Nuclear photographic emulsion - a photographic emulsion for recording the trajectories (tracks) of charged particles. The French physicist A. Becquerel detected the radioactivity of U salts in 1896 from the blackening they caused in an ordinary photographic emulsion. In 1910 the Japanese physicist S. Kinoshita established that the grains of Ag halide in an ordinary photographic emulsion become capable of development if an α-particle has passed through them. In 1927 L. V. Mysovsky and his co-workers made plates with an emulsion layer 50 μm thick and used them to observe the scattering of α-particles by the nuclei of the emulsion. In the 1930s the manufacture began of special nuclear photographic emulsions (with standard properties), with which the tracks of slow particles (α-particles, protons) could be recorded. In 1937—38 the Austrian physicists M. Blau and H. Bombacher, and A. P. Zhdanov with his co-workers, observed in nuclear photographic emulsions the disintegration of nuclei caused by cosmic radiation. In 1945—48 relativistic nuclear photographic emulsions appeared, for recording relativistic particles. The nuclear photographic emulsion method became an accurate quantitative method of research.
Nuclear photographic emulsions differ from ordinary photographic emulsions in two respects: the ratio of the amount of Ag halide to the gelatin is 8 times greater; the thickness of the layer is as a rule 10-100 times greater and sometimes reaches 1000—2000 μm. The grains of Ag halide in the emulsion usually have an average linear size of 0.08 – 0.30 μm.
Charged particles passing through a nuclear photographic emulsion create sensitive centres in some of the Ag halide grains lying in their path (a latent image). After development these grains turn into crystallites of metallic Ag, which are opaque and, once the nuclear photographic emulsion has been fixed, form a chain of black grains along the track of the particle. The tracks of particles are observed with microscopes at magnifications of 200—2000.
In nuclear physics, nuclear photographic emulsions are usually used in the form of layers glued to glass backings. In research on high-energy particles (at accelerators or in cosmic rays) the emulsion layers are sometimes stripped from the backing and assembled into large stacks of several hundred layers. The volume of the stacks reaches tens of litres — a practically continuous photosensitive mass is formed. After exposure the individual layers are glued to glass backings and processed. The positions of the layers are marked, and this makes it possible to follow the trajectory of a particle through the whole stack, passing from layer to layer.
The range of a particle of charge Q and velocity v in a nuclear photographic emulsion before the particle stops is proportional to the mass M of the particle. At a sufficiently high velocity the grain density (the number of developed grains per unit length of track) is g~Q2/v2. If the grain density is too great, the grains merge into a continuous black track. In this case (especially at large Q) a measure of the velocity of the particle may be the number of secondary so-called δ-electrons, which form characteristic branches along the track. Their density is also ~Q2/v2. If Q=e (the charge of the electron) and v~c, then the track of the particle in a relativistic nuclear photographic emulsion has the form of a broken line of 15—30 black grains per 100 μm of path.
The scattering of particles can be measured in a nuclear photographic emulsion. The mean angular deflection per unit path is q>~Q/pv (p is the momentum of the particle). A nuclear photographic emulsion can be placed in a very strong magnetic field and the momentum of the particle and the sign of its charge measured, which makes it possible to determine Q, M and v. The advantages of the nuclear photographic emulsion method as a track detector of particles are its high spatial resolution (phenomena separated by a distance of ~1 μm can be distinguished, which for a relativistic particle corresponds to flight times of ~ 10-16 s) and the possibility of accumulating rare events over a long period. Pi-mesons were discovered by the nuclear photographic emulsion method, and the interaction of π- and K-mesons after they stop was detected. With the help of the nuclear photographic emulsion it proved possible to estimate the lifetime of the π0-meson, to detect the decay of the K-meson into 3 pions, and to discover the ∑-hyperon, hypernuclei and the anti-lambda hyperon. The composition of the primary cosmic radiation was investigated by the nuclear photographic emulsion method, and it was proved that besides protons it contains nuclei of He and of heavier elements, up to Fe. In the 1950s international expeditions were organised to lift many-litre emulsion stacks on balloons into the high layers of the atmosphere and to various geomagnetic latitudes. Parts of the stacks were distributed among dozens of laboratories around the world, which worked to agreed programmes. This made it possible to accumulate large statistics in a short time and led to some of the discoveries listed above.
Although bubble chambers have displaced the nuclear photographic emulsion in research on high-energy particles, the latter is still in use. Nuclear photographic emulsions are also used in autoradiography: a small quantity of radioactive atoms is introduced into the structure of the object under study, these atoms reveal their presence by their decays, and a nuclear photographic emulsion placed near the object can indicate where they are. To increase the resolution and the sensitivity of the method, the nuclear photographic emulsion is sometimes applied in liquid form directly to the object, or thin layers of nuclear photographic emulsion stripped from the backing are used. Both the total blackening can then be measured and individual tracks recorded, reaching a spatial resolution of ~1 μm.