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Stable Dosimeter Based on a CdS Single Crystal
It is known that CdS crystals can be used as counters of elementary particles and as dosimeters for X-rays[^1],[^2],[^3]. However, numerous experiments have shown that such dosimeters are extremely unstable.
It turns out that placing a photoresistor made from a CdS single crystal in a vacuum leads to such high constancy and reproducibility of the photocurrent that it becomes possible to use CdS single crystals as reliable dosimeters for X-rays[^4].
The method for manufacturing a vacuum X-ray dosimeter consists of the following.
Among CdS crystals grown from the gas phase, specimens are selected according to the following criteria, which ensure the highest quality of the dosimeter:
1) single crystals as perfect as possible, with a length of 5–15 mm, a width of 1–2 mm, and a thickness of 0.01–0.05 mm;
2) crystals with a small dark-current value (not more than \(10^{-7}\) A at a drawing voltage of 30 V);
3) crystals with the greatest possible rectifying effect.
From Current Literature
The crystal selected in this way is glued to a glass bead fused to two soldered-in electrodes, and then, in order to clean the surface, is subjected to electron bombardment. Gold electrodes are deposited in vacuum on the cleaned specimen in such a way as to connect the crystal with the electrodes soldered into the bead. To eliminate the influence of air, which manifests itself in the occurrence of prolonged changes both in the magnitude of the photocurrent (at constant illumination) and in the magnitude of the dark current, the specimen mounted on the bead is placed in an evacuated glass vessel. The dosimeter, assembled in the form of an electron tube with two leads on the base, has a height of 3 cm and a diameter of 0.6–1.5 cm. If the dosimeter is illuminated immediately after assembly, it turns out that the excited photocurrent is small. In order to achieve a considerable increase in conductivity upon illumination of the dosimeter, it must first be
Electrical circuit of the dosimeter:
B — rectifier, L — glow-discharge lamp.
subjected to forming. Forming consists in the mounted specimen being slightly heated (for example, by passing current through it) and thereby losing the water adsorbed on the surface.
Photoresistors made of CdS by the method described above retained their properties and stability in operation for many months.
In vacuum CdS photoresistors, the strong fluctuations of photocurrent disappear, and their inertia is considerably smaller than that of photoresistors not enclosed in vacuum (“air” ones).
If, when an “air” photoresistor is illuminated with modulated light, a change in the polarity of the applied voltage leads to a delayed rise of the current to the maximum value, then for vacuum photoresistors the polarity of the applied voltage is immaterial in the sense indicated above. An important advantage of a CdS photoresistor as an x-ray dosimeter is the fairly large magnitude of the photocurrent, which makes it possible to use ordinary measuring instruments for its measurement.
Despite the fact that vacuum CdS dosimeters are sufficiently stable in operation, it has so far not been possible to obtain an unambiguous dependence of the magnitude of the photocurrent on the degree of “hardness” of the x-ray beam (at constant beam intensity).
The x-ray dosimeter is mounted as a portable instrument, which can be moved on a frame in two mutually perpendicular directions. The electrical circuit of the dosimeter is given in the figure.
The indicator is a galvanometer with a shadow pointer. The instrument is powered by alternating current from the mains. The drawing voltage is about 30 V.
Such a simple and compact instrument can easily be used and widely applied in industry and medicine for determining the intensity of X-ray radiation.
In conclusion, we would like to draw attention to the following circumstance.
As B. T. Kolomiets has shown, not only single crystals but also polycrystalline CdS samples are good photoconductors in the visible region of the spectrum. In this connection, the question of whether polycrystalline CdS samples can be used as X-ray dosimeters is of undoubted interest.
I. Ts.
References Cited
- R. Hofstadter, UFN 39, 426 (1949).
- R. Frerichs, Phys. Rev. 72, 594 (1947).
- H. Kallmann, R. Warminsky, Ann. d. Phys. 4, 69 (1948).
- H. Simon, Ann. d. Phys. 12, 45 (1953).