Page:Radio-activity.djvu/201

 The writer showed that the absorption of the γ rays from radium was approximately proportional to the density of the substance traversed. A more detailed examination of the absorption of these rays in various substances has been recently made by McClelland. The curve (Fig. 44) shows the decrease of the ionization current in a testing vessel due to the β and γ rays with successive layers of lead. It is seen that the β rays are almost completely stopped by 4 mms. of lead; the ionization is then due entirely to the γ rays.

In order to leave no doubt that all the β rays were absorbed, the radium was covered with a thickness of 8 mms. of lead, and measurements of the coefficient of absorption λ were made for additional thicknesses. The average value of λ was calculated from the usual equation I/I_{0} = e^{-λd}, where d is the thickness of matter traversed. The following table shows the value of λ, (I) for the first 2·5 mms. of matter traversed (after initially passing through 8 mms. of lead), (II) for the thickness 2·5 to 5 mms., (III) for 5 to 10 mms., (IV) 10 to 15 mms.

TABLE A.

+-++++-+ +-++++-+ +-++++-+
 * Substance |    I     |   II   |   III  |   IV  |
 * Platinum   |  1·167   |        |        |       |
 * Mercury    |   ·726   |  ·661  |  ·538  | ·493  |
 * Lead       |   ·641   |  ·563  |  ·480  | ·440  |
 * Zinc       |   ·282   |  ·266  |  ·248  | ·266  |
 * Aluminium  |   ·104   |  ·104  |  ·104  | ·104  |
 * Glass      |   ·087   |  ·087  |  ·087  | ·087  |
 * Water      |   ·034   |  ·034  |  ·034  | ·034  |

In the above table, the absorption in aluminium, glass and water was too small to determine with accuracy the variation of λ with distance traversed. It will be observed that, for the denser substances, the coefficient of absorption decreases with the distance through which the rays have passed. This indicates that the rays are heterogeneous. The variation of λ is more marked in heavy substances.