VII. On a new secondary radiation of positive rays

M. Wolfke
1918 The London Edinburgh and Dublin Philosophical Magazine and Journal of Science  
U P to now, two secondary radiations of positive rays have been known: the slow electron rays T and the very soft X-rays recently discovered by J. J. Thomson $, which are probably caused" by a retardation of the ions when encountering a rigid body. Some years ago Chadwick w and Russell]l showed that a-rays are capable of exciting the ~/-radiation of heavy elements. So tar, however, it has not been known whether positive rays were capable of producing a similar effect. Now tiffs question is of
more » ... fs question is of very vital interest, for if it can be proved that positive rays are able to excite characteristic rays, it then becomes possible to obtain a better insight into the connexion between the process of excitation and the chemical nature and charge of the exciting particles. And such an investigation might throw new light on the questions relating to the mechanism of excitation and emission of X-spectra. These considerations have prompted me to investigate the question as to whether positive rays are capable of exciting the characteristic X-radiation. The experimental method was based upon similar principles to that employed by Chadwick 82 in his investigation of the excitation of the characteristic ~/-radiation of gold by a-rays. Through a channel of circular section 10 mm. wide a pencil of positive rays was let fall upon a circular opening provided in a brass box. This opening is divided into two halves, and each of these is covered up by a double foil made up of one foil of a heavy metal, say tin or lead, and of a second foil of a light metal, say aluminium, laid over the first one. These foils are so placed that over one half of the opening the heavy-metal foil is on the outside with the alumininm foil turned towards the inside of the box, while the second similar double foil is so arranged over the other half of the * 60 Dr. M. Wolfke on a New opening as to have its aluminium side outwards. Behind these foils the photographic plate is i)laced. Thus on one half of the opening the positive rays fall upon a heavv-metal surface, and on the other half upon an aluufinium "surface. The charaeteristi-ra(liation of the hea,vy metal being more intense and harder than that of aluminiun:, it, reaches the photographic plate wifh an intensity not perceptibly diminished. On the other hand, the characteristic radialion of aluminium is weak and soft and is absorbed to a large extent by the layer of heavy metal throu;~h which it has {o pass. Therefore, if' it is true that the eharacteristie radiation of the heavy meted is excited by positive rays, then the impression produced on the pllot.ographic plate must be stronger beneath dmt half of the ol)ening where lhe positive rays fall upon heavy metal, and less strong beneath the other half, where they encounter an alnminium foil. The higher the intensity of excitation of the characteristic rays, the more pronounced will he this difference in the strength of the impression obtained. Such secondary cathode rays as might have been produced by the positive rays and the X-rays in the channel itself were deflected behind the channel, and prevented from entering the opening in the box by means of a field of suitlcient power created between channel and box. In order to eliminate the effect that might have been produced by any irregularity el'thickness in the two foils, every test made Was checked by a second exposure with the same foils as in the first case, but turned about so as to have their relative position reversed. In order to avoid too strong heating of the foils by these powerful positive rays, the exposures were intermittent so that short exposures alternated with longer breaks. Two heavy metals, tin and lead, were treated. ]?he foils used were "016 rain. thick in tim ease of' tin and "0.298 mm. in the ease of lead; the thickness of the almninimn toil was "007 ram. The experiments have shown that when acted upon by positive rags, either of these metals emitted a penetratim! radiation of .[air intensity which is probably its characteristic radiation. When tin was tested, all photographs without exception showed a very marked contrast--that is, where the positive rays fell upon the tin surface, the darkening of the plate was strongly pronounced, while the other half of the circular imprint showed but a faint darkening. The photograph obtained is seen in the annexed figure, and this result was in accordance with anticipation. 61 Several similar exposures were made with an induction-coil and others with a,~ influence-machine. The duration of exposure was varied bt~tween 2"5 and 22 minutes, the potential between 25 and 40 ram. spark-gap, and the pressure between "0007 and "0037 ram. mercury. The annexed photograph was obtained with an influencemachine and an exposure of 22 minutes. The spark-gap was 25-30 ram. and the pressure "0037 ram. mercury. When photographs were taken with lead foil, with potentials ranging between 25 ram. and 40 ram. spark-gap, the darkening of the plate was only faint, and no difference was visible in intensity of the impression on either half of the image. As soon, however, as the spark-gap was increased to 45 mm. a very distinct contrast became visible in the darkening of the two halves. The intensity of the photograph was greatest on that half where the positive rays fell upon the lead surface, and there only on that part where the intensity of the positive rays would be a maximum. From this it would seem that the energy required for the excitation of characteristic X-rays has a lower limit, just as Duane, Hunt, Hull, Webster and others have observed in the case of cathode rays in a Coolidge tube. For cathode rays the relation r has been established, where e is the minimum energy of the electron necessary for exciting ~he K-series, and v the maximum frequency of the line Kfl~ that corresponds to this series. Supposing this relation to hold also for the excitation of characteristic rays by positive particles, then to excite the K~ line of tin (X='432.10 -s era.) the voltage required wouhl have to exceed 57 KV. In the described experiments, however, the 62 Pro[. Barton and Miss Browning on maximum width of spark-gap used was 45 ram., the spheres being 31 mm. in diameter; so that according to tests made by C. Miiller* the voltage applied could not have exceeded 50 KV. The K-series could not then have been excited, and it is probable that the wave-length excited belonged to the L-series of either metal. I propose to inveatigate further the wave-lengths and other properties of these radiations. A report of such studies will be published shortly. Summary of Results.
doi:10.1080/14786440108635736 fatcat:xj25kj2ykzhzjlhgybam7zloj4