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187

have been more completely absorbed by the glass than the

VOL. LXXXVIII., No. 2290.

RADIO-ACTIVE SUBSTANCES.*

By Mdme. SKLODOWSKA CURIE.
(Continued from p. 177).

CHAPTER III. (continued).

Proportion of B-Rays in the Radiation of Radium. As I have already mentioned, the proportion of B-rays increases with increase of distance from the source of radiation. These rays never occur alone, and for great distances the presence of y-rays is always discernible. The presence of very penetrating, undeflected rays in the radiation of radium was first observed by M. Villard. These rays constitute only a small portion of the radiation measured by the electrical method, and their presence escaped our notice in our first experiments, so that we believed falsely that the radiation at great distances contained only rays capable of deflection.

The following are the numerical results obtained with experiments made by the electrical method with an apparatus similar to that of Fig. 5. The radium was only separated from the condenser by the surrounding air. I shall indicate by the letter d the distance from the source of radiation to the condenser. The numbers of the second line represent the current subsisting when the magnetic field is acting, supposing the current obtained with no field equal to 100 for each distance. These numbers may be considered as giving the percentage of the total a- and y-rays, the deflection of the a-rays having been scarcely observable with the conditions employed.

At great distances there are no a-rays, and the undeflected radiation is therefore of the y kind only. Experiments made at short distances:

1

d, in centimetres Percentage of undeflected rays 74 56 33 Experiments made at long distances with a product considerably more active than that which was used for the preceding series :

{

{

The following experiment shows that a thin sheet of aluminium (oor m.m. thick) absorbs principally a-rays. The product being placed 5 c.m. from the condenser, the proportion of rays other than 8, when the magnetic field is acting, is about 71 per cent. When the same substance is covered with the sheet of aluminium, the distance remaining the same, the radiation transmitted is found to be almost totally deflected by the magnetic field, the a-rays having been absorbed by the aluminium. The same result is obtained when paper is used as the absorbing screen.

The greatest part of the radiation of radium consists of a-rays, which are probably emitted principally by the superficial layer of the radiating matter. When the thick. ness of the layer of radiating matter is varied, the intensity of the current increases with this thickness; the increase is not proportional to the thickness for the whole of the radiation; it is, moreover, more considerable for the B-rays than for the a-rays, so that the proportion of B-rays increases with the thickness of the active layer. The source of radiation being placed at a distance of 5 c.m. from the condenser, it is found that for a thickness equal to 0'4 m.m. of the active layer, the total radiation is given by the number 28, and the proportion of the B-rays is 29 per cent. By making the layer 2 m.m. thick, i.e., five times as thick, a total radiation equal to 102, and a proportion of B-rays equal to 45 per cent are obtained. The total radiation which exists at this distance has therefore been increased in the ratio of 3.6, and the B-radiation has become five times as strong.

The preceding experiments were made by the electrical method. When the radiographic method is used, certain results seem to be in contradiction with what precedes. In the experiments of M. Villard, a beam of radium rays, subjected to the action of the magnetic field, was received on to a pile of photographic plates. The undeflected and penetrating y-beam passed through all the plates, leaving its trace on each. The deflected B-beam produced an impression on the first plate only. This beam appeared therefore to contain no rays of great penetration.

On the contrary, in our experiments a beam which is propagated in the air contains at the greatest distances accessible to observation about 9/10 of B-rays, and the same is the case when the source of radiation is enclosed in a little sealed glass vessel. In M. Villard's experiments, these deflected and penetrating B-rays did not affect the photographic plates beyond the first, because they are to a great extent diffused in all directions by the first solid obstacle encountered, and no longer form a beam. In our experiments the rays given off by radium and transmitted through the glass of the vessel were also probably scattered by the glass, but the vessel being very small would itself act as a source of B-rays at its surface, and we were able to follow the course of the latter to a great distance from the vessel.

The cathode rays of Crookes tubes can only traverse very thin screens (aluminium screens of oor m.m. thickness). A beam of rays striking the screen normally is scattered in all directions; but the diffusion becomes less with diminishing thickness of the screen, and for very thin screens the emerging beam is practically the prolongation of the incident beam.

The deflected B-rays of radium behave in a similar manner, but the transmitted beam experiences, for the same thickness of screen, a much slighter modification. According to the experiments of M. Becquerel, the very readily deflected B-rays of radium (those with a relatively small velocity) are powerfully scattered by an aluminium screen of thickness o'i m.m.; but the penetrating and less deflected rays (rays of the cathode kind of great velocity) pass through this screen without being sensibly diffused, whatever be the inclination of the screen to the direction of the beam. The B-rays of great velocity penetrate without metres), and in this the curvature of the beam produced diffusion a much greater thickness of paraffin (several centiby the magnetic field can be traced. The thicker the