Technical Note of the Institute of Radium research No. 179, Vienna, 1925.
Proceedings of the Vienna Academy of Sciences, Mathematical and Physical Sciences Section, Section IIa, Volume 134, 1925.
Marietta Blau
(with 3 Figures)
It is desirable that the experiments on atomic disintegration, performed by G. Kirsch, H. Petterson, et al. at our institute, be further proven by other objective measurements besides the subjective scintillation counting, which is the only method used up until now. Therefore, I have performed several experiments to observe the possibly existing effect of H particles on photographic materials.
Based on the scintillation and ionization effects of H-rays shown by Bose and MacAulay, it was considered quite possible that their interaction with photographic material would exist. Muehlenstein mentioned, in his paper 'Les treces des particules', published in 1922, the possibility of a photographic detection of H-rays, but his experiments, which were not explicitly described in the paper, did not show any positive results.
The difficulty of the photographic detection of H-radiation lies in the small numbers of H-particles emitted even by a relatively intense 
radiation source. With the implied necessary long exposure times, not only are the influence of 
and 
radiation of the radiation source noticeable, but different effects also appear, mentioned in the literature as the Colson-Russell effect, metallic radiation, photo-activity, etc. In addition, after development every photographic plate shows the usual appearance of several black granules even with very careful treatment. This makes the recognition of very weak microscopic black spots on the plate more difficult.
Plate material
Different kinds of plates recommended in the literature were used in the following experiments. The best results are displayed by the 'photomechanical' plate of Jahr (Dresden), first used by Michl to obtain pictures of radiation. In the last of the present experiments, Lainer's positive photographic plates for bright slide pictures were substituted for the Jahr plates several times, showing advantages in several aspects. They possess even smaller granules, are insensitive to red light, which cannot be totally eliminated while treating the plates in the dark room, and reduce the effect of and radiation in comparison to radiation. The disadvantage of these plates is that the granules become more brown and transparent during the developing time, which has to be necessarily shorter than for pictures taken with natural light. This makes the observation of the granules under the microscope more difficult.
Probes
In the majority of the experiments, polonium probes which are created by electrolysis of radium free RaD solution are used as the radiation source. The polonium was electrochemically deposited on gold or platinum cathode strips with a width of 2 to 2.5 mm, which were dipped 4 to 6 mm deep into the solution. The emission of the probe strips varied about between 100 and 550 st. E. (measured at the surface).
The current setting at the electrolysis was chosen in such a way that only polonium but not RaD or RaE could be deposited on the strips. Only in a few cases could it not be avoided that a small amount of RaE appears simultaneously with polonium on the cathode. After a waiting period of 4 to 5 decay times of RaE the experiments were begun. All used probes were free of RaD as was determined from their and activity. The highest strength probe used, of about 550 st. E. per 8 mm2, was created through distillation within a stream of hydrogen. To avoid a possible enrichment of polonium hydride, the palladium strip was then oxidized within a oxygen stream. After the decay of RaE, all polonium probes showed a weak and activity, the transmission probability of which was not examined further on due to the smallness of the effect. This activity can be probably explained by the radiation of polonium, discovered by Russell and Chadwick, and the secondary radiation caused by the polonium's radiation interaction with the substrate of Au, Pt or Pd. The smallest effect appears while using the palladium, especially in respect to the huge activity of the probe. The activity level was 10 -5 st.E., measured with a pot-shaped set up (cylinder with a radius of 10 cm and a height of 18 cm) which included a Wulf electrometer. Finally, radon with 10 to 20 MC, encapsulated in capillary with thin walls and filled with paraffin, was used for a series of measurements, .
Experiments at parallel incidence
It was planned at the beginning of the experiment to run using the method
of Michl to obtain a representation of the photographic effects of radiation. In the case that the a particles strike the photographic plate nearly at parallel, a straight series of points are obtained which can be clearly observed under the microscope, and which represent the tracks of an particle. Following Michl, the length of these series is proportional to the path length of the particle traversing the gel of the plates. Employing the absorption charts of Mardsen, the energy loss of the H-particle in the material follows laws similar to those of particles. This gave us the hope of identifying the tracks of the H-particles, due to their longer path length, by observing the longer series of points on the photographic plate.
At all times paraffin was used as the H-particle radiation source because of its high hydrogen content. Thin cuts of 30 micro-units of paraffin were used, and mounted directly on the probe. In addition, absorption foils with an equivalent absorption distance of 4 cm in air were used, constructed from glowing (glimmer) or copper foil, in order to eliminate the direct influence of the radiation due to the polonium. In the beginning, the optimum condition for obtaining the H-particle tracks with parallel incidence was found by many pre-experiments, which resulted in the following set up, which was also used in principle by Michl.
The probe, enclosed in paraffin and absorption foils, was mounted on cardboard and covered by a thin iron plate with a gap of 0.5 mm width and 2.5 mm length (iron does not react with the photographic plate). Perpendicular to the probe, but parallel and right above the gap, the photographic plate was mounted, supported by an iron block. Opposite to the reacting side of the photographic plate, a glass plate with a spacing between the glass and the photographic plate of the thin cardboard was set up in the way that the radiation entered the space between the two plates. While this guaranteed that the incidence of the radiation was parallel to the plate, it yielded a poor rate of detected events.
A series of experiments demonstrated that glowing (Glimmer) foil was not useful as an absorption material. Under the same conditions, the photographic plate was stronger and more irregularly blackened than with copper foil with the same air equivalence. This can be probably explained by two different effects. First, the glowing foil in addition to being a source of soft, secondary radiation, has less ability for stopping a, b and g radiation. Second, it seems that glowing foils show direct luminescence, because the blackening could be partially reduced by covering the glowing foil with a sheet of aluminum or silver. Therefore, only copper or glowing foil combined with copper was chosen for the following experiments. Under these conditions and the right choice of exposure and developing time, it was possible to see several long series of points on the plate in addition to shorter ones, which could be distinguished from other randomly existing or constructed series of points by their angle of inclination into the photographic plate . The experiment was repeated several times under these same good conditions, but only a total of three clear pictures were obtained, because the plates were mostly damaged at the edges by both scratches and as a result of pressure on the plates. These three pictures, however, didn't shown the convincing clarity of Michl's radiation pictures, as could be expected due to the small numbers of events and the many perturbing influences. From the direction of the traces, showing a picture of the gap in total, the radiation could be identify as particle radiation. The possibility that these longer traces were produced by particles of polonium, transmitted through the paraffin and copper foil, can be excluded, because the air equivalence of paraffin was 3.2 cm and of copper was 3.9 cm, as was determined by measuring their weight along with calibration with a ThC source. (A long distance radiation of 6.1 cm R could be excluded with the selected absorption foils and, even so, the very existence of this radiation is doubtful after the latest experiments of Curie and Yamada). The used foils were examined carefully under the microscope for holes, and the probe along with its foil covers, were tested for scintillation, with the result that no holes could be found, even with a long observation time. Finally, the foils were tested for a contamination before the experiment using the photographic method, with longer exposure time than the duration of the actual experiment.
The length of the series of points can be directly compared with those of particles in Michl's experiment, because Michl used the same kind of plate material. For radiation due to polonium with a path length of 3.9 cm, Michl has found an average length of the series of points of 23 micro-lengths, containing 8 to 10 points each. The H-particles in this experiment have, after absorption in paraffin and copper, a maximum path length of about 9 cm. If they interact with the photographic plate following the same law as the radiation, a length of the series of points of 53 micro-lengths is expected. But, especially since the number of events of fast H-particles is small, one cannot expect to find this length with the small total number of events which are possible with this experimental set up. The longest series of points which was found was 46 micro-lengths, and it was composed of eleven points. In addition, a series of eleven points with a length of 44 micro lengths and about 5 series with a length above 30 micro-lengths were found. Most of the remaining series contains 4 to 7 points with a length of 10 to 20 micro-lengths. It is worth mentioning at this point that one of the series, with a length of roughly 30 micro-lengths, showed a change of its direction at the end of the trace.
Unfortunately, the number of results is too small to get any information about the ionization of H- and particles as a result of the measured ratio of number of points over total length for both types of particles. But it seems that the blackened granules lie closer together in the case of the particles than of the H-particles, which indicates the smaller ionization rate of the H-particle.
To increase the number of events, and to decrease the disturbance due the edge of the plate, the following set up was tested (see Fig. 1), which was used in a similar way by Muehlestein for displaying the traces of a particles. In Fig. 1, AA is a brass plate with an area of 3.6 x 3.6 cm2, and a thickness of 1 mm, in the middle of which a gap with a width of 0.75 mm and a length of 6 mm and an angle of 30 degrees was cut. BB indicates two holes where the rods of the probe base T are mounted. It is built out of a brass wedge, where two metal plates P1 and P2 can be screwed in on the side of the hypotenuse, also at an angle of 30 degrees. The probe strip can be clipped under the metal plates in such a way that the side of the probe is positioned directly over the gap. An iron-tin plate E with a thickness of 0.4 mm is soldered at the lower side of the plate with the tilted gap and has a quadratic hole cut out in the middle with a side length of 1 cm. The slope of the gap was chosen to be 30 degrees because the tracks of particles, which hit the photographic plate at this angle, can be easily identify as a series of points. The inclination of the probe was set equal to those of the gap to obtain the highest amount of fast H particles. On the gap a gauged and hole-free copper plate was placed, with an air equivalence of 3.9 cm, and a layer of paraffin on top of it acting as the radiation source (with an air equivalence of 3.2 cm). For several measurements, a glowing foil with an air equivalence of 2 cm was placed between the copper and the paraffin to make sure that all the radiation of the polonium was absorbed. In this way the characteristic copper radiation was eliminated as a source of error.
This set up produced quite satisfying pictures. Starting from a more blackened strip, which are indicative of particles of perpendicular incidence, lines of short series were seen in the forward direction. The length of the series of points are short (on average 4 to 7 points), which can be explained by the enormously reduced path length of the H radiation which is transmitted through the copper at an angle. The series are indeed significant, and can be easily distinguished from other series which appear randomly on the photographic plate under the influence of light, because the trajectory can be followed point-by-point deeper into the plate. An advantage of these pictures over to the ones discussed above is found in the large total number of series of points. But it must be mentioned that even these pictures didn't show the clarity of those obtained with radiation. With long exposure times - up to 144 hours - even the smallest background of and g radiation was noticeable. Indeed, we obtained the best pictures using the probe made of palladium plated on polonium, which at highest activity showed the smallest transmitted radiation, as was mentioned above.
Finally, several experiments with RaC as primary radiation source were performed to get yield even longer tracks of the H particles on pictures (the probes were similar to emanation capillaries, with inner walls was covered by paraffin). The H particles which are produced by RaC have a mean path length in air of 28 cm, while those due to polonium reach only 16 cm. The photographic plate was protected against the intense and radiation by a massive lead plate of 3 mm thickness, to which the H particles are bent by a well chosen magnetic field and, in addition, by lead wedges and the pole of the magnet itself. To avoid secondary radiation, the poles of the magnet were covered with cardboard, in addition to the lead facing the plate with copper and cardboard. Even so, the experiment failed, because, due to the extremely long exposure time, which had to be chosen to get a reasonable amount of traces, the transmitted radiation produced too large a "veil" to unambiguously recognize the series of points.
Experiments with Perpendicular Incidence
The failure of the last experiment aside, all the pictures indicate a photographic effect of H radiation. The observed series of points, and the picture by itself can only be explained by the effects of particle radiation, which can only be caused by H-particles, as radiation is excluded with a high level of certainty. To explain the results as due to radiation is also unlikely, because such an effect has not yet been observed. It was necessary, however, to examine its possible influence, especially in the radiation due to RaE, because the probes can be polluted by RaE, as mentioned above. This radiation seems to be the main source of primary particles. It was not possible to bend the particles with a magnet, because with this experimental set up H particle could hardly be detected. Therefore, an attempt was made to obtain some information about the probably existing radiation by absorption experiments. For these further results, in experiments with perpendicular motion of the particles, the following set up was used (see Fig. 2). The probe holder is solidly connected to a brass plate of 3 mm thickness and is able to grasp the probe between two strips of cardboard. In the middle of the brass plate, there is a gap 1 mm thick and 6 mm long, in which the probe is mounted. The plate fits exactly into the brass frame R1 where, at the bottom, a 0.4 mm thick iron plate is soldered. This plate has also a gap of 2 mm thick and 4 mm long gap, which is inline with the gap in the brass plate. R1 again sits in a second frame R2, which carries two 0.4 mm iron plates. The upper plate has a central hole of 2 cm2 cut, and a lower one of 1 cm2. Finally R2 fits into R3, in the same way as the two iron plates are mounted. Before the experiments, a copper foil of 3.9 cm air equivalence, calibrated with Thorium C, a glowing foil with 2 cm air equivalence and 30 microns of paraffin was put into the cut out region of R2.
The cut out region of R3 was used for another absorption filter, whose air equivalence was enough to absorb all the H particles. A silver foil was chosen as an absorption filter, because silver doesn't react with the photographic plate even with long exposure times. The air equivalence of the Ag foil was 10 cm, so that the H particle have to transmit, with the copper and glowing foil, through a total of 16 cm air equivalence. Pictures were taken with and without the silver foil under the same conditions, to exclude variations due to different developing conditions. The exposure time was chosen in such a way that, without the silver foil, the blackening of the picture can be seen by eye. With silver foil and the same exposure time, a visual blackening didn't appear, and even under a microscope it was not definitively observed.
Under the assumption that the number of black granules is proportional to the intensity of a particle radiation perpendicular to the plate, the radiation should be noticeable by comparison of both exposed areas, in spite of the uncertainty of this method, the intensity of which is about 20% due to the non-absorbed radiation. In the case that all of the blackening is caused by the radiation of RaE, only up to 50% of the radiation is lost in the silver foil, and a weak blackening has been observed with the silver foil in place. Therefore, it is certain that the total effect of the photographic effect is not produced entirely by the radiation of RaE. In addition, it is worthwhile to discuss how far secondary radiation are involved in the blackening process. For the reason that the radiation is produced by the radiation of the polonium and the emitted radiation of the Au and Pt/Pd layer caused by particles, however, it is difficult to get this information by absorption experiments.
The final proof of whether the blackening is in fact caused by the H particles and not by the secondary b radiation, can be given by an experiment, where under the same conditions the paraffin is substituted with a substance which does not at all, only slightly, radiates H-particles. Heavy metals are principally excluded from consideration as this comparison substance, because they emit x-rays when they are struck by a particles, and therefore introduce a source of error. The same experimental conditions are mostly obtained when a material is chosen which contains, in a similar manner as paraffin, only hydrogen and carbon: naturally hydrogen in a small amount, with a as the optimum choice pure carbon. In respect to this argument, soot was chosen as the comparison substance. Layers of soot were condensed quite homogeneously on glowing foils with 2 cm air equivalence by a using a turpentine flame on an area of about 1 cm2. The thickness of the layer was determined by weighing with a scale, and the weight per unit area was chosen equivalent to paraffin with the same area, about 6/7 of the paraffin weight per unit area. The absolute equivalence of the experimental conditions was given by the stable and well centered set up seen in Figs. 1 and 2. Indeed, only the paraffin with the glowing foil was exchanged with the soot containing an equivalent amount of carbon on a glowing foil with the same thickness (exact up to 1 mm air equivalent). As a special precaution, the order of placement of the paraffin and the soot was varied.
Of course the comparison experiments were done using the same plates. The total number of these experiments was nine, but two failed, because the glowing foil broke during insertion into the frame. In the other seven pictures more intense blackening of the paraffin in respect to soot is visible even to the naked eye. Fig. 3 shows a reproduction of one of the pictures with a soot layer of 3.1 mg/cm2, a glowing foil of 1.59 cm air equivalent, and a paraffin layer of 3.6 mg/cm2 and glowing foil with 1.6 cm air equivalent.
The remaining blackening can be caused by the radiation of RaE, by radiation and secondary radiation. Like the pictures with parallel incidence,
which may also indicate similar traces of H-particles, it seems clear that some H-particles were also detected, which can be probably explained by the fact that the soot may be polluted in some way. Even after the disintegration of carbon atoms, the H radiation may not be excluded, as the experiments of Petterson have indicated. The higher intensity of the blackening on the paraffin pictures can be only explained by the H-radiation emitted within the paraffin. Under the assumption that delta radiation is produced more copiously in paraffin than in the comparison soot layer, it does not affect the photographic plate because delta radiation with a kinetic energy larger than 100 eV was never observed, and therefore should be totally absorbed by the glowing and copper foils. But, the energy of the secondary radiation, which is produced by primary and radiation, is mainly independent of the material, and the intensity is proportional to the absorbed primary radiation, and therefore equal for paraffin and soot. All other factors are the same for both paraffin and soot, and so the experiment seems to be the proof that the photographic effect of interest is due to H-radiation.
Quantitative results about the intensity of H-radiation, etc. could not be obtained with this experimental set up, which is only a qualitative proof of H-radiation. Finally, it is mentioned that some experiments were done with hydrogen used as the H radiation source. The hydrogen was deposited on a palladium plate with 9 micron thickness using water electrolysis. Because the hydrogen layer on the palladium plate also blackens the unexposed photographic plate, a foil of copper or aluminum was also placed over the plate. It was planned to examine the H-radiation by comparing experiments performed with hydrogen deposited on palladium with those performed with plain palladium, baked out in vacuum to purify the surface. However, an increasing intensity of the blackening was seen under the same conditions upon repetition of the experiment, a result which can be explained by pollution of radiation, which was transmitted from the probe through the palladium plate. Whether this pollution is due to a diffusion of polonium hydride, produced by the contact of polonium and free hydrogen, or whether it appears due to a break-up of the palladium, as it is observed with the process of depositing the hydrogen on the palladium, cannot be clarified at present.
Conclusions
To make connection with the experiments concerning atomic disintegration, performed by G. Kirsch, H. Petterson, et al. in this institute, it is desirable to extend the subjective, and so far only successful method of scintillations counting, to other, objective methods of observation. With this goal in mind, the method of photographic detection of H-particles was developed, beginning with the experiments in using the methods of Michl with particles, to obtain pictures of H-particles, which struck the photographic plate with parallel incidence. As with the particles, the results were series of points clearly defining a direction, where each series corresponds to the track of an H-particle. With the aid of absorption experiments and of comparison experiments where the H-radiation source of paraffin was replaced by a layer of soot with an equivalent content of carbon, it was shown that the photographic material blackening can only explained by the effect of H-particles and not by possibly existing radiation.
Footnotes:
-Strahlen von Polonium in der Gelatine derselben Plattensorte den Wert 27-7 p.
Figures:
Figure 1
Figure 2
Figure 3