CONTROLLING A WILSON CHAMBER USING AN INTERNAL COUNTER

The use of uncontrolled Wilson chambers for the study of nuclear disintegrations—“stars”—is limited by the extremely small probability of registering “stars” in the gas or in the walls of the chamber. This is connected with the fact that the number of nuclear disintegrations inside the chamber amounts to only a few per hour, while the effective operating time of the chamber is on the order of 0.01 sec. Indeed, Hazen¹, in 9000 photographs obtained with an uncontrolled Wilson chamber, registered only 58 nuclear disintegrations, of which only two were formed in the gas of the chamber. Consequently, it is necessary to use a controlled Wilson chamber, triggered only when a nuclear disintegration occurs within its volume. The usual method of controlling a chamber by means of Geiger counters or proportional counters arranged in a definite way outside it cannot be used in this case. A nuclear disintegration can be registered by an external counter only if it has occurred outside the Wilson chamber, since the paths of the particles produced as a result of the disintegration are usually short. It is clear that such a disintegration will not be of interest, since in the best case the chamber photograph will show the track of a fast particle that arose in this disintegration, and not the “star” itself. Attempts² have been made to place inside a Wilson chamber an ionization chamber or a counter with sufficiently thin walls. However, this also did not give satisfactory results.

The authors of the paper under review³ proposed an original method of controlling a Wilson chamber by a proportional counter, located

situated within its volume, the working volume of the counter being at the same time the working volume of the Wilson chamber. The arrangement of the counter is shown in the figure. The counter filament, made of platinum or tungsten wire of diameter 0.1–0.17 mm, is fastened to two brass holders passing through holes in the side wall of the chamber. The holes are sealed by means of small rubber plugs, which also serve to tension the filament. The cathode of the counter consists of six steel rods 2 mm in diameter, arranged in the form of a hexagonal prism. The length of the counter is 25 cm, its diameter 4 cm. The counter is installed in a Wilson chamber 28 cm in diameter and 9 cm deep.

Usually the Wilson chamber is filled with technical argon and saturated with vapors of water and ethyl alcohol. Therefore, in the volume of the chamber

there is usually a considerable number of oxygen atoms. As is known, oxygen is an electronegative gas; therefore electrons arising in the chamber when a charged particle passes through it are very likely to form negative ions. Thus, in an ordinary chamber the trace of a particle is a chain of positive and negative ions on which vapor condensation has occurred.

From the theory of proportional counters it is known that gas amplification is produced mainly by electrons near the counter filament, and that the formation of negative ions sharply decreases the gas-amplification coefficient and makes it depend on the place where the particle enters. Therefore, when working with proportional counters, every effort is usually made to remove oxygen from their volume, so as to prevent the formation of negative ions. The Wilson chamber described by us was filled with technical argon (pressure 118 cm Hg), purified of oxygen, and with vapors of very pure ethyl ...

of alcohol. Such a mixture makes it possible to obtain good tracks in the chamber and at the same time is a satisfactory filling for the counter. It turned out that, for several days after the chamber was filled, the magnitude of the pulses registered by the counter continuously decreases. This is connected with the liberation of oxygen and water vapor from the walls and velvet of the chamber, and also with the diffusion of air through the rubber diaphragm. It turned out that, for satisfactory operation of the counter, it is sufficient to refill the chamber once a week.

In the voltage region from 1950 to 2300 V the counter operates as a proportional counter, and at a voltage of the order of 2500 V—as a Geiger counter. The negative voltage pulses arising at the counter anode when electrons are collected on the filament are amplified by a linear amplifier with a high amplification factor (from 500 to \(3 \cdot 10^5\)).

The amplifier has a diode discriminator, which determines the minimum registered pulse magnitude. In the first experiments the internal counter was included in a triple-coincidence circuit (vertical telescope) with two external counters. The output pulse from the coincidence circuit is used to control the Wilson chamber. The voltage is removed from the filament \(20\ \mu\text{sec}\) after registration of a coincidence, i.e., before the positive ions formed along the track of the registered particle have time to move any appreciable distance. Therefore the particle tracks are clearly visible and are not distorted even inside the effective volume of the counter. On the filament, near the track of the registered particle, an ionization “ball” is usually observed, associated with the formation of positive space charge near the filament during the discharge in the counter. The “ball” can be made very small if a low gas-amplification factor is used (with a high radio-engineering amplification factor) and the discharge is interrupted rapidly.

The effective resolving time of such a chamber turns out to be only \(100\ \mu\text{sec}\) instead of \(\sim 0.02\ \text{sec}\) in ordinary chambers. Indeed, if, some time before the passage of the primary particle that caused the expansion, a particle with smaller ionization has passed through the chamber, it will cause a discharge in the counter, but the voltage will not be removed from the filament. Therefore the positive ions formed during the discharge at the filament will move toward the cathode and, by the time of the expansion in the chamber caused by the passage of the registered particle, will no longer form a “ball” near the filament, but a “ring.” If, however, the particle passed through the counter after the passage of the primary registered particle, then, when the voltage has already been removed, it will not cause any discharge in the counter at all, and neither a “ball” nor a “ring” will be visible near the filament in the photograph. Thus, from the photograph of the discharge in the counter one can determine the moment at which the particle passed through the counter.

If the Wilson chamber is controlled only by the pulse from one internal counter, setting, with the aid of the discriminator, such a “threshold” of the circuit that only strongly ionizing particles are registered, then, apart from cases of nuclear disintegrations, ordinary \(\alpha\)-particles from radioactive contaminations will also be registered. A rough estimate shows that at sea level one photograph of a star will correspond to 500 photographs of \(\alpha\)-particles. If, however, two such “open” counters are placed inside the Wilson chamber at a distance of more than 5 cm from one another and the chamber is controlled by coincidences of these counters, then the number of registered stars sharply increases.

The apparatus described can also be used for studying the phenomena of gas discharge in counters. The photographs presented in the article excellently illustrate the local character of the discharge in the proportional region; the propagation of the discharge along the filament in leaps—

shaped—in the region of limited proportionality, and continuous—in the region of self-sustained discharge (a solid cylinder of positive ions around the filament). These photographs show that the most suitable operating region for the internal counter is the proportional region. In this region, as a result of the discharge, only a small “ball” of positive ions is formed, which, first, does not interfere with observation of the track and, second, helps to identify the particles being recorded.

A. G.

Cited Literature

1. W. Hazen, Phys. Rev. 65, 67 (1946).
2. M. J. Cohen, Phys. Rev. 74, 1244 (1948); 75, 1329 (1949).
3. A. L. Hodson, A. Loria, N. V. Ryder, Phil. Mag. 41, 826 (1950).