Very valuable studies of the directions in which photo-electrons are ejected by X-rays have been made recently by Williams, Auger, and Anderson. All of these observers, however, used the C. T. R. Wilson expansion-chamber method which, in spite of its power in working with the individual electron, suffers from the disadvantage that the particular energy level in the atom from which the electron is ejected in general cannot be determined. It is true that in the case of heavy atoms such as xenon and bromine, Auger and Anderson succeeded, through the use of X-rays of particular energies, in distinguishing the electrons thrown out of the K level from those thrown out of the L levels, but to go much further in this direction by the expansion-chamber method (and, for example, to distinguish the L
I
from the L
II
or L
III
electrons) seems hopeless. Consequently, the magnetic spectrograph developed by one of us for studying the velocity of the X-ray electrons as a function of the angle of emission was applied to the problem with the results which it is the purpose of this paper to describe. We, as yet, have not succeeded in determining the actual directions of ejection with the precision which has been attained in the expansion-chamber method, but the resolving power of the apparatus for velocities is so large that the particular level in which each electron group originates is in general quite unambiguous. A description of the apparatus used and the procedure followed has been given in the paper referred to above and need not be repeated here. Ballast lamps of the sort developed at the General Electric Company and sold by the Radio Corporation of America (radiotron UV-886) have proved very useful in holding the current through the solenoid which produces the magnetic field constant during the long exposures (100-200 hours) which are necessary. Eastman X-ray plates were used throughout, as they have been found to be the most sensitive of any so far tried (except Schumann plates which are much too irregular for intensity measurements). The work has been seriously handicapped by the lack of sensibility of the photographic plates for slow electrons and by their rapid falling off in sensibility for electrons of velocities below about 12,000 volts. X-ray tubes with silver, molybdenum and copper anticathodes were used. The characteristic radiation from copper is, however, in spite of its intrinsic intensity, too soft to eject electrons with sufficient velocity to give photographic results in a reasonable length of time with the apparatus used. All the results shown below consequently were obtained with the characteristic rays of either silver or molybdenum.
The use of fricke dosimetry for low energy x-rays
