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.
PART VI.
113. Comparison of the ionization produced by the α and β rays. With unscreened active material the ionization produced between two parallel plates, placed as in Fig. 17, is mainly due to the α rays. On account of the slight penetrating power of the α rays, the current due to them practically reaches a maximum with a small thickness of radio-active material. The following saturation currents were observed[181] for different thicknesses of uranium oxide between parallel plates sufficiently far apart for all the α rays to be absorbed in the gas between them.
Surface of uranium oxide 38 sq. cms.
| Weight of uranium oxide in grammes per sq. cm. of surface | Saturation current in amperes per sq. cm. of surface |
|---|---|
| . | |
| ·0036 | 1·7 × 10-13 |
| ·0096 | 3·2 × 10-13 |
| ·0189 | 4·0 × 10-13 |
| ·0350 | 4·4 × 10-13 |
| ·0955 | 4·7 × 10-13 |
The current reached about half its maximum value for a weight of oxide ·0055 gr. per sq. cm. If the α rays are cut off by a metallic screen, the ionization is then mainly due to the β rays, since the ionization produced by the γ rays is small in comparison. For the β rays from uranium oxide it has been shown (section 86) that the current reaches half its maximum value for a thickness of 0·11 gr. per sq. cm.
Meyer and Schweidler[182] have found that the radiation from a water solution of uranium nitrate is very nearly proportional to the amount of uranium present in the solution.
On account of the difference in the penetrating power of the α and β rays, the ratio of the ionization currents produced by them depends on the thickness of the radio-active layer under examination. The following comparative values of the current due to the α and β rays were obtained for very thin layers of active matter[183]. A weight of ⅒ gramme of fine powder, consisting of uranium oxide, thorium oxide, or radium chloride of activity 2000, was spread as uniformly as possible over an area of 80 sq. cms. The saturation current was observed between parallel plates 5·7 cms. apart. This distance was sufficient to absorb most of the α rays from the active substances. A layer of aluminium ·009 cm. thick absorbed all the α rays.
| Current due to α rays | Current due to β rays | Ratio of currents β/α | |
|---|---|---|---|
| Uranium | 1 | 1 | ·0074 |
| Thorium | 1 | ·27 | ·0020 |
| Radium | 2000 | 1350 | ·0033 |
In the above table the saturation current due to the α and β rays of uranium is, in each case, taken as unity. The third column gives the ratio of the currents observed for equal weights of substance. The results are only approximate in character, for the ionization due to a given weight of substance depends on its fineness of division. In all cases, the current due to the β rays is small compared with that due to the α rays, being greatest for uranium and least for thorium. As the thickness of layer increases, the ratio of currents β/α steadily increases to a constant value.
114. Comparison of the energy radiated by the α and β rays. It has not yet been found possible to measure directly the energy of the α and β rays. A comparison of the energy radiated in the two forms of rays can, however, be made indirectly by two distinct methods.
If it be assumed that the same amount of energy is required to produce an ion by either the α or the β ray, and that the same proportion of the total energy is used up in producing ions, an approximate estimate can be made of the ratio of the energy radiated by the α and β rays by measuring the ratio of the total number of ions produced by them. If λ is the coefficient of absorption of the β rays in air, the rate of production of ions per unit volume at a distance x from the source is
where q₀ is the rate of ionization at the source.
The total number of ions produced by complete absorption of the rays is
Now λ is difficult to measure experimentally for air, but an approximate estimate can be made of its value from the known fact that the absorption of β rays is approximately proportional to the density of any given substance.
For β rays from uranium the value of λ for aluminium is about 14, and λ divided by the density is 5·4. Taking the density of air as ·0012, we find that for air
λ = ·0065.
The total number of ions produced in air is thus 154q₀ when the rays are completely absorbed.
Now from the above table the ionization due to the β rays is ·0074 of that produced by α rays, when the β rays passed through a distance of 5·7 cms. of air.
Thus we have approximately
Therefore about ⅙ of the total energy radiated into air by a thin layer of uranium is carried by the β rays or electrons. The ratio for thorium is about ¹⁄₂₂ and for radium about ¹⁄₁₄, assuming the rays to have about the same average value of λ.
This calculation takes into account only the energy which is radiated out into the surrounding gas; but on account of the ease with which the α rays are absorbed, even with a thin layer, the greater proportion of the radiation is absorbed by the radio-active substance itself. This is seen to be the case when it is recalled that the α radiation of thorium or radium is reduced to half value after passing through a thickness of about 0·0005 cm. of aluminium. Taking into consideration the great density of the radio-active substances, it is probable that most of the radiation which escapes into the air is due to a thin skin of the powder not much more than ·0001 cm. in thickness.
An estimate, however, of the relative rate of emission of energy by the α and β rays from a thick layer of material can be made in the following way:—For simplicity suppose a thick layer of radio-active substance spread uniformly over a large plane area. There seems to be no doubt that the radiations are emitted uniformly from each portion of the mass; consequently, the radiation, which produces the ionizing action in the gas above the radio-active layer, is the sum total of all the radiation which reaches the surface of the layer.
Let λ1 be the average coefficient of absorption of the α rays in the radio-active substance itself and σ the specific gravity of the substance. Let E1 be the total energy radiated per sec. per unit mass of the substance when the absorption of the rays in the substance itself is disregarded. The energy per sec. radiated to the upper surface by a thickness dx of a layer of unit area at a distance x from the surface is given by
The total energy W1 per unit area radiated to the surface per sec. by a thickness d is given by
if λ1d is large.
In a similar way it may be shown that the energy W2 of the β rays reaching the surface is given by
where E2 and λ2 are the values for the β rays corresponding to E1 and λ1 for the α rays. Thus it follows that
λ1 and λ2 are difficult to determine directly for the radio-active substance itself, but it is probable that the ratio λ1/λ2 is not very different from the ratio for the absorption coefficients for another substance like aluminium. This follows from the general result that the absorption of both α and β rays is proportional to the density of the substance; for it has already been shown in the case of the β rays from uranium that the absorption of the rays in the radio-active material is about the same as for non-radio-active matter of the same density.
With a thick layer of uranium oxide spread over an area of 22 sq. cms., it was found that the saturation current between parallel plates 6·1 cms. apart, due to the α rays, was 12·7 times as great as the current due to the β rays. Since the α rays were entirely absorbed between the plates and the total ionization produced by the β rays is 154 times the value at the surface of the plates,
Now the value of λ1 for aluminium is 2740 and of λ2 for the same metal 14, thus
This shows that the energy radiated from a thick layer of material by the β rays is only about 1 per cent. of the energy radiated in the form of α rays.
This estimate is confirmed by calculations based on independent data. Let m1, m2 be the masses of the α and β particles respectively and v1, v2 their velocities.
Now it has been shown that for the α rays of radium
The velocity of the β rays of radium varies between wide limits. Taking for an average value
it follows that the energy of the α particle from radium is almost 83 times the energy of the β particle. If equal numbers of α and β particles are projected per second, the total energy radiated in the form of α rays is about 83 times the amount in the form of β rays.
Evidence will be given later (section 253) to show that the number of α particles projected is probably four times the number of β particles; so that a still greater proportion of the energy is emitted in the form of α rays. These results thus lead to the conclusion that, from the point of view of the energy emitted, the α rays are far more important than the β rays. This conclusion is supported by other evidence which is discussed in chapters XII and XIII, where it will be shown that the α rays play by far the most important part in the changes occurring in radio-active bodies, and that the β rays only appear in the latter stages of the radio-active processes. From data based on the relative absorption and ionization of the β and γ rays in air, it can be shown that the γ rays carry off about the same amount of energy as the β rays. These conclusions are confirmed by direct measurement of the heating effect of radium, which is discussed in detail in chapter XII.