XXXVIII. Production of Fluorescent Röntgen Radiation. By J. Crosby Chapman, B.Sc, Tutor in Mathematics at King's College, London, late Research Student of Gonville and Caius College, Cambridge [Communicated by Professor C. G. Barkla.]. TO satisfy the corpuscular theory, Professor Bragg [citation redacted] has recently suggested that not only ionization is an indirect phenomenon due to [beta] rays, but that in addition the fluorescent X-rays themselves are possibly due also to action of the [beta]-rays. That is, the fluorescent X-radiation is a tertiary and not a secondary effect, being merely due to the expelled 360 [header] electrons themselves colliding with other atoms, and in this way producing the peculiar type of radiation known as fluorescent X-rays. This theory, that the secondary homogeneous radiation results from the bombardment of atoms by ejected corpuscles, has been discussed in a research published with Mr. Piper [citation redacted], as well as in a later paper f. The object of the experiment, suggested by Professor Barkla, and described in the later paper, was to test this theory directly. Allowing X-rays to fall on bromine vapour (C 2 H 5 Br), it was found that the presence of C0 2 in the first case, and H 3 in the second case,, as the gas separating the ethyl bromide molecules, made next to no difference in the amount of secondary radiation emitted by the bromine when excited by X-rays. Whereas on this theory, since the C0 2 , which is a heavy gas, must absorb many of the expelled corpuscles, while a light gas cannot do this, the CO1 ought to interfere with the conversion of the cathode rays into bromine X-rays, and thus a greater intensity or secondary radiation be obtained in the second case than in the first. This was not found to be so. The results obtained were : — Secondary radiator .... C 2 H-,Br. Intensity of secondary radiation with H 2 separating C 2 H 5 Br molecules. Intensity of secondary radiation with C0 2 separating C 2 H 5 Br molecules The calculations given in the paper show that, if the theory is correct that the expelled corpuscles from bromine do, by subsequently bombarding fresh bromine atoms, make the latter emit homogeneous radiation, there should be a notice- able difference in the intensity of the radiation in the two cases. From the fact that the radiations were of the same intensity, the conclusion naturally followed that this theory of the indirect action of the X-rays in producing fluorescent X-radiation is untenable. This experiment, however, does not seem to Professor Bragg to be decisive. For in a recent paper published with Mr. Porter, he criticises the conclusion stated above, in the following way [citation redacted]: — "This does not seem to me conclusive. If the production of X-rays is a consequence of the encounters of cathode rays with bromine atoms, there will be an ample opportunity for the effect to take place even though the C0 2 molecules are scattered among the bromine atoms. If an electron meets a C0 2 molecule first it is not arrested there, [header] 36 but deflected, and may have hundreds of encounters before it; so that its chance of meeting a bromine atom is practically as great as ever. If it is argued that the cathode ray is ' absorbed ' by the bromine and C0 2 in proportion to weight, it must be answered that whatever ' absorption ' may mean, there is no clear evidence of the universality of Lenard's law." Such a fundamental difference between the action of the C0 2 molecules and the Br atom on the electron, as Professor Bragg has been compelled to ascribe, is purely hypothetical, and until experimental evidence of its truth can be brought forward, it must be treated as an assumption which is merely a convenience when explaining the results of the above experiment from the point of view of the corpuscular theory. With regard to Lenard's law, at the time when the paper was written, and in fact even now, the law is not completely established, but recently it has been shown to be an approximate representation of what actually takes place, even for slowly moving corpuscles, and Professor Bragg himself has published results from which he draws important conclusions which necessitate a far more accurate fulfilment of this law than was needed in the experiment referred to. However, to settle the point as to whether the fluorescent X-rays are produced directly or indirectly, the following experiment was devised, so that in the case of one X-radiator the corpuscles were certainly absorbed in the radiating substance, while in the case of the other radiator, the corpuscles lost their energy in a substance in which it is impossible to produce measurable secondary fluorescent X-radiation. In constructing an experiment to meet this demand two difficulties have at once to be faced. In the first place, if any appreciable fraction of the total amount of corpuscular radiation produced in a sheet of metal is to escape from the metal, the latter must be exceedingly thin, of the order of 10~ 5 cm. Secondly, the method of separating the different parts of the radiator must be such as will readily allow of the whole serving as a convenient secondary radiator. Recently, however, I have shown [citation redacted] that gold with other heavy elements is a most efficient secondary X-radiator; and that its type of radiation [citation redacted] , both in regard to the secondary X-radiation and the [beta]-rays produced, differs in no respect from the type of radiation emitted by an element such as bromine of the more usual group K (elements Cr — I). Now gold in the form of leaf can be obtained in exceedingly thin 362 [header] sheets of the order of thickness required by the experiment. In addition, it has often been proved that carbon is a most inefficient corpuscular radiator, so much so that in all experiments where this type of radiation from the walls of the ionization- chambers is to be minimised, carbon in the form of paper is used for this purpose. So that, instead of air or C0 2 separating the different portions of the radiator, which is in the form of gold leaves, it is possible to use thin paper. Apparatus. — The apparatus, with a few modifications, was similar to that used in previous experiments. Construction of Radiators. The two radiators, the efficiency of which as fluorescent X-ray producers it was required to determine, were constructed as follows: — [figure redacted] Radiator I. Radiator II. Gold leaves separated. Gold leaves together. Continuous line=sheet of paper. Dotted lines =gold leaf. The important point to notice is that both radiators consisted of the same mass of gold and paper, namely, seven gold leaves and sixteen paper sheets, the sole difference between the two radiators being the relative positions of the gold and the paper. In radiator I. we have first two sheets of paper then a gold leaf, then again two sheets of paper and another gold leaf, and so on till the seven gold leaves and the sixteen paper sheets are all used. In radiator II., however, we take first eight sheets of paper, then seven gold leaves altogether, [header] 363 followed by another eight sheets of paper. Each of these two arrangements of gold leaf and paper was mounted between thick aluminium frames. In this way the leaves and paper were gripped around their edges, and with a little care in fixing the outer paper sheets with shellac on to the frames, it was arranged so that the paper in each case was stretched tightly across the frame. The two aluminium frames were pressed together in a vice and melted wax was run around their edges, so that the two remained tightly pressed to one another. The thickness of the effective parts of each radiator, that is, in the centre, where the X-rays struck and where there was no aluminium, was such as could be accounted for by the presence of the paper sheets alone. The desirability of leaving no air-gaps will become apparent in the calculations. A third radiator was made in a similar manner containing only the sixteen paper sheets. It may be added here, that alternate gold leaves, taken from the same book, were used to form the radiators I. and II. respectively. It will be noted that, owing to the arrangement of the paper and the gold leaves and their small absorption, the gold fluorescent radiation was absorbed to approximately the same extent in the two cases. Object of this Construction. Imagine now the same primary beam of X-rays to be passed through radiator I. and radiator II. successively. Consider, first, radiator I : suppose there is being produced in the central gold leaf [delta] a certain amount of corpuscular radiation. It will be shown later that the greater part of the energy of these corpuscles escapes from the leaf. This is the sole assumption made at the present, and it is proved by actual experiment later in the paper. Then in radiator I. the corpuscles which emerge from the central leaf [delta] will at once pass into paper, where they will be totally absorbed, so that their existence as regards acting on gold atoms is at an end. Whereas in radiator II. corpuscles produced at [delta] will have to spend practically the whole of their energy in the gold leaf itself. So that, while in the case of radiator I. the corpuscles spend a large part of their energy in the paper,, in the case of radiator II. the corpuscles spend almost all their energy in the gold. So that, quite apart from any meaning "absorption" may have, we should expect, if the expelled electrons by bombarding other gold atoms produce secondary X-radiation, that the radiator II. would be more 364 [header] efficient than radiator T. in producing such radiation. Two sheets of paper were used to separate the gold leaves in radiator I. so as to ensure in all parts a sufficient thickness to -absorb the whole of the incident corpuscular energy. The research resolves itself into three parts : — (1) to test the efficiency of the two X-radiators ; (2) to determine the penetrating power of the exciting primary beam ; (3) to investigate what fraction of the total corpuscular radiation produced in one gold leaf escapes from that leaf. Efficiency of the two X-radiators. The following were measured : — (a) the efficiency of radiator I. as a fluorescent X-ray producer ; (p) the efficiency of radiator II. as a fluorescent X-ray producer ; (c) the intensity of scattered radiation from the third radiator consisting only of paper. Using the results so obtained, the ratio of the efficiencies of the radiators I. -f II. was calculated. It only remained to show that the radiation which was being measured was actually the gold characteristic radiation. In order to prove this the absorption in aluminium of the radiations from the gold radiators was determined, allowance being made for the scattered radiation from the paper. No correction was applied for the scattered radiation from the gold itself ; this correction is of such a small order that in the experiment it would have served no purpose. The absorption coefficient ([formula redacted]) of these rays in aluminium was equal to 20*6, while the most accurate value of [formula redacted] for gold radiation in aluminium = 21*6, and this is after somewhat tedious corrections have been applied. This near agreement of 20*6 and 21*6 showed that the radiation, the intensity of which was being determined, was actually the gold radiation. The results obtained after subtraction of the scattered radiation from the paper are given below in tabular form. In each case the radiation when radiator I. was used is taken .as the standard. 365 [header] Table I. Intensity of fluorescent X-radiation from radiator I. [gold leaves separated]. B. Intensity of fluorescent X-radiation from radiator II. [gold leaves together]. A. Ratio A B Mean [formula redacted] in Al of primary. [table redacted] Taking mean value Intensity of fluorescent X-radiation from radiator II.. Intensity of fluorescent X-radiation from radiator [formula redacted] in Al of gold characteristic radiation experimental = 20*6 Standard value = 21*6 Penetrating Power of the Primary Radiation. The X-ray bulb furnishing the exciting primary radiation was fitted with a palladium side tube, which enabled the rays leaving the bulb to be made of any desired degree of hardness. The value of the mean penetrating power of the radiation was obtained by finding the absorbability of the radiation scattered by a thin carbon sheet, the scattered radiation from carbon being almost identical with that or' the primary radiation. In order to obtain the very hard rays, not only was the bulb worked at its maximum hardness, but in addition thick aluminium ("19 cm.) was placed in the path of the primary beam so as to absorb all but the hardest rays from the bulb. The values for the absorption coefficient [formula redacted] in aluminium of the primary beam are given in column 4 of Table I. The importance of knowing the order of the penetrating- power of the primary beam is clear from the work of Cooksey [citation redacted], Innes [citation redacted], Sadler [citation redacted], and Beatty [citation redacted], which shows that the velocity [footer] 366 [header] with which the corpuscles are ejected varies directly with the penetrating power of the exciting radiations. Suppose in this experiment it had been possible to use a very soft radiation, say [formula redacted] in aluminium = 60, the corpuscles ejected by this primary would have had relatively low velocities, and would be rapidly stopped, and few would be able to emerge from the gold leaf, so that the total effect would have been that practically all the corpuscular radiation would have been absorbed in the gold leaf, whether radiator I. or radiator II. was used. In this case the experiment as a test of the two theories breaks down. If now instead of a soft radiation the hard primary beam is employed, it can be shown that as large a fraction as 70 per cent, of the total corpuscular energy produced in a gold leaf escapes from the metal. As the only assumption underlying the experiment depends on the fact that a reasonable fraction of corpuscles which have still sufficient velocity to excite the characteristic radiation from the gold shall escape, it was thought advisable to determine this fraction experimentally, though the penetrating power of the corpuscular radiation could easily have been calculated from the figures of Beatty or Sadler. Corpuscular Radiation produced in Gold by the Tin Radiation. Tin serves as a convenient secondary radiator, for it emits in moderate quantity a very hard radiation the absorption coefficient ( [formula redacted] ) of which in aluminium =1'5, a hardness of the order of the primary beam used in this experiment (see Table I. column 4). The object of this part of the research was to determine what fraction of the total energy of the corpuscles produced in a gold leaf by tin radiation is able to escape from the metal itself. This was accomplished in the following manner : — An ionization-chamber (1 cm. thick), kindly lent me by Mr. Philpot, was fitted with an electrode made of fine aluminium wire mounted on an aluminium frame, and this electrode was connected to an electroscope. The back and front of this chamber were made of carbon. Initially when rays passed through the chamber, the ionization, which was small, was almost wholly due to the ionization of the air by the tin [header] 367 radiation. Supposing now on the internal incidence face of this ionization-chamber a single gold leaf is affixed; and the tin rays again passed through the chamber. There is then superimposed on the original effect due to the air, the ionization produced by that fraction of the corpuscular rays produced by the tin radiation which is able to escape from the gold and produce ionization in the air. If now a second leaf be added, and if it happens that corpuscles are able to penetrate more than one thickness of leaf, there should be an increase in the ionization due to that fraction of the corpuscular rays from the first leaf which is able to penetrate the second leaf, and thus ionize the air in the chamber. In this way leaf after leaf was added until further leaves produced no increase in the intensity of ionization. At this point the layer of gold on the internal incidence face could be considered infinitely thick from the point of view of corpuscular rays. In the following table the values of the ionization corresponding to the various numbers of radiating leaves are given. Table II. Metal acting as corpuscular radiator — Gold. Exciting radiation — Tin. Thickness of gold = [formula redacted] cm. No. of gold leaves serving as corpuscular radiator. Intensity of Ionization due to : — (I.) Direct air effect. (II.) Corpuscles expelled from gold. Intensity of corpuscular radiation. [table redacted] From these results a curve (p. 368) is plotted showing the relation between ionization due to corpuscular radiation, and the number of leaves which served as corpuscular radiator. [footer] 368 [header] From measurements of this curve it can be shown at once that the total energy of the radiation produced in the gold leaf by the tin radiation is absorbed to the extent of 52 per cent, (approximately), when it passes through a thickness of gold [figure redacted] equivalent to one leaf. Since the absorption of corpuscular rays in one gold leaf is 52 per cent., this means that half that thickness will absorb something of the order of 30 per cent. Arithmetical calculation gives the following figures when one gold leaf is considered. Gold leaf thickness = [formula redacted] cm. Corpuscular radiation produced by tin characteristic rays. Energy of corpuscular radiation emerging from gold leaf _ iD Total energy of corpuscular radiation produced in gold leaf — And this was the fraction it was required to determine. The following results should be stated as showing the smallness of error due to absorption of the gold X-radiation by the paper and gold leaf. Percentage absorption of gold fluorescent radiation by: — (1) A single thickness of paper = *7 per cent. (2) A single thickness of gold leaf =2*0 „ [header] 369 It is now possible to calculate as completely as the experiment demands what should be the difference in the efficiencies of the two radiators I. and II., supposing that the bombardment theory to be an accurate representation of: fact. From Whiddington's [citation redacted] results, assuming the general law, velocity of ejection of corpuscles by tin radiation [formula redacted] Now velocity of corpuscles when they no longer have the power of producing gold X-rays [formula redacted] [formula redacted] Therefore the minimum fraction of energy which they must lose before they cease to produce gold X-rays is [formula redacted] But it is experimentally found that they lose only 30 per cent, of their energy before they emerge from the gold ; that is, they must still possess the power of producing an intense X-radiation from the gold. The experiment shows, however, that the X-radiation is just as intense when this corpuscular energy is absorbed in paper as when it is absorbed in gold. If the X-radiation produced in gold be taken as proportional to the diminution in energy of the corpuscles in the gold, the ratio of the X-ray efficiencies of the two radiators would be Efficiency of radiator II. Efficiency of radiator I. Correcting for the surface effect in radiator II., this ratio works out at Efficiency of radiator II. _-i.q Efficiency of radiator I. The experimental ratio of the two efficiencies is Efficiency of radiator II. Efficiency of radiator I. No conceivable hypothesis as to the relative efficiencies of corpuscles of varying velocities in producing X-rays can explain this difference between the theoretical and the experimental ratio. So that this experiment seems clearly to 370 [header] prove that the bombardment theory does not represent even to a small extent what takes place when secondary fluorescent radiation is produced. That is, fluorescent radiation cannot, be an indirect effect of [beta]-ray activity. In addition, it will be noticed that in the case of radiator I. there is much less ionization in the gold than is the case in radiator II. So that this experiment seems to negative those theories which assume that it is on recombination o£ the gold atoms that the fluorescent X-radiation is produced. The evidence of this paper strengthens the theory put forward by Professor Barkla [citation redacted], and supported by later paper?, namely, that the atom from which the electron is ejected is the seat of production of the radiation. Summary. The paper deals with Professor Bragg's theory that the fluorescent X-radiation is produced indirectly by the action of the expelled [beta]-rays, and not by the direct action of the primary beam. His criticisms of a previous paper in which an experiment is described to test this theory have been discussed. Another experiment has been performed, in which the theoretical objections he raised have been obviated. The results obtained agree precisely with those given previously, and indicate that Professor Bragg's bombardment theory does not represent even to a small extent the process of production of the fluorescent Röntgen radiation. In conclusion, I wish to express my thanks to Professor Barkla for his continued help and advice. Wheatstone Laboratory, King's College.