Page:Radio-activity.djvu/266

 readily absorbed, the current due to it practically reaches a maximum when the surface of the plate is completely covered by a thin layer of the active material. On the other hand the current produced by the emanation increases until the layer is several millimetres in thickness, and then is not much altered by adding fresh active matter. This falling off of the current after a certain thickness has been reached is to be expected, since the emanation, which takes several minutes to diffuse through the layer above it, has already lost a large proportion of its activity.

With a thick layer of thorium oxide in a closed vessel, the current between the plates is largely due to the radiation from the emanation lying between the plates. The following tables illustrate the way in which the current varies with the thickness of paper for both a thin and a thick layer.

Thin Layer.

Thickness of sheets of paper ·0027.

+-+-+ +-+-+ +-+-+
 * No. of layers | Current |
 * of paper   |         |
 * 0        |   1     |
 * 1        |    ·37  |
 * 2        |    ·16  |
 * 3        |    ·08  |

Thick Layer.

Thickness of paper ·008 cm.

+-++ +-++ +-++
 * No. of layers | Current |
 * of paper   |          |
 * 0      |   1      |
 * 1      |    ·74   |
 * 2      |    ·74   |
 * 5      |    ·72   |
 * 10      |    ·67   |
 * 20      |    ·55   |

The initial current with the unscreened compound is taken as unity. In Table I, for a thin layer of thorium oxide, the current diminished rapidly with additional layers of thin paper. In this case the current is due almost entirely to the [Greek: alpha] rays. In Table II the current falls to ·74 for the first layer. In this case about 26% of the current is due to the [Greek: alpha] rays, which are practically absorbed by the layer ·008 cm. in thickness. The slow decrease with additional layers shows that the emanation diffuses so rapidly through a few layers of paper that there is little loss of activity during the passage. The time taken to diffuse through 20 layers is however appreciable, and the current consequently has decreased. After passing through a layer of cardboard 1·6 mms. in thickness the current is reduced to about one-fifth of its original value. In