From Current Literature

Wilson Diffusion Cloud Chamber

In an ordinary expansion Wilson chamber, a certain time is required for the completion of the processes that ensure the registration of ionizing particles, and therefore in such a chamber continuous observation of radiation is impossible. It is, however, desirable in a number of cases. In this connection, the so-called Wilson diffusion chambers, which continuously maintain a sensitive state, are of interest. Chambers of this type have been described in the literature 1, 2, 3; however, satisfactory results with diffusion chambers have been obtained only recently.

A simple chamber using the diffusion of saturated vapor from a heated surface to a cold one is described in 4. The chamber consists of a glass cylinder, the bottom of which is cooled with dry ice, while the lid, made of cardboard impregnated with alcohol, is at room temperature. Vapors of ethyl or propyl alcohol, diffusing through a noncondensing gas, create near the cold surface a sensitive layer several centimeters thick, in which condensation of droplets on ions occurs. In order to improve the conditions for observing ionization, a black cloth, which was flooded with a thin layer of alcohol, was placed on the bottom of the cylinder through an opening 10 cm in diameter made in the upper lid. In 5–10 minutes after cooling of the chamber bottom with dry ice begins, an equilibrium state is established, and sharp tracks of ionizing particles are observed in the sensitive layer. The chamber remains sensitive as long as a certain temperature difference is maintained between the alcohol-moistened upper lid and the bottom of the chamber. The chamber was used in a vertical magnetic field ($H = 1000$ oersted), and characteristic curved tracks of electrons and positrons with energies of the order of 1 MeV were observed. Recoil protons and nuclear reactions caused by a Be–Po neutron source placed outside the chamber at the level of the sensitive region were also observed. This chamber was also used to view the radiation from very weak artificially prepared and natural radioactive specimens. For this purpose the radioactive source was placed inside the sensitive volume of the chamber. It is interesting to note that, at distances up to 1 mm from the surface of a specimen placed in the sensitive volume, condensation already occurs on ions and, in this way, it is possible to distinguish ionizing particles emitted from the surface of the specimen from particles formed in the gas by $\gamma$ rays or neutrons. Illumination of the sensitive volume by a daylight lamp does not disturb the operation of the chamber; therefore it is possible to observe continuously the radiation of a radioactive specimen placed inside the sensitive volume.

More complex chambers operating on the same principle are described in [5]. The chamber was designed to work with a cyclotron. The upper cover of the chamber is aluminum; to its lower part is attached a perforated aluminum disk covered with cloth impregnated with alcohol, the humidity of which is automatically kept constant during operation. The bottom of the chamber is made of electrolytically blackened aluminum and is cooled by dry ice, which is pressed against the bottom of the chamber by a plate with springs. The walls of the chamber consist of two glass rings, each 30 cm in diameter. The height of the upper ring is 7.5 cm and that of the lower one 10 cm. The upper ring is separated from the lower by an electrically conducting rubber gasket, which is one of the electrodes serving to create a field that removes ions. The other electrode is the upper and lower covers of the chamber, connected to each other by a conductor. This arrangement of the electrodes makes it possible, when an electric field is applied, to remove ions from the upper part of the chamber without pushing them downward into the sensitive layer. Rubber gaskets were used to seal the joints of the bottom and cover. The parts of the chamber were joined by means of anchor bolts. The chamber operated at pressures from 0.1 to 2 atm, and no substantial changes in its operation were detected when the pressure was varied within these limits.

The camera was placed on the upper cover of the chamber above a small window. Electric heaters mounted in the upper cover made it possible to maintain the temperature gradient needed for the best operation of the chamber. When working with argon and methyl alcohol, the sensitive layer along the axis of the chamber had a depth of 7 cm at a cover temperature of +39° C. When the chamber was filled with air and methyl alcohol, a sensitive layer of 7 cm was obtained at a cover temperature of +44° C. In the latter case the chamber operated worse.

As the temperature of the cover is increased, the thickness of the sensitive layer increases; however, raising the temperature above the optimum causes disruption of the normal operation of the chamber.

The intensity of ionization inside the chamber also affects the thickness of the sensitive layer and the processes of droplet formation, but the chamber operates satisfactorily at ionization several times greater than normal ionization at sea level.

When the chamber is used in work with accelerators, periodic cleaning of the sensitive volume is necessary. Cleaning the sensitive volume of drops condensed on ions is performed by applying voltage pulses of the order of 600 V to the chamber electrodes, immediately following the operating cycle of the accelerator. In this case the chamber operates periodically.

The chamber described was also used to record showers produced by cosmic particles, the photographic process being controlled by counters connected in coincidence.

Specifically for the study of cosmic-ray showers, a rectangular chamber was constructed with principal dimensions 22.5 × 22.5 cm and a height of 17.5 cm. The maximum depth of the sensitive layer when using a mixture of methyl and ethyl alcohol with argon was 6 cm at a cover temperature of +34° C. The sensitive volume was illuminated through the side walls; photography was carried out through the front glass on a background of the black wall at the rear side of the chamber. Cleaning of the sensitive volume in this case was also carried out by means of an electric field. Voltage pulses were applied to the chamber electrodes every 45 sec. In construction, the cover and the bottom do not differ in principle from those used in the chamber described above.

For the study of nuclear interactions in gases, a chamber filled with hydrogen at a pressure of 15 cm Hg was used. The chamber was made—

made of stainless steel. The chamber cover was heated, the bottom was cooled with dry ice, and thermocouples were used to monitor the temperature gradient. Observation of ionization and illumination of the tracks were carried out through thick glass windows.

V. K. Lapidevskii and Yu. A. Shcherbakov

References

1. L. G. Hoxton, Proc. Virginia Acad. Sci. Abstr. 9, 23 (1933–1934).
2. R. E. Vollrath, Rev. Sci. Instr. 7, 409 (1936).
3. A. Langsdorf, Rev. Sci. Instr. 10, 91 (1939).
4. T. S. Needls and C. E. Nielsen, Rev. Sci. Instr. 21, 976 (1950).
5. E. W. Cowan, Rev. Sci. Instr. 21, 911 (1950).
6. D. H. Miller, E. C. Fowler, R. P. Shutt, Am. Phys. Soc., New York meeting, February 1 (1951).