Fig. 40.
In this case, PM is the range of the α particles from the lowest layer of the radio-active matter. The current should be constant for all distances less than PM.
For a thick layer of radio-active matter, the curve should be a straight line such as APB.
Curves of the above character should only be obtained when definite cones of rays are employed, and where the ionization vessel is shallow and includes the whole cone of rays. In such a case the inverse square law need not be taken into account.
In the experiments previously recorded (sections [99] and [100]), the ionization was measured between parallel plates several centimetres apart for a large area of radio-active material. Such an arrangement was necessary at the time at which the experiments were made, as only weak radio-active material was available. Measurable electrical effects could not then be obtained with narrow cones of rays and shallow ionization vessels, but this disadvantage is removed by the advent of pure radium bromide as a source of radiation.
The interesting experiments described by Bragg and Kleeman show that the theoretical curves are approximately realized in practice. The chief difficulty experienced in the analysis of the experimental results was due to the fact that radium is a complex radio-active substance and contains four radio-active products each of which gives rise to α rays which have different ranges. The general character of the results obtained from radium are shown graphically in [Fig. 41], curves A, B, C, D.
Fig. 41.
The ordinates represent the distance between the radium and the gauze of the testing vessel; the abscissae the current in the ionization vessel in arbitrary units. Five milligrams of radium bromide were used, and the depth of the ionization vessel was about 5 mms. Curve A is for a cone of rays of angle 20°. The initial current at a distance of 7 cms. is due to the β and γ rays and natural leak. This curve is initially parabolic, and then is made up of two straight lines. Curve B is for a smaller cone, and shows the straight line character of the curve to within a short distance of the radium. Curve C was obtained under the same condition as curve A, but with a layer of gold beater’s skin placed over the radium. The effect of this is to reduce all the ordinates of curve A by the same quantity. This is to be expected on the simple theory already considered. Curve D was obtained when the radium was heated so as to get rid of the emanation and its products. The α particles of greatest range are quite absent and the curve is simpler in character.
