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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Radio amateur dosimeter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Dosimeters Ionizing radiation is dangerous to humans in any dose. In small cases, its impact is very masked - the consequences can appear years later, decades later, and even in the next generations (oncology, genetic damage, etc.). With an increase in the level of exposure, not only the likelihood of such consequences increases, but disturbances occur in the human body that can lead to death in a matter of days, hours, or even directly "under the beam" *. So to know the level of radiation, to be able to at least approximately estimate it does not seem superfluous. Having found an increased level of ionizing radiation, it is natural to inquire about its source. What is it: secretly buried radioactive waste? An accelerator from a nearby research institute? X-ray machine, "shining" in the wrong direction? Isotope "mine" of an enlightened killer? A discarded fire detector? radioactive mineral? Dinosaur bone?.. What is the activity of the discovered? The configuration of its radiation?.. To answer all these questions, we need a device capable of measuring the level of ionizing radiation in some units. Schematic diagram of an amateur radio dosimeter that measures ionizing radiation in the NRF - in units of natural background radiation (Df@15 μR/h), is shown in fig. 74**. The radiation sensor BD1 in the dosimeter is a Geiger counter type SBM20, sensitive to g- and hard b-radiation (see annex 4). Its reaction to the natural radiation background is current pulses following without visible order at an average speed of Na=20...25 imp/min***. The count rate in Geiger counters is linearly related to the level of radiation.
So, for a tenfold increase in its level, the SBM20 counter will respond with a tenfold increase in the counting rate - up to Nrad \u200d 250 ... 20 imp / min. The direct proportionality of the conversion Nrad <->Drad will begin to be violated only at very significant levels of radiation, with the appearance of a large number of pulses separated by a too small, beyond the resolution of the counter, time interval. In the passport of the counter, Nmax is usually indicated - the maximum counting rate. For the SBM4000 counter Nmax=2000 imp/s. And if it preserves the linearity of the transformation Nrad <->Drad at least up to 1 pulses/s, then it will be possible to numerically estimate the radiation fields in the range Drad = (5000...XNUMX) Df by the count rate - more than sufficient for a household appliance. The recommended supply voltage for the SBM20 meter is Upit = 360 ... 440 V. This voltage range accounts for the so-called plateau: changes in Upit within these limits have little effect on the count rate and there is no need to take measures to stabilize it. In any case - in devices of moderate accuracy. The device that converts the voltage of the battery that feeds the dosimeter into a high voltage Upit at the anode of the Geiger counter is based on a blocking generator (T1, VT1, etc.). On the step-up winding I of its transformer, a short - 5 ... 10 μs - pulse with an amplitude of 440 ... 450 V is formed, charging capacitor C1 through diodes VD2, VD1. The pulse repetition rate of the blocking generator F@1/2R6 C3@40 Hz. Each ionizing particle that excites the Geiger counter causes a short, avalanche-like discharge. Arising on the meter load, resistor R1, voltage pulses are fed to a single vibrator (DD10.3, DD10.4, etc.), which forms "rectangular" pulses of duration tf1 from them@R7 C7@0,2 ms and an amplitude sufficient to drive CMOS microcircuits. All time intervals and frequencies needed in the device are generated by the counter DD1. Its master oscillator operates at the frequency of the ZQ1 quartz resonator - 32768 Hz. The counting unit of the dosimeter is composed of three decimal counters DD4, DD5, DD6, whose luminescent indicators HG1, HG2 and HG3 indicate, respectively, "units", "tens" and "hundreds", and one binary counter - DD7, representing "thousands". The outputs of the decimal counters are connected to the corresponding segments of the fluorescent indicators, and the outputs of the DD7 counter are connected to the decimal points of the same indicators, on which "thousands" are displayed in binary code: °°° - "0", °°* - "1", °* ° - "2",..., ** ° - "6", *** - "7" (° - dot "not lit", * - dot "lit"). The capacity of the counting node is thus increased to "7999". The counter DD3 forms the unit of measurement adopted in this device. If its sensor is in the conditions of a normal background radiation, then at the measuring interval tmeas=39 s (this is the duration of "zero" at the output M of the counter DD1), an average of Nf 3/39=(60...20) 25/39@16 impulses. Those. normal, at Nrad@Nf on the counter display will be fixed: "000" if Nrad<16, or "001" if 16 The measuring interval tmeas ends with tind - a 3-second demonstration of the measurement result. It is formed by the counter DD2. For a time t, the input of the counting unit is blocked and the device (VT3, VT4, T2, etc.) is turned on, which converts the supply voltage of the microcircuits into a much lower supply voltage for the incandescences of fluorescent indicators. Its shape is a meander, the frequency is 32768 Hz. The indication interval tind ends with the transfer of all counters of the device to the zero state. And then a new measurement cycle begins. The device is mounted on a single-sided printed circuit board 123x88 mm in size, made of foil fiberglass 1,5 mm thick (Fig. 75). All parts are installed on the board, except for the power switch, sound emitter and Corundum battery. Almost all resistors in the device are of the MLT-0,125 type (R1 - KIM-0,125). Capacitors: C1 - K73-9, C2 - KDU or K2M (for a voltage of at least 500 V), C3, C4 and C5 - K53-1, the rest - KM-6, K10-176, etc ... The Tl transformer is wound on a M3000MN K16x10x4,5 ferrite ring, after smoothing its edges with sandpaper and wrapping it with a thin lavsan or fluoroplastic tape. Winding I is wound first, containing 420 turns of wire PEV-2 0,07. It is placed almost over the entire core, with a gap of 1,5 ... 2 mm between the beginning and end. The winding is carried out almost turn to turn, moving along the core only in one direction. Winding I is also covered with a layer of insulation. Windings II (8 turns) and III (3 turns) are wound with wire PEVSHO 0,15 ... 0,25.
They should be distributed over the core as evenly as possible. When mounting the transformer, it is necessary to observe the phasing of its windings (their beginnings are marked in the diagram with the "•" icon). You should not experiment with this - you can burn the transistor VT1. The T2 transformer is wound on a K10x6x5 ring (ferrite 2000NN). It is prepared for winding in the same way as the core for the T1 transformer. Winding I (400 turns) is wound in two wires (PEV-2 0,07). The end of one half-winding is connected to the beginning of the other, so a midpoint is formed. Winding II contains 17 turns of wire PEV-2 0,25 ... 0,4. Outside, it is recommended to wrap the transformers with plastic tape - a narrow strip cut from sticky PVC. This will protect them from adverse external influences. The transformers are fixed with an MZ screw (thread in the board). Seemingly simpler fastening of the transformer with a wire clip is fraught with danger: the clip can form a short-circuited coil in the transformer; a common mistake, unfortunately. In order to avoid breaking the winding or closing its turns, the fastening must be soft, elastic. The board is mounted on the front panel of the device (high-impact polystyrene, duralumin, etc.), in which a window is cut out against fluorescent indicators. It can be covered with a green filter. On it, in a cutout of the desired size, a ZP-1 or ZP-22 piezo emitter is mounted. And under the LED HL1 make a hole corresponding to its size. The body of the device is a standard plastic box 130x95x20 mm (for example, from under checkers). In order to avoid a noticeable decrease in the sensitivity of the device to soft ionizing radiation, a 10x65 mm cutout must be made in the case wall adjacent to the Geiger counter, which can then be covered with a rare grating. Of course, not all of the above is strictly required. Resistors of the MLT type can be replaced with others of the same size. As VT3, VT4, almost any npn transistors can be taken. If their current gain is small, it may be necessary to slightly reduce the resistance of resistors R9 and R10. It is possible and even desirable to replace the IV3 fluorescent indicators with IV3A, which have a lower filament current. The SBM20 counter is not indispensable either. Any 400-volt Geiger counters with background activity Nf are suitable.@24 pulses/min. In this case, no changes will need to be made to the device circuit. If Nf is different, then between the outputs 1, 2, 4, 8 and 16 of the counter DD3 and the input of the accumulator counter, you need to turn on a diode-resistor decoder, in which, by installing the appropriate diodes, a number should be dialed, possibly closer to 0,65 Nf . The diagram fragment (Fig. 76) shows how to do this for Nf = I6. Here 0,65 Nf@11, which is in binary code and typed in the decoder. On the printed circuit board there is a place for installing a diode-resistor decoder.
Another way is also possible: the required Nph can be obtained by connecting several insensitive Geiger counters in parallel. For example, a "battery" of five counters SBM10 or SBM21 is suitable. The parameters of the most suitable Geiger counters for household dosimeters are given in Appendix 4. Table 12
LED HL1, which turns on when the accumulator counter overflows, i.e. at a very high level of ionizing radiation, it should be red and possibly brighter: AL307KM, AL307LM, etc. The parameters of the transformer T1 are chosen so that when the battery is discharged, the voltage on the Geiger counter remains within the limits of the plateau of the counting characteristic. Table 12 shows the dependence of the counting rate on the supply voltage of the device at a constant activity of the radiation source. Table 13 shows the dependence of the current consumed by the device on the voltage of the power supply. The mass of the device with the battery "Korund" - 225 g. The display of the accumulator counter can also be made on liquid crystal indicators. Schematic diagram of this unit with a display type IZhTs5-4/8 is shown in fig. 77. Since there are four digits in the IZHTS5-4/8 display, the "thousands" counter is made here similarly to the previous ones - on the K176IE4 decimal counter. In a dosimeter with an LCD, of course, no filament voltage generation unit is needed. Therefore, the elements VT3, VT4, T2, R9, R10 can be removed, and DD9.1 and DD9.2 can be used for another purpose (otherwise, their inputs must be connected to "ground" or "+" of the power source). Table 13
The DD7 counter can be saved, but only to generate an alarm: when "8000" appears on the display - a radiation level that is 8000 times higher than the level of natural background radiation - it will turn on an alarm sound and light alarm. Another feature of the LCD is that the signal on its segment must be in the form of a meander. The segment becomes visible (black) if its meander is in antiphase with the meander of the LCD substrate (pins 1 and 34), and remains background, not highlighted if their phases coincide. The K176IE4 counter generates meanders of a "single" and "zero" phase at its outputs, if a reference meander with a repetition rate of several tens or hundreds of hertz is applied to its input S (pin 6). It is possible, for example, to connect the inputs S of all four counters to the output F (frequency 1024 Hz) of the counter QD1. The energy efficiency of a dosimeter with a liquid-crystal display will, of course, be much higher than with a luminescent one. *) Homo sapiens is one of the most sensitive biological species to ionizing radiation. The lethal dose for humans is 600 roentgens. **) Natural background radiation as a kind of test generator makes it possible to calibrate a household dosimetric device, including home-made ones, without resorting to the help of any services. This non-strict unit made it possible at one time to legalize home-made dosimetric devices. ***) Some part of N. must be attributed to the counter itself, in particular, to the effect on it of radioisotopes included directly in its design. In good Geiger counters, this component N. is quite small and is usually not taken into account in household appliances. Publication: cxem.net
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