Photographic Plates with Thick Emulsion Layers

Recently, reports have appeared in the literature of a number of discoveries made as a result of using, in experiments connected with nuclear processes, photographic plates coated with a new thick-layer emulsion. Thus Fowler, Burrows, and Currie discovered the disintegration of a nitrogen nucleus into 4 α-particles according to the formula: \(N^{14} + H^2 \to 4He^4\); Tsien San-Tsiang, Ho Tsei-Wei, Chastel, and Vigneron (Joliot’s laboratory)\(^{2,3}\) observed the third and fourth fragments in the fission of uranium. Finally, Powell, Occhialini, and others\(^{4}\) discovered a heavy meson with mass \(m_\mu = 400\,m_e\) and its transformation into a light meson with mass \(m_\mu = 200\,m_e\), as well as a number of nuclear disintegrations under the action of mesons.

It will therefore be of interest to dwell on the properties of these plates, as described in the paper under review*).

As is known, in order to determine the energy of a particle and its mass it is sufficient to measure the length of the track which it traces while passing through a photographic emulsion. The length of the track is taken to be the distance between the first and last grain in the series of grains making up the track. Ordinary emulsions, however, have the disadvantage that there are large gaps between the grains, as a result of which the measured track lengths for groups of identical particles turn out not to be equal to one another, but fluctuate considerably about some mean value. On the other hand, the particle trajectories themselves, owing to energy loss in collisions with atoms along their path, are not identical either, but obey a statistical law of fluctuations. The magnitude of the fluctuations in this case is 1% for an α-particle possessing an energy of 5 MeV and 2% for a proton of the same energy. In an ordinary emulsion, however, the ambiguity in determining track lengths is considerably greater than the ambiguity caused by fluctuations. Thus, the half-width of the maximum of energy dispersion of a homogeneous group of particles is 0.5 MeV for α-particles and 0.2 MeV for protons, remaining almost constant for energies between 2 and 10 MeV. It is quite obvious that if it were possible to increase the number of developed grains on the track of a particle, this would considerably improve the results obtained in work with plates. Another shortcoming of the old emulsions is the fact that the presence

*) Powell and Occhialini, Journ. Scient. Instr. 23, 5 (1946).

PHOTOGRAPHIC PLATES WITH THICK EMULSION LAYERS

of background caused by γ-rays makes the work difficult and requires highly qualified personnel.

As early as 1939 Powell made an attempt to create a new type of emulsion by doubling the density of silver bromide grains in comparison with emulsions of the old type. This attempt did not yield any noticeable improvement in the quality of the plates. Further work was suspended by the war. In 1945 the work was resumed, and as a result a new emulsion was created, containing eight times more silver halide than ordinary emulsions. With plates coated with this emulsion a number of experiments were carried out. The photographs obtained showed that the background had become weak and that the density of the grains was quite sufficient for the track to be recognized not only at its end, where the ionization is large, but also at its beginning. The mean distance between emulsion grains (reduced to the air equivalent) is 2 mm, the maximum distance is 8 mm. Calculation of the energy of the particles shows that the maximum of scattering has become considerably narrower, and the resolution of the maxima of particles of different mass has become better. (Owing to the fact that the uncertainty in determining the energy, caused by the presence of gaps between the grains in the track, has become much smaller.) For example, for a proton having an energy equal to 12 MeV, the error in determining the energy is approximately 0.3%, and this emulsion proves to be quite suitable for work with particles of such energy. The lower the energy, however, the more noticeable the errors in measuring the track length are, and for work with particles of energy 0.8–2 MeV further improvements of the emulsion are desirable.

A number of remarks must be made concerning work with the emulsion. It is often important to determine exactly the direction of the track. In doing so it must be taken into account that, after the chemical processing to which it is subjected, the emulsion undergoes a certain shrinkage, resulting from the removal of part of the silver bromide during fixing. In old emulsions this change in thickness was approximately 10%. In the new emulsion the percentage of silver bromide in the gelatin is considerably larger and, according to the measurements of the authors of the article, the shrinkage increased to 43%. This value must be corrected somewhat if the refractive index of the emulsion is taken into account. It is small and is approximately equal to the refractive index of oil in immersion objectives. The corrected value of the shrinkage is in good agreement with that calculated on the assumption that the removal of silver halide during fixing should lead to the filling of voids with gelatin.

Of essential importance is the possibility of distinguishing proton tracks from α-particle tracks by differences in track density. This was successfully done by desensitizing the plates before development. For this purpose the plate was dipped for several minutes into a solution of “Desensitol” or some other composition that reduces sensitivity. It was found that as a result of this operation the tracks of α-particles remain almost unchanged, while the grain density of proton tracks is considerably reduced. For example, in a plate containing proton tracks 10 cm long (air equivalent), the distance between grains increased from 1.3 to 3 mm. This makes proton tracks readily distinguishable from α-particle tracks. Good results were obtained when a 2% solution of chromic acid was used as the desensitizing composition. By varying the compositions, one can obtain a reduction of sensitivity also with respect to other particles, as well as to γ-radiation, while retaining sensitivity to particles that have high ionization losses per unit path. The authors point out that an analogous result can also be achieved by changing the developing technique, and refer to the works of Perfilov⁵, who completely eliminated the background of α-particles by using the oxidizing action of K₃Fe(CN)₆.

The increased density of grains in the new emulsion makes it possible to apply the method of bathing the plate in solutions of salts of various elements, with the aim of adding to the emulsion the desired substance without fear of seriously impairing the quality of the tracks of ionizing particles. Such attempts, made with the old emulsion in 1938–1939, were not successful. Repeated in 1945 with the new emulsion, they gave good results. The plate was wetted in a saturated solution of thorium nitrate and, after 50 minutes, was washed and developed. Examination of the developed plate showed the presence on it of several clear tracks of $\alpha$-particles, produced as a result of the decay of ThC′. When the emulsion swells, the distance between the grains increases by approximately a factor of 10, and with emulsions of the former types, when exposed in the wet state, it was not possible to detect tracks. Similar experiments with protons could not be carried out, but experiments with $\alpha$-particles were so convincing that the authors express confidence in the possibility of repeating them for protons as well.

V. Fëdorov

CITED LITERATURE

1. Fowler, Berrous and Kerry, Nature 159, 569, 1947.
2. Tsin San Tsiang, Ho Zah Weg, Chastel, Vigneron, Phys. Rev. 70, No. 6, 1947.
3. Tsin San Tsiang, Ho Zah Weg, Chastel, Vigneron, C. R. 223, 986, 1946; 223, 1119, 1946; 224, 272, 1947.
4. Lattes, Occhialini and Powell, Nature 159, 694, 1947; 160, 453, 1947.
5. Perfilov, DAN, 45, 7, 1944; 47, 9, 1945.