On the Properties of CdS Photoresistors in Ionizing Radiation

In the journal Uspekhi Fizicheskikh Nauk a review by I. Ts. “A Stable Dosimeter Based on a CdS Single Crystal” was published, covering mainly materials published in the foreign literature[^1]. By the time the review appeared, a number of domestic works on the same question had been published, illuminating much more fully the properties of CdS photoresistors when they are irradiated with ionizing radiation.

CdS photoresistors are produced in two types: in the form of single-crystal samples FSK-M (Institute of Physics, Academy of Sciences of the Ukrainian SSR)[^2] and in the form of polycrystalline elements FS-K1 (Leningrad Physico-Technical Institute)[^3],[^4]. Both photoresistors possess considerable sensitivity both to X-radiation and to $\gamma$- and $\beta$-rays up to energies of several MeV. However, the indicated photoresistors differ substantially in their characteristics, which determines the possibility of using them for various purposes of dosimetry.

Fig. 1.

Fig. 2.

Thus, the specific sensitivity of FSK-M, all other conditions being equal, proves to be 7–10 times higher than the specific sensitivity of FS-K1 to ionizing radiation. The maximum permissible operating currents in single-crystal CdS photoresistors, with an active working surface of 1–2 mm², do not exceed 200–300 μA, whereas in polycrystalline samples current values of 500–600 μA can readily be attained, though with a considerably larger working surface. According to recent data, FSK-M can also be manufactured with large...

working surfaces of up to 10–15 mm². In this case, the advantage of higher permissible photocurrents in FS-K1 naturally disappears.

Single-crystal CdS photoresistors have the further advantage over polycrystalline ones of low dark currents, practically equal to zero, and a lower noise level, which makes it possible to obtain stable operation at photocurrents of the order of \(10^{-8}\)–\(10^{-9}\) A and, in individual specimens, \(10^{-10}\) A. In FS-K1 the dark currents in the operating mode reach values of several microamperes.

The advantages of FSK-M are also manifested in their low inertia, which is one or two orders of magnitude lower than the inertia of FS-K1. At considerable intensities of ionizing radiation, the total inertia of FSK-M usually does not exceed 0.1–0.3 sec. At low radiation intensities, with photocurrents of the order of tenths and hundredths of a microampere, its inertia increases tens of times. This dependence of inertia on illumination is almost absent in FS-K1. To reduce inertia in the case of single-crystal photoresistors operating at low levels of ionizing radiation, illumination with the visible region of the spectrum should be used so that the total photocurrent reaches several microamperes.

The indicated photoresistors operate reliably both on direct and on alternating current at voltages from several units to several hundred volts. The possible range of operating voltages in FSK-M is, as a rule, wider than in FS-K1, owing to an increase in the upper limiting values to 600–800 V.

An advantage of polycrystalline CdS photoresistors is the linearity of their volt-ampere (Fig. 1) and dosimetric (Fig. 2) characteristics.

From a comparison of the properties of FSK-M and FS-K1 it follows that the latter photoresistors are desirable to use where direct proportionality between photocurrent and the intensity of the exciting radiation is obligatory, while rapid response is not essential.

The use of CdS photoresistors in various circuits of x-ray dosimeters³ has confirmed the promise of this method for monitoring ionizing radiation.

S. V. Svechnikov

References Cited

1. I. Ts., UFN 51, 416 (1953).
2. V. E. Laksharev, G. A. Fedorus, Izv. AN SSSR, ser. fiz. 16, 81 (1952).
3. B. T. Kolomiets, DAN 83, No. 4, 561 (1952).
4. B. T. Kolomiets, Avt. i tel. 14, 445 (1953).
5. S. V. Svechnikov, ZhTF 22, No. 5, 8, 1305 (1952).
6. S. V. Svechnikov, ZhTF 23, 11, 2094 (1953).
7. S. V. Svechnikov, ZhTF 23, 6, 942 (1953).