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CONTROLLED WILSON CLOUD CHAMBERS FOR RECORDING IONIZING PARTICLES FORMED INSIDE THE CHAMBER
In a controlled Wilson cloud chamber, a particle is recorded only if it produces ionization both in the working volume of the chamber and in the working volume of the ionization device that provides the controlling pulse. Between the moment the particle passes through both instruments and the recording of the track that has been formed, a certain time elapses, which is necessary for collecting the ions in the ionization device, starting the expansion mechanism, growth of the drops, and photography. During this time the ions and drops in the Wilson chamber diffuse away from the place where they were formed.
The growth of drops on ions begins after the expansion mechanism is actuated. The greater the interval of time between the passage of the particle and the expansion, the farther the ions disperse and the more diffuse the track becomes.
The ionization device may be placed either outside the Wilson chamber or inside it. It has been shown that, under certain conditions, the ionization volume of the chamber 1, of a proportional counter 2, or of a counter with a self-sustained gas discharge 3 may be partially or completely combined with the working volume of the Wilson chamber. Thus the Wilson chamber and the ionization chamber may have a common filling and be combined in one instrument—an ionization-Wilson chamber.
The ions and electrons produced by a particle as it passes through the working volume of such an instrument perform two different functions. The electrons, in their motion, provide the controlling pulse, while the positive and negative ions serve as condensation centers after the expansion.
The more negative ions are formed through electron capture, the denser the track, but the smaller the pulse controlling the particle-recording mechanism. Since an electron can be captured along its entire path of motion toward the anode from the place where it was created, some of the negative ions are formed far from the track, which leads to an increase in the fog background.
The probability of electron capture is proportional to the path traversed by it; therefore, in a chamber with electronegative gases, the magnitude of the controlling pulse depends on the place where the primary ionization is formed. Consequently, electronegative gases and vapors, ordinarily used for filling the Wilson chamber, cannot be used
in the ionization–Wilson chamber. In order for the instrument to operate satisfactorily, it is necessary to eliminate the capture of electrons. In such an instrument the ionization chamber must operate on electrons, and the Wilson chamber only on positive ions.
In the work of filling a proportional counter placed in the working volume of a Wilson chamber, ethyl alcohol was used. However, because of the high pressure of the saturated vapor, 44 mm Hg at \(+20^\circ\)C, it is not a sufficiently good filling substance for an ionization chamber.
In the work cited in reference 4, an ionization–Wilson chamber filled with ethyl alcohol operated unsatisfactorily because of a decrease in the pulse amplitude, which was due to electron capture. When the chamber was filled with isoamyl alcohol, no decrease in the amplitude of the pulse produced by \(\alpha\)-particles was observed.
Fig. 1. Longitudinal section of the ionization–Wilson chamber:
\(A\)—reducer, \(B\)—argon, \(C\)—flash lamp, \(D\)—cylindrical lenses, \(E\)—diaphragm, \(G\)—cylindrical ionization chamber, \(H\)—front stop, \(I\)—velvet, \(K\)—rear stop, \(L\)—magnetic valve, \(N\)—stereo camera.
The section of the chamber is shown to scale in Fig. 1. The Wilson chamber is a rectangular vessel made of brass 1.25 cm thick, with glass windows 3.2 cm thick. The useful area covered by the camera was \(25 \times 25\) cm with an illuminated chamber depth of 20 cm.
The chamber was designed for a pressure of \(\sim 5\) atm. Expansion of the gas (99% argon, saturated with vapors of isoamyl alcohol) in the chamber (volume \(F\)) was carried out by connecting the volume \(M\) with the volume \(R\) through the magnetically controlled valve \(L\). The pressures in the volumes \(F\), \(M\), and \(R\) were 2.3, 3, and 1.85 atm, respectively. After expansion, the pressure in \(R\) was restored to the initial value by pump \(P_1\), controlled by the differential manometer \(S\). Pump \(P_2\) maintained a constant pressure in volume \(R\).
The ionization chamber was arranged coaxially with the Wilson chamber in its volume and was a brass cylinder 9.4 cm in diameter and 20 cm long. The central electrode was made of steel with a diameter of 1.5 mm.
The construction of the ionization chamber is seen in Fig. 2.
Cylindrical geometry was used in order to reduce the dependence of the magnitude of the electronic pulse on the place where the ionizing particle passes inside the chamber.
The pulse is amplified and, after the amplitude discriminator, is fed to the input of the control circuit. The electron collection time was 7 μsec. Twelve μsec after the arrival of the signal from the amplitude discriminator, the circuit begins to remove the high potential from the chamber cylinder.
During this process, whose duration was 100 μsec, the positive ions practically do not shift from the place of their formation.
After 0.1 sec after the moment when the particle passes through the chamber, four pulsed lamps are ignited, illuminating the volume located outside the ionization chamber. Each lamp operated from a 48 μF capacitor at a voltage of 1000 V.
The internal volume of the ionization chamber was illuminated by a single pulsed lamp, supplied from a 120 μF capacitor at a voltage of 1100 V. The lamp was located directly beneath the bottom of the Wilson chamber, as shown in Fig. 2.
One of the photographs obtained in this chamber is given in Fig. 3.
The use of an amplitude discriminator for the pulses makes it possible to register only those particles that produce inside the ionization chamber ionization exceeding a predetermined threshold value. For calibration of the discriminator, a polonium source of α-particles, introduced into the ionization chamber, is used. The experiment shows that, at a voltage of 1800 V on the electrodes, the ionization chamber is in the saturation-current regime. At higher voltages the chamber operated unsatisfactorily because of the increase in the noise level, caused by leakage over the insulator.
Fig. 2. Transverse section of the ionization chamber:
A—volume of the ionization chamber, B—central electrode, C—glass rod, D—light guide, E—particle source, F—bottom of the Wilson chamber, G—pulsed lamp, H—copper tube, I—steel wire, L—quartz lead-in, M—latun tube, N—textolite base.
During operation of the ionization-Wilson chamber, the illumination sources and the camera were placed in a light-tight casing covered with sheet copper, for shielding the ionization chamber.
One of the drawbacks of the described instrument is that the ionization volume occupies only part of the working volume of the Wilson chamber. The cylinder of the ionization chamber is opaque, which makes illumination and photography of the tracks difficult.
An ionization-Wilson chamber with a transparent collecting electrode is described in work 5. The chamber, with a volume of 5 l, has three cylindrical electrodes arranged concentrically: a steel electrode (diameter 6 mm), an outer glass electrode (diameter 20.6 cm), and an inner
...also made of glass (diameter 7.75 cm). The wall thickness of the outer cylinder is 0.62 cm, that of the inner one 0.25 cm. The surface of the glass cylinders is coated with a transparent electrically conducting composition, which makes it possible to illuminate the entire volume of Wilson’s chamber.
The outer electrode, formed by the walls of the chamber, and the inner one are grounded; the intermediate cylinder is the collecting electrode and is at a potential of \(+1400\) V. Such a system makes it possible to collect electrons from the entire volume of the chamber.
The magnitude and shape of the pulse arising when ionizing particles pass through the chamber can be determined, and the track formed on the positive ions can be photographed.
This chamber was filled with neon, argon, or krypton and was used for recording stars produced by cosmic radiation\(^6\).
The method considered here for controlling a Wilson chamber with the aid of an ionization chamber placed in its working volume can also be applied to a diffusion chamber (see, for example,\(^{7,8}\)).
In the sensitive region of a diffusion chamber, the growth of droplets on ions begins immediately after the passage of a particle. Since, other conditions being equal, the diffusion rate is smaller the larger the particle size, it is obvious that the track in a diffusion chamber will be sharper than in an expansion chamber, if the time interval between the passage of the particle and its registration is the same in both cases.
Fig. 3.
The controlled diffusion chamber described in work\(^9\) consisted of a glass cylinder 40 cm in diameter and 22.5 cm high. The chamber was filled with argon and operated with vapors of amyl alcohol at a total pressure of 1.4 atm. The height of the sensitive layer was 10 cm.
The ionization chamber was placed in the sensitive region of the diffusion chamber. The outer electrode consisted of six steel rods (rod diameter 1.5 mm, length 22.5 cm), arranged on a circumference 7.5 cm in diameter, and was at a potential of \(-1200\) V. The central electrode (a stainless-steel rod 1.5 mm in diameter) was grounded.
After amplification, pulses with an amplitude discriminator were fed to a circuit that removed the high voltage and triggered the flash lamp.
The delay in firing the lamp relative to the moment at which the particle passed is regulated by a delay circuit.
V. L. and Yu. Sh.
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