A Direct Spectrum of the Structure and Shift of the Compton Line with Helium Gas as the Scatterer

The characteristic $K$ lines from a molybdenum target x-ray tube were scattered almost directly backward by helium gas at fourteen atmospheres in a chamber provided with a celluloid window. Entry and exit of radiation was provided through distinctly separate regions of the window to avoid window scattering. The scattered radiation was spectrally resolved with a Cauchois focusing curved (quartz) crystal spectrograph on a curved photographic film after an exposure of 2059 hours. The structure and breadth of the modified line obtained with a recording microphotometer are in excellent agreement with the theoretical predictions of DuMond's theory in which the modified line is regarded as broadened by the Doppler effect of the motion (momentum) of the electrons in the scattering substance. The momentum distribution for the helium atom and the consequent modified line structure have been computed by B. Hicks. The slight defect in the shift (discovered by Ross and Kirkpatrick) caused by the momentum imparted to the whole mass of the atom in the process of ejecting the recoil electron is also verified. All previous reliable studies with solid scatterers have shown the structure of the modified line much broader than DuMond's theory predicts for the electron momenta to be expected in a free atom of the scattering substance although the functional dependence of the breadth on the scattering angle and the primary wave-length were shown by the authors to be in good accord with their Doppler interpretation. The supposition that this excess breadth for solid scatterers is an effect of the higher electron momenta required in the solid state by the Pauli exclusion principle is thus verified in the present work by the fact that with this gaseous scatterer the modified line breadth shows no such excess over the value computed for the free atom. The authors believe this to be the first time a direct spectrum of radiation scattered by a gas has ever been obtained. A brief review of work to date on the structure and shift of modified scattered radiation is given to serve as a background against which to emphasize the interesting questions answered by this investigation and to point out the erroneous character of currently accepted results of certain previous investigations. The experimental details and the methods of reducing the data are fully given. A discussion is given of focusing x-ray spectrographs in general, operating on the geometrical principle first enunciated by the present authors. Advantages and disadvantages of the transmission type with curved crystalline lamina are discussed. Formulae and convenient methods are given for computing the dispersion at any point from reference lines recorded on the film thus automatically correcting for instrumental aberrations. The relativistic correction to the shift formula as derived by DuMond in 1929 is shown to be too small to require consideration here. Conclusive internal evidence from the extreme faintness of the unmodified scattered lines relative to the modified band, proves that the modified scattering here obtained has been scattered by helium only.