Single and double ionization of helium by high-energy photon impact

Cross sections for production of singly and doubly ionized Helium by impact of photons of energy E{sub {delta}}=1-18 keV have been calculated. Both photoabsorption and Compton scattering are considered. For the ratio R{sub P} of double to single ionization by photoabsorption we obtain the non-relativistic asymptotic high-energy limit 1.66 % in accordance with other authors. The behavior of R{sub P} at high finite energies is proportional to E{sub {delta}}{sup -1} with a proportionality constant which is very sensitive to final-state correlation. The ratio R{sub C} of double to single ionization due to Compton scattering is estimated using the incoherent Compton cross section and assuming that the energy lost by the photon is shared among the electrons in a ratio given by R{sub P}. Our results are consistent with recent synchrotron measurements.