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nuclear emulsion[′nü·klē·ər i′məl·shən]
a photographic emulsion designed to record the trajectories (tracks) of charged particles. It is used in nuclear physics, elementary particle physics, and cosmic ray physics for autoradiography and in nuclear radiation dosimetry.
A photographic emulsion was first used in nuclear physics research by A. H. Becquerel, who first detected the radioactivity of uranium salts in 1896 from the fogging the salts caused on a photographic emulsion. In 1910 the Japanese physicist S. Kinoshita demonstrated that silver halide grains in an ordinary photographic emulsion become capable of development if even a single alpha particle passes through them. In 1927, L. V. Mysovskii and his colleagues (USSR) prepared plates with an emulsion layer 50 micrometers thick and used them to observe scattering of alpha particles by the nuclei of the emulsion. Nuclear emulsions with standardized properties were first manufactured in the 1930’s; they were used to register the tracks of slow particles (alpha particles and protons). In 1937 and 1938, M. Blau and H. Wam-bacher (Austria) and A. P. Zhdanov and his colleagues (USSR) observed the splitting of nuclei in nuclear emulsions caused by cosmic radiation. Between 1945 and 1948 nuclear emulsions were manufactured that were suitable for recording weakly ionizing, singly charged relativistic particles, and the nuclear emulsion technique became a precision quantitative research method.
Two features distinguish nuclear emulsions from ordinary photographic emulsion: the mass ratio of silver halide to gelatin in the former is eight times higher than that of conventional emulsions, and the thickness of the emulsion layer is usually from ten to 100 times greater, sometimes reaching 1,000–2,000 micrometers or more (the standard thickness of commercial photographic emulsions is 100–600 micrometers). The silver halide grains in nuclear emulsions are spherical or cubical in shape, with an average size (depending on the grade of emulsion) usually between 0.08 and 0.30 micrometers.
Charged particles or electromagnetic radiation associated with nuclear reactions affect nuclear emulsions in a manner similar to light. The process of development greatly amplifies the initial weak effect (the latent photographic image), just as the Town-send avalanche in a Geiger counter and the intense development of bubbles in a bubble chamber greatly magnify the weak effects associated with the initial ionization produced by a charged particle. Charged particles usually have high energy and consequently can produce sensitivity centers in the silver halide grains located in their path. After the nuclear emulsion has been fixed, a chain of black grains is formed along the trail of the particle. Particle tracks are observed by a microscope at a magnification of 200–2,000 diameters.
The emulsions used in nuclear physics are usually layers applied to glass supports. In the study of high-energy particles in accelerators or cosmic radiation, the emulsions are sometimes stacked in several hundred layers. The volume of the stacks reaches tens of liters to form a nearly solid photosensitive mass. After exposure, individual layers may be cemented to glass supports and processed in the conventional manner. The position of the layers is precisely marked so that the trajectory of particles passing from layer to layer can be easily traced through the entire stack.
The properties of the track left in the emulsion by a charged particle depend on the particle’s charge Z, velocity v, and mass M. Thus, the residual range of a particle (the length of the track from its beginning to the point where it stops) is proportional to M for some given e and v; at a fairly high velocity v of the particle, the grain density (the number of developed grains per unit length of track) g ~ e2/v2. If the grain density is too high, the tracks merge into a solid black trail. In this case, especially if e is large, the’ number of delta electrons that form characteristic branching tracks can serve as a measure of velocity. Here, the grain density is also ~e2/v2. If e = 1 and v ~ c (where c is the speed of light), the trail of the particle in a nuclear emulsion appears as a broken line of 15–20 black points per 100 micrometers of path length.
Nuclear emulsions can be used to measure the scattering of a particle, that is, the average angular deflection per unit of path length: φ ~ e/pv (where p is the momentum of the particle). A nuclear emulsion may be placed in a strong magnetic field to measure the momentum of a particle and the sign of its charge, as well as its mass and velocity. The advantages of the nuclear emulsion technique include high spatial resolution (effects separated by distances of less than 1 micrometer can be distinguished, which corresponds to a passage time of less than 10−16 sec for a relativistic particle) and the possibility of prolonged, cumulative registration of infrequent events.
The development of modern nuclear emulsions constitutes a great scientific and technical achievement. As the British physicist C. F. Powell has pointed out, the development of improved emulsions opened a new window into nature through which man was able to see for the first time strange and unexpected tracks previously unknown to physicists.
Several important discoveries were made by the nuclear emulsion technique between 1945 and 1955: π-mesons (pions) and the decay sequences π → μ + v, μ → e + v + v were recorded in nuclear emulsions exposed to cosmic rays, and the nuclear interactions of π−-mesons and K–-mesons were also observed. Nuclear emulsions have made it possible to evaluate the lifetime of the π0-meson (10–16 sec), to observe the decay of a K-meson into three pions, to discover the Σ-hyperon and demonstrate the existence of hypernuclei, and to discover the antilambda hyperon (seeHYPERONS). The nuclear emulsion technique has been used to study the composition of primary cosmic radiation, revealing that in addition to protons, primary cosmic radiation contains nuclei of helium and heavier elements up to iron. Since the 1960’s, the technique has been displaced by use of bubble chambers, which provide greater measurement accuracy and permit the use of computers for data processing.
REFERENCESPowell, C. F., P. H. Fowler, and D. H. Perkins. Issledovanie elementarnykh chastits fotograficheskim metodom. Moscow, 1962. (Translated from English.)
A. O. VAISENBERO