NEW INSTRUMENTS AND METHODS OF MEASUREMENT

CHARGED PARTICLE DETECTORS BASED ON SUPPORTLESS EMULSIONS

A. A. Varfolomeev

INTRODUCTION

The method of thick-layer photographic plates has come to be used especially widely in nuclear physics in recent years, when plates were obtained capable of recording charged particles with minimal ionizing power and allowing the angle of multiple scattering of particles to be measured. The use of the photographic method has made it possible to make a number of important discoveries in nuclear physics, especially in the study of various phenomena in cosmic rays.

However, this method has the basic drawback that the thickness of the emulsion layer usually does not exceed one millimeter. This limitation on the thickness of the photographic-emulsion layer of the plate is due to the specific character of the photographic processing of plates and their examination under a microscope.

In using any method for studying the tracks of charged particles (counting grains, measuring the angle of multiple scattering, measuring the density of δ-electrons, etc.), the result is obtained the more accurately, the larger the section of track accessible to measurement. The latter is directly proportional to the thickness of the emulsion layer of the plate, if the tracks are distributed more or less isotropically. (The probability that a track of length \(R\) will fit within a thin layer of thickness \(T\) is equal to \(\sim \dfrac{T}{2R}\).)

It is very important to have emulsion layers of great thickness in studying unstable particles, since complete information on the nature of a decaying particle (on its mass, decay scheme, lifetime, etc.) can be obtained only if its entire track is recorded, up to stopping or to the point of decay in flight. The number of stops in the plate of secondary particles produced by the primary radiation, and of tertiary particles arising in ...

in the decay of unstable secondary particles, will be proportional to their mean path length in the emulsion, and, consequently, to the thickness of the emulsion layer, since the distribution of such particles may be considered isotropic. The number of births of particles inside the emulsion layer is proportional to the volume of this layer, and, consequently, to the thickness \(T\). Thus, the total number of stopping points of particles formed in the emulsion will be proportional to the square of the thickness of the emulsion layer.

Already from these examples it is clear that, except in those cases when directed fluxes of fast particles are being studied (for example, beams of charged particles from accelerators), emulsion layers of large thickness are necessary. Since the mean free path of fast particles in an emulsion, determined by nuclear interaction, is approximately \(25\ \text{cm}\) (\(\sim 100\ \text{g}/\text{cm}^2\)), it is expedient in any case to have layers with a thickness of not less than several centimeters.

The most advantageous form of an emulsion detector for studying unstable particles produced by cosmic rays is a spherical or cubic form*).

1. METHODS OF OBTAINING EMULSION DETECTORS WITH A LARGE WORKING THICKNESS OF THE PHOTOGRAPHIC EMULSION LAYER

There are several ways of increasing the working thickness of emulsion layers. The first method consists in producing still thicker photographic plates. However, this method is associated with great difficulties in the photographic processing of the plates and in viewing them under a microscope. With an increase in the thickness of the emulsion layer of a plate, it becomes considerably more difficult to achieve its uniform development through the depth and to ensure small distortions of the emulsion layer during processing. At the same time the duration of photographic processing also increases**). For viewing plates with an emulsion layer of great thickness, special microscopes with a large working distance of the objective are necessary. The scattering of light in the emulsion layer when viewing plates, which worsens the quality of the image in the microscope, in general sets a fundamental limit to increasing the thickness of the emulsion layer of plates.

All this has led to the fact that plates with an emulsion layer thicker than \(1200\ \mu\) are used very rarely, and in the literature there are only isolated reports\(^{2}\) on the preparation of emulsion layers of \(2\ \text{mm}\).

*) The mean path length of fast isotropically distributed particles in a detector having the form of a convex polyhedron is equal to \(4V/S\) for particles entering the detector from outside, and \(2V/S\) for particles formed in the emulsion (\(V\) is the volume of the emulsion detector, \(S\) is its surface).

**) According to Dainton’s data\(^{1}\), processing a plate with an emulsion layer \(1\ \text{mm}\) thick takes \(\sim 15\) days.

Overlaying two emulsion plates onto one another made it possible to extend the limits of the working thickness of the emulsion layer only by a factor of 2 in comparison with photographic plates, i.e., only up to 2–3 mm.

The method of assembling separate plates into a stack^3 proved expedient in the study of such phenomena in cosmic rays as “jets” and heavy (multiply charged) particles of the primary component, since in these cases the particles could be traced through several plates, despite the fact that one emulsion layer was separated from another by glass. In studies of individual light particles, however, which have a considerable probability of being scattered in glass, this method of increasing the volume of the emulsion detector of charged particles is, of course, inapplicable.

The only effective method of increasing the working thickness of the emulsion detector to the practically necessary value has proved to be the method that has recently received the name of the emulsion chamber.

A stack is assembled from separate unsupported emulsion layers. After irradiation of the stack it is taken apart, and the layers are processed and examined separately. The thickness of the layers is chosen so that their processing and examination will not be difficult (200–600 μ).

At present, descriptions of several variants of emulsion chambers^4–10 are available in the periodical literature.

The use of a stack of unsupported photoemulsion layers as a charged-particle detector became possible after two main experimental tasks had been solved:

1. The development of methods for producing layers and methods for processing them that do not introduce large distortions.

2. The development of effective methods for tracing the tracks of charged particles through several layers.

The various variants of the emulsion chamber differ from one another in the particular way in which these tasks are solved. In view of the interest presented by the emulsion-chamber method, it seems advisable to give a brief review of some methodological details connected with the production, processing, and examination of such stacks of emulsion layers.

2. PREPARATION OF STACKS OF UNSUPPORTED PHOTOEMULSION LAYERS

Unsupported photoemulsion layers are prepared from the same emulsion as photographic plates. To improve its mechanical properties, special additives may be introduced into it.

Most foreign investigators use ready-made emulsion layers supplied by the Ilford firm. Concrete ...

no descriptions of methods for pouring such emulsion layers in the published literature.

The method of pouring emulsion layers proposed and developed in the Soviet Union by V. V. Alpers in 1948 has fully justified itself. The emulsion is poured onto carefully leveled, horizontally positioned and cooled cuvettes with rims, made of organic glass. After solidification the emulsion is transferred to a drying cabinet, in which air circulates at a temperature of \(\sim 25^\circ\text{C}\). The dried emulsion is removed from the cuvettes (after cutting the edges) and cut with the aid of a punch in the form of circles. The diameter of the circles adopted recently is 10 cm, with layer thicknesses of 300–400 \(\mu\).

A similar method of pouring emulsion layers was described by Demers\(^5\). The melted emulsion was poured onto a horizontally positioned cuvette, which consisted of a sheet of glass (measuring \(78 \times 53\ \text{cm}^2\)) with lucite rims glued along the edges. At the same time all precautions were taken against the formation of air bubbles in the emulsion, since the latter lead to distortion of the layers. The emulsion was dried in a weak air current at a temperature of \(25^\circ\text{C}\) and a humidity of \(67 \pm 3\%\). (With a dry emulsion-layer thickness of 300 \(\mu\), drying took 2 days.) The dried emulsion was removed from the glass, and separate pieces of the required dimensions were cut from the resulting emulsion sheet.

The most common size of layers in an emulsion chamber is \(15 \times 15\ \text{cm}^2\) with a thickness of 600 \(\mu\). Descriptions are encountered of emulsion chambers with layers measuring \(5 \times 10\ \text{cm}^2\) and \(10 \times 15\ \text{cm}^2\), with thicknesses from 300 \(\mu\) to 600 \(\mu\).

For assembling emulsion layers into a stack, two methods were usually used. In one method the layers were interleaved with thin sheets of paper, preventing them from sticking to one another\(^6,8\). This led to the fact that even when the stack was tightly compressed, the layers were separated from one another\(^8\) by distances up to 100–150 \(\mu\). Such gaps between layers are highly undesirable, since they considerably hinder the process of tracing tracks through the stack.

In those cases where the emulsion was sufficiently dry and nonsticky, the layers were placed directly on top of one another without spacers\(^5,7,10\). In order, as far as possible, to reduce the air gaps between layers and to facilitate separation of the layers after exposure, Orir\(^ {11}\) proposes sharpening the edges of the emulsion layers.

The assembled stack was usually clamped between two plates of hard material (metal, bakelite, etc.) and either wrapped with adhesive tape or tightened with bolts. In this way it was possible to achieve air gaps between layers in a stack without spacers\(^5,10\) of \(\sim 10\ \mu\). With circular layers, screw-tightening cassettes can be used.

3. METHODS OF PHOTOGRAPHIC PROCESSING OF PHOTOEMULSION LAYERS

At the present time two methods are used for the photographic processing of emulsion layers.

One of the methods consists in the fact that, after the stack is disassembled, the layers are glued onto specially prepared glass plates*) and are processed like ordinary photographic plates \(^{6—9}\).

In the other method the layers are processed in a free state, not glued to a backing, \(^{4, 5, 8, 10}\). Using the second method, the Bombay group of physicists processed layers in glass cuvettes \(^{3}\). The bottom of the cuvettes was lined with filter paper, which prevented the layers from sticking to the glass. In the warm stage of development, a glass plate was carefully placed under the paper and then, together with the emulsion layer lying on it, was placed on a warm metal surface. During the entire processing the layers were not touched by hand.

When processing by the second method, fairly good results were obtained when the layers, during processing, were placed in specially made “nests” of organic glass, with a bottom of mesh fabric (canvas).

During processing, a stack of such nests was rocked up and down (by hand or mechanically) so that for a considerable part of the time the layers were in the solution in a suspended state. The layers were removed from the nests only for the warm stage of development.

Orear \(^{10}\), in processing unbacked layers by the second method, placed them in special cuvettes with a bottom made of Teflon fabric. For uniformity of development, in the cold stage the layers were from time to time turned and inverted. (The entire processing, except for the warm stage of development, was carried out at a temperature of \(5^\circ\)C, and therefore the layers were hard and elastic.)

Less successful should be considered the method of developing emulsion layers without a backing under conditions in which they are suspended by the edges \(^{3}\), since this method introduces relatively large distortions of the layers. The layers stretch even at a very low temperature (\(0—2^\circ\)C) throughout the entire processing.

When layers are processed in a free state, diffusion of the solutions into the layer occurs from both sides; therefore the processing regimes in this case may be the same as those adopted for plates with an emulsion-layer thickness \(1.5—2\) times smaller.

*) In all the cases described, glass plates prepared by the Ilford firm were used. For methods of gluing layers onto glass, see \(^{6—9, 11}\).

A. A. BARFOLOMEEV

4. DISTORTIONS OF PHOTOEMULSION LAYERS INTRODUCED BY PROCESSING, AND METHODS FOR REDUCING THEM

After the processed emulsion layers have dried, it may turn out that, in addition to shrinkage of the emulsion due to the removal of silver bromide from it during fixing, shear deformations have occurred in the layers during processing, the layers have changed their shape or linear dimensions—in other words, distortions of the layers, distortions, have occurred.

Every photographic processing of emulsion layers is subject to the requirement that it introduce the least distortions (with proper development), since only with sufficiently small distortions is it possible to make reliable measurements along particle tracks and effectively trace tracks through the stack.

The literature contains descriptions of a number of measures that in one way or another reduce distortions (deformations) of layers arising as a result of their processing. Some of these measures must certainly be borne in mind when processing an emulsion chamber, in order to ensure small deformation of the layers.

a) When processing layers glued onto glass, large distortions may be caused by bubbles, which form in the course of processing the plates (usually at the final stage of fixing). The cause of bubble formation is apparently small air bubbles trapped between the emulsion layer and the glass.

As measures against bubble formation, Powell[^6] recommends degassing by boiling the glycerin solution used for gluing the layers; Stiller[^7] advises thoroughly wetting the surface of the glass and of the layer during gluing and carrying out the gluing more rapidly; Daniel[^9] proposes using freshly prepared glass plates, little exposed to air, for gluing. The number of bubbles can thereby be reduced to an acceptable figure: 1 bubble per \(\sim 75\ \mathrm{cm}^2\) of emulsion[^9], or even[^6][^7] per \(\sim 800\ \mathrm{cm}^2\).

b) With the first method of processing it is necessary to provide that, in the course of gluing the layers, they do not change their dimensions. For this, for example, it is necessary to reduce to a minimum the interval of time for which the layer is immersed in the glycerin solution in water before gluing[^6], and the temperature of the solution should not be high \((<10^\circ\mathrm{C})\). Otherwise the layers become soft and, when pressing them to the glass, one may considerably change their dimensions.

It is possible to replace altogether the operation of immersing the layers in water by wetting the surfaces of the layer and the glass with cotton wool impregnated with the appropriate solutions[^7][^8].

c) To reduce the distortions introduced by the processing of layers, it is recommended to avoid abrupt changes in the concentration of salts, the temperature, and the pH of solutions ^7.

d) A number of authors note that distortions of layers are certainly smaller if all their processing is carried out at low temperature ^5,6,10. This applies especially to the second method of processing layers (without a substrate). It is therefore no accident that O’Rear ^10 carries out all the processing (with the exception of the warm stage of development) at a temperature of \(\sim 5^\circ\)C, and Demers ^5 even at \(0—2^\circ\)C (including development).

e) The Bombay group ^8,9 came to the conclusion that distortions of layers can be reduced if, during processing, the plates are placed strictly horizontally and are not touched by hand.

f) The greatest distortions of layers arise during fixing, washing, and drying of the layers. This is due to the fact that at these stages of processing the emulsion layer undergoes large changes in volume.

Distortions caused by the fixing process can be reduced, first, as was already noted above, by lowering the fixing temperature to \(\sim 5^\circ\)C ^7,10 or even to \(0—2^\circ\)C ^8 and, second, by using tanning fixing solutions ^5,8.

g) Drying of unsupported layers constitutes a whole problem. Drying in an aqueous solution of alcohol should be regarded as the most successful method ^2,8,10.

A 3–6 percent concentration of glycerin is usually maintained in the solution. Approximately every 4 hours the alcohol concentration is increased by 10–20% until the water is completely replaced by alcohol. (In this process the layers may become smaller in size than before processing.) For final drying, the layers are removed from the alcohol and either pasted onto glass or clamped between two sheets of smooth cardboard ^8,10. In the latter case the dried layers are resilient and elastic. For ease of viewing they may be pasted onto glass by one method or another ^8,10.

Demers ^5 uses a somewhat different method of drying unsupported layers. After washing in water, the layers are placed for 10 minutes in a 10 percent aqueous solution of alcohol with an admixture of glycerin in an amount of \(\sim 8\%\) by weight. The layers are then laid on glass and pressed to it with a roller. Finally, the layers are dried in a weak air current for one day. With this method of drying, the linear dimensions of the dried layers turn out to be several percent larger than the original ones.

h) Plates (layers pasted onto glass) can be dried in the same way as unsupported layers, in alcohol ^8,9. Stiller ^7 dried plates in a drying cabinet, surrounding them with a protective ring of other plates. The humidity of the air in the cabinet was gradually changed from 100 to 50%. The results obtained in this way proved to be quite satisfactory (see below).

Of course, everything listed above does not yet exhaust all the possibilities for reducing the deformation of emulsion layers introduced by processing. There still remains much that is unclear in the question of the causes of layer deformations. However, even now one can draw certain conclusions about the methods and results of processing emulsion layers.

When processing in the state in which they are mounted on glass, the emulsion layers remain unchanged both in shape and in area. The number of bubbles causing large distortions of the layers can be reduced to such an extent that they will not practically interfere with the work. As Stiller[^7] notes, with all the precautionary measures he adopted, the processed layers mounted on glass have local shear deformations somewhat larger than in ordinary plates, and only in isolated cases are the deformations the same as in plates ($\sim 50$ kovans1) processed under the same conditions.

The Bombay group succeeded in reducing the deformation of the layers by the first processing method initially from 110 kovans[^8] to 80, and then even to 30 kovans[^9].

Thus, emulsion layers can be processed by the first method in such a way that the deformations in these layers (of the order of 30–50 kovans) differ little in magnitude from the deformations in ordinary thick-layer photographic plates. Such processing results make it possible to use all methods for studying the tracks of charged particles (grain counting, counting of $\delta$-electrons, measurement of ranges, etc.), with the possible exception of the method of multiple scattering for very fast particles ($p\beta > 10^{10}$ ev/sec).

With the second method of processing emulsion layers (without a backing), local distortions are, as a rule, considerably smaller than in the first case and are approximately equal to 10 kovans. This is apparently explained by the fact that, in the first processing method, stresses arise in the layer when the emulsion swells, since one of the surfaces of the layer is fixed, whereas the other is free. In the second method of processing, such stresses do not arise, because both surfaces are free.

Such small distortions in the layer ($\sim 10$ kovans) make it possible to use the method of multiple scattering even for very fast particles. However, as was already noted above, during drying, unsupported layers usually shrink strongly, and therefore the final dimensions of the layers in area may differ substantially from the initial ones. Thus, with careful drying in alcohol, in the Bombay—

of the group[^8] the final linear dimensions of the layers were 2 mm smaller than the original ones—\(10 \times 15\ \mathrm{cm}^2\).

Even if one tries to change the dimensions of the layers during drying by changing the concentration of alcohol (adding water in order to increase the size of the layer), it is still difficult to obtain the required layer size with an accuracy better than 0.1 mm.

The presence of such large changes in the linear dimensions of the layers greatly complicates the work of following tracks from layer to layer and makes the second method of processing emulsion layers of little applicability in cases where the number of layers in the stack is large.

5. FOLLOWING TRACKS OF CHARGED PARTICLES THROUGH SEVERAL LAYERS

The advantages of a stack of unbacked photographic emulsion layers as a detector of charged particles can be fully utilized only when there exists an effective method for following tracks through the stack, including tracks of minimum ionization.

To facilitate the transition from one layer to another in the study of tracks, marks are usually made on the layers (before processing), by which one can judge the relative position of the layers in the stack during its exposure. Thus, in the Orr emulsion chamber[^10], when the stack was assembled, small pieces of thin wire were clamped between the layers; these left indentations in the layers that were visible even after processing.

For moving from layer to layer, it appears more convenient to have, as marks on the layers, a coordinate grid of thin lines with a cell size of several millimeters. Such a grid can be applied, for example, by irradiating the stack with X-rays through a narrow (15–30 \(\mu\)) slit between two thick layers of lead glass.

As was already indicated above, when layers without a backing are processed (by the second method), large volume distortions arise; therefore, in this case, when searching for the continuation of a track, one usually uses the coordinates of this track relative to some nearest mark on the adjacent surfaces of the layers. For a more accurate determination of the point where the track enters the neighboring layer, one can use its coordinates relative to some track of a heavy particle passing through both layers not far from the point under investigation. The coordinates of the continuation of the track under study can then be determined with an accuracy[^10] of up to \(\sim 10\ \mu\). If one orients oneself by several nearby tracks, the probability of an erroneous transition becomes much less than 1%. When stacks are irradiated with cosmic rays, one can, in general, use as marks

make use of tracks of heavy particles, if there are sufficiently many such tracks$^{8}$.

However, the method of moving from layer to layer by coordinates relative to some mark takes comparatively much time.

When processing layers glued to glass, their overall dimensions, as already noted above, are preserved; it therefore makes sense to achieve such a position of the emulsion layers relative to the edges of the plates that the layers would be arranged on the microscope stage in an identical manner. The adjacent regions of neighboring layers that were in contact during irradiation will then have the same coordinates (with an accuracy determined by the adjustment of the plates and by distortions of the layers). In this case the transition from layer to layer will be carried out by simply replacing the plates on the microscope stage and searching for the track under investigation in the microscope field of view.

To facilitate adjustment of the plates, it is useful to put a number of marks on the layers when they are in a stack. Powell$^{6}$ recommends putting a series of lines on all the layers at once by irradiating the stack with X-rays from the side of the lateral faces of the layers. Using the marks obtained (of width $\sim 200\,\mu$), one can trim the edges of the plates so that the relative arrangement of the plates on the microscope stage is the same as in the stack, with an accuracy of up to $\sim 100\,\mu$. Stiller$^{7}$, in order to put on marks, irradiated with X-rays a stack covered with a lead plate in which there were small holes. On all the layers (if the X-rays were of sufficient hardness) marks in the form of round dots were obtained, by which one could orient oneself when trimming the edges of the plates.

The Bombay group$^{8}$, after processing the plates (layers fixed on glass), glued them to rectangular transparent frames with a slowly drying adhesive. The plate could be shifted relative to the frame for $\sim 30$ min. During this time the frame with the plate was placed on the microscope stage, and the required position of the plate on the frame was selected. With some skill, by orienting oneself by the tracks of heavy particles, it was possible to arrange the plates so that the relative position of the plates on the microscope stage differed from their relative position in the stack by no more than $10\,\mu$. Taking into account distortions of the layers and the gaps between them in the stack ($\sim 100$—$150\,\mu$), this meant that the coordinates of the entry and exit of tracks from neighboring layers could differ by $50\,\mu$. Under such conditions it is still difficult to trace tracks of minimum ionization.

When going over to a large number of layers in a stack (to 125, and then to 200), the Bombay group had to improve somewhat the design of the chamber and the adjustment of the plates$^{9}$, so as not to spend too much time on tracing tracks through

bundle. Instead of the paper interlayers used previously, nets made of thin (18 μ in diameter) nylon threads were placed between the layers. The gaps between the layers were thereby reduced to 25–35 μ. The threads were first impregnated with an α-active (polonium) solution, and therefore traces of the threads (a coordinate grid) remained on the layers. The ranges of the α-particles were, in this case, sufficiently large for the grid to be clearly visible, and sufficiently short that their traces did not impair visibility inside the layer.

After processing, the plates were glued, as in the preceding case,^8 onto frames; the coordinate grid now served as the reference. Some intersection on the surface of the layer was fixed by the objective of a microscope on whose stage the plate was placed; another intersection on the same surface was fixed by a second, movable microscope placed close to the first. The neighboring plate was glued to the frame in such a way that intersections of the coordinate grid from the same sections of the threads as in the first case appeared in the fields of view of the microscopes. Such control of the position of the plate by means of two microscopes made it possible to glue the plates in the same position with an accuracy of up to 5 μ.

Owing to the reduction of distortions of the layers during processing (see above), the reduction of the gaps between the layers to 25–30 μ, and the more accurate mounting of the plates (with the aid of two microscopes), the coordinates of the exit of a track from a layer differed from the coordinates of its entrance into the neighboring layer, on average, by no more than 20–30 μ. This greatly accelerated the process of transition from layer to layer and made it possible to follow reliably even tracks of minimum ionization. Without these conditions a detector with such a large number of emulsion layers would have been of little effectiveness.

6. POSSIBILITIES OF THE EMULSION CHAMBER

At present, the technical difficulties encountered in using a bundle of unsupported photoemulsion layers as a detector of charged particles have largely been overcome. Methods have been developed for the photographic processing of the bundle that introduce distortions of the emulsion layers almost as small as in the case of processing ordinary photographic plates with an emulsion layer of the same thickness. In any case, when working with an emulsion chamber processed in the corresponding manner, one can use all the methods ordinarily employed in work with photographic plates for studying tracks of charged particles, up to the measurement of the angle of multiple scattering of not very fast particles \([p\beta < (5—10)\,10^9\ \text{eV}/c]\).^9

In the study of unstable particles, such a limitation for measuring the angle of multiple scattering is not a great hindrance,

since, nevertheless, reliable information about new particles is obtained only when it is possible to register the stopping of a particle in the emulsion (for which the emulsion chamber is designed), while, on the other hand, particles arising in the decay of unstable particles, because of the comparatively small energy release in the decay, will not possess so large an energy that this limitation would be significant.

With the already developed technique of the emulsion chamber it is possible to assemble stacks of any thickness. At present, the limitation on the thickness of an emulsion chamber may be due, in essence, to such causes as the impossibility of examining excessively large volumes of emulsion, the limited quantity of emulsion, the permissible weight of the emulsion chamber, the finite dimensions of the apparatus used in the manufacture and processing of the emulsion chamber, etc., but not to fundamental circumstances, as was the case with photographic plates.

An illustrative example of the advantages and practical possibilities of increasing the thickness of an emulsion detector in the form of a stack of layers is provided by the latest version of the emulsion chamber of the Bombay group9. The chamber consisted of 200 emulsion layers, each of size \(15 \times 15 \text{ cm}^2\) and thickness \(600 \mu\). Thus the emulsion detector was a block of dimensions \(15 \times 15 \times 12 \text{ cm}^3\) and weighed about \(11 \text{ kg}\). The mean possible path \(\overline{L}\) in the emulsion for particles born in such a chamber is \(4.60 \text{ cm}\), whereas in the case of photographic plates it was equal to the thickness of the emulsion layer. In comparison with the first block8 of 24 layers, each \(600 \mu\) thick and of size \(10 \times 15 \text{ cm}^2\), in the latter case6 the volume was increased by a factor of 12.5, and the path \(\overline{L}\) by a factor of 4.

Taking into account that the number of stoppings or decays of particles in the emulsion is proportional to the mean possible path of the particles in the chamber (so long as the latter does not become greater than the maximum possible path of the particles, determined by their energy or lifetime), and that the number of births of such particles in the emulsion is proportional to the volume of the block, one may suppose that in the latter block approximately 50 times more stoppings (decays) of unstable particles formed in the emulsion can be recorded than in the first block. In other words, this means that such unstable particles as heavy mesons and hyperons can be recorded in numbers 2–3 times greater than have been recorded up to now in total*).

With the minor complications caused by the necessity of passing from layer to layer, the emulsion-chamber method has great advantages in comparison with photographic plates—

*) Recently it has become known that the Bristol group has made and processed an emulsion chamber weighing about \(60 \text{ kg}\).

...especially where large working thicknesses of emulsion are required. It is therefore no accident that, although the first reports in the foreign literature on the preparation of a stack of emulsion layers appeared only in 1950,^4 by 1953 most institutes studying unstable particles were using stacks of supportless layers instead of photographic plates. The emulsion-chamber method made it possible for the first time to record two^13,14 and then three^15,16 stopped $\pi$-mesons arising in the decay of $\tau$-mesons, and thus to determine the sign of a $\tau$-meson decaying at rest and, with good accuracy, the energy released in its decay. An emulsion chamber was used for the first recording^17 of the decay scheme $\tau^+ \to \pi^+ + \pi^0 + \pi^0$; a $\tau$-meson produced under laboratory conditions,^18 the pair production of a charged $K$-particle with a hyperon,^13,14 and a number of other interesting phenomena that could not be detected by the photographic-plate method.

Taking all the foregoing into account, one may state with confidence that the emulsion-chamber method will become more widespread with each passing day, especially where large working thicknesses of emulsion layers are required.

References Cited

1. A. D. Dainton, A. R. Cattiker, W. O. Lock, Phil. Mag. 42, 396 (1951).
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3. H. L. Bradt, B. Peters, Phys. Rev. 74, 1828 (1948).
4. P. Demers, Canad. Journ. of Phys. A28, 628 (1950).
5. P. Demers, Canad. Journ. of Phys. 32, 538 (1954).
6. C. F. Powell, Phil. Mag. 44, 219 (1953); Fowler, Bagneres Conference (1953).
7. B. Stiller, M. M. Shapiro, F. W. O’Dell, Rev. Sci. Instr. 25, 340 (1954).
8. D. Lal, Jash Pal, B. Peters, Proc. Ind. Ac. Sci. A38, 277 (1953).
9. R. R. Daniel, G. Friedman, Jash Pal, B. Peters, Proc. Ind. Ac. Sci. A40, 151 (1954).
10. J. Orear, Rev. Sci. Instrum. 25, 1023 (1954).
11. Georges Kayas, Journ. phys. rad. 14, 621 (1953).
12. M. G. E. Cosyns, G. Vanderhaegne, Bull. Centre Phys. Nucleaire No. 15 (April 1953).
13. D. Lal, Jash Pal, B. Peters, Phys. Rev. 92, 438 (1953).
14. D. Lal, Jash Pal, B. Peters, Proc. Ind. Ac. Sci. A38, 398 (1953).
15. G. Belliboni, B. Sechi, G. T. Lorn, Nature 173, No. 4415 Suppl., VIII (1954).
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1. Shear deformations of emulsion layers can be measured, independently of the layer thickness $T$, by the coefficient $k = K/T^2$, where $K$ is the vector of shear deformation of points on the surface of this layer. If the magnitude $K$ is expressed in microns and $T$ in millimeters, then the coefficient $k$ will be expressed in conventional units—kovans, so named after the two authors who proposed this method of measuring deformation[^12]. ↩