XL The Röntgen Radiation from Substances of Low Atomic Weight, By Charles A. Sadler, D.Sc, Lecturer and Demonstrator in Physics, University College, Reading, late (Hirer Lodge Fellow in the University of Liverpool, and PAUL MeshAM, M.Sc, Lecturer and Demonstrator in Physics, University of Liverpool [Communicated by the Authors. Part of the expenses of this Research have been defrayed by a Government Grant.]. IT was found by early investigators that when a beam oP X rays Fell upon a substance an X radiation was excited in it. This secondary radiation, as it was called, varied with the nature of the substance in which it originated. With elements of low atomic weights, the excited radiation was approximately of the same type as the exciting beam. More recently [citation redacted], Barkla and Sadler have shown that from sub- stances of atomic weight higher than 40, the secondary radiation is characteristic of the substance and consists in the main of one or more homogeneous beams. In the earlier experiments investigators were considerably handicapped by having to work with beams which were very heterogeneous and whose character was continually liable to variation. For example, when the secondary radiation from some particular element of low atomic weight was examined it was found to depend upon the kind of primary beam used. The discovery of the homogeneous beams offered an enormous simplification of the problem, since the character of these beams is quite definite and can be reproduced with certainty at any time. There is, however, one serious drawback in their use. The intensity of the homogeneous beams can only be a comparatively small fraction of the intensity of the primary beam exciting them. Under the most favourable circumstances the radiation scattered by a substance of low atomic weight is o£ feeble intensity, and when the exciting radiation is considerably weakened, as it is when we use homogeneous beams, special arrangements have to be made to measure the intensity and other properties of the scattered radiation. The authors made use of the special form of electrometer described in previous papers, and found that by taking- reasonable precautions they were able to obtain consistent results. Carbon was chosen as a suitable substance to serve as a radiator. In accordance with the results obtained by those who had [header] 139 used ordinary primary beams, we expected to find that the scattered radiation from carbon excited by a homogeneous beam would be itself homogeneous and of the same character as the primary, i. e. if we placed a thin sheet of absorbing material in the path of the scattered beam the percentage diminution in intensity would be equal to that occurring if the same sheet were used to cut down the primary beam. A reference to fig. 1, a diagrammatic representation of the [figure redacted] apparatus used for making measurements, shows that the tertiary rays from the carbon enter the ionization chamber, on the whole, by more oblique paths than is the case with the secondary radiation, which consists of a nearly parallel beam. A correction must be made for this obliquity, since the length of path of the oblique rays in their passage through the absorbing substance is greater than that of rays which traverse the absorber perpendicularly, and thus a correspondingly greater relative absorption takes place. On the other hand, some of the oblique rays have only a small range within the ionization chamber, and thus do not contribute much to the ionization. (An electroscope of the usual Wilson type received a small pencil of rays from the primary beam. In making measurements of the absorption of the tertiary beam, readings of the primary electroscope and the tertiary electrometer were taken before and after the introduction of the absorber into 140 [header] the path of the tertiary radiation. When testing the secondary radiation readings were taken on the primary and secondary electroscopes. By a suitable interpolation the corresponding readings on the secondary electroscope and the tertiary electrometer could be obtained at any time.) Calculations based on careful drawings gave the order of correction to be expected, but its experimental determination was deemed more satisfactory. For this purpose a plate of pure copper was substituted for the carbon, and the absorption coefficient in aluminium of the oblique tertiary homogeneous radiation from it (excited by a more penetrating secondary beam) was found and compared with a nearly parallel homogeneous beam from copper when used as a secondary. The correction obtained in this case was in very close agreement with that obtained when other oblique homogeneous tertiaries of a more penetrating type were compared with the corresponding secondary parallel beams. It was found when absorption experiments were carried out on the lines indicated above, that when all such corrections had been applied there was strong evidence that the radiation excited in carbon was heterogeneous, and distinctly less penetrating than the primary exciting beam. The results are given in the following table: — Table I. Element giving sec. radiation. Previous absorption i of ter. radiation. Percentage absorption of tertiary. Percentage absorption of secondary. Different thicknesses of aluminium were used as absorbers for the different beams, in order to keep down the absorption [header] 141 of the tertiary to approximately the same figure in each case. The tertiary ionization was not sufficiently intense to enable the analysis to be pushed to extreme limits. A glance at column 3, however, shows that the signs of heterogeneity are quite pronounced, while a comparison between columns 3 and 4 indicates sufficiently the degree of softening which has taken place. The observed effects might be reasonably explained by any of the following hypotheses : — (1) That there were impurities present in the carbon, presumably of a kind which would furnish secondary radiations of a softer type than that of the secondary exciting beam. The presence of such impurities would account for both the heterogeneity and the softer character of the scattered radiation as a whole. It was evident that the substances to be looked for were traces of metals of higher atomic weight than carbon or of their salts. (2) That the exciting secondary radiation was imperfectly homogeneous and that a preferential scattering by the carbon of a soft constituent, which might be supposed to be present, took place. (3) That carbon itself emits a homogeneous radiation which is feeble in character but very much softer than any of the exciting beams used. These hypotheses were examined in detail. Presence of Impurities. A piece of the carbon used as radiator was carefully analysed, and was found to contain traces of iron but no perceptible traces of any other metallic substances. The amount of iron present was extremely small. To test how far this impurity and that of any other metallic compounds might explain the observed results, two similar carbon blocks were taken, one preserved intact and the other ground to a powder, and digested in a mixture of hot strong nitric and hydrochloric acids for several hours. The mixture was allowed to settle and the clear solution poured off. The residue was washed and further digested with strong nitric acid. Water was added, and the liquid was then filtered, and this residue carefully washed with distilled water and allowed to dry. The powder was made up into a radiator on a frame (this frame was carefully shielded from the secondary exciting beam) between two layers of tissue-paper. The two carbon radiators were now compared in their behaviour when subjected to the same secondary beam. 142 [header] Identical results were obtained in each case within the limits o£ experimental error (a radiator made up of several sheets of tissue-paper behaved in a similar manner to a block of carbon). Other substances besides carbon were examined, but not in such detail, e. g. a block of paraffin wax gave a softening effect, though not quite to the same extent as a similar block of carbon . We were forced to conclude that this first hypothesis was no longer tenable. We had satisfied ourselves that in the powdered carbon no appreciable amount of metallic substance remained after treatment. The behaviour of paraffin and of paper appeared to negative the possibility of the softening in carbon being due to organic impurities. For had this been the case, we would have expected the softening to have been much more pronounced in paraffin than in carbon. Heterogeneity of the Secondary Beams. It has been shown in previous papers that the secondary radiation from substances like copper is nearly all of the characteristic type. It was pointed out that the radiation was remarkably homogeneous. This has been demonstrated in two ways. The first, the absorption method, showed that a thin sheet of aluminium, say, absorbed as nearly as possible the same percentage of a parallel secondary beam from copper, when the beam was reduced to only 5 per cent, of its original components by a suitable thickness of aluminium, as of the original beam. This method of analysis is not so sensitive as it might appear to be at first sight. For a nearly pure homogeneous beam with a slight admixture of other constituents differing slightly in hardness from the main beam would behave in much the same way when the absorption was pushed to this extent only. The second method [citation redacted] depends upon the fact that the fluorescent radiation is only excited when the exciting radiation is of a more penetrating type than itself. Now a sheet of pure iron subjected to the radiation from a similar sheet of iron emits an extremely feeble radiation compared with that produced when it is subjected to a more penetrating radiation. But if the secondary radiation from iron had contained an appreciable amount of a soft constituent, this soft constituent would have been excited by the normal fluorescent radiation from iron. On the other hand, if the iron radiation possesses a constituent harder than the normal, it would excite the normal constituent from the second plate. The radiation so excited when iron radiation falls upon iron is as stated above very feeble in amount, and is on the whole slightly softer [header] 143 than the normal iron radiation. We must conclude, therefore, that if other homogeneous components coexist with the normal in the radiation from iron, they form relatively unimportant parts of the whole. If, therefore, the softened character of the radiation from carbon is to be explained in terms of the soft residual component of the beams from iron, copper, &c, we must assume that a substance like carbon is a much more efficient scatterer for soft radiation than for hard, especially as the softening effect becomes more pronounced when the hard exciting beams are used. In order to test this point, we used the secondary radiation from tin, which is known to consist of a very penetrating radiation, together with a component of appreciable intensity of a much softer type. If this latter is scattered to a greater extent by the carbon than the former, then since, if the secondary beam from tin is cut down by plates of aluminium before falling on the tertiary radiator, the soft radiation in the secondary beam disappears first, we should get a correspondingly rapid diminution in the intensity of the tertiary. Tho results of such an experiment are given in Table II. Table II. Relative value of Intensity of tertiary secondary radiation Tin radiation on Carbon. Substance in path of sec. beam. Deflexion of primary electroscope. Deflexion of tertiary electroscope. Deflexion of secondary electroscope. [table redacted] 144 [header] It will be seen from columns 5 and 6 that while the secondary beam is cut down about 25 per cent, by a piece of aluminium "02 cm. in thickness, the tertiary radiation is not perceptibly reduced, while three thicknesses of *01 Al reduces the tertiary radiation by less than 5 per cent. But the soft radiation is practically all absorbed by the first two pieces of aluminium, for the first sheet absorbs 20 per cent., the second 7 per cent., while the third only 4 per cent, of the beam, which is about the amount by which a beam consisting solely of the hard constituent would be reduced by such a thickness of aluminium. But the ratio of the original ionizations produced by the soft and hard components respectively is at least as high as 1*5. If this relatively high proportion of soft radiation takes so small a share in the excitation of radiation from carbon, we are probably justified in concluding that the softened character of the carbon radiation cannot be explained by reference to any lack of homogeneity in the secondary beams used in these experiments. Characteristic Radiation from Carbon. That the effect was not due to corpuscular radiation from the carbon reaching the tertiary ionization chamber can be easily demonstrated, for the shortest distance between these two was at least 3 cm. But it was shown by one of us* that the most penetrating type of corpuscular radiation excited by a Röntgen beam failed to penetrate a distance of more than 2 cm. in air at atmospheric pressure. The experiment with tin as secondary suggested that hard rays were most effective in exciting radiation in carbon. It now became necessary to consider our third hypothesis — the possibility of there being a homogeneous radiation from carbon superposed upon the scattered. It was evident that if we had a mixture of two homogeneous radiations— -the scattered homogeneous secondary and the homogeneous tertiary characteristic of carbon — we could by suitable absorptions by sheets of aluminium find the absorption coefficient in Al of this latter component. For let I1 and I2 be the relative intensities of the two homogeneous radiations. If [alpha] be the absorption by 1 cm. of air of the radiation I1 and [beta] that of I2 , the absorption in the electroscope of these two radiations will be proportional to [formula redacted] and [formula redacted], [header] 145 and the total ionization in the tertiary electroscope may be written [formula redacted] (A and k being constants of the electroscope ; A including a correction for obliquity of the rays.) [formula redacted] Let I1 and I 2 ' be the intensities after the first absorption by a plate of aluminium. The total ionization in the electroscope will then be [formula redacted] If [lambda]1 be the absorption coefficient of the unknown homogeneous radiation, and X 9 that of the homogeneous secondary radiation, in aluminium, and x 1 the thickness of the first aluminium absorber, [formula redacted] If [lambda] = the 1st apparent absorption coefficient of the mixed beam, [formula redacted] Similarly, if i1 and i 2 be the intensities, after a further absorption by a second plate of aluminium of thickness x. 2 , and [lambda]' the second apparent absorption coefficient, [formula redacted] Substituting, we have [formula redacted] and [formula redacted] from (1) [formula redacted] [footer] 146 [header] and from (2) [formula redacted] and hence [formula redacted] If [formula redacted] if the same thickness of absorbing material be used in obtaining- the same successive absorptions, [formula redacted] and [formula redacted] and equation (3) reduces to the simpler form, [formula redacted] All the quantities in this equation are known, and since [formula redacted] [lambda]1 the unknown absorption coefficient of the second radiation may be calculated. To test the order of accuracy to be obtained by using this formula, a mixed radiator of Cu and Fe was made up and subjected to the radiation from As, the mixed tertiary beam in this case consisting essentially of the homogeneous radiations characteristic of Fe and Cu. A sheet of paper, placed in the path of the tertiary beam, gave a first absorption of 37'0 per cent. A similar sheet of paper was then placed in the path of the beam, and a second absorption found with the first sheet of 35'5 per cent, (this method ensures that x is the same for each absorption). Taking the absorption coefficient of the Cu radiation as known to be 129, and supposing that of the Fe radiation to be unknown, we have [formula redacted] substituting these values in the above equation, [formula redacted] From this, we get as a value of the absorption coefficient for the Fe radiation, [formula redacted] against a known value for Fe radiation of 239. [header] 147 Other test cases were taken, and the formula found to be reliable to within about 10 per cent. To ensure accuracy it was necessary to secure that no one constituent had been completely absorbed by the two absorbing layers. It was important also that the difference between the two apparent coefficients should be as great as possible. Having regard to these two points a piece of carbon was taken and subjected to homogeneous beams of: varying hardness. Assuming that the tertiary radiation in each case consisted of secondary radiation scattered as a homogeneous beam of unaltered hardness, together with a radiation characteristic of carbon, the absorption coefficient of the latter in aluminium was calculated from the observed absorptions. The results of these experiments are given in the following table : — Table III. Source of Secondary radiation. [lambda] for Secondary radiation. Calculated value of [lambda] for the soft Tertiary radiation. [table redacted] Note. — The values of [lambda] in column 2 are for radiations filtered through appropriate thicknesses of aluminium. In the case of those secondary beams known to contain a soft constituent, this was removed by first passing the radiation through absorbing sheets of aluminium before it fell upon the tertiary radiator. A glance at column 3 shows at once that after making due allowance for experimental error there is no semblance of agreement between the values of [lambda]. We must then abandon our hypothesis as to the composition of the tertiary beam. There is no precedent for supposing that if a characteristic radiation were emitted by carbon it would be other than homogeneous. Moreover, experience justifies us in concluding that in such a case its degree of hardness would be constant and independent of the hardness of the exciting radiation. [footer] 148 [header] It is well known that some substances have more than one type of homogeneous radiation. It is possible that carbon is one of these, though the balance of evidence on the whole makes it improbable that an explanation of the phenomena lies here. The authors incline more to the belief that an actual modification of the X ray occurs in its passage through matter, a general softening taking place. Other evidence favouring this view is accumulating along rather different lines in a separate investigation, the details of which are not yet ripe for publication. One thing appears to be firmly established, viz., the harder the rays, the more profoundly are they modified in their passage through matter. An extension of an experiment described in an earlier paragraph seems to bear upon this point. It was pointed out that the radiation from tin consisted of two parts, a soft component having a value for [lambda] in aluminium about 40-50 and a hard component for which [lambda] is about 4*0. It was also shown that the intensity of the tertiary radiation excited in carbon depended chiefly upon the hard component. On testing the quality of the tertiary radiation, it was found that this too was practically unchanged when the soft component was suppressed. It was found impossible to determine to what extent the tertiary excited by the hard component from tin was heterogeneous, owing to the feebleness of the ionization it produces. The disturbing effects of stray radiations become more pronounced when dealing with a residual ionization. No claim for accuracy is put forward with regard to the values of [lambda] given in column 3, Table III. For if the radiation prove to be non-homogeneous it is obvious that they can only represent average values. There can be no doubt, however, that a homogeneous beam is not scattered solely as a homogeneous beam, even if at all. The experiments described do not enable us to determine whether there is a homogeneous radiation emitted by carbon. It is highly probable that such a radiation does exist, though of feeble intensity. It is hardly necessary to point out that a clear solution of this problem is bound to have an important bearing on the theory of the production of an X ray. Incidentally, the bearing of these results upon such questions as the polarization and the general distribution of a scattered radiation will need to be carefully considered. It is perhaps only fair to mention that, as far as we can judge, the primary heterogeneous beams used in the earlier experiments on scattering were generally of a soft type. [header] 149 When we consider that the constituents of any one such beam would have a wide range of hardness and that the hardest constituents are more effective in transforming the incident radiation, it is not to be wondered that on the whole the scattered radiation behaved as a primary weakened in intensity only. The authors recognize that their experimental data are somewhat fragmentary and fail to provide a complete solution of this peculiarly elusive problem. It was thought, however, that the results already obtained contained sufficient novelty to warrant publication. Summa?*y. It is shown that : (1) A homogeneous beam when scattered by a substance of low atomic weight is transformed into a softer type of radiation. (2) The harder the exciting beam, the greater is the intensity of the scattered radiation. (3) The harder the exciting beam, the more profound is the change in its quality. In conclusion we wish to thank Professor Wilberforce for placing the resources of the laboratories at our disposal. The George Holt Physical Laboratories. January 1912.