Compton double-to-single ionization ratio of helium at 57 keV

The authors have measured the Compton double-to-single ionization ratio of helium using an ion time-of-flight spectrometer along with monochromatized synchrotron radiation of 57 keV. This photon energy is higher than in all former published experiments and probes the Compton ionization alone since the photoionization gives only a negligible contribution to the total cross section. Comparing their results, which is (1.25 ± 0.3)%, with theoretical calculations and measurements at lower energies shows that this energy is most likely still not high enough to confirm the value of the asymptotic high-energy limit experimentally. A similar results was obtained in a recent experiment using a different method