RadiatorDensitySecondary RadiationSec. Rad. / DensityAluminium ·085 cm. λ
Mercury13·614710·8
Lead11·414112·418·5
Copper8·8799·020
Brass8·4819·621
Iron (wrought)7·8759·620
Tin7·4739·920·3
Zinc7·07911·3
Granite2·75420·012·4
Slate2·65320·412·1
Aluminium2·64216·124
Glass2·54417·624
Cement2·44719·613·5
Brick2·24922·313·0
Ebonite1·13229·126
Water1·02424·021
Ice·922628·2
Paraffin solid·91718·821
„ liquid·851618·8
Mahogany·5621·438·223
Paper·4?21·05222
Millboard·4?19·44820·5
Papier-mâché...21·9
Basswood·3620·75722
Pine·3521·86221
X ray screen 75·2 23·6

The secondary radiation not only comes from the surface of the radiator but from a considerable depth. The amount of secondary rays increases with the thickness of the radiator, and, in the case of glass and aluminium, reaches a practical maximum for a plate about 3 mms. thick.

In the above table, the secondary radiation arises from both the β rays and γ rays together. When the β rays were cut off by a layer of lead 6·3 mms. thick, placed between the radium and the radiator, the effect on the electroscope was reduced to less than 20 per cent. of its former value, showing that the β rays supplied more than 80 per cent. of the secondary radiation. The following table shows the relative amount of secondary rays from different substances when exposed to β and γ rays together and to γ rays alone. The amount from lead in each case is taken as a standard and equal to 100. The amount of secondary radiation found by Townsend from soft X rays is added for comparison.

Secondary Radiations.

Radiatorβ and γ raysγ raysRöntgen
Lead100100100
Copper5761291
Brass5859263
Zinc57...282
Aluminium303025
Glass313531
Paraffin1220125

It will be observed that the relative amounts are about the same for the γ rays alone as for the β and γ rays together. On the other hand, the amount of secondary radiation set up by X rays is very different, lead for example giving much less than brass or copper. The secondary rays from the γ rays alone are slightly less penetrating than for the β and γ rays together, but are far more penetrating than the secondary radiation from the X rays examined by Townsend.

The amount of secondary radiation set up by the β and γ rays is mainly independent of the state of the surface of the radiator. About the same amount is obtained from iron as from iron filings; from liquid as from solid paraffin; and from ice as from water[[180]].

Becquerel has shown that the secondary rays set up by the β rays are deflected by a magnet and consist of negatively charged particles (electrons). It has been pointed out in [section 52] that the cathode rays are diffusely reflected from the metal on which they fall. These secondary rays consist in part of electrons moving with about the same velocity as the primary, and in part of some electrons with a much slower speed. The secondary rays set up by the β rays of radium have on an average less penetrating power than the primary rays, and consequently less velocity than the primary rays. It must be remembered that the β rays from radium are very complex, and consist of electrons projected with a considerable range of velocities. The secondary rays are, on an average, certainly more penetrating than the most easily absorbed β rays emitted from radium, and probably move with a velocity of about half that of light.

It is still uncertain whether the secondary rays are produced by the action of the primary rays on matter, or whether they consist of a portion of the primary rays whose direction of motion has been deflected in their passage through matter, so that they emerge again with diminished velocity from the surface.

112. Magnetic deflection of secondary rays from γ rays. It has been seen that the secondary rays set up by the γ rays alone are very similar in character to those caused by the β rays. This result was still further confirmed by Eve, who showed that the secondary rays produced by the γ rays are readily deflected by a magnetic field. The experimental arrangement is shown in [Fig. 46].