A Precision Measurement of the Change of Wave-length of Scattered X-rays

According to the quantum theory, the change of wave-length due to scattering is δγ=(h/mc) (1-cosφ) where h is the radiation constant, m the mass of an electron, c the velocity of light and φ the angle of scattering. In the present experiments φ was made as large as possible (169^∘{}) by placing the slits and calcite crystal in a lead box fastened to the Coolidge tube. Mo Kα and Kβ radiation was scattered from paraffin back to the crystal and then to a photographic plate; Zr Kα rays for comparison were obtained from a Zr radiator. The best plate required an exposure of 48 hr. The change of wave-length was measured 16 times by means of a special microphotometer. After certain corrections were made, the shift came out (.04825 ±{}.0002)A; the theoretical value is.04798 ±{}.0001. This agreement is an excellent confirmation of the theory. This effect may also be used to obtain a measurement of m which is independent of e/m. The value of the mass of the electron thus computed from these measurements is (8.99 ±{}.034) ×{} 10^-28 gm in agreement with the mean result from deflection experiments, whereas the spectroscopic value of e/m gives 9.04.