Photoelectric effect of soft X-rays

For the investigation of soft X-rays, Richardson and Bazzoni employed the method of ejection of photoelectrons from a metal plate under the action of the rays. Subsequent investigations in this region were carried out by Richardson and Chalkin, Kurth, Mohler and Foote, Horton, Davies and Andrewes, Thomas, Compton and Thomas, and also by others by the same method, with the view to determining voltages at which excitations of characteristic soft X-rays occur. More recently Richardson and Robertson have employed this method for studying the relative soft X-ray emissivities of a large number of elements. Other methods have also been employed, among which the spectroscopic method of analysis may be mentioned.§ In this method, a pressure lower than 10-3 mm. can in the present state of things be hardly secured. It has been found that the radiation from the residual gas at a pressure of 1.10-5 mm. is approximately equal to that of soft X-rays from a carbon target. Further, the gas radiation increase linearly with pressure. So it can be easily understood what an important rôle the residual gas may be playing in the present spectroscopic methods.