PHOTOELECTRIC METHOD FOR MEASURING TRACK DENSITY IN NUCLEAR EMULSIONS

Determining the number of developed grains in the tracks of charged particles plays an important role in studies of nuclear processes using thick-layer emulsions. If the ionizing power of a particle is small, then the grains can be counted under a microscope. The technique of such counting has been brought to a high degree of perfection.^{1,2} In modern electron-sensitive emulsions of the type G5, particles with increased specific

FROM CURRENT LITERATURE

by ionization create such dense tracks that individual grains overlap and grain counting is unreliable or impossible.

Some authors3 in this case resort to the method of partial development (selective development, or underdevelopment) in order to improve the distinguishability of grains in the tracks of strongly ionizing particles. However, this can lead to loss of the tracks of weakly ionizing particles and to the impossibility of a comparative study of fast and slow particles in one and the same emulsion.

Determination of the specific ionization by the number of δ-electrons4 in the tracks of strongly ionizing particles suffers from the drawback that it is sometimes difficult to decide whether a given δ-electron is associated with the track of a multiply charged particle. Moreover, if the number of δ-electrons exceeds 20 per 100 μ of track, counting becomes very difficult and the errors increase considerably.

The method of exposing a stack of plates5, in which high-sensitivity and low-sensitivity emulsions alternate, also has disadvantages: it is impossible to expect a linear dependence of grain density on energy loss in low-sensitivity emulsions, which necessitates careful calibration of each batch of plates. In addition, special development of these plates is required, and the tracks have to be followed over their entire path through the stack of plates.

The authors of the paper under review6 attempted to solve this problem in another way, proceeding from the fact that even in very dense, completely developed tracks in electron-sensitive emulsions there should exist some variation of density along the track and a difference in the densities of tracks of particles of different velocity and charge. The authors succeeded in detecting these differences and in establishing the dependence between velocity, mass, and residual range, an analogous dependence obtained by the method of grain counting. For this purpose they used a photoelectric method for measuring the amount of light absorbed by the track.

The optical part of the apparatus consists of a microscope with a Huygens eyepiece. In the image plane in the eyepiece there is a glass scale with a division value of 0.1 mm, which makes it possible to determine the displacement of the track. Directly above the scale there is a movable slit, also situated in the image plane. The slit is positioned parallel to the image of the track so that it passes through its center. Observation of the track during these operations is carried out through a side eye lens in the eyepiece tube. The divergent beam of light from the slit falls on the cathode of a photomultiplier. Since the developed grains are opaque, the measured photocurrent is proportional to the area of the slit not shielded by the image of the track. After reading the photocurrent on a galvanometer with a sensitivity of \(10^{-8}\) a/division, the track is moved perpendicular to the length of the slit until it leaves the slit, and the photocurrent of the free slit is measured. The difference between these two readings is a measure of the area of the image of the track and of the density of grains in it. The width of the slit is chosen depending on the width of the track. It must exceed the width of the image of the track by a factor of 3–4. Measurements of the tracks of mesons, protons, and heavier particles in G-5 emulsion were made with slits of width 0.15–0.20 mm, which corresponds to 1.5–2.0 μ in the plane of the object (the average width of a proton track is about 0.5 μ). The length of the slit is determined by various factors—the greater it is, the weaker the influence of statistical fluctuations of grain density7 and the smaller the number of readings in measuring the whole track. A short slit is necessary for curved tracks or tracks going steeply in the emulsion. The authors always used a slit corresponding to 30 μ of track. From the galvanometer readings in the absence and in the presence of the track and from the dimensions of the slit, it is easy to determine the area and the average width of the section of track.

for a gap. By carrying out measurements for successive segments of a track, it is possible to find the same quantities for any residual range. In these units of area or mean width the authors express the density of tracks.

Preliminary measurements have shown that the photoelectric density of a track is proportional to the density determined by the method of counting grains, that the reproducibility of the measurements is quite satisfactory, and that the light flux and the readings of the galvanometer are proportional to the area of the gap. These results indicate the reliability of the new method.

Fig. 1.

Figure 1 shows a histogram demonstrating the distribution of the total area of the last 400 μ of tracks of μ- and π-mesons, protons, deuterons, tritons, α-particles, and lithium nuclei. In constructing the histogram, a correction was introduced for the change in track density with depth of their location in the emulsion. This correction can be determined experimentally by the same photoelectric method. Thus, the new method makes it possible to identify the indicated particles even for comparatively short residual ranges. A necessary condition is uniform development and correction for the depth of the track.

Fig. 2.

Figure 2 shows the dependence of the track density on the residual range in G5 emulsion for certain groups of particles from the histogram

Fig. 1. The density represents the average width of the track in hundredths of a micron.

Using the new method, the authors found that the mean diameter of a developed grain in the G5 emulsion is equal to \(0.59\,\mu\), and established that this diameter does not depend on the grain density along the track, in agreement with the well-known fact of complete development of the grain irrespective of the energy imparted to it (provided the grain is at all capable of being developed).

The photoelectric method, the foundations of which are set forth in the paper under review, was improved in subsequent works\(^{9,10}\). In paper\(^{9}\) the authors made an oscillographic recording of the output voltage of a photomultiplier. The scattering of light by the emulsion does not permit photometric measurement of tracks going deep into the emulsion or situated deep below its surface. The authors succeeded in overcoming this difficulty by measuring the distribution of absorption across the width of the track (perpendicular to its length). The half-width of the absorption curve obtained in this way does not depend on the depth of the track in the emulsion and is an exact measure of the ionization of the particle. This method, even in the case of semitransparent emulsions, is the most suitable of the existing methods for determining the specific ionization of particles with energy losses above \(300\,\text{MeV}/e\,\text{cm}^{2}\) and with velocities greater than \(10^{10}\,\text{cm/sec}\). For specific energy losses between 30 and \(100\,\text{MeV}/e\,\text{cm}^{2}\), photometry is of little use, since it has low sensitivity. Multiply charged particles producing short tracks (\(\geq 200\,\mu\)) ending in the emulsion can also be identified by the method of measuring the half-width when other methods are inapplicable. These results were obtained for tracks of 98 particles with specific ionization ranging from the ionization of relativistic \(\alpha\)-particles to the ionization of iron nuclei.

In the case of singly charged particles with mass of order 1, analysis of the photometric curves is greatly complicated. The authors of the second paper\(^{10}\) proposed another method, by means of which they intend to identify short tracks even in the case of protons, \(\alpha\)-particles, and mesons. The current from the photomultiplier was amplified and fed to the terminals of a short-period galvanometer, whose deflections were recorded photographically on light-sensitive paper in the form of a curve with ordinates proportional to the deflections. The problem reduces to determining the true absorption of light by the track and the exact area of the track. For this it is necessary to take into account the effect of light scattering by the emulsion for each section of the track studied. The change in absorption across the width of a given section of track was determined with the aid of a series of slits of increasing width, for example from 0.08 to \(1.9\,\mu\) (width in the object plane). For a slit already narrower than the central part of the track, the deflection of the galvanometer would be zero if there were no scattering and no breaks in the track. In practice the galvanometer always gives a certain deflection, increasing with the width of the slit. As a result of a theoretical and experimental analysis of this phenomenon, the authors established a dependence that permits determination of the true area of the track with an experimental correction for the effect of scattering and breaks in the track.

A. Kh.

CITED LITERATURE

1. C. Lattes, G. Occhialini, C. Powell, Proc. Phys. Soc. 61, 173 (1948).
2. P. Fowler, Phil. Mag. 41, 169 (1950).
3. G. Stevens, Fundamental mechanisms of phot. sensitivity, London, 1951, p. 310.

1. L. Voyvodic, Canad. J. Res. A 28, 315 (1950); collection “The Photographic Method in Nuclear Physics,” IL, 1952.
2. H. Bradt, B. Peters, Phys. Rev. 80, 943 (1951); collection “The Photographic Method in Nuclear Physics,” IL, 1952.
3. S. v. Friesen, K. Kristiansson, Ark. f. Fys. 4, 505 (1952).
4. P. Hodgson, Brit. J. Appl. Phys. 3, 11 (1952).
5. K. Mees, The Theory of the Photographic Process, Moscow–Leningrad, 1949.
6. M. Ceccarelli, C. Zorn, Phil. Mag. 43, 356 (1952).
7. M. Della Corte, M. Ramat, Nuovo Cimento 9, 605 (1952).