Page:Radio-activity.djvu/209

 radium is, however, readily shown by the photographic method. We have already seen (section 93) that very low velocity electrons accompany the [Greek: alpha] rays from radium or radio-tellurium spread on a metal plate. These electrons are probably liberated when the [Greek: alpha] rays escape from or impinge upon matter, and the number emitted depends upon the kind of matter used as a screen. The differences shown in the above table when the screens were interchanged are explained simply in this way.

+-+-++ +-+-++
 * | Thickness | Current |
 * Screens employed |  in mms.  | observed |
 * Aluminium        |   0·01    |          |
 * Cardboard        |   0·005   |   17·9   |
 * Cardboard        |   0·005   |          |
 * Aluminium        |   0·01    |    6·7   |
 * Aluminium        |   0·01    |          |
 * Tin              |   0·005   |  150     |
 * Tin              |   0·005   |          |
 * Aluminium        |   0·01    |  126     |
 * Tin              |   0·005   |          |
 * Cardboard        |   0·005   |  13·9    |
 * Cardboard        |   0·005   |          |
 * Tin              |   0·005   |   4·4    |
 * Tin              |   0·005   |          |
 * Cardboard        |   0·005   |  13·9    |
 * Cardboard        |   0·005   |          |
 * Tin              |   0·005   |   4·4    |
 * Cardboard        |   0·005   |          |
 * Tin              |   0·005   |   4·4    |

Fig. 45.

111. Secondary rays produced by β and γ rays. An examination of the amount and character of the secondary radiation emitted by various substances, when exposed to the