The velocity of the α particle is thus reduced only a out 36 per cent. of its initial value when it fails to produce any action on the photographic plate.
Now Bragg has shown (section 104) that the α particle produces approximately the same number of ions per cm. of path in air over its whole range. Consequently, the simplest assumption to make is that the energy of the α particle is diminished by a constant amount in traversing each layer of foil. After passing through 12 layers the kinetic energy is reduced to 41 per cent. of the maximum. Each layer of foil thus absorbs 4·9 per cent. of the maximum energy. The observed kinetic energy of the α particle after passing through successive layers of foil, and the value calculated on the above assumptions, are shown in the following table.
Number of layers Observed Calculated
of aluminum foil energy energy
0 100 100
5 73 75
8 58 61
10 53 51
12 41 41
The experimental and theoretical values agree within the limits of experimental error. We may thus conclude, as a first approximation, that the same proportion of the total energy is abstracted from the α particles in passing through equal distances of the absorbing screen.
Range of ionization and photographic action in air.
The abrupt falling off of the photographic impression after the rays
had passed through 12 layers of foil suggested that it might be
directly connected with the corresponding abrupt falling off of the
ionization in air, so clearly brought out by Bragg. This was found to
be the case. It was found experimentally that the absorption in each
layer of aluminium foil was equivalent to that produced by a distance
of ·54 cms. of air. Twelve layers of foil thus corresponded to 6·5 cms. of
air. Now Bragg found that the α rays from radium C ionize the air
for a distance 6·7 cms., and that the ionization then falls off very
rapidly. We may thus conclude that the α rays cease to affect the
photographic plate at the same velocity as that at which they cease to
ionize the gas. This is a very important result, and, as we shall see
later, suggests that the action on the photographic plate is due to an
ionization of the photographic salts.
