358 Taylor — Retardation of Alpha Rays oy Metals. 



With the polonium set at a definite distance from the ioniza- 

 tion chamber, it was found that, when the metal sheet was 

 moved away from the polonium toward the ionization chamber, 

 the ionization increased. This increase in the ionization was 

 attributed to the alpha particle having a greater velocity (or 

 range) upon entering the chamber when the sheet was near the 

 chamber than it had when the sheet was at a distance from the 

 chamber. Hence the metal sheet did not cut down the range 

 of the particle so much when the sheet was at a distance from 

 the polonium as it did when near the polonium. As a prelim- 

 inary to more extensive experiments by this method, two tests 

 were made to ascertain whether a scattering of the rays could 

 explain the increase in the ionization observed when the metal 

 sheets were moved away from the polonium towards the 

 ionization chamber. 



First test. — Any marked scattering of the rays by the foils 

 would change the shape of the cone of rays and especially the 

 form of the top portion of the cone. The slope of the top, 

 or nearly horizontal portion, of the Bragg ionization curve, as 

 well as the value of the maximum ionization, depend upon the 

 form of the cone of rays arriving at the ionization chamber. 

 Thus, if scattering of the rays exist to a very marked degree, 

 it might be expected that differences between the slope and 

 form of the two Bragg curves obtained with and without the 

 metal foil over the polonium could be readily detected. With 

 polonium as the source of rays, numerous determinations of 

 the Bragg curves, both with and without the various foils over 

 the polonium, were made. A study of these curves showed 

 them to run parallel to each other and to give the same value 

 of the maximum ionization. The effect of putting the foils 

 over the polonium was merely to diminish all the ordinates of 

 the curves by the same amount. 



Second test. — An iris diaphragm whose circular opening 

 could be adjusted to any desired diameter between 0*5 and 

 5.50ms was constructed of thin sheets of brass and placed 

 directly below the ionization chamber. The center of the 

 opening of the diaphragm was directly below the center of the 

 ionization chamber. With the source of rays (radium C) at 

 such a distance from the ionization chamber that the chamber 

 cut the top portion of the Bragg curve, the ionization was meas- 

 ured for various distances of the metal sheets above the source 

 of rays ; first with the diaphragm open and then with the open- 

 ing in the diaphragm of such diameter as to just limit the geo- 

 metrical beam of rays, or to cut off the edge of the beam. For any 

 given position of the sheet above the source of rays, the ioniza- 

 tion was always greater when the diaphragm was completely 

 open than it was when the diaphragm just limited the beam. 



