SBM-20 counter in a food dosimeter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Dosimeters

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In a food dosimeter (see article "Product dosimeter"in "Radio", 2000, No. 4, pp. 30, 31; No. 5, pp. 40-42) uses a high-sensitivity mica Geiger counter SBT-10, which has a large flat "window" that can be brought very close to the surface However, today this counter is practically unavailable to a radio amateur due to the increased price.

Another, the most common counter in our country - SBM-20 - is a cylinder made of very thin (0,05 ... 0,07 mm) stainless steel. Perceiving ionizing radiation with its entire surface, it can also serve as a food dosimeter sensor, if the shape of this counter is taken into account in the design of the food cuvette. It is also necessary to take into account its relatively low radiation sensitivity.

The product cuvette is made in the form of an open cubic box with an internal space of 60x60x60 mm. The SBM-20 counter is glued into the cuvette so that it takes the central position in it (Fig. 1).

Since here the counter comes into direct contact with the controlled product, after the measurement is completed, its surface must be thoroughly washed. This must be done not only because the remaining radioactivity will introduce an error in the next measurement, but also because even the thinnest residual layer of this product forms an additional filter on the surface of the counter, which will affect its radiation sensitivity.

For washing, a lid with fittings (Fig. 2) is installed on the cuvette, which is fastened to the cuvette with several rubber rings. One of the fittings is connected through a rubber hose to a tap with hot or cold water, the other - outlet - is left at the drain. After washing, the cuvette is dried, and possible traces of moisture are removed from the anode insulator of the counter. The lid can also be used for measurements - it will not allow the liquid product to overflow.

The cuvette can be glued from a sheet of impact-resistant polystyrene with a thickness of 2...2,5 mm. Glue - pieces of the same polystyrene, which are dissolved in dichloroethane or in some other suitable solvent. The glue fixing the meter itself should only be waterproof. Fittings can be polystyrene (then they are glued) or metal. Convenient tubes from lighting equipment with M10x1 thread. For them, holes are made in the lid with the appropriate thread. A rubber gasket can be placed between the cuvette and the lid to prevent water leakage when washing the cuvette.

For the board of the measuring head, which was mounted together with the meter (see article), you need to make a separate case. The counter is connected to the measuring head of the product dosimeter with two multi-colored (in order not to be mistaken in polarity) conductors 10 ... 15 cm long. size and conclusions for SBM-20: 06,5x6,5 mm).

The procedure for calibrating a food dosimeter with such a sensor does not differ from that described. It is necessary to make at least ten measurements with an empty cuvette (radiation background control) and the same number with a cuvette filled with an exemplary emitter.

In a direct experiment, the following was obtained: radiation background - Nf±ΔNf=520±17, where Nf is the number of pulses collected in 31 min 39 s (this is, recall, the exposure in a food dosimeter), ΔNf is the standard deviation from Nf. The same, but with a cuvette filled with potassium bromide (its natural radioactivity Scr = 9700 Bq/kg): NKBr± ΔNKBr=8290±70.

Let us determine the radiation sensitivity of a food dosimeter with such a sensor:

K \u9700d Sqr / (Nkvr - Nf) \u829d 520 / (1,25 - XNUMX) \uXNUMXd \uXNUMXd XNUMX Bq / kg imp.

Hence the measurement accuracy of background values: KΔNF = 1,25-17 = 21 Bq/kg Accordingly, the accuracy of measurements, estimated by the difference between two such values ​​([background + product] - [background]), will be no worse than 2КΔNF = 42 Bq/kg For SBT-10, we recall, 2КΔNF = 40 Bq/kg.

In other words, in terms of the ability to detect radiation contamination (hard β and γ), the SBM-20 counter immersed in the product is almost as good as the SBT-10 counter located above its surface.

Author: Yu.Vinogradov, Moscow

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