L. Further Experiments on Delta Rays. By Norman Campbell, Sc.D. [Communicated by the Author.] 1. In three recent papers [citation redacted] the problem of the velocity of the delta rays and its dependence on the velocity of the exciting alpha ray and the material from which they are emitted has been discussed. At the end of the last paper it was pointed out that most of the phenomena which had been observed could be reconciled with the view that the delta rays possessed no finite initial velocity greater than a small fraction of a volt and that this view, if it could be accepted, would simplify greatly the theory of ionization. The general method of experiment had been to determine the relation of the current flowing between two plates, from which delta rays were being emitted, to the potential difference between those plates, and there was some evidence that this relation was determined as much by the energy required to drag the slowest rays away from the place where they originated as by the energy required to prevent the fastest rays leaving that place. But all observers agree that delta rays can leave a p]ate struck by alpha rays, even when that plate is at a positive potential of some volts with regard to its surroundings. The rays must, therefore, be leaving the plate with a finite velocity, and if, as has been suggested, this velocity is not conferred on them in the act of emission, some source of energy competent to confer it after they have been emitted must be found. When the suggestion was put forward, it appeared that such a source of energy might be found in a 528 [header] process analogous to voltaic action, for the acquirement of a spontaneous positive charge by a body struck by alpha rays had never been observed except when the electrodes were of different materials. The first of the miscellaneous experiments here described was designed to test this hypothesis. 2. There is one remarkable feature which distinguishes radioactivity and its effects in ionization from almost all other processes : they are independent of the temperature. If the relation of the current to the potential difference were determined by secondary causes unconnected with radioactivity and ionization, then that relation should change with the temperature ; but if it is determined by the primary causes of emission of the delta rays, then it should be independent of the temperature. The experiment was made by insulating about 1 mm. apart two parallel copper plates, one of which was covered with polonium, in an evacuated glass vessel. The vessel was strongly heated during exhaustion to drive off air from the solid contents. The relation of the current 'to the potential difference between the plates was then determined (1) at room temperature, (2) when the vessel was heated to the softening point of the glass, (3) when the vessel was immersed in liquid air. Not the smallest difference could be detected between the relations for (1) and (3) ; in (2) all the currents were about 3 per cent, greater than in (1), a difference which could be attributed to the fact that gas had not been completely eliminated from the vessel, or possibly to the beginnings of thermionic emission. It may be safely concluded that the emission of delta rays is independent of the temperature, and that the velocity of those rays is determined by processes directly connected with the action of alpha rays on the atoms they strike. It is necessary to imagine that the rays really start with an initial velocity of some volts. 3. Definite evidence that the form of the relation between current and potential difference was influenced by the effect of the field in dragging delta rays out of the electrodes had been produced only in the case when one of the electrodes was covered with soot. It appeared that if this action was important, the relation ought to be determined by the electric field between the plates rather than by the potential difference between them. Experiments were accordingly made with the apparatus described in the last of the three papers to discover whether the relation was changed when the distance between the electrodes (and, therefore, the field for a [header] 529 given potential difference) was changed. The electrodes were covered with aluminium, and the distance between them could be altered in the ratio of 1 to 2 '5 (approx.) without violating the necessary geometrical conditions. The following table shows that the relation between current and potential difference is independent of the field strength, and that, when the electrodes are of aluminium, the form of the Table I. P.D. Current. Electrodes 05 era. apart. Electrodes- 12 cm apart. [table redacted] relation is determined by the velocity of the rays. It is not proved that, in this case, the " simple theory " of that relation, given in the first paper, is adequate, for the possibility of a reflexion of the rays, which depends upon the velocity, has to be taken into account Nevertheless, these two experiments do much to remove the uncertainties which were previously attached to interpretations of the experiments based on that theory ; it appears that some small proportion of the rays, at least, must have an initial velocity as great as 10 volts . 4. It was concluded from the previous work that the relation between current and potential difference, and therefore the distribution of the initial velocities of the rays, was independent of the velocity of the exciting alpha rays. The velocity of the exciting rays had been varied only by inserting absorbing layers in the path of the rays from polonium. A variation over a wider range was now obtained by substituting for the polonium the active deposit of thorium obtained from a preparation of mesothorium. The range of the polonium rays is 3'9 cm., that of the rays from ThB and ThP 5*0 and 8*6 cm. The experiments 530 [header] mentioned in the last paragraph and many others have been repeated with the substitution of the thorium deposit for polonium ; in no case has any alteration due to the substitution been detected. It may be taken as quite certain that the velocity of the delta rays is perfectly independent of that of the alpha rays by which they are excited. 5. Many of the difficulties in interpreting the results of the experiments described in the first two papers arose from the fact that in all cases the rays proceeded from both of the electrodes between which the current was measured. It was difficult to decide whether the effect of an increase in the potential of one electrode was due to a stopping of the rays proceeding from that electrode or to an increase in the number proceeding from the other. It appeared that, if only an apparatus could be obtained in which the rays proceeded from one electrode only, then it would be possible to determine accurately the maximum speed of the rays at least, for it would be that corresponding to the positive potential on that electrode which is sufficient to prevent any current passing. At length the apparatus shown in fig. 1 was designed and seemed to fulfill the necessary conditions. [figure redacted] The active material, which in this case was the active deposit of thorium, is deposited on the under side of the ring O. The alpha rays from it can excite delta rays at the interior surface of the box A, and these rays can emerge through the hole in the top of the box and reach the other electrode D ; but the alpha rays cannot strike any ether part of the apparatus, so that there is no other source of delta rays. Both electrodes were, of course, contained in a vessel which could be exhausted. Preliminary experiments showed that, if the hole in the top of A were covered with the thinnest possible aluminium foil, no detectable current could be made to pass between [header] 531 the electrodes. It was clear that the desired conditions were attained, in that all delta rays were produced within A and that no disturbance was introduced by the [beta] and [gamma] rays emitted by the active deposit. 6. If there were no air in the vessel and if none of the delta rays were reflected at the electrode, the relation of the current to the potential difference between A and D would give directly the distribution of the velocities of the rays. When the potential difference is zero, all the rays emerging through the hole strike D ; when the potential of A is V above that of D, all the delta rays having an initial velocity less than Y will be prevented from reaching D. Accordingly, if iv is the current flowing to D corresponding to the potential [formula redacted] should be the proportion of the rays having a velocity greater than V. When V is very great i Y should be zero, and it should be zero also for all values of V when a magnetic field of sufficient strength is established in a direction parallel to the surface of D, so as to prevent any of the rays from A reaching D. The values of the current when Y is negative do not concern us ; they merely indicate how many electrons can be dragged from inside A by an external field. Fig. 2. A & D both Aluminium. Aluminium; D, Soot. [figure redacted] Figs. 2 and 3 show the nature of the curves determined experimentally. In fig. 2 both A and D were of aluminium; 532 [header] in fig. 3 A was of aluminium while D was covered with soot; in each case curve 1 represents the readings without a magnetic field, 2 with a magnetic field, and 3 the difference between 1 and 2. The magnetic field was of such a strength that a further increase in it produced no further effect. A correction is made in every case for the activity of the thorium deposit, so that the readings are comparable. 7. It will, be noted that in the curves 1 the current for large values of V is not zero, but a finite positive quantity. The existence of a positive current would seem at first sight necessarily to indicate that positive ions were travelling from A to D, for the preliminary experiments with the hole covered with aluminium leaf show that no delta rays are liberated from D. These ions are naturally attributed to the presence of gas remaining in the vessel, but their number is unexpectedly large. Observations with the vessel containing gas at measurable pressures showed that the current due to the presence of air at the pressure indicated by the McLeod gauge should be only O'l per cent, of the current carried by the delta rays, whereas it appears to be some 15 per cent. It is probable that some of the gas adhering to the surface of A is only liberated when the alpha rays strike it and ionize it ; a similar explanation of some of his observations has been put forward by Pound in a recent paper,[citation redacted] and it is doubtless correct. But if the positive current is due to this cause, it should be unaffected by the action of a magnetic field ; for experiments at higher pressures showed that such a field had no influence on the current due to the ionization of the air, at least for potential differences greater than 2 volts. In fig. 3, when D was covered by soot, the application of a magnetic field does not change the magnitude of the positive current, but when D is of aluminium the magnetic field decreases the positive as well as the negative current. Similar measurements, when D was of brass and gold, showed that it w T as only when D was covered with soot that the positive current was unchanged by the magnetic field, and it was also noted that in this case the positive current was somewhat smaller than the others. 8. There seems only one explanation possible of the change of the positive current with the magnetic field. In his work en the reflexion of slow electrons, v. Baeyer [citation redacted] has shown that .electrons with speeds of the order of 10 volts excite so many secondary rays at the surface of metals on which they are incident, that it is possible for more electrons to leave such [header] 533 a surface than fall upon it. If a fast delta ray which can travel against a potential difference of 10 volts excites at D more than one secondary ray, then there will obviously be a positive current to D which will be stopped when the delta rays are prevented from reaching D by the application of a magnetic field. And since v. Baeyer found that soot shows practically no reflexion, the difference which has been noted between soot and metals is immediately clear. In a similar manner it may be possible to explain a curious phenomenon which, though it was suspected in the previous experiments, was not mentioned for lack of certainty. It now appears that it has also been observed by Pound (loc. cit.) and there can be no further question as to its reality. In such measurements as were described in the first two papers, especially when the electrodes were covered with aluminium, it was found that an increase in the potential difference between the electrodes beyond the 16 volts necessary to attain saturation to 40 or even 100 volts produced a small but noticeable decrease in the current [citation redacted]. The only explanation I can offer of this decrease is that a delta ray from the negative electrode sometimes attains a sufficient speed in travelling through the 16 volts to enable it to liberate a secondary ray which can travel back to the negative electrode against the field: such an action presents no difficulty on the score of energy, for the rays start from the negative electrode with a finite speed. The number of such secondary rays liberated would increase with the potential difference between the electrodes so long as the power of exciting secondary rays increased with increase in the velocity; v. Baeyer has shown that a stage is reached later when the power of exciting secondary rays decreases with increase in velocity ; but I have never been able to observe in the cases mentioned a second increase of the current with the potential difference. 9. If the explanation offered is correct, it is clear that the form of the curves will be materially influenced by the reflexion of the delta rays at the electrode D, unless this electrode is covered with soot. In all other cases the quantity [formula redacted] will not represent accurately the proportion of the rays which have a velocity greater than V, for the number retained by D will depend, not only on the number of rays striking it, but also on the number reflected from it. But unless A is also covered with soot, we introduce another source of error in covering D with soot rather than with the same material [footer] 534 [header] as A ; there will be a Volta difference of potential between the electrodes, and the actual P.D. will not be that read on the voltmeter. It is possible that a way out of this dilemma might be found by the use of some of the methods described recently by Gompton [citation redacted] in his work on the influence of the Volta effect on the measurement of the speed of the photoelectric electrons. But even those methods were not completely successful, and the results which have been already attained appear to me so conclusive that a further elaboration of the apparatus would not be justified. In any case, it would be very difficult to obtain extremely accurate measurements, for in the neighbourhood of V = the current changes so rapidly with V and with the geometrical form of A, that the exact reproduction of a curve is almost impossible. The apparatus is also extremely sensitive to small magnetic fields ; if a piece of soft iron of 30 c.c. volume is placed near the electrode the current for V = is changed about 5 per cent. It is certain that the earth's magnetic field has some influence on the shape of the curves. The quantity i 10 was taken as standard, because saturation appears to be attained with about 10 volts, so that [formula redacted] represents the whole current, and [formula redacted] that part of it carried by rays of which the speed is less than V. Since only comparative values are important the choice of a standard is unimportant. 10. The results of the measurements are given in Table II. Table II. P.D. Current. D same as A. D covered with Soot. [table redacted] [header] 535 The unit of P.D is 1*035 volts, and the quantities termed " current " are the values of [formula redacted] for curves obtained, like those marked 3 in figs. 2 and 3, by subtracting the values of the current with the magnet on from those with the magnet off; thus they are corrected for the presence of residual air. It will be seen that it is quite impossible to say precisely for what P.D. saturation is attained. The curves when D is covered with soot appear to require a higher potential for saturation than the others, but I am sure that the apparent saturation in the latter case is due to the influence of reflexion which has already been noted ; it will be observed that in one case the current for V = 20 is smaller than for V=10. The uncertainty attaching to any single measurement of current does not exceed '005, but it was found that, owing to the causes just mentioned, readings could not be reproduced with this accuracy when the apparatus had been taken down and put up again. The influence of reflexion at the electrode D in the first half of the table and that of the Volta effect in the second half make it unlikely that, even if the distribution of speed among the delta rays from different substances were the same, all the numbers in the same row of the table would be the same. But it will be seen that there is a very general agreement between the figures for different metals, and such divergencies as there are in one half of the table are not reproduced in the other. The figures for gold in the first half are less than those for aluminium, in the second half they are greater, I have no hesitation in ascribing these divergencies to the influence of reflexion of the Volta effect and of unavoidable small changes in the apparatus, and I have no hope of producing more satisfactory evidence that there is no material difference in the quality of the delta rays emitted by different substances. It is true that the differences between the various materials is greater here than in the experiments described in the first two papers, which were not regarded as conclusive as to the identity of the rays, but the sources of error are now definitely known, whereas before it was uncertain whether the measurements gave any information at all as to the speed of the delta rays, 11. Accepting this conclusion some further questions require consideration. Bumstead [citation redacted] has suggested that the delta rays are emitted from an air film on the surface of the materials and not from the materials themselves, so that it is only natural that they [footer] 536 [header] should appear always of the same quality. It seems to me sufficient evidence against this view that different materials obviously reflect the rays differently: if the rays can penetrate the air film (from which doubtless some rays come) when they are incident on the material, surely some of the rays excited in the material must be able to emerge. It is just possible, however, that the differences which have been observed with different materials are caused only by a difference in the state of the air film on their surfaces. But I have never found that the most drastic treatment for removing air films has produced any considerable change in the form of the curve. Secondly, we may ask, What is the velocity of the delta rays? I think it is clear from these experiments that there is no definite velocity and that rays are emitted with widely different speeds ; some, probably about half of them, seem to have a velocity of less than 1 volt, while others seem to have speeds as great as 10 or 20 volts. It is to be remembered that the rays are emitted at all angles with the surfaces of the electrode, whereas it is only the component of their velocity perpendicular to the electrodes which is effective in carrying them across. But this fact is not sufficient to account for the apparent heterogeneity of the rays. It is easy to show that if the rays were emitted with the same speed of V volts, but in all directions, we should have (when V lies between and V ) [formula redacted] The curve should touch the axis of potential at V = 0, and should have a marked " knee " at the point where saturation is attained ; the actual curves exhibit neither of these features. On the other hand, heterogeneity of the rays may possibly be due to a loss of velocity by those rays which are not produced actually in the surface in passing through the intervening layer. Since the rays are completely stopped by the thinnest material layers obtainable, it seems impossible to test this hypothesis. 12. Some further experiments have been made with the apparatus described in the paper on " Ionization by Alpha Rays " (loc. cit.) on the current through a. layer of gas at low pressure contained between parallel electrodes and ionized by alpha rays. The electrodes were covered with aluminium ; in one series they were about [formula redacted] mm. apart, in [header] 537 another about [formula redacted] mm. ; the results given below apply equally to both series. In the former experiments measurements were never made with a potential difference less than that required to saturate the delta ray current when the vessel was completely exhausted. Accordingly the variations of current noted can never have been due to any effect of the field in stopping the delta rays from one electrode from reaching the other. It appeared that it might be possible to obtain some information as to the delta rays liberated in the gas, and not at the electrodes, by measurements with much smaller potential differences. Let us suppose that the delta rays are liberated uniformly throughout the gas by the alpha rays, that they are all projected from the atoms with the same velocity of V volts, but that their velocities are equally distributed in all directions ; let us suppose further that the pressure of the gas is so low that very few encounters occur between the delta rays and the molecules, and that the positive ions have initial velocities so low that they all travel to the negative electrode under the smallest potential differences with which we are concerned. Then a simple calculation will show that, if N delta rays and N positive ions are liberated per second in the gas, the relation between Y and the current i will be [formula redacted] if V is less than V , [formula redacted] if V is greater than V . It is to be noted that the tangents to the two portions of the curve are coincident at the point where they cut, so that the curve has no " knee/' but if the relation between i and V turned out to be that predicted an estimate of the value of V could be obtained. 13. If collisions between the delta rays and the molecules occurred, this theory would not be applicable. It was thought that, if the current for all P.D/s was found to be proportional to the pressure of the gas (and therefore to N) for all pressures under a certain limit, it might be assumed that the effects of such collisions were negligible. However, in the experiments on air it was found that the current for all P.D.'s between 1 and 15 volts was accurately proportional to the pressure, until a pressure of 4 mm. was reached with the smaller 538 [header] distance between the electrodes, or a pressure of 1*5 with the larger. At the highest of these pressures collisions between the rays and the molecules must have been frequent, for (1) calculation shows that the mean free path of the electrons is not large compared with the distance between the electrodes, and (2) when P.D.'s of some 50 volts were employed, unmistakable signs of ionization by collisions appeared. It is clear, therefore, that the measurements cannot be expected to conform to the theory given, but, though no explanation of them can be given as yet, they seem of sufficient interest to be mentioned. In fig. 4 the relation is plotted between the P.D. and the current expressed as a fraction of that which would be carried [figure redacted] by all the ions liberated by the alpha rays, i. e. [formula redacted]. The value of N was extrapolated from that found at higher pressures, when a true saturation current could be obtained and Was found to be accurately proportional to the pressure. The effect of the delta rays from the electrodes was eliminated in all cases by subtracting from the current at any given pressure that obtained with the same P.D. when the pressure Was zero. It will be seen that the curve is curiously like that obtained with much higher pressures and higher P.D.'s, where saturation is obtained over a certain range of P.D/s and the current subsequently increases as ionization by collision enters. But the " saturation " which is obtained in this case occurs when only about 0'7 of the ions produced reach the electrodes. [header] 539 14. When hydrogen was substituted for air, still more strange results were obtained. In this case, the current proved not to be proportional to the pressure for any finite range at low pressures. The addition of a very small quantity of gas to the previously exhausted vessel did not increase the current at all; in some cases it appears even to decrease it. But the most remarkable feature occurred when the relation between the current and the P.D. was examined for these low pressures. In Table III., column 1 gives the relation for a Table III. Current. P.D. [table redacted] pressure of 1*1 mm. and a distance between the electrodes of about 1-^ mm., column 2 gives the relation for a pressure zero, and column 3, which is the difference between 1 and 2, the relation for the part of the current which would be expected to be due to the gas only. It will be observed that for a certain range of low potentials the current seems to decrease with an increase of potential. No explanation is offered at present of this result, which is typical of those obtained with hydrogen, or of the results with air; they are recorded merely in order to show that there are many things connected with conduction through ionized gases which still wait to be explained. Summary and Conclusions. 2-4. It is shown that, in the case of a current carried by delta rays from metals, the relation between the current and the P.D. is independent of the temperature of the electrodes; also that the current is determined rather by the P.D. between the electrodes than by the electric field in the space between them. It is concluded that the delta rays must be emitted originally with a finite velocity and that the velocity which 540 [header] they have been found to possess is not dee, as was suggested in a former paper, to secondary causes. 5-11. Experiments with a new form of apparatus have proved to the satisfaction of the author that there is no difference in the velocities of delta rays emitted from different materials. The conclusion formerly announced that these Velocities are also independent of the velocity of the exciting alpha ray receives much more complete confirmation. Evidence is quoted against the view that the delta rays from all materials appear to be the same, because they come from a layer of air on the surface of those materials and not from the materials themselves. The same experiments show that the delta rays are very heterogeneous in velocity ; most of them appear to have Velocities less than 3 volts, but a very few may have velocities as high as 10 or 20 volts. It is possible, but not probable, that this heterogeneity is imposed on originally homogeneous rays by passage through the surface layers of the material from which they come. 11-13. The results of some measurements on the currents through air and hydrogen at low pressures and under low P*D/s are quoted. They exhibit remarkable features of which no explanation is offered. If the conclusions of this paper are accepted, the ivestigation enters on a new phase. If the delta rays are emitted with velocities which depend neither on the nature of the material from which they come nor on the velocity of the rays which excite them, on what do those velocities depend ? It seems that either we must imagine that they depend on some structure common to all materials, or on the nature of the helium atom of which all alpha rays consist. Some decision between the alternatives may be obtained by examining the process of ionization by agents other than alpha rays. Several recent investigators have spoken of the " delta rays liberated by Röntgen rays," but there appears to be no experimental evidence whether the electrons ejected from atoms by these agents possess finite velocities at all similar to those possessed by the electrons liberated by alpha rays. Endeavours will now be made to obtain such experimental evidence. The University of Leeds, May 1912. Note added Aug. 22.' — Experiments which will be described in the next number of the Phil. Mag. show that Röntgen rays liberated delta rays precisely similar to those liberated by alpha rays.