PART V.
Secondary Rays.
109. Production of secondary rays. It has long been known that Röntgen rays, when they impinge on solid obstacles, produce secondary rays of much less penetrating power than the incident rays. This was first shown by Perrin and has been investigated in detail by Sagnac, Langevin, Townsend and others. Thus it is not surprising that similar phenomena should be observed for the radiation from radio-active substances. By means of the photographic method, Becquerel[[177]] has made a close study of the secondary radiations produced by radio-active substances. In his earliest observations, he noticed that radiographs of metallic objects were always surrounded by a diffuse border. This effect is due to the secondary rays set up by the incident rays at the surface of the screen.
The secondary rays produced by the α rays are very feeble. They are best shown by polonium, which gives out only α rays, so that the results are not complicated by the action of the β rays. Strong secondary rays are set up at the point of impact of the β or cathodic rays. Becquerel found that the magnitude of this action depended greatly on the velocity of the rays. The rays of lowest velocity gave the most intense secondary action, while the penetrating rays gave, in comparison, scarcely any secondary effect. In consequence of the presence of this secondary radiation, the photographic impression of a screen pierced with holes is not clear and distinct. In each case there is a double photographic impression, due to the primary rays and the secondary rays set up by them.
These secondary rays are deviable by a magnetic field, and in turn produce tertiary rays and so on. The secondary rays are in all cases more readily deviated and absorbed than the primary rays, from which they arise. The very penetrating γ rays give rise to secondary rays, which cause intense action on the photographic plate. When some radium was placed in a cavity inside a deep lead block, rectangular in shape, besides the impression due to the direct rays through the lead, Becquerel observed that there was also a strong impression due to the secondary rays emitted from the surface of the lead. The action of these secondary rays on the plate is so strong that the effect on the plate is, in many cases, increased by adding a metal screen between the active material and the plate.
The comparative photographic action of the primary and secondary rays cannot be taken as a relative measure of the intensity of their radiations. For example, only a small portion of the energy of the β rays is in general absorbed in the sensitive film. Since the secondary rays are far more easily absorbed than the primary rays, a far greater proportion of their energy is expended in producing photographic action than in the case of the primary rays. It is thus not surprising that the secondary rays set up by the β and γ rays may in some cases produce a photographic impression comparable with, if not greater than, the effect of the incident rays.
On account of these secondary rays, radiographs produced by the β rays of radium in general show a diffuse border round the shadow of the object. For this reason radiographs of this kind lack the sharpness of outline of X ray photographs.
110. Secondary radiation produced by α rays. Mme Curie[[178]] has shown by the electric method that the α rays of polonium produce secondary rays. The method adopted was to compare the ionization current between two parallel plates, when two screens of different material, placed over the polonium, were interchanged.
These results show that the α rays of polonium are modified in passing through matter, and that the amount of secondary rays set up varies with screens of different material. Mme Curie, using the same method, was unable to observe any such effect for the β rays of radium. The production of secondary rays by the β rays of radium is, however, readily shown by the photographic method. We have already seen ([section 93]) that very low velocity electrons accompany the α rays from radium or radio-tellurium spread on a metal plate. These electrons are probably liberated when the α rays escape from or impinge upon matter, and the number emitted depends upon the kind of matter used as a screen. The differences shown in the above table when the screens were interchanged are explained simply in this way.
| Screens employed | Thickness in mms. | Current observed |
|---|---|---|
| Aluminium | 0·01 | |
| Cardboard | 0·005 | 17·9 |
| Cardboard | 0·005 | |
| Aluminium | 0·01 | 6·7 |
| Aluminium | 0·01 | |
| Tin | 0·005 | 150 |
| Tin | 0·005 | |
| Aluminium | 0·01 | 126 |
| Tin | 0·005 | |
| Cardboard | 0·005 | 13·9 |
| Cardboard | 0·005 | |
| Tin | 0·005 | 4·4 |
Fig. 45.
111. Secondary rays produced by β and γ rays. An examination of the amount and character of the secondary radiation emitted by various substances, when exposed to the β and γ rays of radium, has recently been made by A. S. Eve[[179]]. The general experimental method employed is shown in [Fig. 45].
The electroscope ([Fig. 45]) was placed behind a lead screen 4·5 cms. thick, which stopped all the β rays and absorbed the greater proportion of the γ rays from the radium tube placed at R. On bringing near a plate of matter M, the primary rays fell upon it and some of the secondary rays, emitted in all directions, passed into the side of the electroscope, which was covered with aluminium foil of thickness ·05 mm. Before the plate M was placed in position the rate of discharge of the electroscope was due to the natural leak and the γ rays from R, and the secondary radiation from the air. On bringing the radiator M into position, the rate of discharge was much increased, and the difference between the rate of movement of the gold-leaf in the two cases was taken as a measure of the amount of secondary rays from M. The absorption of the secondary rays was tested by placing an aluminium plate ·85 mm. thick before the face of the electroscope.
The secondary rays were found to be fairly homogeneous, for the intensity fell off according to an exponential law with the distance traversed. The value of the absorption constant λ was determined from the usual equation
where d is the thickness of the screen. The table given below shows the results obtained when thick plates of different substances of the same dimensions were placed in a definite position at M. The secondary radiation from fluids was obtained by a slight alteration of the experimental arrangements.
Thirty milligrammes of radium bromide were used, and the results are expressed in terms of the number of scale divisions passed over per second by the gold-leaf.
It will be noticed that the amount of secondary radiation follows in most cases the same order as the densities, and is greatest for mercury. The value of (secondary radiation)/density is not a constant, but varies considerably, being greatest for light substances. The absorption constant of the secondary rays from different radiators is not very different, with the exception of substances such as granite, brick, and cement, which give out secondary rays of nearly twice the penetrating power of other substances.
β and γ rays.
| Radiator | Density | Secondary Radiation | Sec. Rad. / Density | Aluminium ·085 cm. λ |
|---|---|---|---|---|
| Mercury | 13·6 | 147 | 10·8 | |
| Lead | 11·4 | 141 | 12·4 | 18·5 |
| Copper | 8·8 | 79 | 9·0 | 20 |
| Brass | 8·4 | 81 | 9·6 | 21 |
| Iron (wrought) | 7·8 | 75 | 9·6 | 20 |
| Tin | 7·4 | 73 | 9·9 | 20·3 |
| Zinc | 7·0 | 79 | 11·3 | |
| Granite | 2·7 | 54 | 20·0 | 12·4 |
| Slate | 2·6 | 53 | 20·4 | 12·1 |
| Aluminium | 2·6 | 42 | 16·1 | 24 |
| Glass | 2·5 | 44 | 17·6 | 24 |
| Cement | 2·4 | 47 | 19·6 | 13·5 |
| Brick | 2·2 | 49 | 22·3 | 13·0 |
| Ebonite | 1·1 | 32 | 29·1 | 26 |
| Water | 1·0 | 24 | 24·0 | 21 |
| Ice | ·92 | 26 | 28·2 | |
| Paraffin solid | ·9 | 17 | 18·8 | 21 |
| „ liquid | ·85 | 16 | 18·8 | |
| Mahogany | ·56 | 21·4 | 38·2 | 23 |
| Paper | ·4? | 21·0 | 52 | 22 |
| Millboard | ·4? | 19·4 | 48 | 20·5 |
| Papier-mâché | ... | 21·9 | ||
| Basswood | ·36 | 20·7 | 57 | 22 |
| Pine | ·35 | 21·8 | 62 | 21 |
| 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.
| Radiator | β and γ rays | γ rays | Röntgen |
|---|---|---|---|
| Lead | 100 | 100 | 100 |
| Copper | 57 | 61 | 291 |
| Brass | 58 | 59 | 263 |
| Zinc | 57 | ... | 282 |
| Aluminium | 30 | 30 | 25 |
| Glass | 31 | 35 | 31 |
| Paraffin | 12 | 20 | 125 |
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].
Fig. 46.
A small electroscope was mounted on one side of a lead platform 1·2 cms. thick, which rested on a lead cylinder 10 cms. high and 10 cms. in diameter. The radium was placed at the bottom of a hole reaching to the centre of the cylinder.
On applying a strong magnetic field, at right angles to the plane of the paper, so as to bend the secondary rays from the platform towards the electroscope, the rate of discharge was much increased. On reversing the field, the effect was much diminished. Since the γ rays are not themselves deflected by a magnetic field, this result shows that the secondary radiation is quite different in character from the primary rays, and consists of electrons projected with a velocity (deduced from the penetrating power) of about half the velocity of light. We have already pointed out that the emission of electrons from a substance traversed by the rays will account sufficiently well for the charge observed by Paschen, without the necessity of assuming that the γ rays carry a negative charge of electricity.
The secondary radiation set up by Röntgen rays, like that due to the β and γ rays, consists in part of electrons projected with considerable velocity. These three types of rays seem about equally efficient in causing the expulsion of electrons from the substance through which they pass. We have seen that the X and γ rays are, in all probability, electromagnetic pulses set up by the sudden starting or stopping of electrons, and, since these rays in turn cause the removal of electrons, the process appears to be reversible. Since the β rays pass through some thickness of matter before their energy of motion is arrested, theory would lead us to expect that a type of soft X rays should be generated in the absorbing matter.