compton opacity (K ) is small and independent of p.