XI. Secondary Radiation from a Plate exposed to Rays from Radium. By A. Stanley Mackenzie, Ph.D., Munro Professor of Physics, Dalhousie University, Halifax, N.S. [Communicated by the Author.] The following experiments were made to examine more carefully than has been done the secondary radiation from the back side of a plate bombarded by the rays from radium, and to see what light a comparison of this radiation with that from the front side would throw on the mechanism involved in the production of secondar[gamma] rays ; and, further, to see what evidence it would give of the secondary radiation of penetrating type, of which other experiments seem to show the existence. In order to restrict to a definite bundle the beam of radium rays employed, and to shield as far as possible the rest of the apparatus from rays leaving the radium in other directions, the radium was put at the apex of a conical opening of 19° half-angle in a massive block of lead. The accompanying diagram (fig. 1) will make the arrangement clear. A hole was bored through the lead block along the axis of the cone, and the radium (5 mg. in a glass tube) was put in its place by being inserted in the end of a brass rod which fitted the hole. The lead block had a circular cross-section of 10*7 cm. diameter, and its greatest length was 16 cm. As the radium was inserted to a depth of 7*7 cm. in the block, no rays could emerge from it without passing through at least 5 cm. of [header] 177 lead, except through the conical opening. The shape of the block was so arranged, with a corner sawed off at a, and a prolongation added at b, that no secondary radiation from the sides of the conical opening, nor from the absorbing plates A, nor from the surfaces at that end of the block, [figure redacted] could enter the ionization-cell I, as the direction of the dotted line ad will show. A channel c was cut in the protuberance b to hold the ends of the plates A, which were used as absorbing layers to cut down the primary radiation. A conical frustum of lead C (shown in position in the diagram) could be inserted in the conical opening when still further reduction of the beam was required. The height of the frustum used was 1*18 cm. The axis of the block made an angle of 34° with the horizontal. The intensity of the secondary radiation was measured by allowing the rays to enter an ionization-cell I which was connected with a Wilson tilted-electroscope. This cell was a brass cylinder of 24*2 cm. and 7*2 cm. diameter, having a central wire insulated by sulphur with an earthed guard-ring. The mouth of the cell was covered by thinnest aluminium-leaf. The central wire was connected to the gold-leaf of the electroscope, which was situated about a metre distant, and the connecting wire was surrounded by an earthed metallic conductor extending from the earthed guard-ring to the box of the electroscope. The cell I and [footer] 178 [header] the plate of the electroscope were brought to the required high potential by connecting with one pole of a battery of small storage-cells, the other pole of which was earthed, in the usual way. The axis of the cylinder made an angle of 23° with the horizontal. The radiation entering the ionization-cell could be absorbed by screens S placed against the aluminium end of the cell. The position and inclination of the cylinder were so arranged that the radium lay in the plane of the aluminium-leaf, as shown by the dotted line cd : this is to reduce to a minimum the secondary radiation set up in the screen S by the [gamma] rays from the radium, and was sufficient to make any leak due to this cause so small as to be negligible. When a plate was to be bombarded it was put either in position R or in position T. Position R was such as to make equal angles with the central axes of the lead block and the ionization-cylinder, in which position the reflected radiation entering the cell was found to be a maximum. It is inclined toward the lead block about 10° from the vertical. The distance from the radium to the plate measured in the line of the axis of the block was about 13 cm., and from the aluminium-leaf to the plate, measured along the axis of the cylinder, about 6 cms. Position T was horizontal, and the lower surface of the plate 8 mm. below the corner of the block b. The distances from radium and aluminium, measured as before, were 18 and 12 cm, respectively. The plates were 30 x 40 cm. in area, and of various thicknesses. It was found that the same effect was produced by a single plate of given thickness as by a pile of plates of the same aggregate thickness. The secondary radiation from the front side of a plate has been carefully investigated by Eve [citation redacted], McClelland [citation redacted], Allen [citation redacted], and others, and the chief properties of these rays are now beyond doubt. The interpretation of the results of experiments of this kind is very difficult on account of the complex character of the radiations involved, and what seems to be a penetrating radiation may be easily confused with one which is only of a tertiary nature, and so on. It was hoped that in the apparatus as here set up the various radiations could be kept fairly well distinct. The stream of radiation which enters the cylinder, especially when either the reflecting plate R the transmitting plate T is used, is made up of many components, consisting of (1) [gamma] rays direct from radium, and the secondar[gamma] rays they [header] 179 produce ; (2) the secondar[gamma] rays sent out from the air, due to the passage through it of the primar[gamma] rays ; (3) the secondar[gamma] rays from the lead ; (4) the air-rays produced by (3) ; and (5) various tertiar[gamma] rays, &c. The first of these is always present, and with the so-called spontaneous ionization constitutes an invariable amount of leak, which is always added to that from other sources. The second group was first studied, that is, the rays from air due to the passage through it of [beta] and [gamma] rays. A few numbers are given in Table I. to show the power of the primar[gamma] rays of different penetrability to produce these air-rays. The numbers denote the rate of leak as observed by the movement of the gold-leaf of the electroscope. In order to preserve similarity of conditions and ensure greater accuracy, the numbers actually observed were for the time in seconds required for the leaf to move over 20 scale-divisions, and these numbers divided by 20 are added in brackets to give a general idea of the actual magnitude of the times involved. The leak as given is 100 times the reciprocal of the above time, or the number of scale-divisions passed over by the leaf in 100 seconds. The leak is given for the full unobstructed beam, and also for the rays left when various plates of lead A (and C) are interposed in the path of the beam, over a large range of thicknesses from '02 to 15*4 mm. The method was susceptible of an accuracy of 1/2 of 1 per cent., and where necessary to interpretation the numbers can be relied on to that decree ; in general an accuracy of 1 per cent, may be claimed for them. Table I. Thickness of lead plate A in mm. Time in sec. for 1 scale-div. Leak - no. of scale-div. In 100 sec. [table redacted] The leak 1'89 for an absorbing layer A of 15"4 mm. is the smallest noted during the course of the experiments, and is useful as an index when asking whether the leak is as small in any given circumstances as there is a possibility of its being. From the table it is seen that, although the easily absorbable [footer] 180 [header] rays are the most effective in producing these air-rays, there is an appreciable amount of radiation set up by the [gamma] rays, or, at least, set up after a layer of lead is interposed sufficient to cut out all the [beta] rays ; equal to several per cent, of the maximum. It will, however, be shown later that there is a large radiation sent out from the side of the lead plate which is not bombarded, what we may call for convenience "transmitted" rays; and although the apparatus is so arranged that these transmitted rays cannot directly enter the ionization-chamber, yet they may produce air-rays to an unknown extent, and it is impossible now to estimate how much of the leak which we observe when a plate say of 6 mm. is at A is due to the [gamma] rays which strike the air after getting through the plate, and how much is due to the rays which are set up in the lead and then strike the air. If these numbers are plotted, it will be seen that the law of absorption over the whole range is very far from being an exponential one, being too steep at first and then too flat. But before trying to find an equation to the curve, it would be necessary to subtract from the numbers given the leak which is always present due to spontaneous ionization and the direct [gamma] rays and their secondary effects. This I have not found a way of estimating with any accuracy. The leak when the radium is out of range and out of the room (a room never contaminated by having had open radium in it) is small) being about '21 on the scale adopted. In order to test the type of these rays as to penetrability, the leak was observed with lead screens S of various thicknesses over the aluminium end of the cylinder. The results are found in the second column of Table II. It will be seen that these air-rays are mainly of the easily absorbable kind, and that about two-thirds of the whole is absorbed by lead-foil '02 mm. in thickness, and five-sixths by 1/4 mm. of lead. One of the most interesting results is that the remaining one-sixth is of a seemingly very penetrating type, like [gamma] rays, the leak through 11 mm. of lead being only about 3 per cent, less than through ^ mm. The question as to whether these are [gamma] rays or not will be considered later. By referring to Table I. it will be seen that as a rough statement we may say that the proportion of the beam of secondar[gamma] rays of any given penetrability is about the same in order of magnitude as the effectiveness of primar[gamma] rays of that penetrability in producing secondary air-rays. These results are in the main in agreement with those found by others. [header] 181 Thickness of screen S in mm Thickness of plate A Giving the leak when air is exposed to [beta] + [gamma] rays [gamma] rays [beta] rays Thickness of plate A Diving the leak when plate R 1.8 mm is exposed to [beta] + [gamma] rays [gamma] rays [beta] rays [table redacted] In these observations both [beta] and [gamma] rays were present to affect the air, and in order to determine whether both soft and penetrating rays were produced by both [beta] and [gamma] rays, another set of observations was taken with the [gamma] rays only failing upon the air. To be perfectly certain that no [beta] rays would be present the cone C of 11*8 mm. of lead was put in the opening, and for fear the chance of its not fitting exactly might allow some stray [beta] rays to emerge, a plate of 3*6 mm. of lead was put at A. The total, 15'4 mm., is more than enough to ensure the passage of nothing but [gamma] rays. The results are given in the third column, and the differences of the second and third columns will show the leaks due only to the [beta] rays ; these are found in the fourth column. It will be seen that the radiations set up in air by [beta] rays are practically all of the soft kind and are absorbed by 1 mm. of lead. To return to the observations on the effects of the [gamma] rays, as given in column 3, which are of considerable interest. The numbers are at first sight rather surprising and anomalous. With no screen the rate of leak is 1*89 ; when a screen S is put in the path of the rays we expect a reduction of the leak due to absorption of the radiation by the screen; but we find that a layer of ^mm. thickness increases the leak to 1*91, and that the leak increases with increasing thickness of screen, until for 1 mm. the leak is 1*96, nearly 4 per cent, greater than with no screen. For still greater thickness of screen the leak decreases again, but even after a plate 182 [header] 11 mm. thick is put before the ionization-chamber the leak is still 1*92, or nearly 2 per cent, greater than without any plate. In other words, you can increase the leak in a vessel in certain circumstances by merely making the walls thicker. I£ the rays here approaching the cylinder are of an absorbable type, we could explain this phenomenon by saying that when they meet the plate they start up both a radiation of similar type as well as a very penetrating type, the sum o£ the two being a maximum for a lead plate 1 mm. thick, more only acting as an absorber. If they are of the [gamma] type, we could say that they themselves pass easily through the thickest screen used, and at the same time start up a secondary radiation in the cylinder of ionizing activity more than enough to make up for the absorption of the original rays. We shall have evidence later of the existence of both these phenomena, but the latter explanation is probably the main one. In order to put this peculiar behaviour to a further test, a plate of 6 mm. thickness was put at A instead of the cone and plate used before ; this must absorb all but a few of the very fastest [beta] rays and let through a quantity of [gamma] radiation cut out before. The results were similar to those just described, but the changes were not so great. The leak with no screen at S was 1'92 ; with S^mm. thick the leak was not measurably different from that with no screen ; with S '225 mm. the leak increased to 1*96 ; and later decreased again. The proportion of less penetrable secondary air-rays due to the impact of the less penetrable primary rays has begun to mask the effect due to the rays coming from the impact of the very penetrable ones ; and there must be a certain thickness of plate A for which the phenomenon in question would just disappear. It is not present when the whole beam is used. Further discussion of this behaviour is reserved until experiments on "transmitted" rays are described. Finally, with regard to the data of columns 2 and 3, we may say that the [gamma] rays passing through air produce no great amount of radiation except the peculiar kind just described, and that practically all the absorbable radiation produced is by the [beta] rays, and that it cannot penetrate more than 1 mm. of lead ; the leak shown in column 2 for thicknesses greater than 1 mm. is that due to the [gamma] rays referred to, since the numbers are the same as the corresponding ones in column 3 within the limits of accuracy of the experiments. Before giving the results obtained from investigating the secondary rays reflected from varying thicknesses of lead bombarded by the primary rays, the proportion of the [header] 183 secondary beam from lead due to the [beta] and to the [gamma] rays and the penetrating powers in each case may be referred to and compared with the corresponding results for air just described. These results are given in the last three columns of Table II. for a reflector of thickness 1*8 mm., which was found to be more than sufficient to produce the maximum effect. In the first place, the [beta] rays produce a greater proportion of penetrating radiation from lead than from air ; for whereas in the case of air only one-fifth of the stream of rays gets through ^ mm. of lead, now one-half of it does so. As before, the leak does not fall to the possible minimum of 1*89 for an absorbing screen of 11 mm., but after the thickness of S reaches beyond 1 mm. the leak remains in the neighbourhood of 1*95 for the [gamma] rays, and "06 for the [beta] rays. Another obvious result is that the [gamma] rays now produce considerable absorbable radiation, sufficient to mask entirely the peculiar effect noticed in the case of air. The final leak after 11 mm. of lead is interposed is 1"91, and is diminishing very slowly with increasing thickness of lead, and is nearly 3 per cent, greater than that we know to be a possible minimum. The last column of the table shows that the secondary rays set up by [beta] rays are still effective in the case of the greatest thickness of screen S used, that is, are able to produce by striking lead rays of the highly penetrating kind. Secondary Radiation from the Front Surface of a Lead Plate. "Reflected Rays." The plate was put in position R, of the diagram,, and various thicknesses were used in order to find when the intensity of the reflected beam reached a maximum. The position of the front surface of each plate was always the same. Observations were made with no absorbing plate at A, and with 15"4 mm. of lead at A, and their differences taken, as before. Table III. contains the results. Table III. Thickness of reflector in mm No plate at A 15.4 mm lead at A Difference [table redacted] 184 [header] From this we see that the [beta] rays cease increasing their effect after a thickness of lead of about 1/4 mm. is reached. But for the [gamma] rays it requires more than 6 or 7 mm. before the limit is reached. An important conclusion from this is that some of the secondary rays from lead produced by [gamma] rays passing into it are of a highly penetrating character and can return through 6 or 7 mm. of lead. Moreover, we saw before from Table II. that the [gamma] rays produce comparatively little absorbable radiation as compared with that from the [beta] rays : so that it seems that a large part of secondary radiation is always of a similar type to the primary. Curves A and B of fig. 2 show the growth of the secondary radiation with [figure redacted] A. Reflected radiation from [beta] rays. B. Reflected radiation from [gamma] rays. C. Transmitted radiation from [beta] rays. D. Transmitted-radiation from [gamma] rays. thickness for the [beta] and [gamma] rays respectively. They are drawn to different scales, but it will be seen that they follow the same general law. They suggest the equation [formula redacted] but the rise is too steep and they flatten too quickly after passing the knee. [header] 185 Secondary Radiation from the Bach Surface of a Lead Plate. "Transmitted Rays." The plate was put in position T of the diagram. The position of the bottom surface of each plate was always the same. The results are in the following Table. Table IT. Thickness of reflector T in mm. Leak when exposed No plate at A. 15.4 mm. Lead at A Difference 1.8 mm lead at A .225 mm. Lead at A [table redacted] An examination of these numbers discovers a rather surprising result. The [beta] rays which ceased giving "reflected" rays after the thickness of lead traversed reached ^ mm., now give "transmitted" rays up to the limiting thickness used, 15*6 mm., and the intensity even for this thickness is relatively great. The result was so unexpected that it was considered it must be due to the more absorbable [gamma] rays cut out by the 15*4: mm. of lead. To test this some measurements were made with a thickness of the absorbing plate A only 1*8 mm., which would keep back only the more absorbable [beta] rays. The results are appended to Table IV., and show that these [beta] rays which had been cut out must give a very decided stream of "transmitted" rays. For instance, with a plate T of thickness 9*2 mm. the leak when all the [beta] and [gamma] rays struck the plate was 2*38, and when the rays which could not pass through 1*8 mm. of lead were cut out the leak was only 2"31, showing that a leak of '07 was due to the easily absorbable [beta] rays. That is, the rays which cannot pass through 1*8 mm. of lead will yet start up rays from the far side of a plate of lead 9*2 mm. thick, when they strike the near side of it. Even more astonishing is the evidence of the last row of numbers in Table IV., which shows that even the softest [beta] rays (those totally absorbed by 186 Prof. A. Stanley Mackenzie on Secondary Radiation \ mm. of lead) cause these transmitted rays in a 9 - 2 mm. plate. Since the reflected rays due to incidence of [beta] rays do not increase after 1/4 mm. of lead is used, one would expect the transmitted rays due to the same [beta] rays to cease increasing after about double that amount ; but there seems to be no limit of even that order of magnitude. Another important result is shown by the numbers of the second row of the Table, the "transmitted" [gamma] ray leak. For the smallest thickness used, lead-foil ^ mm. thick, the leak is 2'05 ; as the thickness is increased the leak increases instead of decreasing, as was the case for [beta] rays, and as one might at first sight expect. The leak reaches a maximum for a thickness of about f mm. of lead and thereafter decreases steadily, but remains large for a thickness of even 15*6 mm. These very penetrating [gamma] rays are evidently not very largely absorbed by ^ - mm. of lead, and it requires many times that thickness before their maximum effect is brought out. The existence of this maximum for the [gamma] ray leak suggests that there should be a similar one for the [beta] ray leak. The values were accordingly plotted, and are found in fig. 2 as curves D and C respectively. The shape of the curve C leads one to expect that there is a maximum for a thickness of about 1/50 mm., and I have drawn that part of the curve on that conjecture. It would then be very like the curve whose equation is [formula redacted] . Referring back to the discussion of the results in Table II., it will be seen that the anomalous behaviour of the air-rays due to [gamma] rays with increasing thickness of screen S there observed, is precisely the same as that we have just been considering ; in that case also the leak increased with increasing thickness of screen up to 1 mm. and then decreased, but very slowly, even after a thickness of 11 mm. was used. This gives further evidence that we were there dealing with [gamma] rays entering the ionization-chamber, and hence that these [gamma] rays were made by [gamma] rays striking air. In the case of lead, Table II. shows that [beta] rays also can set up these secondary [gamma] rays. It remains to account for the radiation from the back of a thick lead plate the front surface of which is struck by easily absorbable primary [beta] rays. It is usually stated that these primary rays are practically all absorbed by 2 mm. of lead, and we might have expected some little effect from the back of a 4 mm. plate ; but instead we find a relatively very large effect with 10 and 15 mm. It seems reasonable to [header] 187 conclude that the process is not a single action, that the primary ray is not simply deflected, nor simply absorbed (consider, for instance, the production of [beta] rays by [gamma] rays, and the reverse); but that the mechanical process is something more like a convective transference o£ energy, in a manner suggested by electrolytic convection. As a working hypothesis one might say that the atoms which are bombarded absorb energy until internal instability is reached, and then a sort of explosion takes place with the expulsion of electrons and [gamma] rays ; these in turn provide energy to be absorbed by the next layer ; and so on through the plate. Since the limiting layer required for maximum "reflected" rays at the angle here employed is small compared with the maximum layer capable of giving "transmitted" rays, we would have to assume that the propagation of the energy obeyed a law somewhat similar to that known for the propagation of wave-energy, a maximum in the direction of and a minimum in the opposite direction to the stream of entering energy. The primary beam of rays would produce, as it were, an E.M.F. in the direction of the beam. The relatively large intensity of these "transmitted" rays would seem to have a bearing on the method of investigating spontaneous ionization. The general results of this paper are in agreement with those of Cooke and others on the influence of the secondary rays due to the radiations coming from outside the ionization-vessel ; bat they do not seem to fall in with some of the observations of Campbell [citation redacted] on the influence of plates of ordinary materials placed before the thin aluminium side of his testing-vessel. The method of measuring the absorption of a beam of radiation caused by a plate of given material, by noting the change of ionization in a vessel when different thicknesses of that material are put in the path of the beam, gives not the absorbing power of the material, at least in the sense one is apt to associate with the term, but the difference between what it absorbs and what it radiates ; and as the latter quantity can be quite large, the method is open to objections. Physical Laboratory, Dalhousie University. May 9th, 1907.