Continuous Photoelectric Absorption Cross Section of Helium

Abstract

The continuous photoelectric absorption cross section of helium has been measured in the spectral region extending from 180 to 600 A with greater accuracy and the observations are found to agree with the calculations of Huang and Stewart and Wilkinson. A grazing incidence spectrometer with a photomultiplier was used for a single measurement at 180 A while the remaining measurements were carried out in a normal incidence spectrometer utilizing photographic techniques. Whereas in previous experiments the absorbing gas sample was allowed to fill the entire spectrometer chamber, in the current measurements the gas was confined to a small cell provided with sufficiently transparent windows. The use of an absorption cell reduces contamination and facilitates the measurement of gas pressures. The results indicate that the cross section varies from a value of 0.98±0.04 Mb at 180 A to a value of 7.7±0.3 Mb at the absorption edge located at 504 A.