ty,’ Pe 
Penetrating Radium Rays. 73 
Sufficient thickness of lead is placed permanently between 
Rand the tube a to absorb all the B rays, so that the ionization 
observed is due entirely to y rays, and measures the intensity 
of these rays. The substance to be examined is placed in 
successive layers in front of the tube a. In this way, curves 
are drawn showing the intensity of the y rays, after passing 
through various thicknesses of different substances. 
40 milligrams of pure radium bromide were used, placed 
10 ems. from the testing-tube a. 
Observations. 
In the first place, the thickness of lead required to stop ‘all 
8 rays was determined by observing the ionization produced 
in the tube a, with nothing between the radium vessel R and 
the tube, and then putting between successive layers of lead ; 
the vessel R itself stops most of the @ rays. Tig. 3 
shows that all the 8 rays are stopped by 4 mms. of lead. In 
Fig. 3. 
oe) 
— | | 
Sitensity of Madietion 
Mm p roy) 
oO (oo) (o) 
dis 
10 . 20 50 
Thickness Oo Lead tr? . 777TRS. 
all the following experiments a thickness of 8 mms. of lead 
was kept in position between the radium and the testing- 
tube, a thickness which ensured that all 6 rays were 
stopped. 
