ABSTRACTS

HYDRAULIC PARTICLE COUNTER AND ITS APPLICATION

The Geiger counter (in one design or another), together with the Wilson chamber, is at present one of the principal instruments by means of which the problems of atomic and nuclear physics are studied. It is enough to recall the remarkable use of Geiger counters for detecting positrons in the apparatus of Blackett and Occhialini.* There are many different designs of Geiger counters; however, in all previous types the current pulse produced as a result of the discharge is too weak to cause a noticeable action—for example, to make a loudspeaker sound or to turn an electric numerator—and for this the use of amplifiers is necessary.

By using the properties of a water jet, it has been possible to develop a particle counter that requires no special amplifiers and at the same time is extremely simple. Its construction is as follows: from a thin glass tube a stream of water flows out under slight pressure. At a distance of 2–3 centimeters from the end of the tube the stream breaks up into drops. Somewhat above this part of the stream, the bottom of a tin can is placed. Near the edge of the tube, at the side and at a distance somewhat less than one millimeter, the end of a thick wire is placed. A voltage of the order of 2,000 V is applied to the wire through a large resistance (about \(100\,000\,000\ \Omega\)), and it is adjusted so that a spark discharge just does not occur.

When a particle passes between the stream and the wire, a discharge flashes, and the voltage instantly falls. Owing to the fall in voltage, the attraction of the stream to the wire changes, and the stream experiences a shake, clearly noticeable from the click it produces when striking the tin membrane.

The energy needed to obtain the acoustic effect is supplied by the water stream, and the spark is only the regulator. During the discharge the water stream experiences not only a shake but also a slight deflection, which can easily be used for objective registration. One may directly act with the stream upon a light pen that records the appearance of particles on a strip of paper, or else, with its aid, close an electric contact and obtain strong pulses of current, enabling one, for demonstration purposes, to light a lamp or, in measurements, to actuate a numerator.

One may, abandoning the mechanical action of the stream, use optical registration. In this case a light beam is directed perpendicular to the stream at the place where the stream still retains its transparency. The illuminated part of the stream is imaged on the slit of a photographically registering drum. Such an arrangement makes it possible not only to count particles, but also to observe the very process of deflection of the stream during individual discharges. In this way it has been possible to show that the deflections are identical for particles of different kinds. Registration of particles by this method proceeds reliably and practically without interference.

The sharp interruption of the discharge is explained in this counter by the deflection of the stream and thereby by the instantaneous increase of the discharge distance. In addition,

* See Uspekhi fizicheskikh nauk, XIII, 491, 1933.

in this counter there is no change with time in the properties of the surface of one of the electrodes, which is observed in ordinary counters, since the surface of the jet is renewed continuously. Owing to this, the action of the counter is considerably more stable.

The hydraulic counter described can be used to register individual photoelectrons. By illuminating a water jet with a mercury lamp, one can observe individual discharges similar to those obtained from elementary particles. In this way it is possible to count individual electrons knocked out by ultraviolet light from the surface of the water. In this, of course, only electrons knocked out from the part of the jet located opposite the wire are counted. By inserting a quartz filter that retains the shortest ultraviolet radiation causing the photoelectric effect from the surface of water, one can greatly reduce the number of particles. Until now it has been believed that ultraviolet rays with wavelength \(\lambda = 200\,m\mu\) do not in general cause a photoelectric effect in water. The new method, which for the first time made it possible to observe directly the elementary photoelectric effect in liquids, showed that water is sufficiently photoactive. This observation permits the supposition that the ultraviolet rays contained in the solar spectrum knock photoelectrons out of water and water vapor and, consequently, take part in the creation of electric charges in the atmosphere (Helvet. Physica Acta, 7; 360—367, 514—517, 1934, Naturwiss. 45, 761, 1934).

K. Wulfson,