Page:Radio-activity.djvu/404

 where I_{T} is the maximum activity observed, which is reached after an interval T. Since the activity finally decays according to an exponential law (half value in 28 minutes), one of the values of λ is equal to 4·13 × 10^{-4}. As in the case of thorium and actinium, the experimental curves do not allow us to settle whether this value of λ is to be given to λ_{2} or λ_{3}. From other data (see section 226) it will be shown later that it must refer to λ_{3}. Thus λ_{3} = 4·13 × 10^{-4} (sec)^{-1}.

The experimental curve agrees very closely with theory if λ_{2} = 5·38 × 10^{-4} (sec)^{-1}.

The agreement between theory and experiment is shown by the table given below. The maximum value I_{T} (which is taken as 100) is reached at a time T = 36 minutes.

In order to obtain the β-ray curve, the following procedure was adopted. A layer of thin aluminium was placed inside a glass tube, which was then exhausted. A large quantity of radium emanation was then suddenly introduced by opening a stopcock communicating with the emanation vessel, which was at atmospheric pressure. The emanation was left in the tube for 1·5 minutes and then was rapidly swept out by a current of air. The aluminium was then removed and was placed under an electroscope, such as is shown in Fig. 12. The α rays from the aluminium were cut off by an interposed screen of aluminium ·1 mm. thick. The time was reckoned from a period of 45 seconds after the introduction of the emanation.

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 * Time in | Theoretical value | Observed value |
 * minutes |   of activity    |  of activity   |
 * 0    |        0          |       0        |
 * 10    |       58·1        |      55        |
 * 20    |       88·6        |      86        |
 * 30    |       97·3        |      97        |
 * 36    |      100          |     100        |
 * 40    |       99·8        |      99·5      |
 * 50    |       93·4        |      92        |
 * 60    |       83·4        |      82        |
 * 80    |       63·7        |      61·5      |
 * 100    |       44·8        |      42·5      |
 * 120    |       30·8        |      29        |