SETUP FOR MEASURING γ-CURRENTS IN SOLIDS

S. M. Ryabykh, Yu. A. Zakharov

(Presented by the scientific seminar of the Department of Radiation Chemistry and the Laboratory of Chemical Technology of the Scientific Research Institute of Physics and Technology)

In studying the decomposition of solid substances under the action of ionizing radiation, important information on the mechanism of radiolysis can be obtained by measuring the electrical conductivity of a sample before irradiation, during irradiation, and after irradiation has ceased.

The setup used for measuring electrical conductivity under irradiation must satisfy a number of requirements. Since substances subjected to radiolysis are often dielectrics or high-resistance semiconductors, their dark conductivity is small, and the γ-currents initiated by radiation are also small \((10^{-9}—10^{-17} a)\). Therefore, the sensitivity of the measuring circuit must be no less than \(10^{-13} a\) in current. The circuit must have low inertia so that it is possible to record the change in γ-current at the beginning of irradiation and after irradiation has ceased (the kinetics of the rise and decay of γ-conductivity).

The design of the setup must provide the possibility of carrying out investigations over a wide temperature range. Pickups induced by irradiation should be reduced to a minimum.

In the present work a setup is described for measuring γ-currents in pressed tablets of solid substances, one which, in our view, largely satisfies the requirements set forth above.

Fig. 1. Electrical circuit of the setup for measuring γ-conductivity of solids

The schematic diagram (Fig. 1) of the setup does not differ from those used earlier [1—3]. The recording instrument was a U1-2 direct-current amplifier, whose inertia did not exceed

1.5 sec. The current sensitivity of the circuit is \(10^{-14}\) A. The recording part of the setup is located behind shielding, and the remote unit of the amplifier is in the irradiation chamber. The latter was protected by a massive lead housing. All supply leads were made as coaxial cables, whose shielding was carefully grounded.

The design of the setup is shown in Fig. 2.

Fig. 2. Setup for measuring \(\gamma\)-conductivity of solids:
1—lead collimator; 2—electrodes; 3—shielding cylinder; 4—Teflon washers; 5—vacuum cell; 6—shield

The measuring electrodes 2 (preferably made of a noble metal) and the thermocouple are placed inside a vacuum glass cell (5), connected by a ground joint to the vacuum system. By carrying out the experiment in vacuum it is possible to avoid significant parasitic currents caused by ionization of air. Sometimes it is possible to avoid these pickups by carefully grinding the electrode and the specimen (3) to one another; however, this is not always possible.

During the experiment, a vacuum of no worse than \(10^{-6}\) torr was maintained in the cell, achieved by a combination of an RVN-20 fore-vacuum pump and a glass diffusion oil pump. It was found that, beginning at a pressure of \(10^{-2}\) torr, the magnitude of the pickup does not depend on pressure; that is, the parasitic emf in the pressure range below \(10^{-2}\) torr is due only to electron emission from the electrodes under irradiation.

For thermostating, the cell is equipped with a water jacket through which water or oil from a thermostat flows.

For operation at temperatures below \(0^\circ\)C, a modified design of the vacuum cell is proposed (Fig. 3). Liquid nitrogen or any other cooling mixture is poured into volume (A). The vacuum

the cell is placed in a grounded shield made of brass foil. The tablet, whose diameter in our case was 10 mm, is clamped around its perimeter between the electrodes by means of sliding Teflon washers. A beam of radiation 4 mm in diameter is cut out by a lead collimator (1), which was oriented so that it fell at the center of the tablet. In this way it is possible to avoid irradiation of the electrodes, which

Fig. 3. Vacuum cell for measuring the γ-conductivity of solids at low temperatures.

[in figure:] to the vacuum system

would lead to a parasitic e.m.f.; at the same time the collimator shields the lead-in wires from radiation. Since the radiation beam is parallel to the electrodes, it is possible automatically to “cut off” the so-called “direct” γ-current caused by the flux of fast electrons moving in the direction of the radiation. This part of the γ-current can be measured if the applied electric field is perpendicular to the radiation beam [4].

Despite the use of the collimator, the parasitic e.m.f. arising from irradiation of the electrodes by scattered radiation is still rather large (≈ −0.3 V). To reduce it, a shielding cylinder (3), insulated from ground and from the electrodes, was used (proposed in [2]). Under the action of the radiation the cylinder acquires a charge approximately equivalent to the charge on the electrodes, which leads to their mutual compensation. With the use of the shielding cylinder, the magnitude of the parasitic e.m.f. was reduced to −0.05 V. In the temperature interval from 0°C to 100°C, the magnitude of the pickup does not depend on temperature.

The apparatus described was used to measure γ-currents in lead azide [5] and proved to be sufficiently sensitive and stable in operation.

REFERENCES

1. F. T. Farmer. Nature, 150, 521 (1942).
2. V. G. Tsukerman. PTE, No. 6, 192 (1965).
3. G. Kramer, M. Ghalla, J. Canfield. J. Chem. Phys., 45, 1346 (1966).
4. R. A. Meyer, F. L. Bouguet, R. S. Alper. J. Appl. Phys., 27, 1012 (1956).
5. Yu. A. Zakharov, S. M. Ryabykh, N. P. Kharchenko. This collection, p. 71.