Presentation on the topic of the geiger counter in physics. Presentation in physics on the topic: "Experimental methods for the study of particles." Input multivibrator circuit
ESSAY
" Geiger–Muller counter"
Operating principle
a) Counter and switching circuit. Geiger-Muller counter, along with scintillation counter, in most cases it is used to count ionizing particles and, above all, s-particles and secondary electrons arising under the action of g-rays. This counter usually consists of a cylindrical cathode, inside of which a thin wire is stretched on insulators along its geometric axis, which serves as an anode. The gas pressure inside the tube is usually on the order of 1 Z10 atm.
Schematic diagram of the meter connection is given in fig. Voltage is applied to the meter U, which for the most commonly used counters reaches 1000 in; resistance is connected in series with the counter R. The voltage drop that causes R during the passage of current through the meter, can be determined by the appropriate measuring device. An amplifier is most often used for this purpose; for simple experiments, a string electrometer can also be used. Dotted container FROM is 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 incorrectly connected, the meter can be rendered unusable.
b) Discharge mechanism. The operation of the described circuit depends significantly on the magnitude of the voltage U. At very low voltages, the ions formed in the gas between the cathode and the anode under the action of charged particles move towards the electrodes so slowly that some of them have time to recombine before they reach 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, there is a voltage pulse equal to AU= = ne/S, which decreases with time as
/>. In this area extending from U$ up to voltage Upt, the counter acts like a conventional ionization chamber.
At voltage Upi the field strength in the immediate vicinity of the anode becomes so large that the number of primary ions produced by the ionizing particles increases due to the impact ionization. Instead of h primary electrons arrive at the anode pA electrons. Gas amplification factor BUT, increasing with increasing voltage, in the "proportional region" between Upl and Up1 does not depend on primary ionization; therefore, the number of voltage pulses that arise, for example, on the resistance R under the action of a strongly ionizing 6-particle and one fast s-particle, will be related to each other as the primary ionizations of those and other particles. At voltage USA gain A= i, and at the upper boundary of this region it can reach a value of 1000 or more. At a voltage higher UR, gain BUT is no longer dependent on primary ionization, so that the momenta arising from weakly and strongly ionizing particles are more and more equalized. At Ugl– threshold voltage, "counter plateau" or "Geiger region" - all pulses have almost the same value, regardless of primary ionization. At voltages higher than the not very clearly defined voltage Ug2 , appears a large number of false impulses, which eventually turn into a continuous discharge.
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Schematic diagram of the inclusion of the counter
Amplitude characteristic of the counter depending on the voltage
The counters described below operate in the Geiger region between Ug1 and Ug2 .
Highly difficult process discharge 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 that appeared in this cloud free electrons they reach the anode in a very short time, while at a gas amplification factor of 1000 the slower positive electrons still slightly move 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 terminates. However, during IO-4 sec positive ions move to the cathode and usually, when neutralized, form secondary electrons there. These photoelectrons move towards the anode and cause a new avalanche there; as a result, delayed discharges or an oscillating corona discharge may occur. The appearance of ions with negative charges or metastable states of the atom can also be the cause of such interference. It is believed that the counter of charged particles meets its purpose only if it is possible to suppress these post-discharges. For the latter, it is necessary either to lower the voltage on 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 the leakage resistance L large enough, then a charge equal to pae, drains so slowly that the voltage again reaches the threshold value required to trigger the counter only after all 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-extinguishing counters which give discharge pulses only a few ten-thousandths of a second long , obtained by filling 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 appear to be energized by interfering ions or metastable noble gas atoms by dissociation; therefore, practically no new electrons appear and there are no interfering post-discharges. Since the quenching gas gradually decomposes mainly due to dissociation, such counting tubes become unusable after IO7-IO9 discharges.
d) Characteristics of the counter. To check the quality of the counter, find the quantity N voltage pulses arising on the resistance R with constant meter irradiation depending on the voltage on the meter U. As a result, the characteristic of the counter 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 a few 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 is further increased to the end of the Geiger region. This, of course, does not ideally hold; 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 in the characteristic.
The following requirements are imposed on good meters: 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 in voltage; the characteristic must be constant for a long time and in a sufficient area independent of temperature; the sensitivity for β-particles should be practically 100%; each counter-particle passing through the sensitive spaces must be registered. It is desirable that the counter has a low threshold voltage and gives 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 meter switching circuit.
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B) Manufacture of counters
a) General provisions. Great care and cleanliness is necessary in the manufacture of counters; for example, small dust particles, or fragments of electrodes, or small amounts of foreign gases, such as water vapor, can already render the meter unusable. But even if these requirements are met, not every counter is successful, so that, depending on various circumstances, the counting of particles can occur with a greater or lesser error. An important role in the manufacture of the counter is played by the absence of dust, thorough cleaning of the electrodes and glass tube from fat and other contaminants and good vacuum technology. In order for the tube to have a long service life, the filling gas must always be kept clean. For this purpose, it is best to use glass tubes with fused electrodes, which are best annealed in vacuum. Since it is sometimes impossible to avoid joints on glue, it is at least necessary to use glue with a low vapor pressure. and insignificant solubility in organic gases added to the filler gas to quench the discharge.
The counters described below can operate as proportional counters at the appropriate voltage if a linear amplifier with a sufficiently high gain is connected between the counting tube and the counting device.
b) Filling with gas. 1) Gas pressure. The average specific ionization by fast electrons for most gases is about 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 run length of approximately 2 cm probably formed at least one pair of ions in the counter and thus causing a signal in the meter, a minimum pressure of about 50 mm rt. Art. Upper limit pressure is most often set at this level; at a higher pressure, the operating voltage on the meter would have to be set too high.
2) Non-self-extinguishing counters. In non-self-extinguishing meters, by selecting a suitable gas for their filling and appropriate circuit parameters, it is possible to bring the dead time to a value less than 10-4 sec. Successful fillers are the noble gases, which need not, of course, be exceptionally 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 low, so it should be used at a pressure of at least 200 mm rt. Art.; helium can be used up to atmospheric pressure; so it is suitable for counter with very thin windows. Operating voltage even at atmospheric pressure is about 1100 in. Particularly suitable gases are argon and neon, which have a high specific ionization and a relatively low operating voltage. The addition of up to 10% hydrogen has proven to be 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 negative ions being formed at the cathode. If used as filler carbon dioxide, then 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 very suitable for filling counters. All gases must be thoroughly dried, since negative ions are especially easily formed in water vapor. Vapors should also be avoided. organic matter; they can occur, for example, when using glue.
The filling gas used in proportional meters is 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 an air-insulated meter tube.
3) Self-extinguishing counters. For self-extinguishing meters, the dead time is usually a few ten-thousandths of a second. For the manufacture of high-quality self-extinguishing meters, it is necessary that both the filler and the quenching gas are very clean, since even minor impurities can disrupt the quenching process.
Most often, a mixture of argon and 5–10% ethanol is used as a filler at a total pressure of about 100 mm rt. Art. The higher the alcohol content, the less flat the counter plateau is. Traces of water vapor or air, as well as slight nitrogen pollution, lead to the deterioration of the plateau. In the presence of alcohol vapors, due to their dissociation under the action of discharges, the counter plateau worsens over time, and the operating voltage increases. good counters in melted glass tubes after IO8-10 "discharges fail and must be refilled. Counters made using organic glue are even less stable. Since such counters cannot be annealed, 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 agent, 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 has a value between 800 and 12U0 in, if the counter is used as quenchers for vapors of organic substances.
Of the diatomic gases, only halides can be used as a quenching additive to noble gases; this additive should be only a few thousandths, otherwise negative ions will be formed that disrupt the quenching process. Since the halogen molecules do not decompose, the life of the counter is not limited in this respect. Particularly suitable for filling counters is, according to Libzon and Friedman, neon, which is added to a mixture of four parts of argon with one part of chlorine in an amount of 0.1–1%. With a total pressure of 200 to 500 mm rt. Art. the value of the operating voltage lies in the range from 250 to 600 in. Argon with the addition of a few thousandths of bromine or neopa 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. You can also use stainless steel and brass. Metal tubes are well polished inside and thoroughly cleaned with alcohol or acetone before installation. Turned on a lathe or polished metals, immediately after processing, spontaneous electron emission is detected, which gradually disappears. Therefore, it is recommended to warm up the mechanically processed cathodes before assembling the counter or leave it to lie in the air for a day.
For reliable cleaning of copper cathodes, in particular in non-self-extinguishing meters, a mixture of the same parts of 50% is used. nitric acid and 90% sulfuric acid, 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 the tube is heated 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 constant integral part filler is oxygen, then the cleaned cathodes after intensive heating in air or oxygen are covered with a thin film of oxide. Heating under nitric oxide to form a dark purple film is also recommended.
Some metals, such as aluminum and lead, are sometimes difficult to use as cathode material. 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-rays is achieved by making the cathode wall thickness approximately equal to the path length of the secondary electrons in the given material. The sensitivity of the counter for radiation, i.e. the fraction 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 counters at 10 cab and 2.6 mev approximately the same; its minimum lies between 200 and 300 kee and is about 0.1%. Heavy metal cathodes, such as lead or gold, have a sensitivity that decreases unevenly from 3–4% at 10 kee up to about 0.8% at 600 kee, and then increases again to 2% at 2.6 Mev Anodes. As anodes, it is best to use tungsten wire with the same diameter along the entire length. It is also possible to successfully use wires made of other metals, such as kovar, stainless and ordinary steel. Since the working voltage increases with increasing wire diameter, it is necessary to use the thinnest possible wire: the lower limit of the diameter is about 0.08 mm; with a diameter greater than 0.3 mm, no good plateau.
In order to fuse the wire into the glass wall of the meter or into the glass insulator, the corresponding segments of wires with a thickness of 0.5–1 mm for melting into glass. Before installation in the meter, the wire must be thoroughly cleaned; never touch the wire with your fingers. It is better to ignite all of it in a high vacuum or in an atmosphere of hydrogen. If the design of the meter is such that both ends of the wire protrude, then the wire is annealed 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 metal pins, which protrude slightly into the cathode; it is possible to limit the length of the wire using welded-on glass beads or glass rods.
In proportional counters, to prevent small discharges in the direction of the anode along the surface of the insulator, it is recommended that the anode input be surrounded by a protective ring, the potential of which is constant and approximately equal to the anode potential.
glass counter
e) Form of counters. Below are instructions for self-manufacturing meters.
1) Dimensions. Counters can be very different in shape and size, which is explained by the wide variety of their applications. In most cases, counters with a cathode diameter between 5 and 25 mm and anode wires from 2 to 20 Cjh; in studies, for example, of cosmic rays, much longer counters are used. In general, the length of the counter should be many times 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-extinguishing 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.
Entrance to a metal meter with soldered metal plug, glass insulator and metal base.
Liquid counter
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; it is also possible to 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 are also suitable for this purpose, which are obtained by applying a layer of aquadag. Before applying the metal or graphite layers, it is necessary to clean the glass tube very carefully with a solution of potassium dichromate in sulfuric acid or some other similar cleaner, since it is necessary that the layer adheres well to the glass; otherwise, if small films are separated from the layer, the counter will quickly become unusable. The lead to the cathode is made in the form of a thin wire fused into a glass tube. For a soft sodium glass tube with a wall thickness of less than 0.8 mm the graphite layer can be applied to the glass tube from the outside: the conductivity of the thin layers of glass is sufficient to allow the current to pass through the wall.
Counter with thin mica bottom
Since most of the cathodes, already under the action of visible light, emit a small amount of photoelectrons, which actuate the counter, it is necessary during measurements to carefully protect the counters with screens from the action of light rays. Glass cases are best coated with an opaque, highly insulating varnish or ceresin, into which an opaque, fat-soluble dye is added. .
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3) Metal counters. The meter is most simply made from a metal tube, both ends of which are closed with well-fitted insulators glued with picein or, if they will work at high temperature, araldite. Brass pins drilled along the length from 3 to 4 mm thick 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 to their outer ends. In addition, a thin glass tube is installed in one of the insulators for pumping out 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 life of the meter is not important. It is better to use plexiglass, trolley and similar materials; however, more suitable materials for insulators are glass or ceramic materials such as porcelain, steatite, and the like. With 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 the brass plugs that end the metal counter. The anode wire is fused in the same way as in glass tubes. On fig. in addition, a metal base is shown attached to the meter, with a plug-in 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 counters for v-particles. Due to the insignificant penetrating ability of the particles for them studies require very thin-walled counters. p-particles with energy 0.7 mevno longer slip through the glass or aluminum thickness 1 mmor through copper thick 0,3 mm. With tube diameter from 10 before 15 mmmore glass counters can be evacuated 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 should have a pitch equal to several mm, and consist of three wires running in parallel.
The counter for research of 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 injected into the narrow space between the tubes. In this case, the liquid must 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 sheet of mica, as shown in Fig. Mica foil is placed on a heated and evenly lubricated flange, fixed at the end of the meter tube, and pressed with a hot metal ring, also lubricated with glue. Mica window with a diameter of 20 to 25 mm stable up to a thickness of approximately 2 to 3 mg/cm2 , those. rounded 0.01 mm. Wire thickness 0.2 mm strengthened in the counter with only 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, the glass tube is heated from the welded end over a length of 1–2 cm to almost complete softening; then its welded 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 inside of the tube is fused to the outside wall; then the tube breaks off approximately at the place shown in the figure by the dashed line, and the edge of the tube is melted.
Making a thin glass window
C) Amplifiers for counters
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 ones will be described here.
For self-extinguishing counters, the pulses are fed to the measuring circuit either directly or through a preamplifier, which in the simplest case consists of one pentode or two triodes with resistive-capacitive coupling between the stages. The pulses entering the circuit are converted into pulses of equal size and shape. For this, for example, a thyratron can be used in a flip-flop circuit in which the capacitor Sz discharges through the thyratron as soon as the grid voltage under the action of positive pulses exceeds the blocking voltage. The negative blocking voltage is typically about 5% of the anode voltage; To ensure reliable quenching, the grid voltage is set 5–10 below the thyratron turn-off voltage. Helium-filled thyratrons have a response time of about 10 ~ 5 sec, and filled with argon - a slightly longer time.
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Thyratrons are very expensive, so in most cases, especially when high resolution is required, vacuum tube triggers are used. An example of such
device is shown in fig. Both triodes share a common resistance in the cathode circuit; in steady state, current flows through the first triode , while the second triode is locked by the grid voltage being negative with respect to the cathode. A negative pulse from the counter, amplified by the first triode, is fed in positive polarity to the grid of the second triode and unlocks the lamp. The first triode is locked due to cathode coupling and remains in this state until the positive charge on the capacitance in the second grid circuit drains through the leakage resistance, as a result of which the circuit returns to its steady state. This occurs every time a pulse is counted that exceeds the threshold value by approximately 1 in; on the anode of the second triode, a negative rectangular pulse of 50 vi with a duration of 100 microsec serves to control the scaling scheme. Double triodes of the type 6SN71 are best used as amplifying tubes in this circuit, but, of course, corresponding individual triodes can also be used.
A similar circuit, which simultaneously serves as a damping circuit, is shown in Fig. Here, at steady state, the current flows through the second lamp while the first lamp is off.
Input multivibrator circuit
Impulse from the meter through capacitors with a capacity of 0.001 microf and 27 pf enters the grid of the second lamp and leads to a “tipping”, so that at the same time a negative rectangular pulse of approximately 270 V appears on the anode of the first lamp, which is supplied as a quenching pulse to the counter thread through the coupling capacitor, as a result of which its voltage drops to zero. The duration of rectangular pulses is adjustable from 150–430 microsec with variable resistance 5 Mom. A negative pulse to control the subsequent scaling circuit is taken from the voltage divider in the anode circuit of the first lamp, while a positive pulse from the voltage divider of the second lamp is used to control the mechanical counter.
Input circuit as damping circuit
According to F. Droste, in the diagram shown in fig. you can also make a quenching circuit if the meter cathodes are not grounded, but connected to the anode of the input lamp; in this way, a quenching impulse of at least 200 in.
b) Calculation schemes and mechanical counters. Conventional electromechanical counters are used to count pulses. However, to match the resistance of the meter coil with the output resistance of the terminal lamp of the amplifier, it is necessary to increase the number of turns of the coil so that its resistance is several thousand ohm. The easiest way to use for this purpose is a telephone meter, in which a 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 the thyratron or output lamp, the power of which is sufficient for the operation of the meter. A 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 counter armature is attracted at rest, and is released when a pulse appears.
Due to the relatively large response inertia of mechanical counters, even at counting rates of about 100 pulses per minute, significant miscalculations occur.
Mechanical counters with low inertia can only be manufactured at high cost. It is much easier to achieve reliable results if you include a scaling circuit in front of the counter, which transmits to the mechanical counter, say, only every second pulse. If you turn on in series h such circuits, then only every 2nth pulse will go to the mechanical counter. On fig. two widely used conversion schemes are given. A circuit using the principle of a symmetrical multivibrator has, in contrast to the asymmetric circuits shown in Fig. two stable states in which, as the case may be, one lamp is closed while the other is conducting. Dual diodes are included in the circuit to cut off positive pulses. Their cathodes are under the potentials 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 locked triode. The potential of the anode of another triode is much lower than the potential of the cathode of the diode and, through the coupling capacitor, enters the grid of the unlocked triode . This triode is closed, and the circuit enters the second stable state, in which it remains until the arrival of the next counting pulse. Several of these triggers are connected in series as shown in the figure. Setting the zero of the recalculation scheme is carried out by breaking for a short time the key, indicated on the scheme by the word "zero". Thus, before the start of counting, the second trigger lamps are open. On neon lights GL, 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 that occurs at the second anode is not transmitted to the second trigger. On the second pulse, the first trigger returns to its initial state again, the neon lamp "1" goes out, a negative pulse on the second anode causes the second trigger to overturn, and the neon lamp "2" lights up.
Let's assign the numbers 1, 2, 4, 8, 16, etc. to the neon lamps of successive triggers. Then the total number of pulses received by the input-cell counting circuit, the last of the cells of which controls a mechanical counter through the end lamp, will be equal to the reading of this counter multiplied by 2, plus the number indicated 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.
Recalculation 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. It is possible to obtain a reading proportional to the average counted number of pulses per unit time if, for example, the average anode current of the thyratron is measured in the circuit shown in Fig. The inertia of the device, which is necessary to reduce the current fluctuations associated with the statistical distribution of pulses, can be obtained if a galvanometer with a series-connected resistance of several com shunted with a large capacitor with the highest possible insulation resistance. This instrument is calibrated to imp/min by comparing his readings with those of the scaling circuit. In addition, a number of capacitors are provided Cs, C4 and resistance Rs different sizes, which can be switched on by means of a switch. In this way, you can change the area
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measurements over a wide range. If an ordinary output lamp 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. The voltage on the meter for accurate measurements must be kept as constant as possible. This is done, for example, by stabilizing a series of small glow-discharge lamps connected in series, which consume little current. The meter amplifier often works satisfactorily also with an unregulated voltage; however, it is better to stabilize its anode voltage.
D) Statistical errors and their correction
a) Statistical errors. If for a certain time calculated N pulses, then the average statistical error of this result is equal to ±X ~N. Due to the presence in environment cosmic rays and radioactivity, each counter, even in the absence of a radiation source, gives a small background . This background can be significantly reduced by shielding the meter on all sides with a layer of lead or iron a few centimeters thick. For each measurement, the background must first be determined. If for the same time in the presence of a radiation source 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 equal to
b) Correction for limited resolution. If the most inertial element of the counting device has a resolution time h seconds and the average count rate is N"imp/sec, then the true average counting rate
Therefore, for example, at the average value N" = = 100 imp/sec and resolution time f= 10~s sec the miscalculation is 10% of the total number of pulses.
"Neutrino" - Upward ?L=up to 13000 km?. P(?e??e) = 1 – sin22?sin2(1.27?m2L/E). 5. May 13, 2004. ??. p, He … The Second Markov Readings May 12-13, 2004 Dubna - Moscow. Neutrino oscillations. 2-?. ?. Atmospheric neutrinos. S.P. Mikheev. S.P. Mikheev INR RAS. What do we want to know. 3. Up/Down Symmetry. ?e.
"Methods for registration of elementary particles" - Tracks elementary particles in thick film emulsion. Methods of observation and registration of elementary particles. The space between the cathode and the anode is filled with a special mixture of gases. R. Emulsions. Method of thick-layer photographic emulsions. 20s L.V. Mysovsky, A.P. Zhdanov. The flash can be observed and fixed.
"Antiparticles and antimatter" - There should be an equal number of stars of each kind in the world, - Paul Dirac. With the same unidirectionality of time, the relation of matter and antimatter to space-time is different "simplification" of Nature. The positron was discovered in 1932 using a cloud chamber. Refutation of Dirac's theory or refutation of the absolute symmetry of matter and antimatter.
"Methods of Observation and Registration of Particles" - Wilson Charles Thomson Fig. The space between the cathode and the anode is filled with a special mixture of gases. Piston. Registration of complex particles is difficult. Cathode. +. Wilson is an English physicist and member of the Royal Society of London. Wilson chamber. Application of the counter. glass plate. Gas-discharge Geiger counter.
"Discovery of the proton" - Discoveries predicted by Rutherford. Silina N.A., teacher of physics, secondary school No. 2, Redkino village, Tver region. determines the relative atomic mass chemical element. Mass and charge number of an atom. The number of neutrons in the nucleus is indicated. Discovery of the proton and neutron. Isotopes. What are isotopes? To the study of the structure of the nucleus.
"Physics of elementary particles" - In all interactions, the baryon charge is conserved. Thus, the Universe surrounding us consists of 48 fundamental particles. Quark structure of hadrons. Chadwick discovers the neutron. Antimatter is matter composed of antinucleons and positrons. Fermions are particles with half-integer spin (1/2 h, 3/2 h….) For example: electron, proton, neutron.
In total there are 17 presentations in the topic
slide 1
Experimental methods for studying particles. Geiger Counter Municipal educational institution"Average comprehensive school No. 30 of the city of Belovo "Completed by: Voronchikhin Valery, Makareikin Anton Pupils of the 9th "B" class Supervisor: Popova I.A., teacher of physics Belovo 2010slide 2
Geiger counter The widespread use of the Geiger-Muller counter is explained by its high sensitivity, the ability to register various kinds of radiation, the comparative simplicity and low cost of installation. The counter was invented in 1908 by Geiger and improved by Muller. The sensitivity of the counter is determined by the composition of the gas, its volume, and the material (and thickness) of its walls.
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The principle of operation of the device Geiger counter consists of a metal cylinder, which is the cathode, and a thin wire stretched along its axis - the anode. The cathode and anode are connected through resistance R to a high voltage source (200-1000 V), due to which a strong electric field arises in the space between the electrodes. Both electrodes are placed in a sealed glass tube filled with rarefied gas.
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If the electric field strength is high enough, then the electrons acquire a sufficiently large energy over the mean free path and also ionize gas atoms, forming new generations of ions and electrons that can take part in ionization. An electron - ion avalanche is formed in the tube, as a result of which there is a short-term and sharp increase in the current strength in the circuit and the voltage in the resistance R. This voltage pulse, indicating that a particle has entered the counter, is recorded by a special device.
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The Geiger counter is mainly used to register electrons, but there are models that are also suitable for registration - gamma quanta.
Gas discharge Geiger counter
R To amplifier Glass tube Anode Cathode The gas-discharge counter has a cathode in the form of a cylinder and an anode in the form of a thin wire along the axis of the cylinder. The space between the cathode and the anode is filled with a special mixture of gases. A voltage is applied between the cathode and anode.
Scintillation counter
Cherenkov counter Scheme of the Cherenkov counter: on the left - the cone of Cherenkov radiation, on the right - the device of the counter. 1 - particle, 2 - particle trajectory, 3 - wave front, 4 - radiator, 5 - PMT (the development of an avalanche of secondary electrons caused by a photoelectron is shown), 6 - photocathode.
cloud chamber cloud chamber. A container with a glass lid and a piston at the bottom is filled with saturated vapors of water, alcohol or ether. As the piston descends, due to the adiabatic expansion, the vapors cool and become supersaturated. A charged particle passing through the chamber leaves a chain of ions in its path. The vapor condenses on the ions, making the trace of the particle visible.
The first charged particle detector, the cloud chamber, was created on April 19, 1911. The chamber was a glass cylinder with a diameter of 16.5 cm and a height of 3.5 cm. The top of the cylinder was covered with glued mirror glass, through which the traces of particles were photographed. Inside was the second cylinder, in it - a wooden ring, lowered into the water. Evaporating from the surface of the ring, it saturates the chamber with water vapor. The vacuum pump created a vacuum in a spherical container connected to the chamber by a tube with a valve. When the valve was opened, a rarefaction was created in the chamber, water vapor became supersaturated, and on the traces of charged particles they condensed in the form of fog strips (which is why in foreign literature the device is called the cloud chamber - "fog chamber")
bubble chamber. The container is filled with well-purified liquid. There are no centers of vapor formation in the liquid, so it can be superheated above the boiling point. But the passing particle leaves behind an ionized trail, along which the liquid boils, marking the trajectory with a chain of bubbles. Modern chambers use liquid gases - propane, helium, hydrogen, xenon, neon, etc. In the picture: a bubble chamber designed at FIAN. 1955–1956 bubble chamber
Photograph of the collision of sulfur and gold ions in a streamer (a type of spark) chamber. The tracks of charged particles born during collisions in it look like chains of separate non-merging discharges - streamers.
spark chamber
Particle track in a narrow-gap spark chamber Particle tracks in a streamer spark chamber
Method of thick-layer photographic emulsions Charged particles create latent images of motion traces. The length and thickness of the track can be used to estimate the energy and mass of the particle. The emulsion has a high density, so the tracks are short.
We got acquainted with the description of the devices most widely used in the study of elementary particles and in nuclear physics.
A cloud chamber can be called a “window” to the microworld. It is a hermetically sealed vessel filled with water vapor or alcohols close to saturation.
The cloud chamber has played a huge role in the study of 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) - 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 mainly used to register photons and y-quanta.
The counter registers almost all the electrons falling into it.
Registration of complex particles is difficult.
bubble chamber
The bubble chamber was invented by Donald Glaser (USA) in 1952. For his discovery Glaser received Nobel Prize in 1960. Luis Walter Alvarez improved Glaser's bubble chamber by using hydrogen as the superheated liquid. And also for the analysis of hundreds of thousands of photographs obtained during studies using a bubble chamber, Alvarez first applied computer program which allowed data to be analyzed at a very high speed.
The bubble chamber uses the property of a pure superheated liquid to boil (form vapor bubbles) along the flight path of a charged particle. A superheated liquid is a liquid heated to a temperature greater than the boiling point for the given conditions.
The superheated 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 register a charged particle. After photographing the tracks, the pressure rises to the previous value, the bubbles “collapse” and the camera is ready for operation again.
