Interstellar Absorption of Cosmic X Rays

Improved results on the total photo-ionization cross section of atomic helium are given. The total cross section includes contributions from simultaneous ionization and excitation of ${\mathrm{He}}^{+}$ and double ionization; the combined effect of these two processes adds about 10% to the normal photo-ionization process where ${\mathrm{He}}^{+}$ is left in the ground state. A lower abundance (10.92 based on 12.00 for hydrogen) is adopted for helium, based on recent radio determinations. In calculating the opacity due to $K$-shell photo-ionization of heavy elements, a lower abundance (8.00) is also adopted for neon. Brief mention is made of the effects of irregularities in the density distribution of the interstellar gas on the problems of both x-ray and radio-wave absorption.