Radiation sensor in the security system. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Radiation sensor in the security system. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Dosimeters

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A schematic diagram of a sensor that continuously monitors the radiation situation in the area where it is located is shown in Fig. 78.

Transistor VT1, pulse transformer T1, etc. make up a blocking generator, pulses from the step-up winding I of which, through diodes VD1, VD2, charge capacitor C1 to a voltage of +360...440 V. This capacitor is the power source for the Geiger counter BD1. The voltage pulse that appears on the anode BD1 at the moment of its excitation by an ionizing particle (see oscillogram) is fed to the input of the shaper (DD1.1, DD1.2, etc.), which converts it into a “rectangular” one with a duration of R6 C5@1 ms, and then to input C of counter DD2.

The output of the sensor is output "32" of the DD2 counter, on which signal 1 - a sign of radiation danger - will appear only if Nt - the total number of pulses received at the C-input of the counter during time T - reaches 32. Since Nt depends on Nf - average counting rate of a Geiger counter under natural background radiation conditions and DNф - its fluctuations arising due to the brevity of measurements, then the duration of the measuring interval T should be such that the probability of occurrence Nт+DNt=32 as a result of any measurement would turn out to be negligible (due to the nature of the processes occurring here, it cannot be equal to zero).

Radiation sensor in security system
Rice. 78. Radiation sensor in the security system

So, for example, if in a Geiger counter having Nf=20 pulses/min (this is established by long-term measurements), with exposure T=1 min fluctuations DNt max=10, i.e. do not exceed the limits of 10 pulses/min (this is set during the measurement process Nph), then Nt max=20+10<32 and such a sensor will not raise a false alarm.

It may seem that the measurements could be easily sped up. Taking, for example, T=1/2 min and making the sensor output “16” of the DD2 counter. But if the “constant component” of the background in this case decreases by half and amounts to Nф/2=10, then its fluctuations are only Ц2 once, before DNt max/Ц2 = 10 /Ц2=7. In this case, Nт max=10+7>16 and such a sensor will from time to time give a radiation hazard signal without any reason. With all the ensuing consequences. First of all, distrust of him.

In cases where there is a need to detect the short-term appearance of a radiation source (moving, for example, in a passing car), take a Geiger counter, which has greater radiation sensitivity, or use several insensitive counters, dialing the required sensitivity by their number (Geiger counters can be connected in parallel - at low background count rates, their radiation sensitivities are summed up).

Radiation sensor in security system
Rice. 79. Reset pulse generator to radiation sensor

Appendix 4 shows the main parameters of domestically produced Geiger counters, among which you can choose the most suitable one for solving a particular problem.

Transformer T1 may be the same here as in Storozhe-R. The schematic diagram of the pulse generator that sets the pause T at the R input of the DD2 counter is shown in Fig. 79, a. Although this, as a rule, is not necessary - almost always the security system has an electronic clock with a multi-digit frequency divider and the desired sequence of short pulses can be generated as shown in Fig. 79, b.

Publication: cxem.net

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