WILSON DIFFUSION CLOUD CHAMBER WITH A HEATED BOTTOM AND COOLED LID

An attempt to create a stably operating chamber in which the diffusion of vapor through a noncondensing gas would proceed from bottom to top was undertaken as early as 1939 [1]. As the vapor source there served a surface of butyl alcohol poured onto the bottom of a vessel, the vapors of which diffused through hydrogen. However, this chamber operated unsatisfactorily—

creatively because of spontaneous condensation of vapor near the cooled surface, which, apparently, is explained by an unsuccessful combination of the gases used and the vapor. The use of other fillers made it possible to obtain a stably operating diffusion chamber with a heated bottom².

One of the chambers described in work³ is shown schematically in the figure. The bottom of the chamber consists of two glass disks and is heated by water circulating in the gap between them. The metal cover is cooled by alcohol flowing through a soldered copper tube attached to it. The inner surface of the upper cover is made conical in order to ensure drainage of the condensate forming on it. The diameter of the glass cylinder of the chamber is 25 cm. The height of the chamber was varied from 3.8 to 15.5 cm,

and no significant changes in the operation of the chamber were found. The electric field was usually applied between the bottom and the cover. Photography was carried out through the glass bottom of the chamber; the authors present photographs of recoil-proton tracks from a Be–Po neutron source. The chamber was filled with helium; the vapor source was a mixture of 90% propanol and 10% water poured onto the bottom. The experiment shows that when the upper cover is cooled below −10° C, stable operation of the chamber is disrupted by convection.

In a chamber with vapor diffusion from bottom to top, the possibility of convection imposes restrictions on the choice of the temperatures of the upper and lower covers.

This, in turn, limits the possibilities of a diffusion chamber with a cooled cover in comparison with an ordinary chamber with a cooled bottom and vapor diffusion from top to bottom.

In chambers with a cooled bottom, under certain conditions one can obtain a volume sensitive to tracks almost equal to the volume of the chamber. Thus, for example, in a chamber 25 cm in diameter and 14.5 cm high, with a working mixture consisting of 70% methyl alcohol and 30% water, the height of the sensitive layer reached 12 cm at temperatures of the upper and lower covers of +50° C and −60° C, respectively. In a chamber with vapor diffusion from bottom to top, however, the maximum sensitive layer had a thickness of only 3.4 cm. This result was obtained with a chamber 13 cm high, filled with helium, with a working mixture consisting of 50% propyl alcohol and 50% water; the upper cover was cooled to −10° C, and the bottom was heated to +60° C.

A comparison of chambers with different directions of diffusion shows that, if in a chamber with diffusion from bottom to top the temperatures of the heating and cooling surfaces are the same as in a chamber with the opposite direction of diffusion, then the thickness of the sensitive layer and the quality of the tracks in both chambers are approximately the same. However, the appearance of convection currents when the upper cover is deeply cooled does not allow such a chamber to operate at temperatures that give the best result in ordinary chambers with vapor diffusion from top to bottom. This explains the fact that photographs of tracks obtained in a chamber with vapor diffusion from bottom to top are inferior in quality to photographs obtained in a chamber with a cooled bottom.

A diffusion chamber can be very simple in construction. But in a chamber with vapor diffusion from bottom to top, as in a chamber with a cooled bottom, the constancy of the working mixture must be maintained. Since the vapors condensed at the upper cover return back to the bottom, it is not necessary to supply the chamber from outside with the liquid serving as the vapor source.

Diffusion chambers operate stably if the density of the gas-and-vapor mixture increases in the direction from the chamber cover toward its bottom. Therefore a chamber with a heated bottom and a cooled cover should operate stably with vapors having a high molecular weight and diffusing through a noncondensing gas with a low molecular weight. In this case, the increase in vapor density at the lower heated surface may be sufficient for the total density of the mixture to increase in the direction from the chamber cover toward its bottom.

V. L. and Yu. Shch.

Cited Literature

1. A. Langsdorf, Rev. Sci. Instr. 10, 91 (1939).
2. O. H. Weedle and C. E. Nielsen, Phys. Rev. 81, No. 2, 324 (1951).
3. C. E. Nielsen, T. S. Needels and O. H. Weedle, Rev. Sci. Instr. 22, No. 9 (1951).