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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Product dosimeter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Dosimeters Ordinary household dosimeters reliably register background radiation and ionizing radiation from macroobjects (for example, walls). However, they are not suitable for food testing, which remains relevant in a number of regions of the country. The design of the dosimeter offered to the attention of readers allows to solve this problem to some extent. Pay special attention to the calibration of the instrument. Without reliable calibration, such a device can be considered as an indicator, the readings of which will be the basis for further actions: do not buy the product, take the risk of buying, check for SES.
Recently, the author of the article happened to visit the radiation hygiene department of one of the district SESs in Moscow, where something like this conversation took place: - Can I test a can of instant coffee for radiation contamination? - And why did you decide that it was contaminated? - This device (I am demonstrating the described dosimeter) showed 900 Bq/kg. - How did you calibrate it? - Potassium bromide. (After some thought, my interlocutor asked me to show my ID). - Oh, the press! You might be in trouble... - Why? After all, I visited you in the early 90s, showed my instruments. You introduced me to your technique, to the norms adopted at that time for the permissible contamination of various food products with cesium and strontium-90 ... - No no. It couldn't be! “But now it doesn’t matter so much. The press reported that those norms of radiation contamination of food products are outdated and new ones are in force today. Could you introduce me to them? - No. - What about my coffee? - You know, we have a lot of work now ... This concludes our "conversation". Much has changed in the years since the Chernobyl tragedy. Dosimeters, which then recorded radiation that was many times higher than the natural radiation background. today demonstrate almost complete well-being. But is it? Indeed, over all these years, the activity of strontium-90 and cesium-137 - two of the "famous" trio of Chernobyl radioisotopes - has decreased by only a quarter, and we will never see a decrease in the activity of the third - plutonium-239: its half-life exceeds 24000 years. The reason for the current apparent well-being is simple: rains, groundwater, winds, fires, various biological processes, soil erosion have reduced the concentrations of radioisotopes. Spread over large areas, they became hardly noticeable against the background of the natural radiation of the Earth and Space. As sources of external radiation, such emitters no longer pose the same danger to humans. But getting into his body through the alimentary tract and respiratory tract, approaching the vital tissues as close as possible, they are able to leave such a “trace” on them that could not have arisen even with the strongest external radiation. Therefore, the relatively weak radiation contamination of food products cannot be ignored. Below is a dosimeter capable of detecting this kind of pollution and assessing its level. The device consists of a counting unit and a measuring head. The basis of the counting unit (Fig. 1) is a five-digit counter, made on microcircuits DD1 - DD5. Its status is displayed on the HG1 liquid crystal display. The four least significant decimal places are displayed in the usual way - in the form of numbers. The indication of the older one (tens of thousands) is in binary code using decimal points: (· - the decimal point is visible). Thus, the maximum number that can be fixed in such a counter is 159999. Looking ahead, we note that such a not-too-convenient binary-decimal reading will be required only when calibrating the device; in real measurements, the counter DD5 usually remains in the zero state.
Chips DD6 and DD7 set the time during which the pulses coming from the measuring head are counted. A six-bit counter DD6 counts the fronts at the output M of the counter DD7 (according to positive differences, the first of which appears at the 39th second of the first minute interval). The internal counter oscillator DD7 is stabilized by a ZQ1 quartz resonator. When indicated in Fig. 1 turn on DD6 (the input of the inverter DD10.2 is directly connected to the output 32 of the counter DD6) the measurement will last 31 min 39 s. After this time, the count stops (at the input 12 of the DD9.1 element, a prohibition signal appears log. 0) and an audible signal is turned on, announcing the end of the measurement. log signal. 1, at the input 2 of the element DD9.4, allows the passage of a one-kHz meander, taken from the output F of the counter DD7, to the amplifier DD 10.4-D010.6 and the load connected to it in paraphase - the piezoelectric BF1. With a very high radioactivity of the tested product, the counter DD1 - DD5 may already overflow during the measurement. At the same time, a log signal will appear at the output 16 (vyv. 11) of the counter DD5. 1, which will turn on not only the sound, but also the alarm light signal - the transistor VT1 will turn on the HL1 LED. In alarm mode, zeros are displayed on the display. When you press the SB1 "Start" button, a pulse with a duration of tnyck = 10.1R0,7 C4 = 3 ms is generated at the output of the inverter DD6. It enters the inputs R of all counters and transfers them to the initial zero state. On transistors VT2. VT3 and VD1 zener diode, a stabilizer is assembled, which maintains the dosimeter supply voltage practically unchanged when its power source is rather deeply discharged. Schematic diagram of the measuring head is shown in fig. 2. On the transistor VT4. pulse transformer T1 and elements R14, C6. C8, VD2-VD4 assembled converter. It includes a blocking generator. on the winding L3 of the transformer of which short (tnip = 5 ... 10 μs) pulses with amplitude UL3 = (Uc5 0.2)n3 / n2 are formed (Uc5 is the supply voltage of the converter, n2 and n3 are the number of turns in the windings L2 and L3) When n3 \u420d 2 and n6 \u3d 440 Ul1 \u14d 6 V. These pulses, following with a frequency Fimp \u10d 3 / R4 C420 \u430d 8 Hz, through diodes VD1, VDXNUMX charge capacitor CXNUMX to a voltage of + XNUMX ... XNUMX V, which becomes a power source Geiger counter BDXNUMX.
A shaper is assembled on the DD11 chip. It converts a signal with a steep front and a gentle decay, which occurs at the anode of the Geiger counter at the moment of its excitation by an ionizing particle, into a pulse with a duration tcch = 0,7R18 C10 = 0.35 ms, suitable for transmission to the counting unit via a simple three-wire line. The counting unit is mounted on a board made of double-sided foil fiberglass with a thickness of 1,5 ... 2 mm (Fig. 3).
The foil on the side of the parts is retained almost completely and is used mainly as a common wire. To skip details, it has selections - circles with a diameter of 1,5 ... 2 mm (not shown in the figure). The points of connection with the common wire of the "grounded" terminals of capacitors, resistors and other elements are indicated by solid black squares. The blackened squares with a light dot in the center show the connections to the common wire of certain fragments of the installation, as well as the conclusions of 7 microcircuits DD1 - DD6. DD8 - DD10 and pin 8 of the DD7 chip. Under the indicator, a continuous layer of foil has been removed, and such squares indicate contact pads and holes for transition from layer to layer. Solder pieces of tinned wire into these holes. The correct position of the indicator board is established before its installation. To do this, taking the board by the substrate and touching the soldering iron tip to one or another of its outputs, "set fire" to the corresponding segment of the indicator. The board of a measuring head is shown on fig. 4, the foil under the details is also almost completely preserved.
The Geiger counter SBT10 (SBT10A) has ten separate anodes, their conclusions (1 - 10) are connected to each other by soldering. The connection of the cathode of the counter (pin 11) with the foil of the common wire must also be soldered. Resistors KIM-0,125 (R2. R15) and MLT-0,125 (the rest) are used in the dosimeter. Capacitors C4, C5 - imported oxide (Ø6x13 mm), C6 - K53-30. C8 - K73-9. C9 - KD-2. the rest - KM-6, K10-176, etc. LED HL1 - any, better than a red glow. In the transformer T1, a ring magnetic circuit with dimensions of 16x10x4,5 mm made of M3000NM ferrite is used. The sharp edges of the ring should be removed with sandpaper, and then wrapped with a thin Teflon or Mylar tape. Winding L3 is wound first, it contains 420 turns of wire PEV-2 0,07. The winding is carried out almost turn to turn. A gap of 1 ... 1,5 mm is left between its beginning and end. The L3 winding itself is covered with a layer of insulation, and the L1 winding is wound on top of it with a large step (six turns of PEVSHO 0.15 wire). Then, on this winding, the L2 winding is placed (two turns of the same wire). The windings must be arranged around the ring as evenly as possible and so that their conclusions are as close as possible to the corresponding mounting contacts of the board. To avoid damage to the transformer, it is mounted on the board between two elastic washers. When desoldering the windings, it is important not to make a mistake in their phasing (the dots in Fig. 2 mark the ends of the windings that enter the hole of the magnetic circuit on one side). An error in phrasing will disrupt the operation of the converter. The counting unit board is mounted on a front panel made of impact-resistant polystyrene with dimensions of 122x92x2.5 mm. A polystyrene corner with dimensions of 55x29x17 mm is glued onto it, forming a compartment for the Korund battery. Polystyrene rails are glued to the corner, forming grooves into which the counting unit board will be inserted. A vertical stand 14 mm high, having a thread for an M2 screw, is glued to the front panel. With this screw, through a hole with a diameter of 2.1 mm (see Fig. 3), the board is attached to the front panel. In a convenient place on the panel, a PD9-1 power switch is mounted (not shown in Fig. 1). In the appropriate places of the panel, holes are drilled for the SB1 button and the HL1 LED. A hole with a diameter of 30 mm is cut out under the piezo emitter, onto which a decorative grille is glued on top. A general view of the board mounted on the front panel is shown in fig. 5.
As a housing for the counting unit, you can use a plastic box of suitable dimensions (for example, from under checkers measuring 125x95x23 mm). Beforehand, a groove 2,5 mm deep is cut inside it, in which the front panel will be fixed. The measuring head is mounted in a housing with an internal partition, which is made of sheet high-impact polystyrene 2 mm thick. Its plan dimensions are 94x73 mm, height - 60 mm. The counter is mounted on the partition so that its mica "window" is directed to the cuvette with the test product. The converter board is also attached to the same partition. The depth of the measuring cuvette must be at least 25 mm, its dimensions in plan are 94X73 mm. The cuvette is glued from the same polystyrene sheet. The dosimeter described here uses the "thick layer" measurement method, when the radiation from the lower layers of the product in the cuvette is significantly attenuated or completely absorbed by the upper layers and practically does not affect the readings of the Geiger counter. The "thick layer" method, which makes it possible to estimate the radiation contamination of a product in Bq/kg without weighing it, is widely used by dosimetric control services. The surface of the product filling the cuvette should be as close as possible to the mica "window" of the counter (in the author's version of the dosimeter, this distance is 5 mm). Since the relative position of the controlled sample and the counter affects the measurement result, the design of the measuring head must provide for its precise fixation on the cuvette. Setting up the dosimeter comes down to setting the voltage at the output of the stabilizer within 6,3...6,7 V. It depends on the ratio R11/R10 and is specified by selecting one of these resistors. If desired, the dosimeter blocks can be checked separately. If the input of the counting unit (pin 13 DD9.1) is connected to pin. 4 counters DD7 and press the button SB1, then after 31 minutes 39 seconds the display should show the numbers 1899 - the number of seconds in the measurement interval. The measurement time can be significantly reduced, but only when checking the counting unit. If the input (pin 9) of the inverter DD10.2 is connected to output 4 (pin 5) of the counter DD6. then it will be equal to 3 min 39 s, and when a conjunctor (diode-resistor circuit "I") is connected between them, any measurement interval can be set with an accuracy of up to a minute in the range from 39 s to 62 min 39 s. So, for example, the duration of the measurement when using the conjunctor. shown in fig. 6 will be equal to 55 min 39 s. On the printed circuit board (Fig. 3) there is a place for installing a resistor and conjunctor diodes.
To test the transmitter offline, you will need an oscilloscope in standby mode (sweep 5...10 ms). Its input is connected to the output of the head, and if it is in good condition, pulses of positive polarity with a duration of ~0,35 ms with an amplitude equal to the supply voltage appear on the oscilloscope screen, following without visible order with an average frequency of 1 ... 2 Hz. If you have a static voltmeter with a scale of 1 kV (for example, C50), you can check the supply voltage of the Geiger counter (on capacitor C8). It should be within 360 ... 430V. The manufactured dosimeter needs to be calibrated. How can this be done without outside help? First of all, let's determine the level of natural background radiation. To do this, we put the measuring head on an empty cuvette or filled with water and perform at least 10 measurements one after the other. After that, we calculate the average value of the obtained values - Nf - the number corresponding to the level of natural radiation background, and according to the deviations of each measurement from Nf - the root mean square error - ΔNF [1] - the inaccuracy in determining Nf, the root cause of which is the brevity of the measurement. In a direct experiment, Nf = 3500,ΔNf = 60 was obtained. An exemplary source of radiation will be required to evaluate the radiation sensitivity of the instrument. In this capacity, substances containing potassium are used. The thing is. that the natural mixture of potassium isotopes also contains potassium-40 (0.0118%), a β,γ-emitting radioisotope with a half-life of over a billion years. Its high and stable activity, related to the total mass of potassium, is 29600 Bq/kg [2]. It is this circumstance that makes it possible to use a chemical compound with a known and sufficiently large "share" content of potassium as a test object in the calibration of such dosimetric instruments. Here are some of such compounds KCI - potassium chloride, its activity Skcl = 15700 Bq/kg; K < is 29600 Bq/kg [2]. It is this circumstance that makes it possible to use a chemical compound with a known rather high "share" content of potassium as a test object in the calibration of such dosimetric instruments. Here are some of such compounds KCI - potassium chloride, its activity Skcl = 15700/kg; KBr bromide Ckbr = 9700 K2CO03 potassium carbonate is 29600 Bq/kg [2]. It is this circumstance that makes it possible to use a chemical compound with a known rather high "share" content of potassium as a test object in the calibration of such dosimetric instruments. Here are some of such compounds KCI - potassium chloride, its activity Skcl = 15700/kg; KBr bromide Ckbr = 9700 K2C03 potassium carbonate Br - potassium bromide, CkBr = 9700 Bq/kg; K2C03 - potassium carbonate (potash). SC2CO3 = 16800 Bq/kg (all substances are without crystallization and adsorbed water; if there is any doubt about this, the substance is calcined or dried). Let us fill the measuring cuvette to the brim with an exemplary emitter, for example, potassium bromide, and perform a series of measurements. After averaging the results and calculating the error, we will have: NKBr±ΔNKBr . In a direct experiment, NKBr = 31570, ΔNKBr = 120 were obtained. Let us determine the radiation sensitivity of the device: K = CkBr / (NkBr - Nf) = 9700 / (31570 - 3500) = 0,35 Bq / kg and estimate the measurement inaccuracy in Bq / kg activity of weak emitters: K·ΔNf = 0,35·60 = 20 Bq/kg. Thus, having fixed Nprod - the indication of the dosimeter, in the cuvette of which the test product is located, and Nf - the background level "for today", and calculating their difference, for example, Nprod - Nf = 1000, we will establish that the calculated radiation contamination of the product is K( Nnpod - NF) = 0.35 1000=350 Bq/kg. and the actual differs from the calculated one by no more than K·2ΔNF = ±40 Bq/kg. For a household food dosimeter, this accuracy is quite sufficient. But it can be increased. For example, due to the duration of the measurement (however, it grows rather slowly: with an increase in exposure by n times, the accuracy increases only by Vn). The accuracy of measurements will increase if they are carried out under conditions of low background radiation, for example, underground at a depth of 30...40 m (in the metro). It is possible to reduce the radiation background only in the volume of the measuring head by placing it, for example, in a thick-walled (>3 cm) lead container. The underground and the lead must, of course, be radiation-free. Thus, the measurement accuracy can be increased several times. And in conclusion - about the natural (!) radioactivity of products. Its root cause is the same potassium contained in almost every one of them [3]. The table shows the natural (potassium - 40) specific radioactivity of a number of food products [2]. It must be subtracted from the dosimeter readings. Natural (potassium-40) specific radioactivity of food products, Bq/kg
Literature
Author: Yu.Vinogradov, Moscow
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