Note: the page numbering is take from "The Collected Works of Lord Rutherford of Nelson," vol. I
The remarkable radiation emitted by uranium and its compounds has been studied by its discoverer, Becquerel, and the results of his investigations on the nature and properties of the radiation have been given in a series of papers in the Comptes Rendus.* He showed that the radiation, continuously emitted from uranium compounds, has the power of passing through considerable thicknesses of metals and other opaque substances; it has the power of acting on a photographic plate and of discharging positive and negative electrification to an equal degree. The gas through which the radiation passes is made a temporary conductor of electricity and preserves its power of discharging electrification for a short time after the source of radiation has been removed.
The results of Becquerel showed that Röntgen and uranium radiations were very similar in their power of penetrating solid bodies and producing conduction in a gas exposed to them; but there was an essential difference between the two types of radiation. He found that uranium radiation could be refracted and polarized, while no definite results showing polarization or refraction have been obtained for Röntgen radiation. It is the object of the present paper to investigate in more detail the nature of uranium radiation and the electrical conduction produced. As most of the results obtained have been interpreted on the ionization theory of gases which was introduced to explain the electrical conduction produced by Röntgen radiation, a brief account is given of the theory and the results to which it leads.
In the course of the investigation, the following subjects have been considered:
§ 1. Comparison of methods of investigation.
§ 2. Refraction and polarization of uranium radiation.
* Comptes Rendus, 1896, pp. 420, 501, 559, 689, 762, 1086; 1897, pp. 438, 800.
§ 3. Theory of ionization of gases.
§ 4. Complexity of uranium radiation.
§ 5. Comparison of the radiation from uranium and its compounds.
§ 6. Opacity of substances for the radiation.
§ 7. Thorium radiation.
§ 8. Absorption of radiation by gases.
§ 9. Variation of absorption with pressure.
§ 10. Effect of pressure of the gas on the rate of discharge.
§ 11. The conductivity produced in gases by complete absorption of the radiation.
§ 12. Variation of the rate of discharge with distance between the plates.
§ 13. Rate of recombination of the ions.
§ 14. Velocity of the ions.
§ 15. Fall of potential between two plates.
§ 16. Relation between the current through the gas and electromotive force applied.
§ 17. Production of charged gases by separation of the ions.
§ 18. Discharging power of fine gauzes.
§ 19. General remarks.
§ 1. Comparison of Methods of Investigation
The properties of uranium radiation may be investigated by two methods, one depending on the action on a photographic plate and the other on the discharge of electrification. The photographic method is very slow and tedious, and admits of only the roughest measurements. Two or three days' exposure to the radiation is generally required to produce any marked effect on the photographic plate. In addition, when we are dealing with very slight photographic action, the fogging of the plate, during the long exposures required, by the vapours of substances* is liable to obscure the results. On the other hand, the method of testing the electrical discharge caused by the radiation is much more rapid than the photographic method, and also admits of fairly accurate quantitative determinations.
The question of polarization and refraction of the radiation can, however, only be tested by the photographic method. The electrical experiment (explained in § 2) to test refraction is not very satisfactory.
§ 2. Polarization and Refraction
The almost identical effects produced in gases by uranium and Röntgen radiation (which will be described later) led me to consider the question whether the two types of radiation did not behave the same in other respects.
In order to test this, experiments were tried to see if uranium radiation could be polarized or refracted. Becquerel** had found evidence of polarization and
* Russell, Proc. Roy. Soc., 1897.
** Comptes Rendus, 1896, p. 559.
refraction, but in repeating experiments similar to those tried by him, I have been unable to find any evidence of either. A large number of photographs by the radiation have been taken under various conditions, but in no case have I been able to observe any effect on the photographic plate which showed the presence of polarization or refraction.
In order to avoid fogging of the plate during the long exposures required, by the vapours of substances, lead was employed as far as possible in the neighbourhood of the plate, as its effect on the film is very slight.
A brief account will now be given of the experiments on refraction and polarization.
Refraction. A thick lead plate was taken and a long narrow slit cut through it; this was placed over a uniform layer of uranium oxide; the arrangement was then equivalent to a line source of radiation and a slit. Thin prisms of glass, aluminium, and paraffin-wax were fixed at intervals on the lead plate with their edges just covering the slit. A photographic plate was supported 5 mm. from the slit. The plate was left for a week in a dark box. On developing a dark line was observed on the plate. This line was not appreciably broadened or displaced above the prisms. Different sizes of slits gave equally negative results. If there was any appreciable refraction we should expect the image of the slit to be displaced from the line of the slit.
Becquerel* examined the opacity of glass for uranium radiation in the solid and also in a finely-powdered state by the method of electric leakage, and found that, if anything, the transparency of the glass for the radiation was greater in the finely divided than in the solid state. I have repeated this experiment and obtained the same result. As Becquerel stated, it is difficult to reconcile this result with the presence of refraction.
Polarization. An arrangement very similar to that used by Becquerel was employed. A deep hole was cut in a thick lead plate and partly filled with uranium oxide. A small tourmaline covered the opening. Another small tourmaline was cut in two and placed on top of the first, so that in one half of the opening the tourmalines were crossed and in the other half uncrossed. The tourmalines were very good optically. The photographic plate was supported 1 to 3 mm. above the tourmalines. The plate was exposed four days, and on developing a black circle showed up on the plate, but in not one of the photographs could the slightest difference in the intensity be observed. Becquerel* stated that in his experiment the two halves were unequally darkened and concluded from this result that the radiation was doubly refracted by tourmaline, and that the two rays were unequally absorbed.
§ 3. Theory of Ionization
To explain the conductivity of a gas exposed to Röntgen radiation, the theory** has been put forward that the rays in passing through the gas produce
* Comptes Rendus, 1896, p. 559.
**J. J. Thomson and E. Rutherford, Phil. Mag., November 1896.
positively and negatively charged particles in the gas, and that the number produced per second depends on the intensity of the radiation and the pressure.
These carriers are assumed to be so small that they will move with a uniform velocity through a gas under a constant potential gradient. The term ion was given to them from analogy with electrolytic conduction, but in using the term it is not assumed that the ion is necessarily of atomic dimensions; it may be a multiple or submultiple of the atom.
Suppose we have a gas between two plates exposed to the radiation and that the plates are kept at a constant difference of potential. A certain number of ions will be produced per second by the radiation and the number produced will in general depend on the pressure of the gas. Under the electric field the positive ions travel towards the negative plate and the negative ions towards the other plate, and consequently a current will pass through the gas. Some of the ions will also recombine, the rate of recombination being proportional to the square of the number present. The current passing through the gas for a given intensity of radiation will depend on the difference of potential between the plates, but when the potential difference is greater than a certain value the current will reach a maximum. When this is the case all the ions are removed by the electric field before they can recombine.
The positive and negative ions will be partially separated by the electric field, and an excess of ions of one sign may be blown away, so that a charged gas will be obtained. If the ions are not uniformly distributed between the plates, the potential gradient will be disturbed by the movement of the ions.
If energy is absorbed in producing ions, we should expect the absorption to be proportional to the number of ions produced and thus depend on the pressure. If this theory be applied to uranium radiation we should expect to obtain the following results:
(1) Charged carriers produced through the volume of the gas.
(2) Ionization proportional to the intensity of the radiation and the pressure.
(3) Absorption of radiation proportional to pressure.
(4) Existence of saturation current.
(5) Rate of recombination of the ions proportional to the square of the number present.
(6) Partial separation of positive and negative ions.
(7) Disturbance of potential gradient under certain conditions between two plates exposed to the radiation.
The experiments now to be described sufficiently indicate that the theory does form a satisfactory explanation of the electrical conductivity produced by uranium radiation.
In all experiments to follow, the results are independent of the sign of the charged plate, unless the contrary is expressly stated.
§ 4. Complex Nature of Uranium Radiation
Before entering on the general phenomena of the conduction produced by uranium radiation, an account will be given of some experiments to decide whether the same radiation is emitted by uranium and its compounds and whether the radiation is homogeneous. Röntgen and others have observed that the x-rays are in general of a complex nature, including rays of wide differences in their power of penetrating solid bodies. The penetrating power is also dependent to a large extent on the stage of exhaustion of the Crookes tube.
In order to test the complexity of the radiation, an electrical method was employed. The general arrangement is shown in Fig. 1.
The metallic uranium or compound of uranium to be employed was powdered and spread uniformly over the centre of a horizontal zinc plate A, 20 cm. square. A zinc plate B, 20 cm. square, was fixed parallel to A and 4 cm. from it. Both plates were insulated. A was connected to one pole of a battery of 50 volts, the other pole of which was to earth; B was connected to one pair of quadrants of an electrometer, the other pair of which was connected to earth.
Under the influence of the uranium radiation there was a rate of leak between the two plates A and B. The rate of movement of the electrometer needle, when the motion was steady, was taken as a measure of the current through the gas.
Successive layers of thin metal foil were then placed over the uranium compound and the rate of leak determined for each additional sheet. The table (p. 174) shows the results obtained for thin Dutch metal.
In the third column the ratio of the rates of leak for each additional thickness of metal leaf is given. Where two thicknesses were added at once, the square root of the observed ratio is taken, for three thicknesses the cube root. The table shows that for the first ten thicknesses of metal the rate of leak diminished approximately in a geometrical progression as the thickness of the metal increased in arithmetical progression.
|THICKNESS OF METAL LEAF 0.00008 CM.|
LAYER OF URANIUM OXIDE ON PLATE
|Leak per min. in|
|Ratio for each layer|
It will be shown later (§ 8) that the rate of leak between two plates for a saturating voltage is proportional to the intensity of the radiation after passing through the metal. The voltage of 50 employed was not sufficient to saturate the gas, but it was found that the comparative rates of leak under similar conditions for 50 and 200 volts between the plates were nearly the same. When we are dealing with very small rates of leak, it is advisable to employ as small a voltage as possible, in order that any small changes in the voltage of the battery should not appreciably affect the result. For this reason the voltage of 50 was used, and the comparative rates of leak obtained are very approximately the same as for saturating electromotive forces.
Since the rate of leak diminishes in a geometrical progression with the thickness of metal, we see from the above statement that the intensity of the radiation falls off in a geometrical progression, i.e. according to an ordinary absorption law. This shows that the part of the radiation considered is approximately homogeneous. With increase of the number of layers the absorption commences to diminish. This is shown more clearly by using uranium oxide with layers of thin aluminium leaf (see table, p. 175).
It will be observed that for the first three layers of aluminium foil, the
intensity of the radiation falls off according to the ordinary absorption law, and that, after the fourth thickness, the intensity of the radiation is only slightly diminished by adding another eight layers.
|THICKNESS OF ALUMINIUM FOIL 0.0005 CM.|
|Number of layers|
of Aluminium foil
|Leak per minute in|
The aluminium foil in this case was about 0.0005 cm. thick, so that after the passage of the radiation through 0.002 cm. of aluminium the intensity of the radiation is reduced to about one-twentieth of its value. The addition of a thickness of 0.001 cm. of aluminium has only a small effect in cutting down the rate of leak. The intensity is, however, again reduced to about half of its value after passing through an additional thickness of 0.05 cm., which corresponds to one hundred sheets of aluminium foil.
These experiments show that the uranium radiation is complex, and that there are present at least two distinct types of radiation--one that is very readily absorbed, which will be termed for convenience the a radiation, and the other of a more penetrative character, which will be termed the b radiation.
The character of the b radiation seems to be independent of the nature of the filter through which it has passed. It was found that radiation of the same intensity and of the same penetrative power was obtained by cutting off the a radiation by thin sheets of aluminium, tinfoil, or paper. The b radiation passes through all the substances tried with far greater facility than the a radiation. For example, a plate of thin cover glass placed over the uranium reduced the rate of leak to one-thirtieth of its value; the b radiation, however, passed through it with hardly any loss of intensity.
Some experiments with different thicknesses of aluminium seem to show, as far as the results go, that the b radiation is of an approximately homogeneous character. The following table gives some of the results obtained for the b radiation from uranium oxide:
|Rate of Leak|
The rate of leak is taken as unity after the a radiation has been absorbed by passing through ten layers of aluminium foil. The intensity of the radiation diminishes with the thickness of metal traversed according to the ordinary absorption law. It must be remembered that when we are dealing with the b radiation alone, the rate of leak is, in general, only a few per cent of the leak due to the a radiation, so that the investigation of the homogeneity of the b radiation cannot be carried out with the same accuracy as for the a radiation. As far, however, as the experiments have gone, the results seem to point to the conclusion that the b radiation is approximately homogeneous, although it is possible that other types of radiation of either small intensity or very great penetrating power may be present.
§ 5. Radiation emitted by different Compounds of Uranium
All the compounds of uranium examined gave out the two types of radiation, and the penetrating power of the radiation for both the a and b is the same for all compounds.
The following table shows the results obtained for some of the uranium compounds.
|THICKNESS OF ALUMINIUM FOIL 0.0005 cm.|
|Proportionate Rate of Leak|
|Number of layers of Aluminium foil||Uranium metal||Uranium Nitrate||Uranium Oxide||Uranium Potassium Sulphate|
Fig. 2 shows graphically some of the results obtained for the various uranium compounds. The ordinates represent rates of leak, and the abscissae thicknesses of aluminium through which the radiation has passed.
The different compounds of uranium gave different rates of leak, but, for convenience of comparison, the rate of leak due to the uncovered salt is taken as unity.
It will be seen that the rate of decrease is approximately the same for the first layer of metal, and that the rate of decrease becomes much slower after four thicknesses of foil.
The rate of leak due to the b radiation is a different proportion of the total amount in each case. The uranium metal was used in the form of powder, and a smaller area of it was used than in the other cases. For the experiments on uranium oxide a thin layer of fine powder was employed, and we see, in that case, that the b radiation bears a much smaller proportion to the total than for the other compounds. When a thick layer of the
oxide was used there was, however, an increase in the ratio, as the following table shows:
|Rate of Leak|
|Number of layers of Aluminium foil||Thin layer of Uranium Oxide||Thick layer of Uranium Oxide|
The amount of the a radiation depends chiefly on the surface of the uranium compound, while the b radiation depends also on the thickness of the layer. The increase of the rate of leak due to the b radiation with the thickness of the layer indicates that the b radiation can pass through a considerable thickness of the uranium compound. Experiments showed that the leak due to the a radiation did not increase much with the thickness of the layer. I did not, however, have enough uranium salt to test the variation of the rate of leak due to the b radiation for thick layers.
The rate of leak from a given weight of uranium or uranium compound depends largely on the amount of surface. The greater the surface, the greater the rate of leak. A small crystal of uranium nitrate was dissolved in water, and the water then evaporated so as to deposit a thin layer of the salt over the bottom of the dish. This gave quite a large leakage. The leakage in such a case is due chiefly to the b radiation.
Since the rate of leak due to any uranium compound depends largely on its amount of surface, it is difficult to compare the quantity of radiation given out by equal amounts of different salts: for the result will depend greatly on the state of division of the compound. It is possible that the apparently very powerful radiation obtained from pitchblende by Curie* may be partly due to the very fine state of division of the substance rather than to the presence of a new and powerful radiating substance.
The rate of leak due to the b radiation is, as a rule, small compared with that produced by the a radiation. It is difficult, however, to compare the relative intensities of the two kinds. The a radiation is strongly absorbed by gases (§ 8), while the b radiation is only slightly so. It will be shown later (§ 8) that the absorption of the radiation by the gas is approximately proportional to the number of ions produced. If, therefore, the b radiation is only slightly
* Comptes Rendus, July 1898, p. 175.
absorbed by the gas, the number of ions produced by it is small, i.e. the rate of leak is small. The comparative rates of leak due to the a and b radiations is thus dependent on the relative absorption of the radiations by the gas as well as on the relative intensity.
The photographic actions of the a and b radiations have also been compared. A thin uniform layer of uranium oxide was sprinkled over a glass plate; one half of the plate was covered by a piece of aluminium of sufficient thickness to practically absorb the a radiation. The photographic plate was fixed about 4 mm. from the uranium surface. The plate was exposed 48 hours, and, on developing, it was found that the darkening of the two halves was not greatly different. On the one half of the plate the action was due to the b radiation alone, and on the other due to the a and b radiations together. Except when the photographic plate is close to the uranium surface, the photographic action is due principally to the b radiation.
§ 6. Transparency of Substances to the two Types of Radiation
If the intensity of the radiation in traversing a substance diminishes according to the ordinary absorption law, the ratio r of the intensity of the radiation after passing through a distance d of the substance to the intensity when the substance is removed is given by
r = e¯ld
where l is the coefficient of absorption and e = 2.7.
In the following table a few values of l are given for the a and b radiations, assuming in each case that the radiation is simple and that the intensity falls off according to the above law:
for the a radiation
for the b radiation
The above results show what a great difference there is in the power of penetration of the two types of radiation. The transparency of aluminium for the b radiation is over one hundred times as great as for the a radiation. The opacity of the metals aluminium, copper, silver, platinum for the b radiation follows the same order as their atomic weights. Aluminium is the most transparent of the metals used, but glass is more transparent than aluminium for the b radiation. Platinum has an opacity sixteen times as great as
aluminium. For the a radiation, aluminium is more transparent than Dutch metal or tinfoil.
For a thickness of aluminium 0.09 cm. the intensity of the b radiation was reduced to 0.25 of its value; for a thickness of copper 0.03 cm. the intensity was reduced to 0.23 of its value. These results are not in agreement with some given by Becquerel,* who found copper was more transparent than aluminium for uranium radiation.
The b radiation has a penetrating power of about the same order as the radiation given out by an average x-ray bulb. Its power of penetration is, however, much less than for the rays from a 'hard' bulb. The a radiation, on the other hand, is far more easily absorbed than rays from an ordinary bulb, but is very similar in its penetrating power to the secondary radiation** sent out when x-rays fall upon a metal surface.
It is possible that the a radiation is a secondary radiation set up at the surface of the uranium by the passage of the b radiation through the uranium, in exactly the same way as a diffuse radiation is produced at the surface of a metal by the passage of Röntgen rays through it. There is not, however, sufficient evidence at present to decide the question.
* Comptes Rendus, 1896, p.763.
** Perrin, Comptes Rendus, cxxiv, p. 455; Sagnac, Comptes Rendus, 1898.
[§ 7 to § 18. have been deleted.]
§ 19 General Remarks
The cause and origin of the radiation continuously emitted by uranium and its salts still remain a mystery. All the results that have been obtained point to the conclusion that uranium gives out types of radiation which, as regards their effects on gases, are similar to Röntgen rays and the secondary radiation emitted by metals when Röntgen rays fall upon them. If there is no polarization or refraction the similarity is complete. J. J. Thomson* has suggested that
* Proc. Camb. Phil. Soc., vol. ix, pt. viii, p. 397 (1898).
the regrouping of the constituents of the atom may give rise to electrical effects such as are produced in the ionization of a gas. Röntgen's* and Wiedemann's** results seem to show that in the process of ionization a radiation is emitted which has similar properties to easily absorbed Röntgen radiation. The energy spent in producing uranium radiation is probably extremely small, so that the radiation could continue for long intervals of time without much diminution of internal energy of the uranium. The effect of the temperature of the uranium on the amount of radiation given out has been tried. An arrangement similar to that in § 11 was employed. The radiation was completely absorbed in the gas. The vessel was heated uo to about 200° C; but not much difference in the rate of discharge was observed. The results of such experiments are very difficult to interpret, as the variation of ionization with temperature is not known.
I have been unable to observe the presence of any seconday radiation produced when uranium radiation falls upon a metal. Such a radiation is probably produced, but its effects are too small for measurement.
In conclusion, I desire to express my best thanks to Professor J. J. Thomson for his kindly interest and encouragement during the course of this investigation.
September 1, 1898
* Wied. Annal., lxiv (1898)
** Zeit. f. Electrochemie, ii., p. 159 (1895).