THE METHOD OF “DRY” TEMPERATURE DEVELOPMENT AND THE CHOICE OF EMULSION TYPE WHEN WORKING WITH LARGE DOSES OF THERMAL NEUTRONS

Intense beams of thermal neutrons from nuclear reactors make it possible to study relatively rare nuclear reactions and to determine minute quantities of certain elements. Thick-layer emulsions serve as a good detector of the charged particles that arise in these processes. The following applications may be mentioned: 1) quantitative determination of uranium by the method of fission by thermal neutrons[^1]. The amount of uranium is determined from the number of tracks of fission fragments in the emulsion; 2) quantitative determination of lithium and boron from the number of tracks of $\alpha$-particles from the reactions $(n,\alpha)$ proceeding under the action of thermal neutrons on Li and B[^2][^3]; 3) study of reactions $(n,\alpha)$ or $(n,p)$ with a very small effective cross section[^4], proceeding under the action of thermal neutrons on medium and heavy nuclei.

In all the cases indicated, it is necessary to use large neutron doses, of the order of $10^{12}$ neutrons/cm$^2$. Secondary processes associated with such—

...with certain doses, create a strong veil (background), making observation difficult. The authors of the reviewed work[^5] set themselves the aim of developing optimal conditions for irradiation and development of emulsions that would make it possible to distinguish and measure the tracks of fission fragments, $\alpha$-particles, and protons against a minimal background.

The authors point to the following sources of background from isolated grains and tracks that interfere with observation. Fast neutrons, always present in a beam of thermal neutrons, give tracks of recoil protons knocked out of the hydrogen atoms of the gelatin. $\gamma$-rays, always accompanying thermal neutrons, liberate secondary electrons (photoelectrons, Compton electrons, or electron pairs), which give tracks or isolated grains depending on the type of emulsion. The atoms of bromine and silver in the emulsion capture thermal neutrons, becoming radioactive isotopes emitting $\beta$- and $\gamma$-rays, which fog the emulsion. Each capture is accompanied by the emission of $\gamma$-photons with energies between 4 and 8 Mev, which in turn create secondary electrons in the emulsion. A large number of protons arise in the reaction $N^{14}(n,p)C^{14}$ on thermal neutrons. Finally, the materials surrounding the emulsion, under the action of neutrons or $\gamma$-rays, also emit electrons, $\gamma$-rays, protons, or $\alpha$-particles, creating undesirable tracks or isolated grains. The main factor making observation difficult is the background from electrons of various origins; protons knocked out of nitrogen, and recoil protons, interfere much less. As long as electrons produce isolated tracks or separate rare grains, observation of the tracks of fission fragments, $\alpha$-particles, and protons is still possible by underdeveloping the grains affected by the electrons. However, at large doses each grain is affected by such a large number of electrons that selective development becomes impossible, and a continuous electron background completely precludes observation of individual tracks. Rough calculations show that 700 relativistic electrons lose in $1\,\mu$ of emulsion as much energy as one $\alpha$-particle with an energy of 5 Mev.

In the experimental part of the work the authors determined the minimum neutron doses that still make it possible to distinguish individual particle tracks. To obtain ideal conditions for irradiation, the emulsion should be placed outside the reactor in a beam of neutrons scattered laterally by paraffin or graphite: this would reduce the effect of $\gamma$-rays and fast neutrons, which have rectilinear trajectories. To eliminate the formation in the emulsion of the radioactive isotope of silver, the plates should be surrounded by silver foil, which would retain the resonant neutrons. Next, the emulsion must be protected from the electrons emitted by this silver foil, with the aid of lead foil. Finally, it is necessary that, after all these conditions have been met, the neutron flux remain sufficiently large that the irradiation time be less than 10 min. (since the half-lives of the radioactive isotopes of bromine in the emulsion are 18 min. and 4.4 hours). The neutron source was the nuclear reactor at Châtillon (France), operating with heavy water, into which rods of uranium oxide were immersed. This boiler gives thermal-neutron fluxes too weak for the ideal conditions mentioned to be created.

Experiments showed that it is best to place the plates near the tank in a graphite reflector, and to weaken the action of $\gamma$-rays with lead.

Plates in a sheath of black paper or graphite (such sheaths give a minimum number of secondary electrons) were placed in a lead block with dimensions equal to those of the radial channel of the reactor. The entire block was surrounded by a graphite reflector. The plates were irradiated for 5–10 min. in neutron fluxes at various places in the reactor. The fluxes were measured by means of activation of a manganese detector, dis...

placed near the emulsion. After irradiation with neutrons, the emulsions were exposed to polonium $\alpha$-particles and immediately developed. In each experiment the visibility of the $\alpha$-particle tracks, the background density due to $\beta$- and $\gamma$-rays, and the track density of recoil protons were evaluated; for various emulsions and different developers. The following results were obtained.

To obtain the minimum background one should use an emulsion of the minimum thickness compatible with the task at hand. Hence follows the advantage of the emulsion method in determining the content of $\mathrm{C}$ in emulsions over impregnation methods, which require an increase in emulsion thickness when it is desired to increase the number of nuclei under investigation. Emulsions with minimum sensitivity to electrons should be used, i.e., fine-grained emulsions. In this respect Ilford D1 and E1 emulsions are better than C2. However, at doses above $10^{11}$ neutrons/$cm^2$ the number of colored background grains becomes so large that even in D1 and E1 emulsions the visibility of $\alpha$-particle tracks is very low. In this case C2 emulsions give denser $\alpha$-particle tracks and make it easier to distinguish $\alpha$-particles and protons.

The optimal developing conditions depend on the neutron dose received by the emulsion. The use of doses below $10^9$ neutrons/$cm^2$ is not associated with any difficulties. Up to doses of $10^8$ neutrons/$cm^2$ one may use a G5 emulsion $200\,\mu$ thick. Between $10^8$ and $5\cdot10^8$ neutrons/$cm^2$ the thickness should be about $100\,\mu$, and between $5\cdot10^8$ and $10^9$—about $50\,\mu$.

For doses greater than $10^9$ neutrons/$cm^2$, G5 emulsions are unsuitable. In the range $10^9$—$10^{11}$ neutrons/$cm^2$ C2, E1, and D1 emulsions up to $200\,\mu$ thick may be used, applying the method of selective underdevelopment.^6

In the range from $10^{11}$ to $3\cdot10^{12}$ neutrons/$cm^2$ it was possible to obtain sharp $\alpha$-particle tracks, readily distinguishable from protons, in C2 emulsion $50\,\mu$ thick and sufficiently transparent for convenient observation. For this purpose a method of vigorous surface development was devised. Of the four tested developers (ID-19, amidol,^7 glycine, and hydroquinone), the purely hydroquinone developer proved the best. Its composition was: solution A: $K_2CO_3$—50 g, $Na_2SO_3$—10 g, 10% $KBr$—5 $cm^3$, water—up to 500 $cm^3$; solution B: hydroquinone—0.5 g, water—up to 500 $cm^3$; for use, equal volumes of A and B are mixed. The optimal development procedure with these solutions is as follows: immediately after irradiation the plate is transferred for 30 min to a cold chamber at $3^\circ C$. It is then immersed in a warm ($30^\circ C$) developer for 5 min. The contrast of the tracks is increased still further by increasing the temperature difference, for example when going from solid carbon dioxide to $38^\circ C$. However, such a regime is dangerous because of possible reticulation of the gelatin.

It is very interesting that, in the new developing method, the authors of the paper under review replaced the cold stage of impregnation with developer, used in the known method of “temperature development,”^8 by a “dry” cold stage, preserving the “thermal shock” when the plate is transferred to the warm developer and thereby achieving purely surface development.

A. Kh.

CITED LITERATURE

1. J. Curie, H. Faraggi, Comptes Rendus 232, 959 (1951); collection “Radiography,” IL, 1952.
2. A. Fica, Comptes Rendus 233, 1684 (1951).
3. M. Hillert, Nature 168, 39 (1951); collection “Radiography,” IL, 1952.
4. H. Faraggi, Ann. Physique 6, 325 (1951).

1. H. Faraggi, A. Bonnet, M. Cohen, J. phys. et rad. 13, suppl. No. 7—9, 105 A (1952).
2. Téledgi, Zunti, Helv. Phys. Acta 23, 745 (1950).
3. C. Dilworth, G. Occhialini, L. Vermaesen, Bull. Centre Phys. Nucl., Bruxelles, No. 13a, February 1950.
4. C. Dilworth, G. Occhialini, L. Vermaesen, Fundamental mechanisms of phot. sensitivity. London, 1951, p. 297.