Small-sized indicator of radioactivity. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The described indicator was developed from improvised parts in 1986 after Chernobyl. The goal was to make a small-sized, simple, but quite sensitive indicator of environmental and food pollution. It is known that a person is constantly exposed to radioactive irradiation, both cosmic and terrestrial, the sources of which are radon gas released from the earth's crust, various radioactive minerals found in the soil, building materials, watches and instruments with luminous hands and dials, especially released in the first half of the last century, when radium was used in them. Radiation sources are still used today, for example, in smoke detectors. This problem is described in detail in [1].

Many self-made indicators of radioactivity, for example, described in [2], allow you to notice only a fairly significant excess of the radiation level over the natural background, which is extremely uneven. At a low level of irradiation, flashes of a light indicator or clicks of a sound indicator occur at random intervals, from fractions of a second to units and even tens of seconds. Therefore, when calculating them “in the mind”, it is difficult not to go astray and underestimate or overestimate the danger of the observed level of exposure. For reliability, you have to repeat the procedure several times, accurately observing its duration according to the stopwatch.

True, a slight excess of the background is practically safe for humans under external influence. However, when a radioactive substance gets inside, the picture changes dramatically. Particularly harmful are alpha particles emitted by such a substance, which, for example, has entered the lungs with dust. They intensively destroy surrounding tissues.

The proposed indicator is able to detect very small excesses of the background. He made it possible, for example, to detect radioactive contamination of some samples of tea, dry herbal tea and condensed milk, which could not be determined by counting the flashes of the LED.

The diagram of the indicator is shown in the figure. It consists of a high voltage source, radioactive particle detector (Geiger counter), pulse counter, pulse expander, timer and LED indicators.

Diagram of the radioactivity indicator (click to enlarge)

The device uses a Geiger counter SBT-11 (BD1), since of all the small-sized ones I have, only it, thanks to thin mica (20 ... 25 microns) covering the sensitive window, is capable of registering particles with low energy.

The high voltage source for powering the Geiger counter is assembled according to the blocking oscillator circuit on the transistor VT1, pulse transformer T1 and a rectifier with voltage doubling on diodes VD2, VD3 and capacitors C3, C4.

The current pulses arising in the Geiger counter when radioactive particles or gamma radiation quanta pass through it cause voltage pulses across the resistor R5. Diode VD4 limits the amplitude of these pulses. They go to the input 10 of the counter DD1, and through the diode VD5 - to the pulse expander on the field-effect transistor VT2, causing clearly visible flashes of the HL1 LED. A significant increase in the average frequency of these outbreaks signals a dangerous level of radioactive radiation.

Two nodes are implemented on the K176IE5 (DD1) chip: a counter of pulses generated by a Geiger counter and a timer. After turning on the supply voltage, the counters of the DD1 microcircuit are set to zero by the pulse generated at its input R when charging the capacitor C7. Then, a separate counting of pulses entering input 10 and pulses of the internal microcircuit generator begins, the frequency-setting elements of which are capacitors C8 and C9 and resistors R12 (tuner) and R13.

The generator, together with the second counter of the DD1 microcircuit, forms a timer, a sign of the expiration of the time interval counted by which is the turning on of the HL2 LED connected to the output 9 of the microcircuit.

The HL3 LED connected to output 15 of the first counter turns on when more than 128 Geiger counter pulses have been accumulated in this counter. With a normal background radiation level, the HL2 LED should turn on earlier than HL3, and when it is exceeded, vice versa. This is achieved by adjusting the generator frequency with a tuning resistor R12.

The shorter the time interval between turning on the indicator switch SA1 and lighting the LED HL3, the more intense the radiation. At its high intensity, the HL3 LED blinks, and the blinking frequency increases in proportion to the intensity, and then the flashes merge into a continuous glow. Resistor R9 is used to completely discharge the capacitor C5 when the power is off.

The indicator is assembled in a metal case measuring 120x40x30 mm, all parts are located on the circuit board. To install the Geiger counter SBM-11, a conventional panel for a seven-pin fingertip radio tube is provided. The sensitive window of the counter is covered with a hinged protective cover. The switch and LEDs are located at the end of the case. The indicator is powered by the Krona battery, also located inside its case.

The pulse transformer T1 is wound on a ring of size K17,5x8,2x5 made of ferrite 2000NM. Winding I - 8 turns of PEV-2 wire with a diameter of 0,3 mm, winding II - 3 turns of the same wire, and winding III - 250 turns of PEV-2 wire with a diameter of 0,12 mm. Winding III is wound on the ferrite ring first. It must be well insulated (for example, with fluoroplastic tape) from the ring and from the windings I and II wound over it. It is necessary to strictly observe the phasing of the windings I and II indicated in the diagram. If the blocking oscillator is not energized, the leads of one of these windings should be swapped.

Diodes KD510A can be replaced by any pulse, for example KD522B. Resistor R6 - KIM-0,125 or imported, tuning resistor R12 - SP-38a, the rest - MLT-0,125. Capacitors C3 and C4 - ceramic tubular KT-1 group H70, C5 - any oxide, the rest of the capacitors - ceramic or film. The LEDs indicated in the diagram can be replaced with modern high-brightness ones. Switch SA1 - sliding PD9-1.

Establishing the indicator comes down to setting a high voltage of 390 V (permissible limits 320 ... 460 V) by selecting resistors R1 and R2 and setting the measurement time with a tuning resistor R12. High voltage should be measured with a voltmeter with a high input resistance - 10 MΩ or more.

The measurement time should be such that in the absence of any sources of radiation near the device (except for the natural background), the HL2 LED turns on a little earlier than HL3. It must be taken into account that the background is not constant, so this adjustment will have to be made repeatedly.

In the counting mode, the indicator consumes a current of 0,8 ... 0,9 mA.

Literature

1. Radiation. Doses, effects, risk. Per. from English. Yu. A. Bannikova. - M.: Mir, 1990.
2. Nechaev I. Indicator of radioactivity. - Radio, 2014, No. 10, p. 35, 36.

Author: G. Zakomorny

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