ABSTRACT
" Geiger–Muller counter"
Operating principle
a) Counter and switching circuit. A Geiger–Muller counter, along with a scintillation counter, is in most cases used to count ionizing particles and, above all, particles and secondary electrons generated under the influence of rays. This counter usually consists of a cylindrical cathode, inside of which a thin wire is stretched along its geometric axis on insulators, serving as an anode. The gas pressure inside the tube is usually on the order of 1 Z10 atm.
The schematic diagram for switching on the counter is shown in Fig. Voltage is supplied to the meter U, which for the most commonly used counters reaches 1000 V; resistance is connected in series with the counter R. The voltage drop that causes R when current passes through the meter, can be determined by an appropriate measuring device. An amplifier is most often used for this purpose; for simple experiments, a string electrometer can also be used. Capacity indicated by dotted line WITH represents the total capacitance of the circuit connected in parallel with the resistance R. It is necessary to pay attention to the fact that there is always a negative voltage on the cylinder, since if the poles are connected incorrectly, the meter can be rendered unusable.
b) Discharge mechanism. The action of the described circuit depends significantly on the voltage value U. At very low voltages, the ions formed in the gas between the cathode and the anode under the influence of charged particles move towards the electrodes so slowly that some of them manage to recombine before reaching the electrode. But at a voltage higher than the saturation voltage U 5, all ions reach the electrodes, and if the time constant of the circuit is much greater than the collection time of the ions, then, due to the resistance R, a voltage pulse occurs equal to AU= = ne/S, which decreases over time, like
/>. In this area extending from U$ to tension Upt, the counter acts like a regular ionization chamber.
Under tension Upi the field strength in the immediate vicinity of the anode becomes so high that the number of primary ions produced by the ionizing particles increases due to impact ionization. Instead of h primary electrons arrive at the anode pA electrons. Gas Gain Factor A, increasing with increasing voltage, in the “proportional region” between UPl And Up1 does not depend on primary ionization; therefore, the numbers of voltage pulses that arise, for example, at the resistance A under the influence of a strongly ionizing b-particle and one fast b-particle, will relate to each other as the primary ionizations of both particles. Under tension USY gain A= i, and at the upper boundary of this area it can reach a value of 1000 or more. At voltage higher UR, gain A no longer depends on primary ionization, so that the pulses arising from weakly and strongly ionizing particles are increasingly equalized. At Ugl– threshold voltage, “counter plateau” or “Geiger region” - all pulses have almost the same magnitude, regardless of primary ionization. At voltages higher than the not very clearly defined voltage Ug2 , a large number of false pulses appear, which eventually turn into a continuous discharge.
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Schematic diagram of switching on the counter
Amplitude characteristic of the meter depending on voltage
The counters described below operate in the Geiger region between Ug1 And Ug2 .
The very complex discharge process in the plateau region can be described approximately as follows. Electrons generated during primary ionization create a dense cloud of ions in the immediate vicinity of the anode as a result of the combined action of impact ionization and photoionization by ultraviolet light quanta. Due to the high speed of movement, the free electrons that appear in this cloud reach the anode in a very short time, while at a gas gain of 1000, the slower positive electrons still move slightly away from their places of origin. Since a positive space charge arises directly around the wire, the field strength there for 10 ~6 sec or less decreases so much that impact ionization becomes impossible, and the electron avalanche immediately ends. However, during IO-4 sec positive ions move to the cathode and usually form secondary electrons there when neutralized. These photoelectrons move towards the anode and there cause a new avalanche; As a result, delayed discharges or oscillating corona discharge may occur. The appearance of ions with negative charges or metastable atomic states can also cause such interference. It is believed that the counter of charged particles meets its purpose only if it is possible to suppress these after-discharges. For the latter, it is necessary either to reduce the voltage at the meter for a sufficiently long time after the discharge, or to select suitable gases to fill the meter.
c) Discharge extinction. The voltage on the meter decreases each time it is triggered by an amount
If leakage resistance L large enough, then the range is equal to pAe, drains so slowly that the voltage again reaches the threshold value required to trigger the counter only after all the positive ions have disappeared; Only after this dead time can the counter again be considered ready to count the next particle. It is known from experiments that, for example,
Self-quenching counters that produce discharge pulses lasting only a few ten-thousandths of a second , obtained by filling the meters with a polyatomic gas, such as methane, or by adding such a gas to a noble gas, if the latter is introduced into the meter. These gases apparently gain energy from interfering ions or metastable noble gas atoms upon dissociation; therefore, practically no new electrons appear and no interfering after-discharges occur. Since the quenching gas gradually decomposes mainly due to dissociation, such counting tubes become unusable after IO7–IO9 discharges.
d) Characteristics of the meter. To check the quality of the counter, find the quantity N voltage pulses arising on the resistance R with constant irradiation of the meter depending on the voltage on the meter U. As a result, the meter characteristic is obtained in the form of a curve shown in Fig. Voltage U", at which the first pulses begin to be observed depends on the threshold voltage of the measuring device used, which in most cases is several tenths of a volt. As soon as the pulse height exceeds the threshold value, it will be counted, and with a further increase in voltage N should remain constant as the voltage increases further until the end of the Geiger region. This, of course, does not work perfectly; on the contrary, as a result of the appearance of individual false discharges, the plateau has a more or less pronounced smooth rise. In meters operating in the proportional region, it is possible to obtain an almost horizontal plateau of the characteristic.
The following requirements apply to good counters: the plateau should be as long and even as possible, i.e., if the area between Ug, And Ug2 should be equal to at least 100 V, then the increase in the number of pulses should be no more than a few percent for every 100 V tension; the characteristic must be unchanged for a long time and in a sufficient range independent of temperature; The sensitivity for particles should be practically 100%, i.e. Each counter-particle passing through the sensitive spaces must be registered. It is desirable that the meter have a low threshold voltage and produce large voltage pulses. Below we will dwell in detail on the extent to which these qualities of the counter depend on the filler, the type and shape of the electrodes, and the switching circuit of the counter.
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B) Manufacturing of meters
a) General provisions. Great care and cleanliness are required in the manufacture of meters; for example, small specks of dust, or fragments of electrodes, or small amounts of foreign gases, such as water vapor, can already render the meter unusable. But even when these requirements are met, not every counter is successful, so depending on various circumstances, particle counting may occur with a greater or lesser error. An important role in the manufacture of the meter is played by the absence of dust and thorough cleaning of the electrodes. And glass tube for grease And other contaminants and good vacuum technology. In order for the tube to have a long service life, the filling gas must be kept clean at all times. For this purpose, it is best to use glass tubes with fused electrodes, which can be annealed better in a vacuum. Since it is sometimes impossible to avoid glue joints, it is at least necessary to use an adhesive with low vapor pressure And insignificant solubility in organic gases added to the filler gas to extinguish the discharge.
The counters described below, at the appropriate voltage, can operate as proportional counters if a linear amplifier with a sufficiently high gain is connected between the counting tube and the counting device.
b) Gas filling. 1) Gas pressure. The average specific ionization by fast electrons for most gases is approximately 20 to 100 ion pairs per cm mileage at atmospheric pressure; it is inversely proportional to pressure. In order for such an electron to have a path length of approximately 2 cm probably formed at least one pair of ions in the counter And this would cause a signal in the meter, a minimum pressure of approximately 50 is required mm rt. Art. The upper pressure limit is most often set at this level; at higher pressures the operating voltage on the meter would have to be set too high.
2) Non-self-extinguishing meters. In non-self-extinguishing meters, by selecting a suitable gas for filling them and the corresponding circuit parameters, it is possible to bring the dead time to a value less than 10-4 sec. Successful fillers are noble gases, which, of course, do not have to be exclusively pure; it is better to add a certain amount of another gas to them to eliminate the metastable states of noble gas atoms that appear after the discharge.
The specific ionization of helium is very small, so it should be used at a pressure of at least 200 mm rt. Art.; helium can be used up to atmospheric pressure; therefore it is suitable for counters with very thin windows. The operating voltage even at atmospheric pressure is about 1100 V. Particularly suitable gases are argon and neon, which have high specific ionization and relatively low operating voltage. The addition of up to 10% hydrogen has proven extremely successful, and a small amount of mercury vapor can eliminate metastable states; but the addition of oxygen should be avoided due to the danger of the formation of negative ions at the cathode. If carbon dioxide is used as a filler, the formation of negative ions can be avoided by adding CS2 to it. Negative ions appear in large quantities in the air, so it is not suitable for filling meters. All gases must be thoroughly dried, since negative ions are especially easily formed in water vapor. Organic vapors should also be avoided; they can occur, for example, when using glue.
Argon with the addition of a few percent CO2 and, in particular, pure methane, which at atmospheric pressure slowly and continuously flows from a steel cylinder through a pressure reducing valve into a meter tube isolated from air, are used as a filling gas in proportional meters.
3) Self-extinguishing meters. For self-quenching counters, the dead time is usually several ten-thousandths of a second. To produce high-quality self-extinguishing meters, it is necessary that both the filler and the quenching gas be very clean, since even minor contamination can disrupt the quenching process.
Most often, a mixture of argon and 5–10% ethyl alcohol is used as a filler at a total pressure of about 100 mm rt. Art. The higher the alcohol content, the less smooth the meter plateau is. Traces of water vapor or air, as well as slight nitrogen pollution, lead to deterioration of the plateau. In the presence of alcohol vapor, due to their dissociation under the influence of discharges, the plateau of the meters deteriorates over time, and the operating voltage increases. Good counters V in fused glass tubes, after IO8–10" discharges they fail and must be refilled. Meters made using organic glue are even less stable. Since such meters cannot be calcined, leaving them on a vacuum pump, a discharge is passed through them for 1-2 days; at first they are filled only with alcohol vapor so that the surface of the glue is saturated with alcohol. Only in the following days they are actually filled with gas.
In addition to alcohol, a number of other organic gases or vapors can also be used as a quenching impurity, for example, methylal 2), formic-ethyl ether, methane, xylene, carbon tetrachloride, sulfuric ether, ethylene, etc. The service life of the meters, depending on the properties of the vapors included in the filler, ranges from 10" to IO9 discharges. Methane can also be used as an independent meter filler.
With an anode wire diameter of 0.1, the gas pressure is from 50 to 120 mm rt. Art. the threshold voltage ranges between 800 and 12U0 V, if the meter uses vapors of organic substances as quenchers.
Of the diatomic gases, only halogens can be used as a quenching additive for noble gases; this additive should be only a few thousandths, since otherwise negative ions will be formed, disrupting the quenching process. Since the halogen molecules do not decompose, the service life of the counter is not limited in this regard. According to Libzon and Friedman, neon is especially suitable for filling counters, which is added to a mixture of four parts argon with one part chlorine in an amount of 0.1–1%. With a total pressure of 200 to 500 mm rt. Art. The operating voltage ranges from 250 to 600 V. Argon with the addition of a few thousandths of bromine or neop with chlorine also gives a low threshold voltage; however, the plateau in this case is less good.
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c) Cathodes. Copper is the most suitable material for cathodes; in addition, graphite, silver, gold and platinum can be used; They are used, in particular, in glass counters in the form of thin coatings. Stainless steel and brass can also be used. Metal tubes are well polished inside and thoroughly cleaned with alcohol or acetone before installation. Metals turned on a lathe or ground show spontaneous electron emission immediately after processing, which gradually disappears. Therefore, it is recommended to warm up the mechanically processed cathodes before assembling the meter or leave them in the air for 24 hours.
For reliable cleaning of copper cathodes, in particular in non-self-quenching meters, a mixture of equal parts of 50% nitric acid and 90% sulfuric acid is used, which is diluted with 5–10 parts of water. After treatment with this composition, the cathode is washed 5–10 times with water, and finally with distilled water; then heat the tube for about 2 hours in a high vacuum at a temperature of 350–400 ° C. If the filler contains an admixture of hydrogen, then the copper cathodes are reduced in hydrogen; if oxygen is a constant component of the filler, then the cleaned cathodes, after intense heating in air or oxygen, are covered with a thin film of oxide. It is also recommended to heat it in an atmosphere of nitrogen oxide until a film is formed that is colored dark purple.
Some metals, such as aluminum and lead, are sometimes difficult to use as cathode materials. But if, despite this, they still have to be used, then the inside of the tube is covered with aquadag or a thin layer of copper, depositing it by evaporation in a vacuum. If it is necessary to solder brass plugs into an aluminum tube, then the ends of the tube are clad with copper.
The optimal sensitivity of the counter for studying X-ray needles is achieved by making the thickness of the cathode wall approximately equal to the path length of secondary electrons in a given material. The sensitivity of the counter for radiation, i.e. the proportion of quanta counted by the counter in relation to all quanta entering the counter depends on the material of the cathodes and on the radiation energy. The sensitivity of aluminum cathodes decreases from 2% at an energy of 10 kee to about 0.05% at 100 energy kee and then increases again by 1.5% at 2.6 Aiae. Sensitivity of copper or brass meters at 10 kab and 2.6 Mev approximately the same; its minimum lies between 200 and 300 kee and is about 0.1%. Cathodes made of heavy metals, such as lead or gold, have a sensitivity that decreases unevenly from 3–4% at 10 kee to about 0.8% at 600 kee, and then increases again to 2% at 2.6 Mav Anodes. It is best to use tungsten wire with the same diameter along its entire length as anodes. You can also successfully use wires made of other metals, such as kovar, stainless steel and regular steel. Since the operating voltage increases with increasing wire diameter, it is necessary to use the thinnest wire possible: the lower limit of the diameter is about 0.08 mm; with a diameter greater than 0.3 mm, there is no longer a good plateau.
To fuse the wire into the glass wall of the meter or into the glass insulator, appropriate sections of wire with a thickness of 0.5–1 are welded to both ends of the wire by spot welding mm for fusing into glass. Before installation in the meter, the wire must be thoroughly cleaned; Under no circumstances should you touch the wire with your fingers. It is better to calcinate it all in a high vacuum or in a hydrogen atmosphere. If the design of the meter is such that both ends of the wire protrude outward, then the wire is calcined immediately before filling the meter with gas. To obtain a certain effective length of the anode, both ends of the wire are enclosed in thin glass capillaries or in metal pins that protrude slightly into the cathode; the wire can be limited in length using fused glass beads or glass rods.
In proportional counters, to prevent small discharges towards the anode along the surface of the insulator, it is recommended to surround the anode input with a protective ring, the potential of which is constant and approximately equal to the anode potential.
Glass counter
e) Shape of meters. Below are instructions for making counters yourself.
1) Dimensions. Counters can be very different in shape and size, which is explained by the wide variety of their applications. In most cases, meters with a cathode diameter between 5 and 25 are used. mm and anode wires with lengths from 2 to 20 Cjh; When studying, for example, cosmic rays, much longer counters are used. In general, the length of the counter should be many times greater than its diameter. Since the dead time of the counter increases approximately in proportion to the square of the cathode diameter, it is better to use several small-diameter counters connected in parallel instead of one large-diameter counter; for example, instead of a one-meter counter with a diameter of 3 cm you can use a complex of seven counters, each with a diameter of 1 cm, which are fused into one glass tube and have a common gas filling. In very long self-quenching meters, a shorter dead time can be obtained if the anode wire is divided into several parts by fusing small glass beads with a diameter of approximately 0.5 mm.
Entry into a metal meter with a soldered metal plug, glass insulator and metal base.
Liquid meter
2) Glass counters. The simplest glass counter is shown in Fig. The cathode is a thin-walled metal or carbon tube fused into a glass tube, with ends well rounded or slightly curved outward; You can also deposit a thin layer of metal on the inner walls of a glass tube using vacuum evaporation or chemical deposition. In particular, thin graphite layers, which are obtained by applying a layer of aquadag, are also suitable for this purpose. Before applying metal or graphite layers, it is necessary to clean the glass tube very thoroughly using a solution of potassium dichromate in sulfuric acid or another similar cleaner, since it is necessary that the layer adheres well to the glass; otherwise, if small films separate from the layer, the counter will quickly become unusable. The connection to the cathode is made in the form of a thin wire fused into a glass tube. For a soft soda glass tube with a wall thickness of less than 0.8 mm a graphite layer can be applied to the outside of a glass tube: the conductivity of thin layers of glass is sufficient to allow current to pass through the wall.
Counter with thin mica bottom
Since most cathodes, already under the influence of visible light, emit a small amount of photoelectrons that drive the counter, it is necessary to carefully protect the counters with screens from the action of light rays during measurements. It is best to coat glass covers with a light-proof, well-insulating varnish or ceresin, into which an opaque, fat-soluble dye is added. .
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3) Metal counters. The simplest way is to make a counter from a metal tube, both ends of which are closed with well-fitted insulators glued with picein or, if they will operate at high temperatures, with araldite. Brass pins drilled along the length with a thickness of 3 to 4 are installed in the insulators in the center mm with well rounded edges, protruding several mm inside the tube. The anode wire is pulled through the holes in the pins and soldered at their outer ends. In addition, a thin glass tube is installed in one of the insulators for pumping and filling the meter. Ebonite easily releases gas, which quickly renders the meter unusable; therefore, such insulators should only be used in those cases where the service life of the meter is not important. It is better to use plexiglass, trolitol and similar materials; however, more suitable materials for insulators are glass or ceramic substances such as porcelain, soapstone, etc. For glass insulators, the use of glue can be avoided by using glass tubes with metal tubes fused to them. These glass tubes can be soldered with their metal ends into brass plugs that terminate the metal meter. The anode wire is fused in the same way as in glass tubes. In Fig. In addition, a metal base is shown attached to the meter, with a plug pin for connection to the shielded cable that leads to the amplifier. Ceramic insulators can be coated with copper around the edges and soldered to metal cathodes.
4) Thin-walled particle counters. Due to the low penetrating ability of particles for their research requires very thin-walled counters. b-particles with energy 0.7 Mevno longer kicked through glass or aluminum thickness 1 mmor through copper thick 0,3 mm. With tube diameter from 10 before 15 mmmore glass counters can be pumped out And aluminum , if the wall is very uniform in thickness. Thin aluminum tubes are best made from duralumin, while thick flanges can be reinforced at the ends of the tube to increase stability. If the gas filler contains halogens, then it is recommended to insert a stainless steel wire spiral almost close to its walls as a cathode into a thin-walled glass tube; the spiral must have a pitch equal to several mm, and consist of three parallel wires.
A meter for studying liquids is shown in Fig. A thin-walled glass tube is fused to the outer glass tube of the meter so that liquid can be introduced into the narrow interstitial space between the tubes. In this case, the liquid should fill this space to the upper end of the meter tube . In order to increase the efficiency of counting low-energy electrons, it is necessary to have a very thin window in the counter tube, for example from a mica sheet, as shown in Fig. The mica foil is placed on a heated flange, evenly lubricated with glue, mounted on the end of the meter tube, and pressed with a hot metal ring, also lubricated with glue. Mica window with diameter from 20 to 25 mm stable to a thickness of approximately 2 to 3 mg/cm2 , those. rounded 0.01 mm. Wire thickness 0.2 mm is fixed in the meter only at one end; directly behind the window it ends in a glass bead with a diameter of 1–2 mm.
The glass window can be made with a thickness of 10 to 15 mg\cmG. For this purpose, the glass tube is heated from the fused end over a length of 1–2 cm until almost completely softened; then its melted end is heated very strongly and air is drawn into the tube as quickly as possible so that it takes the shape shown in Fig. The inner part of the tube is fused to the outer wall; then the tube breaks off approximately at the place shown in the figure by the dashed line, and the edge of the tube melts.
Making a thin glass window
B) Amplifiers for meters
a) Input circuit. To register and count the number of voltage pulses appearing on the resistance R counter, a large number of schemes have been developed, of which only some of the simplest will be described here.
In self-quenching counters, pulses are supplied to the measuring circuit either directly or through a pre-amplifier, which in the simplest case consists of one pentode or two triodes with resistive-capacitive coupling between the stages. Pulses entering the circuit are converted into pulses equal in size and shape. For this purpose, for example, a thyratron can be used in a trigger circuit in which the capacitor NW discharges through the thyratron as soon as the grid voltage under the influence of positive pulses exceeds the blocking voltage. The negative blocking voltage is usually approximately 5% of the anode voltage; To ensure reliable quenching, the grid voltage is set 5–10 times lower than the thyratron shut-off voltage. Thyratrons filled with helium have a response time of about 10 ~ 5 sec, and those filled with argon take a slightly longer time.
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Thyratrons are very expensive, so in most cases, especially when high resolution is required, triggers on vacuum vacuum tubes are used. An example of this
device is shown in Fig. Both triodes have a common resistance in the cathode circuit; in a steady state, current flows through the first triode , while the second triode is locked with a grid voltage negative relative to the cathode. A negative pulse from the counter, amplified by the first triode, is applied in positive polarity to the grid of the second triode and unlocks the lamp. The first triode, due to cathodic coupling, is locked and remains in this state until the positive charge on the capacitance in the second grid circuit flows through the leakage resistance, as a result of which the circuit returns to its stable state. This occurs for each counted pulse whose value exceeds the threshold value by approximately 1 V; on the anode of the second triode there is a negative rectangular pulse of 50vi with a duration of 100 μsec serves to control the conversion circuit. It is best to use double triodes of the 6SN71 type as amplification tubes in this circuit; however, you can, of course, use the corresponding individual triodes.
A similar circuit, which simultaneously serves as a damping circuit, is shown in Fig. Here, in steady state, current flows through the second lamp while the first lamp is closed.
Input multivibrator circuit
Pulse from the counter through capacitors with a capacity of 0.001 mkf and 27 pf arrives at the grid of the second lamp and leads to a “rollover”, so that a negative rectangular pulse of approximately 270 V appears at the anode of the first lamp, which is supplied as a quenching pulse to the meter filament through the coupling capacitor, as a result of which its voltage drops to zero. The duration of rectangular pulses is adjustable within the range of 150–430 μsec using variable resistance 5 Mom. The negative pulse for controlling the subsequent conversion circuit is removed from the voltage divider in the anode circuit of the first lamp, while the positive pulse from the voltage divider of the second lamp is used to control the mechanical counter.
Input circuit as quench circuit
According to F. Droste, in the diagram shown in Fig. you can also make a damping circuit if the cathodes of the meter are not grounded, but connected to the anode of the input lamp; in this way a damping pulse of at least 200 is obtained V.
b) Conversion circuits and mechanical counters. Conventional electromechanical counters are used to count pulses. However, to match the resistance of the counter coil with the output resistance of the amplifier's final tube, it is necessary to increase the number of turns of the coil so that its resistance is several thousand ohm It is easiest to use a telephone meter for this purpose, in which the coil with a relatively small number of turns is replaced by a coil with a number of turns from 5000 to 10,000. The meter, together with capacitors with a capacity of 0.01 to 0.1, is included in the anode circuit of a thyratron or output lamp, the power of which is sufficient to operate the meter. The positive pulse from the voltage divider in the previous circuit is fed to the thyratron, while the terminal triode or heptode can also be controlled by a negative pulse if the quiescent current of these lamps is chosen in such a way that the meter armature is attracted at rest and released when a pulse appears.
Due to the relatively large inertia of mechanical counters, significant miscalculations occur even at counting speeds of about 100 pulses per minute.
Mechanical meters with low inertia can only be manufactured at great expense. It is much easier to achieve reliable results if you include a conversion circuit in front of the counter, which transmits, say, only every second pulse to the mechanical counter. If you turn it on in series h such circuits, then only every 2n pulse will arrive at the mechanical counter. In Fig. Two widely used conversion schemes are given. A circuit using the principle of a symmetrical multivibrator has, in contrast to the asymmetrical circuits shown in Fig. two stable states in which, according to circumstances, one lamp is closed while the other conducts current. Double diodes are included in the circuit to cut off positive pulses. Their cathodes are at the potential of the anodes of the trigger lamps, so the filament of the heated cathodes of these diodes must be powered from a separate source. A negative pulse is applied to the anode of only the gated triode. The potential of the anode of the other triode is significantly lower than the potential of the cathode of the diode and passes through the isolation capacitor to the grid of the unlocked triode . This triode is turned off, and the circuit goes into a second stable state, in which it remains until the next counting pulse arrives. Several such triggers are connected in series as shown in the figure. Setting the zero of the recalculation circuit is carried out by breaking for a short time the key indicated in the diagram by the word “zero”. Thus, before counting begins, the second trigger lamps are open. On neon lights G.L., connected to the anodes of the first trigger lamps, there is no voltage. At the first pulse, a current passes through the first lamp of the first trigger, the neon lamp “1” lights up, but the positive pulse arising on the second anode is not transmitted to the second trigger. With the second pulse, the first trigger returns to its initial state, neon lamp "1" goes out, a negative pulse on the second anode causes the second trigger to overturn, and neon lamp "2" lights up.
Let us assign the numbers 1, 2, 4, 8, 16, etc. to the neon lamps of successive triggers. Then the total number of pulses received at the input of the cell counting circuit, the last of the cells of which controls a mechanical counter through the final lamp, will be equal to the reading of this counter multiplied by 2" plus the number shown by the burning neon bulbs. So, for example, if the first, fourth and fifth lights are on, then you need to add the number 25.
Conversion scheme
Simple ten-day counting circuits can also be assembled from commercially available special counting lamps, such as ElT1dekatron, trachotron or EZh10.
c) Average value indicator. You can obtain a reading proportional to the average counted number of pulses per unit time if, for example, you measure the average anode current of the thyratron in the circuit shown in Fig. The inertia of the device, which is necessary to reduce current fluctuations associated with the statistical distribution of pulses, can be obtained if a galvanometer with a series-connected resistance of several com bypass with a large capacitor with the highest possible insulation resistance. This device is calibrated in imp\min by comparing its readings with the readings of the conversion circuit. In addition, a number of capacitors are provided Cs, C4 and resistances Rs of various sizes, which can be switched on as desired using a switch. In this way you can change the area
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measurements over a wide range. If a conventional output tube is used instead of a thyratron, then the anode quiescent current flowing through the galvanometer must be compensated. Other schemes for counting the average number of pulses per minute can be found in the literature.
d) Voltage stabilization. For accurate measurements, the voltage on the meter must be kept as constant as possible. This is done, for example, by stabilizing a series of small glow discharge lamps connected in series, consuming little current. The meter amplifier often works satisfactorily also with unstabilized voltage; however, it is better to stabilize its anode voltage.
D) Statistical errors and their correction
a) Statistical errors. If for a certain time it is calculated N pulses, then the average statistical error of this result is ±Х ~N. Due to the presence of cosmic rays and radioactivity in the environment, each counter, even in the absence of a radiation source, produces a small background . This background can be significantly reduced by shielding the meter on all sides with a layer of lead or iron several centimeters thick. For each measurement, the background must be determined in advance. If for the same time in the presence of a radiation source it is calculated N impulses, and without it N pulses, then the radiation effect is N– N pulses, and the average statistical error of this value is
b) Correction for limited resolution. If the most inertial element of the counting device has a resolution time h seconds and the average counting rate is N"imp/sec, then the true average count rate
Therefore, for example, with an average value N" = = 100 imp/sec and resolution timef = 10~s sec the miscalculation is 10% of the total number of pulses.
Slide 1
Slide 2
Slide 3
Slide 4
Slide 5
The presentation on the topic “Geiger Counter” can be downloaded absolutely free of charge on our website. Project subject: Physics. Colorful slides and illustrations will help you engage your classmates or audience. To view the content, use the player, or if you want to download the report, click on the corresponding text under the player. The presentation contains 5 slide(s).
Presentation slides
Slide 1
Slide 2
Geiger counter, Geiger-Müller counter - a gas-discharge device for automatically counting the number of ionizing particles entering it. It is a gas-filled capacitor, which breaks through when an ionizing particle passes through a volume of gas. Invented in 1908 by Hans Geiger. Geiger counters are divided into non-self-quenching and self-quenching (not requiring an external discharge termination circuit)
Slide 3
Geiger counter in everyday life
In household dosimeters and radiometers produced in the USSR and Russia, meters with an operating voltage of 390 V are usually used: “SBM-20” (slightly thicker in size than a pencil), SBM-21 (like a cigarette filter, both with a steel body, suitable for hard β- and γ-radiation) “SI-8B” (with a mica window in the body, suitable for measuring soft β-radiation)
Slide 4
Geiger-Muller counter
A cylindrical Geiger-Muller counter consists of a metal tube or a glass tube metallized from the inside, and a thin metal thread stretched along the axis of the cylinder. The thread serves as the anode, the tube as the cathode. The tube is filled with rarefied gas; in most cases, noble gases are used - argon and neon. A voltage of hundreds to thousands of volts is created between the cathode and anode, depending on the geometric dimensions of the electrode material and the gaseous environment inside the meter. In most cases, widespread domestic Geiger counters require a voltage of 400 V.
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Gas-discharge Geiger counter. The basis of a Geiger counter is a tube filled with gas and equipped with two electrodes to which high voltage is applied. The counter operates based on impact ionization. When an elementary particle flies through the counter, it ionizes the gas, and the current through the counter increases very sharply. The voltage pulse generated at the load is supplied to the recording device.
Slide 5 from the presentation "Particle Research Methods". The size of the archive with the presentation is 956 KB.Physics 9th grade
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A cloud chamber can be called a “window” into the microworld. It is a hermetically sealed vessel filled with water vapor or alcohols close to saturation.
The cloud chamber played a huge role in studying the structure of matter. For several decades, it remained practically the only tool for the visual study of nuclear radiation. In 1927, Wilson received the Nobel Prize in Physics for his invention.
Geiger counter
Geiger counter(or Geiger-Muller counter) is a gas-filled counter of charged elementary particles, the electrical signal from which is amplified due to the secondary ionization of the gas volume of the counter and does not depend on the energy left by the particle in this volume. Invented in 1908 by H. Geiger and E. Rutherford, later improved by Geiger and W. Muller.
Counter application
The Geiger counter is used mainly for recording photons and y-quanta.
The counter registers almost all electrons falling into it.
Registration of complex particles is difficult.
Bubble chamber
The bubble chamber was invented by Donald Glaser (USA) in 1952. Glaser received the Nobel Prize for his discovery in 1960. Luis Walter Alvarez improved the Glaser bubble chamber by using hydrogen as a superheated liquid. And to analyze the hundreds of thousands of photographs obtained from bubble chamber studies, Alvarez was the first to use a computer program that allowed him to analyze the data at very high speed.
The bubble chamber uses the property of a pure superheated liquid to boil (form steam bubbles) along the path of a charged particle. A superheated liquid is a liquid that has been heated to a temperature above its boiling point for the given conditions.
The overheated state is achieved by a rapid (5-20 ms) decrease in external pressure. For a few milliseconds, the camera becomes sensitive and is able to detect a charged particle. After photographing the tracks, the pressure rises to its previous value, the bubbles “collapse” and the camera is ready for use again
Completed by: Andrey Andreyenko
Gomel 2015
Geiger-Muller counter - invented in 1908 by G. Geiger, later improved by W. Muller, who implemented several varieties of the device. It contains a chamber filled with gas, which is why this device is also called gas-filled detectors.
The principle of operation of the meter The meter is a gas-discharge volume with a highly inhomogeneous
electric field. Most often, meters with coaxially located cylindrical electrodes are used:
the outer cylinder is the cathode and a thread with a diameter of 0.1 mm stretched on its axis is the anode. The internal, or collecting, electrode (anode) is mounted on insulators. This electrode is usually made of tungsten, which produces a strong and uniform wire of small diameter. The other electrode (cathode) usually forms part of the meter shell. If the walls of the tube are glass, its inner surface is covered with a conductive layer (copper, tungsten, nichrome, etc.). The electrodes are located in a hermetically sealed tank filled with some gas (helium, argon, etc.) to a pressure of several centimeters to tens of centimeters of mercury. In order for the transfer of negative charges in the counter to be carried out by free electrons, the gases used to fill the counters must have a sufficiently low electron sticking coefficient (as a rule, these are noble gases). To register particles with a short range (α-particles, electrons), a window is made in the counter tank through which the particles enter the working volume.
a - end, b - cylindrical, c - needle-shaped, d - jacketed counter, d - plane-parallel
Geiger counters are divided into non-self-quenching and self-quenching
External discharge suppression circuit.
In gas-filled meters, positive ions travel all the way to the cathode and are neutralized near it, stripping electrons from the metal. These extra electrons can lead to another discharge if steps are not taken to prevent and extinguish it. The discharge in the meter is extinguished by the inclusion of a resistance meter in the anode circuit. In the presence of such resistance, the discharge in the meter stops when the voltage between the anode and the cathode decreases due to the collection of electrons at the anode to values less than those necessary to maintain the discharge. A significant disadvantage of this scheme is the low time resolution, on the order of 10−3 s or more.
Self-extinguishing meters.
Currently, non-self-extinguishing meters are rarely used, since good self-extinguishing meters have been developed. Obviously, in order to stop the discharge in the counter, it is necessary to eliminate the reasons that maintain the discharge after the passage of an ionizing particle through the volume of the counter. There are two such reasons. One of them is ultraviolet radiation generated during the discharge process. Photons of this radiation play a dual role in the discharge process. Their positive role in a self-extinguishing meter
Discharge propagation along the counter filament; the negative role is the ejection of photoelectrons from the cathode, leading to the maintenance of the discharge. Another reason for the appearance of secondary electrons from the cathode is the neutralization of positive ions at the cathode. In a normally operating counter, the discharge should be interrupted at the first avalanche. The most common method of quickly extinguishing a discharge is to add another gas capable of extinguishing the discharge to the main gas filling the meter. A meter with such filling is called self-extinguishing.