IONIZATION DETECTOR OF NUCLEAR RADIATION OPERATING WITHOUT AN EXTERNAL VOLTAGE SOURCE

As is well known, all the usual types of radiation detectors based on measuring the ionization effect (ionization chambers, proportional counters, Geiger counters) require an external voltage source.

Below a new type of ionization detector operating without an external source is described.^1

The operation of the detector is based on the following phenomenon. When radiation ionizes a gas located between two electrochemically dissimilar metals, then, when the electrodes are connected to each other, a current arises, flowing from the electrochemically less active metal to the more active metal. The magnitude of this current depends on the nature of the electrodes, on the type and intensity of the radiation, on the nature and pressure of the gas, and (if gamma rays are incident) on the stopping power of the electrodes.

Thus, for example, if the electrodes are made of gold and lead, the current flows from gold to lead. When gold is replaced by copper, the magnitude of the current decreases, but the polarity remains the same. By oxidizing the surface of the copper electrode, the current can be increased to a value exceeding that obtained with a gold electrode. If both electrodes are made of lead, no current can be detected. On the other hand, replacing gold by aluminum leads to a change in the sign of the current.

Potentiometric measurements have shown that the e.m.f. of such an element depends exclusively on the nature of the electrodes and does not depend on the nature or pressure of the gas, on the geometry of the element, or on the nature of the ionizing rays. Below are given the potentials of several electrodes relative to an aluminum electrode (more precisely, Al₂O₃).

E.m.f. of elements having a negative electrode of aluminum (Al₂O₃)

The order of the electrodes in this table agrees in the main with their order in the usual series of electrochemical potentials.

The magnitude of the current, in contrast to the e.m.f., depends on the density of ionization of the gas located between the electrodes (i.e., on the intensity and nature of the

ionizing rays and on the nature and pressure of the gas), and also on the magnitude of the external load. As the gas pressure is reduced, the current approaches zero.

It is evidently possible to draw a fairly complete analogy between the described element and ordinary galvanic elements; the basic difference consists in the fact that in the latter the source of ions is the dissociated electrolyte, whereas in the “gas” element the ions are produced by external irradiation.

The gamma-ray detector in which the phenomenon described above was used consisted of two coaxial cylinders, the corresponding surfaces of which were coated with a layer of magnesium oxide and lead dioxide. The emf of such an element was 1.24 V. The space between the cylinders was filled with argon to a pressure of 5 atmospheres. The gamma-ray source was placed in the middle of the smaller cylinder and, consequently, the geometrical efficiency exceeded 50%.

The proportionality between the current and the activity of the source was verified with the aid of a series of Co\(^{60}\) preparations of known activity.

At present, gamma-ray detectors are being manufactured whose sensitivity ranges from 0.1 to 10,000 μC over the full scale.\(^2\) The elements described have also been used in X-ray dosimeters and are calibrated in milliroentgens/hour over the range from 0.1 to 10,000 mR/hour over the full scale.

In conclusion it should be noted that the effect described was also used for measuring the potentials of various electrodes and for determining the rate of formation of oxide films.

L. B.

Cited Literature

1. P. E. Ohmart, J. Appl. Phys. 22, 1504 (1951).
2. Rev. Sci. Instr. 23, 144 (1952).