q principle of work scintillation counter

q scintillators

q photoelectronic multipliers

q design of scintillation counters

q Properties of scintillation counters

q Examples of using scintillation counters

q List of used literature

Scintillation counters

The registration method of charged particles using light outbreaks arising from these particles to the screen from sulfur zinc (ZNS) is one of the first methods for registering nuclear emissions.

Back in 1903, Circles and others showed that if we consider the screen from the sulfur zinc, irradiated with A-particles, through a magnifying glass in dark roomThis can notice the appearance of separate short-term flashes of light - scintillations. It was found that each of these scintillation is created by a separate A-particle falling on the screen. A simple device, called the Crox Spintarisk, was built, intended for the A-particle account.

The visual method of scintillation was used in the future mainly for registering A-particles and protons with energy in several million electron control. Separate fast electrons could not register, as they cause very weak scintillations. Sometimes, when irradiated with electrons of the sulfur-zinc screen, it was possible to observe outbreaks, but this happened only when the sulfur zinc crystalline came to the same crystalline simultaneously big number electrons.

Gamma rays no flashes on the screen are caused by creating only a general glow. This allows you to register A-particles in the presence of strong G-radiation.

The visual method of scintillation allows you to register a very small number of particles per unit of time. Best conditions The scintillation account is obtained when their number lies between 20 and 40 per minute. Of course, the scintillation method is subjective, and the results in one way or another depend on individual qualities Experimentator.

Despite the shortcomings, the visual method of scintillation played a huge role in the development of nuclear and nuclear physics. Using it, Rutherford recorded a-particles when scattered on atoms. It was these experiments that led Rostford to the opening of the nucleus. For the first time, the visual method made it possible to detect fast protons, knocked out of nuclear nuclei during bombardment by the A-particles, i.e. The first artificial cleavage of the kernel.

Visual scintillation method had great importance Up to the thirties, when the emergence of new methods for registering nuclear emissions made it forget it for a while. The scintillation method of registration was revived at the end of the forties of the XX century on a new basis. By this time, photoelectronic multipliers were developed (FES), allowing to register very weak light flashes. Scintillation counters were created, with which it is possible to increase the account rate of 10 8 and even more than once compared with the visual method, and can also be recorded and analyzed by energy both charged particles and neutrons and the G-rays.

§ 1. The principle of the scintillation counter

The scintillation counter is a combination of a scintillator (phosphorus) and a photoelectron multiplier (FEU). The meter includes a source electric power FEU and radio equipment, ensuring the gain and registration of pulses of the FEU. Sometimes the combination of phosphorus with FEU is performed through a special optical system (Sveta).

The principle of the scintillation counter is as follows. The charged particle, getting into the scintillator, produces ionization and excitation of its molecules that are through very a short time (10 -6 - 10 -9 seconds) go to a stable state, emitting photons. There is a flash of light (scintillation). Some of the photons falls on the FEU photocathode and knocks out photoelectrons from it. The latter under the action of the applied voltage is focused and sent to the first electrode (DIP) of the electronic multiplier. Further, as a result of the secondary electron emission, the number of electrons is avalanche-like, and a voltage pulse appears at the FEU output, which is then enhanced and registed by radio equipment.

The amplitude and pulse duration at the outlet are determined by the properties of both the scintillator and the FEU.

As phosphors are used:

Ø Organic crystals,

Ø Liquid organic scintillators,

Ø solid plastic scintillators,

Ø Gas scintillators.

The main characteristics of scintillators are: Light output, spectral composition Radiation and duration of scintillation.

When the charged particle passes through the scintillator, there is a certain number of photons with one or another energy. Some of these photons will be absorbed in the volume of the scintillator itself, and other photons with slightly lower energy will be emitted instead. As a result of reabsorption processes, photons will emerge, the spectrum of which is characteristic of this scintillator.

The light yield or conversion efficiency of the scintillator C is called the ratio of the energy of the light outbreak, which goes outward, to the magnitude of the energy E of the charged particle lost in the scintillator,

where is the average number of photons outside the outside, the average energy of photons. Each scintillator emits not mono-energy quanta, but a solid spectrum characteristic of this scintillator.

It is very important that the spectrum of photons emerging from the scintillator coincides or at least partially overlapped with the SPEU spectral characteristic.

The degree of overlapping of the outer spectrum of scintillation with the spectral characteristic. This FEU is determined by the coefficient of coordination

where is the outer spectrum of the scintillator or the spectrum of photons that are out of the scintillator. In practice, when comparing scintillators, combined with TEU data, the concept of scintillation efficiency is introduced, which is determined by the following expression:

Scintillation efficiency takes into account both the number of photons emitted by the scintillator per unit of absorbed energy and the sensitivity of this FEU to these photons.

Usually, the scintillation efficiency of this scintillator is determined by comparing with the scintillation efficiency of the scintillator adopted by the standard.

The intensity of scintillation changes over time on exponential law.

where i 0 is the maximum value of the intensity of scintillation; T 0 is the time constant of the attenuation, determined as the time during which the intensity of scintillation decreases in a time.

The number of light photons n, emitted during T Time after the recorded particle hit, is expressed by the formula

where is the total number of photons emitted during the scintillation.

The fluorescence (highlighting) of phosphorus is divided into two types: fluorescence and phosphorescence. If the flashing occurs directly during the excitation or during the period of time is about 10 -8 seconds, the process is called fluorescence. The interval of 10 -8 seconds is chosen because it is in order of magnitude equal to the lifetime of an atom in an excited state for the so-called permitted transitions.

Although the spectra and the duration of fluorescence do not depend on the type of excitation, the release of fluorescence significantly depends on it. So, when the crystal is excited by a particles, the fluorescence yield is almost an order of magnitude less than when photovating.

Under phosphorescence, they understand the luminescence, which continues considerable time after the cessation of excitement. But the main difference between fluorescence and phosphorescence is not the duration of the afterglow. The phosphorescence of crystal phosphors occurs during the recombination of electrons and holes that occurred during excitation. In some crystals, it is possible to tighten the afterglow due to the fact that electrons and holes are captured by "traps", of which they can free themselves, only receiving additional energy required. Hence the dependence of the duration of phosphorescence from temperature is obvious. In the case of complex organic molecules, phosphorescence is associated with their stay in a metastable state, the probability of transition from which can be small to the ground state. And in this case, the dependence of the damping rate of phosphorescence from temperature will be observed.

§ 2. Scintillators

Inorganic scintillators. Inorganic scintillators are crystals inorganic salts. Practical use In scintillation technology, mainly halogen compounds of some alkali metals have mainly.

The process of the occurrence of scintillation can be represented using the zone theory solid. In a separate atom, which is not interacting with others, electrons are on well-defined discrete energy levels. In the solid body atoms are at close distances, and their interaction is strong enough. Due to this interaction, the levels of external electron shells are split and form zones separated from each other for the prohibited zones. The most external allowed zone filled with electrons is the valence zone. Its above its free zone is a conduction zone. Between the valence zone and the conduction zone there is a prohibited zone, the energy width of which is somewhat electronolt.

If there are any defects, lattice disorders or impurity atoms in the crystal, then in this case the emergence of energy electronic levels located in the forbidden zone. With external effects, for example, when passing through a crystal of a fast charged particle, electrons can move from the valence zone to the conduction zone. The valence zone will remain free places with the properties of positively charged particles with a single charge and called holes.

The described process is the process of excitation of the crystal. The excitation is removed by reverse the transition of electrons from the conduction zone in the valence zone, the electron and holes are recommended. In many crystals, the electron transition from the conduction zone to the valence occurs through intermediate fluorescent centers, the levels of which are located in the forbidden zone. These centers are determined by the presence in the crystal of defects or impurity atoms. When moving electrons in two stages, photons are emitted with an energy of a smaller width of the forbidden zone. For such photons, the probability of absorption in the crystal itself is small and therefore the light output is much more for it than for a clean, unpremissible crystal.

In practice, to increase the light yield of inorganic scintillators, special impurities of other elements are introduced, called activators. For example, in the crystal of iodide sodium, thallium is introduced as an activator. The scintillator, built on the basis of the Naj (TL) crystal, has a large light output. The Naj (TL) scintillator has significant advantages compared to gas-filled meters:

greater registration efficiency (with large crystals, registration efficiency can reach tens of percent);

small duration of scintillation (2.5 10 -7 s);

linear communication between the amplitude of the pulse and the energy of the energy lost by the charged particle.

The last property requires explanation. The luminous yield of the scintillator has some dependence on the specific energy loss of the charged particle.

Fig. 1. Dependence of light output

naj (T1) crystal from particle energy.

With very large values \u200b\u200bthere are significant violations crystal lattice Scintillators, which lead to the occurrence of local extinguishing centers. This circumstance can lead to a relative decrease in the light output. Indeed, experimental facts suggest that for heavy particles, the output is non-linean, and the linear dependence begins to manifest itself only with energy of several million electron-roll. In fig. 1 shows the curves of the dependence C from E: Curve 1 for electrons, curve 2 for A particles.

In addition to these alkaline-halide scintillators, other inorganic crystals are sometimes used: ZNS (TL), CSJ (TL), CDS (AG), CAWO 4, CDWO 4, etc.

Organic crystalline scintillators. Molecular communication forces in organic crystals are small in comparison with the forces acting in inorganic crystals. Therefore, the interacting molecules practically do not perturb energy electronic levels of each other and the process of luminescence of the organic crystal is a process characteristic of individual molecules. Basically electronically, the molecule has several oscillatory levels. Under the influence of the registered radiation, the molecule enters the excited electron state, which also corresponds to several oscillatory levels. Ionization and dissociation of molecules are also possible. As a result of the recombination of the ionized molecule, it, as a rule, is formed in an excited state. Initially excited molecule can be on high levels Excitation and after a short time (~ 10-11 seconds) emits a photon high energy. This photon is absorbed by another molecule, with a part of the excitation energy of this molecule can be spent on the heat movement and the photon eased afterwards will have a smaller energy compared to the previous one. After several cycles of emission and absorption, molecules are formed at the first excited level; They emit photons whose energy can be insufficient to excite other molecules and, thus, the crystal will be transparent to emerging radiation.

Fig. 2. The dependence of the light output

anthracene from energy for various particles.

Thanks to most of The excitation energy is consumed on heat movement, the light output (conversion efficiency) of the crystal is relatively small and is several percent.

The following organic crystals were most common to register nuclear radiation: anthracene, stybene, naphthalene. Anthracene has a sufficiently large light output (~ 4%) and low highlighting (3 10 -8 sec). But when registering severe charged particles, the linear dependence of the intensity of scintillation is observed only with quite large energies of particles.

In fig. 2 shows the graphs of the dependence of the light output C (in arbitrary units) on the electron energy 1, protons 2, deuterons 3 and a-particles 4.

Styben Although it has a somewhat less light yield than anthracene, but it is significantly less than the duration of scintillation (7,09 seconds) than anthracene, which allows it to be used in those experiments where very intensive radiation is required.

Plastic scintillators. Plastic scintillators are solid solutions of fluorescent organic compounds in a suitable transparent substance. For example, anthracene solutions or stybene in polystyrene, or plexiglass. The concentrations of the dissolved fluorescent substance are usually small and make up a few tenths of the percentage of or a few percent.

Since the solvent is much more than a dissolved scine-tillar, then, naturally, the particle recorded produces mainly the excitation of the solvent molecules. The excitation energy is further transmitted by the scintillator molecules. Obviously, the spectrum of the emitting of the solvent must be more rigid than the absorption spectrum of the dissolved substance, or at least coincide with it. Experimental facts show that the excitation energy of the solvent is transmitted by the scintillator molecules due to the photon mechanism, i.e. the solvent molecules emit photons, which are then absorbed by the solute molecules. Another energy transfer mechanism is possible. Since the concentration of the scintillator is small, the solution turns out to be practically transparent for the resulting radiation of the scintillator.

Plastic scintillators have significant advantages compared to organic crystalline scintillators:

Ø The possibility of producing scintillators is very large sizes;

Ø The possibility of introducing the spectrum mixers to the scintillator to achieve a better coordination of its luminescence spectrum with the spectral characteristic of the photocathode;

Ø the possibility of introducing into a scintillator of various substances necessary in special experiments (for example, in the study of neutrons);

Ø the possibility of using plastic scintillators in vacuum;

small tightening time (~ 3 10 -9 sec). Plastic scintillators prepared by dissolving anthracene in polystyrene have the greatest light yield. Good properties The solution is also a solution in polystyrene.

Liquid organic scintillators. Liquid organic scintillators are solutions of organic scinting substances in some liquid organic solvents.

The fluorescence mechanism in liquid scintillators is similar to the mechanism occurring in solid solids and scintillators.

Most suitable solvents It turned out to be xylene, toluene and phenylcyclohexane, and solids Р-thermal, diphenyloxazole and tetraphenylbutadiene. The highest light output has a scintillator made during dissolution

p-thermal in xylene at a concentration of a dissolved substance is 5 g / l.

The main advantages of liquid scintillators:

Ø the possibility of making large volumes;

Ø the possibility of introducing substances in special experiments into the scintillator;

Ø Small flash duration (~ 3 10 -9 sec).

Gas scintillators. During the passage of charged particles through various gases, the appearance of scintillation was observed. The greatest light - exit is heavy noble gases (xenon and krypton). A mixture of xenon and helium also has a large light output. The presence of 10% xenon in helium provides a light output, even more than that of pure xenon (Fig. 3). Insignificantly small impurities of other gases sharply reduce the intensity of scintillation in noble gases.

Fig. 3. Dependence of the light yield of gas

scintillator from the ratio of a mixture of helium and xenon.

It was experimentally shown that the length of the outbreaks in noble gases is small (10 -9 -10 -8 s), and the intensity of the outbreaks in a wide range is proportional to the lost energy of the recorded particles and does not depend on their mass and charge. Gas scintillators have low sensitivity to G-radiation.

The main part of the luminescence spectrum lies in the range of distant ultraviolet, therefore, to bring in accordance with the spectral sensitivity of the FEU light-forming agents are used. The latter must have a high conversion factor, optical transparency in thin layers, low elasticity of saturated vapor, as well as mechanical and chemical resistance. As materials for light creates, various organic compounds are mainly used, for example:

diphenylstilben (conversion efficiency of about 1);

P 1 p'-quaterphenyl (~ 1);

anthracene (0.34), etc.

The light formator is applied with a thin layer on the FEU photocathode. An important transmitter parameter is its highlighting. In this regard, the organic converters are quite satisfactory (10 -9 seconds or several units for 10-s). To increase the lights, the inner walls of the scintillator chamber are usually covered by reflectors (MGO, enamel based on titanium oxide, fluoroplastic, aluminum oxide, etc.).

§ 3. Photoelectronic multipliers

The main elements of the FEU are: a photocatode, a focusing system, a multiplying system (dunododododa), anode (collector). All these elements are located in a glass cylinder, floundered to a high vacuum (10 -6 mm Hg).

For the purpose of spectrometry of nuclear radiation, the photocatode is usually located on the inner surface of the FEU's flat ends. As a material of the photocathode, the substance is selected sufficiently sensitive to the light emitted by scintillators. The highest propagation was the antimony-cesium photocathimods, the maximum spectral sensitivity of which lies with L \u003d 3900¸4200 A, which corresponds to the maxima of the luminescence spectra of many scintillators.

Fig. four. Schematic scheme FEU.

One of the characteristics of the photocathode is its quantum output in, i.e. the probability of pulling out the photoelectron photon photoning on the photocathode. The value of E can reach 10-20%. The properties of the photocathode are also characterized by an integral sensitivity, which is the ratio of a photocathocode (MCA) to a light flow falling onto a photocathode (LM).

The photocathode is applied to the glass in the form of a thin translucent layer. Significant thickness of this layer. On the one hand, for large light absorption, it should be significant, on the other hand, emerging photoelectrons, having very low energy will not be able to leave a thick layer and an effective quantum output may be small. Therefore, the optimal thickness of the photocathode is selected. It is also significant to ensure the uniform thickness of the photocathode so that its sensitivity is the same throughout the area. In scintillation G-spectrometry, it is often necessary to use solid scintillators of large sizes, both in thickness and in diameter. Therefore, it becomes necessary to make FEU with large diameters of photocathodes. In the domestic FEU, photocathodes are made with a diameter of several centimeters to 15¸20 cm. Photoelectrons, knocked out of the photocathode, should be focused on the first multiplying electrode. For this purpose, a system of electrostatic lenses is used, which are a number of focusing diaphragms. To obtain good temporal characteristics of the FEU, it is important to create such a focusing system so that the electrons fall on the first direct with the minimum time variation. Figure 4 shows a schematic device of a photoelectron multiplier. High voltageFueling the FEU, the negative pole joins the cathode and is distributed between all electrodes. The potential difference between the cathode and the diaphragm ensures the focusing of photoelectrons to the first multiplying electrode. Multiply electrodes are called Dinen. Dinododes are made of materials, the coefficient of secondary emissions of which more units (S\u003e 1). In domestic FEU, dinodes are manufactured either as a trough-shaped (Fig. 4), or in the form of blinds. In both cases, dinodes are located in line. The ring-like location of the DIPNOS is also possible. The FEU with a ring-shaped DIPEROV system has better time characteristics. The emitting layer of the distille is a layer of antimony and cesium or a layer of special alloys. The maximum value of S for antimuno-cesium emitters is achieved at an electron energy of 350¸400 eV, and for alloy emitters - at 500¸550 eV. In the first case, S \u003d 12¸14, in the second S \u003d 7¸10. In the working modes of FEU, the value S is slightly smaller. A sufficiently good coefficient of secondary emission is S \u003d 5.

Photoelectrons, focused on the first dyne, knock out secondary electrons from it. The number of electrons leaving the first dynoda is several times the number of photoelectrons. All of them are sent to the second DIPO, where secondary electrons are also knocked out, etc., from the dinod to the dinode, the number of electrons increases in the s time.

Upon passing the entire DIPER system, the flow of electrons increases by 5-7 orders and falls on the anode - collecting electrode FEU. If the FEU works in a current mode, then the anode circuit includes devices that enhance and measuring current. When registering nuclear emissions, it is usually necessary to measure the number of pulses arising from the influence of ionizing particles, as well as the amplitude of these pulses. In these cases, the anode chain turns on the resistance on which the voltage pulse occurs.

An important feature of the FEU is the multiplication coefficient M. If the value S for all distances is the same (with the full assembly of electrons on dinodes), and the number of distances is N, then

			s n \u003d (aue -bu) n

A and B constant, u - electrons energy. The multiplication coefficient M is not equal to the reinforcement coefficient M ", which characterizes the current ratio at the FEU output to the current emerging from the cathode

where S.<1 - коэффициент сбора электронов, характеризующий эффективность сбора фотоэлектронов на первый динод.

The constancy of the reinforcement of the M "FEU, both in time and when changing the number of electrons emerging from the photo of the cathode, is very important. The latter circumstance allows the use of scintillation counters as nuclear emission spectrometers.

About interference in photomultipliers. In scintillation counters, even in the absence of external irradiation, a large number of pulses at the FEU output is possible. These impulses usually have small amplitudes and are called noise. The greatest number of noise pulses is due to the appearance of thermoelectrons from the photocathode or even from the first directs. To reduce the noise of FEU, its cooling is often used. When registering emissions that create large amplitude impulses in the registering circuit, a discriminator does not transmit noise pulses.

Fig. 5. Scheme for suppressing Noise FEU.

1. When registering pulses whose amplitude is compared with noise, rationally use one scintillator with two FEUs included in the coincidence scheme (Fig. 5). In this case, there is a temporary selection of pulses arising from the registered particle. In fact, the flash of light that occurred in the scintillator from the registered particle will fall simultaneously on the ftokathodes of both FEU, and at the same time impulses will appear, forcing the coincidence schemes at the same time. The particle will be registered. The noise impulses in each FEU appear independently of each other and most often will not be registered with the scheme of coincidences. This method allows to reduce its own FEU background by 2-3 orders.

The number of noise pulses is growing with the growth of the applied voltage, first pretty slowly, then the increase increases sharply. The reason for this sharp increase in the background is an autoelectronic emission with sharp edges of the electrodes and the occurrence of reverse ionic communication between the latest dynodes and the FEU photocathode.

In the Anode area, where the current density is the greatest, the occurrence of the glow as residual gas and structural materials is possible. The weak glow arising, as well as the inverse ion connection, determine the appearance of the so-called accompanying pulses, within the time from the main ones by 10 -8 ¸10 -7 sec.

§ 4. Designs of scintillation counters

The following requirements are presented to the designs of scintillation counters:

Ø the best collection of light of scintillation on the photocathode;

Ø Uniform distribution of light by photocathod;

Ø Dimming from the light of foreign sources;

Ø No effect of magnetic fields;

Ø FES amplification stability.

When working with scintillation counters, it is always necessary to achieve the greatest attitude of the amplitude of the signal pulses to the amplitude of noise pulses, which forces optimally use the intensities of flashes arising in the scintillator. Typically, the scintillator is packaged into a metal container closed from one end with flat glass. A layer of material reflecting the light and contributing to the most complete output is placed between the container and scintillator. Magnesium oxide (0.96), titanium dioxide (0.95), gypsum (0.85-0.90) have the greatest reflectivity, aluminum (0.55-0.85).

Special attention should be drawn to thorough packing of hygroscopic scintillators. For example, the most frequently used phosphorus Naj (TL) is very hygroscopic and the moisture in it penetrates yellow and loses its scintillation properties.

Plastic scintillators There are no need to be packaged into hermetic containers, but to increase the lights, you can surround the scintillator reflector. All solid scintillators should have an output window on one of the ends, which is articulated with the FEU photocathode. At the articulation site there may be significant losses of the intensity of the light of scintillation. To avoid these losses between the scintillator and the FEU, Canadian balsam, mineral or silicone oils is introduced and optical contact is created.

In some experiments, for example, when measuring in vacuo, in magnetic fields, in strong fields of ionizing radiation, the scintillator cannot be placed directly on the FEU photocathode. In such cases, the transmission of light from the scintillator on the photocathode is used. The polished rods from transparent materials are used as light lines - such as lusite, plexiglas, polystyrene, as well as metal or plexiglass tubes filled with transparent liquid. Losses of light in the lightweight depend on its geometric sizes and from the material. In some experiments it is necessary to use curved lights.

It is better to use light transitions with a large radius of curvature. Lights can also be aligned with scintillators and FES of different diameters. This uses cone-shaped lights. The articulation of the FEU with a liquid scintillator is performed either through the lighting line or direct contact with the liquid. Figure 6 shows an example of the articulation of the FEU with a liquid scintillator. In various modes of operation on the FEU, voltage is supplied from 1000 to 2500V. Since the FES amplification coefficient is very sharply dependent on the voltage, the power supply should be well stabilized. In addition, self-stabilization is possible.

Power supply to the FEU is performed using a voltage divider, which allows the corresponding potential to each electrode. The negative pole of the power source is connected to the photo and to one of the ends of the divider. Positive Pole and the other end of the divider are grounded. The divisor resistance is selected in such a way that the optimal mode of operation of the FEU is carried out. For greater stability, the current through the divider should be an order to exceed electronic currents going through the FEU.

Fig. 6. The articulation of the FEU with a liquid scintillator.

1-liquid scintillator;

3-light-protective casing.

When operating a scintillation counter in pulse mode at the FEU output, short (~ 10 -8 seconds) pulses, the amplitude of which can be several units or several dozen volts. At the same time, the potentials on the last dinodes may experience sharp changes, since the current through the divider does not have time to fill the charge carried out from the cascade by electrons. To avoid such potential fluctuations, the few last resistances of the divider are shunted by tanks. Due to the selection of potentials on dinodas, favorable conditions for collecting electrons on these dinodes are created, i.e. A specific electronotype system is carried out corresponding to the optimal mode.

In the electronic optical system, the electron trajectory does not depend on proportional change in potentials on all electrodes forming this electronotype system. So in the multiplier, only the gain of the gain changes when its supply voltage changes, but electron-optical properties remain unchanged.

With a disproportionate change in the potentials on FEU dynodes, the conditions of focusing electrons on the site where proportionality is violated, change. This circumstance is used to self-stabilize the FES amplification coefficient. For this purpose, the potential

Fig. 7. Part of the divider schema.

one of the differentials in relation to the potential of the previous dinode is set permanent or using an additional battery or using an additional stabilized divider. Figure 7 shows the part of the divider circuit, where an additional battery is included between di 5 and D 6 dynodes (u b \u003d 90 V). To obtain the best effect of self-stabilization, it is necessary to select the resistance value R ". Usually R" is greater than R 3-4 times.

§ 5. Properties of scintillation counters

Scintillation counters possess the following advantages.

High resolution in time. The pulse duration depending on the scintillators used extends from 10 -6 to 10 -9 seconds, i.e. There are several orders of magnitude less than that of the counters with an independent discharge, which allows much higher account speeds. Another important temporal characteristic of scintillation counters is the low value of the pulse delay after passing the particle being recorded through phosphorus (10 -9 -10-8 seconds). This allows you to use schemes of coincidences with low allowing time (<10 -8 сек) и, следовательно, производить измерения совпадений при много больших нагрузках по отдельным каналам при малом числе случайных совпадений.

High efficiency of registration G-rays and neutrons. To register a G-quantum or neutron, it is necessary that they react with the substance of the detector; At the same time, the resulting secondary charged particle must be registered by the detector. Obviously, the larger the substance on the way of G-rays or neutrons, the greater the likelihood of their absorption, the greater the effectiveness of their registration. Currently, when using large scintillators, the effectiveness of registration of G-rays in several tens of percent is achieved. The effectiveness of registration of neutrons with scintillators with specially injected substances (10 V, 6 Li, etc.) is also much higher than the effectiveness of their registration with gas-discharge counters.

The possibility of energy analysis of the registered radiation. In fact, for light charged particles (electrons), the outbreak intensity in the scintillator is proportional to the energy lost by the particle in this scintillator.

With the help of scintillation counters attached to amplitude analyzers, the spectra of electrons and G-rays can be studied. The situation is somewhat worse with the study of the spectra of heavy charged particles (A-particles, etc.), which create greater specific ionization in the scintillator. In these cases, the proportionality of the intensity of the outbreak of the lost energy is observed not at all sorts of particles and manifests itself only with the values \u200b\u200bof energy, large some values. The nonlinear communication of the amplitudes of pulses with the energy of the particle is different for various phosphors and for different types of particles. This is illustrated by graphs in Fig. 1 and 2.

The possibility of making scintillators of very large geometric sizes. This means the possibility of registration and energy analysis of particles of very large energies (cosmic rays), as well as particles, weakly interacting with substance (neutrino).

The possibility of introducing substances to scintillators with which neutrons interact with a large cross section. To register slow neutrons, use phosphors Lij (TL), LIF, LIBR. With the interaction of slow neutrons with 6 Li, a reaction is 6 Li (N, A) 3 H, in which energy of 4.8 MeV is released.

§ 6. Examples of using scintillation counters

Measurement of the time of the life of the excited states of the nuclei. With radioactive decay or in various nuclear reactions, the generated kernels are often in an excited state. The study of the quantum characteristics of the excited states of the nuclei is one of the main tasks of nuclear physics. A very important characteristic of the excited state of the nucleus is the time of his life t. Knowing this value allows you to receive many information about the structure of the kernel.

Atomic nuclei can be in an excited state of various times. To measure these times, there are various methods. Scintillation counters turned out to be very convenient for measuring the lifetimes of nuclei levels from a few seconds to very small fractions of a second. As an example of using scintillation counters, we will consider the method of detained coincidences. Let the core A (see Fig. 10), by b-decay, turns into a kernel in an excited state, which existed its energy to the sequential emission of two G-quanta (G 1, G 2). It is required to determine the lifetime of the excited state I. The preparation containing isotope A is set between two meters with Naj (TL) crystals (Fig. 8). The pulses arising from the FEU output are fed on the scheme of rapid coincidences with permissive time ~ 10 -8 -10 -7 sec. In addition, pulses are fed to linear amplifiers and further to amplitude analyzers. The latter are configured in such a way that they skip the impulses of a certain amplitude. For our purpose, i.e. For the purpose of measuring the life time of the level I (see Fig. 10), the AAI amplitude analyzer should pass only the pulses corresponding to the energies of the quantum g 1 and the AAII - G 2 analyzer.

Fig.8. Schematic diagram for determining

the lifetime of the excited states of the nuclei.

Next, the pulses from the analyzers, as well as with a quick coincidence scheme, are fed to the slow (T ~ 10 -6 Syuk) scheme of triple coincidences. The experiment examines the dependence of the number of triple coincidences from the magnitude of the time delay of the pulse included in the first channel of the rapid coincidences. Usually, the impulse delay is carried out using the so-called LZ delay variable (Fig. 8).

The delay line should be included in that channel in which the kvaant G 1 is recorded, as it is emitted earlier than quantum G 2. As a result of the experiment, a semi-liter graph of the dependence of the number of triple coincidences from the delay time is based (Fig. 9), and the lifetime of the excited level I is determined along it (just as it is done when determining the half-life with a single detector).

Using scintillation counters with a Naj (TL) crystal and the considered scheme of fast-slow coincidences, it is possible to measure the times of 10 -7 -10 -9 sec. If you use faster organic scintillators, you can measure and smaller times of life of excited states (up to 10-11 seconds).

Fig.9. The dependence of the number of matches from the value of the delay.

Gamma defectoscopy. Nuclear radiation with a large penetrating ability is increasingly used in the technique for detecting defects in pipes, rails and other large metal blocks. For these purposes, the source of G-radiation and the G-ray detector is used. The best detector in this case is a scintillation counter, which has a lot of registration efficiency. The radiation source is placed in a lead container, from which a narrow bundle of the G-ray, illuminating the pipe, extends through the collimator hole. From the opposite side of the pipe is set as a scintillation counter. The source and counter are placed on a movable mechanism, allowing you to move them along the pipe, and also turn around its axis. Passing through the pipe material, the bundle of the G-ray will be partially absorbed; If the pipe is homogeneous, the absorption will be the same everywhere, and the counter will always register the same number (on average) of G-quanta per unit of time, but in some place the pipe there is a sink, then the G-rays in this place will be absorbed Less, account speed will increase. The location of the shell will be detected. There are many examples of similar use of scintillation counters.

Experimental neutrino detection. Neutrinos - the most mysterious of the elementary particles. Almost all properties of neutrinos are obtained from indirect data. The modern theory of B-decay assumes that the mass of neutrinos M n is zero. Some experiments suggest that. Spin neutrino is 1/2, magnetic moment<10 -9 магнетона Бора. Электрический заряд равен нулю. Нейтрино может преодолевать огромные толщи вещества, не взаимодействуя с ним. При радиоактивном распаде ядер испускаются два сорта нейтрино. Так, при позитронном распаде ядро испускает позитрон (античастица) и нейтрино (n-частица). При электронном распаде испускается электрон (частица) и антинейтрино (`n-античастйца).

The creation of nuclear reactors in which a very large number of nuclei are formed with an excess neutron, united the hope of detecting antineutrino. All neutron enforcement cores are disintegrated with emitting electrons, and therefore antineutrino. Near the nuclear reactor with a capacity of several hundred thousand kilowatt, the flow of antineutrino is 10 13 cm -2 · sec -1 - the flow of a huge density, and when choosing a suitable antineutrino detector, it was possible to try to detect them. Such an attempt was carried out by Reyan and Kowan in 1954. The authors used the following reaction:

n + P ® N + E + (1)

this reaction particles-products are positron and neutron that can be registered.

The detector and at the same time a hydrogen target was a liquid scintillator, a volume of ~ 1m 3, with a high hydrogen content saturated with cadmium. Positrons arising in the reaction (1), annihilated in two G-quantum with an energy of 511 keV each and caused the appearance of the first outbreak of the scintillator. Neutron has slowed over several microseconds and captured by cadmium. At the same time, several G-quanta with a total energy of about 9 MeV was emitted by cadmium. As a result, the second flash occurred in the scintillator. Foreign coincidences of two pulses were measured. To register outbreaks, the liquid scintillator was surrounded by a large amount of FEU.

The speed of the delayed coincidences was three reference per hour. From this data it was obtained that the cross section of the reaction (Fig. 1) s \u003d (1.1 ± 0.4) 10 -43 cm 2, which is close to the calculated value.

Currently, liquid scintillation meters of very large sizes are used in many experiments, in particular in experiments on measuring G-radiation fluxes emitted by humans and other living organisms.

Registration of fragments of division. To register fragments of division, gas scintillation counters were comfortable.

Typically, the experiment on the study of the division cross section is set as follows: the layer of the element is studied on some kind of substrate and is irradiated with the neutron flow. Of course, the more the dividing substance will be used, the more division acts will occur. But since usually divided substances (for example, transuranone elements) are A-emitters, the use of them in significant quantities becomes difficult due to the large background from A-particles. And if the acts of divisions are studied using pulsed ionization chambers, it is possible to impulse pulses from A-particles to impulses that have arisen from the fragments of the division. Only a device that has the best time resolution will allow the use of large amounts of felting substance without impulse overlaps. In this regard, gas scintillation counters have a significant advantage compared to pulsed ionization chambers, since the duration of pulses in the last 2-3 orders of magnitude greater than that of gas scintillation counters. The amplitudes of pulses from division fragments are much more than from A-particles and therefore can be easily separated using an amplitude analyzer.

A very important property of the gas scintillation counter is its low sensitivity to the G-rays, since often the appearance of severe charged particles is accompanied by an intense flow G-ray.

Fluorescent chamber. In 1952, Soviet Physists, Zavedsky and others, photographer was photographed by traces of ionizing particles in luminescent substances with sensitive electronofactic converters (EEO). This method of registration of particles called a luminescent chamber has a high resolution in time. The first experiments were produced using CSJ (TL) crystal.

In the future, it began to use plastic scintillators in the form of long thin rods (threads) for the manufacture of a luminescent chamber. The threads are stacked in the form of a stack of rows so that the threads in two neighboring ranks are located at right angles to each other. This ensures the possibility of stereoscopic observation to recreate the spatial trajectory of particles. Images from each of the two groups of mutually perpendicular yarns are sent to separate electronotypes. The threads also play the role of light wires. Light gives only those threads that crosses the particle. This light comes out through the ends of the corresponding threads that are being photographed. Systems with a diameter of individual threads from 0.5 to 1.0 mm are manufactured.

Literature:

1. J.Birks. Scintillation counters. M., IL, 1955.

2. V.O. Ivyazky, I.I. Lomonosov, V.A. Ruzin. Scintillation method in radiometry. M., Gosatomizdat, 1961.

3. Yu.A. Egorov. Stilcyllation method of spectrometry of gamma radiation and rapid neutrons. M., Atomizdat, 1963.

4. P.A. Tishkin. Experimental methods of nuclear physics (nuclear radiation detectors).

Publisher of the University of Leningrad, 1970.

5 G.S. Landsberg. Elementary physics textbook (volume 3)., Science, 1971

From Lij, powdered mixtures, for example, 1 Weight part B2O3 and 5 weight parts of ZNS, span them directly to the FEU's window; You can also use a block diagram of a scintillation spectrometer. 1 - scintillator, 2 - FEU, s - high voltage source, 4 - cathode repeater, D is a linear amplifier, 6 is an amplitude pulse analyzer, 7 - recorder. ZNS, suspended ...

An further increase in voltage N should remain constant with a further increase in the voltage to the end of the Geiger region. This, of course, is not perfectly performed; On the contrary, as a result of the appearance of individual false discharges, the plateau has a more or less pronounced smooth rise. In the counters working in the field of proportionality, it is possible to obtain a practically horizontal plateau characteristics. TO...

Any isotope When writing, the first is always indicated by a mass number of isotope over the string, and then the symbol of the chemical element, and say the opposite: first the element, then the weight of the isotope. Compounds labeled with radioactive isotopes are divided into two groups of substances. First, these are specific chemical compounds, in which one atom (or several) is replaced by an atom of radioactive isotope of the same ...

- The principle of the scintillation counter

- Scintillators

- Photoelectronic multipliers

- Designs of scintillation counters

- Properties of scintillation counters

- Examples of using scintillation counters

- List of used literature

Scintillation counters

The registration method of charged particles using light outbreaks arising from these particles to the screen from sulfur zinc (ZNS) is one of the first methods for registering nuclear emissions.

Back in 1903, Circuits and others showed that if we consider the screen from the sulfur zinc, irradiated with A-particles, through a magnifying glass in the dark room, then it can notice the appearance of separate short-term flashes of light-scintillations. It was found that each of these scintillation is created by a separate A-particle falling on the screen. A simple device, called the Crox Spintarisk, was built, intended for the A-particle account.

The visual method of scintillation was used in the future mainly for registering A-particles and protons with energy in several million electron control. Separate fast electrons could not register, as they cause very weak scintillations. Sometimes, when irradiated with electrons of the sulfur-zinc screen, it was possible to observe outbreaks, but it happened only when the same crystal of sulfur zinc fell at the same time quite large number of electrons.

Gamma rays no flashes on the screen are caused by creating only a general glow. This allows you to register A-particles in the presence of strong G-radiation.

The visual method of scintillation allows you to register a very small number of particles per unit of time. The best conditions for the scintillation account are obtained when their number lies between 20 and 40 per minute. Of course, the scintillation method is subjective, and the results in one way or another depend on the individual qualities of the experimenter.

Despite the shortcomings, the visual method of scintillation played a huge role in the development of nuclear and nuclear physics. Using it, Rutherford recorded a-particles when scattered on atoms. It was these experiments that led Rostford to the opening of the nucleus. For the first time, the visual method made it possible to detect fast protons, knocked out of nuclear nuclei during bombardment by the A-particles, i.e. The first artificial cleavage of the kernel.

The visual method of scintillation was of great importance until the thirties, when the emergence of new methods for registering nuclear radiation forced it for a while to forget it. The scintillation method of registration was revived at the end of the forties of the XX century on a new basis. By this time, photoelectronic multipliers were developed (FES), allowing to register very weak light flashes. Scintillation counters were created, with which it is possible to increase the account rate of 10 8 and even more than once compared with the visual method, and can also be recorded and analyzed by energy both charged particles and neutrons and the G-rays.

§ 1. The principle of the scintillation counter

The scintillation counter is a combination of a scintillator (phosphorus) and a photoelectron multiplier (FEU). The set of the counter also includes the source of the electric power supply of the FEU and radio equipment, which ensures the gain and registration of the pulses of the FEU. Sometimes the combination of phosphorus with FEU is performed through a special optical system (Sveta).

The principle of the scintillation counter is as follows. The charged particle, getting into the scintillator, produces ionization and excitation of its molecules that through a very short time (10 -6 - 10 -9 seconds ) Transfer to a stable state, emitting photons. There is a flash of light (scintillation). Some of the photons falls on the FEU photocathode and knocks out photoelectrons from it. The latter under the action of the applied voltage is focused and sent to the first electrode (DIP) of the electronic multiplier. Further, as a result of the secondary electron emission, the number of electrons is avalanche-like, and a voltage pulse appears at the FEU output, which is then enhanced and registed by radio equipment.

The amplitude and pulse duration at the outlet are determined by the properties of both the scintillator and the FEU.

As phosphors are used:

Organic crystals,

Liquid organic scintillators

Solid plastic scintillators,

Gas scintillators.

The main characteristics of scintillators are: light yield, spectral composition of radiation and duration of scintillation.

When the charged particle passes through the scintillator, there is a certain number of photons with one or another energy. Some of these photons will be absorbed in the volume of the scintillator itself, and other photons with slightly lower energy will be emitted instead. As a result of reabsorption processes, photons will emerge, the spectrum of which is characteristic of this scintillator.

Light yield or conversion efficiency of scintillator C called the energy ratio of light flash , outgoing, to the magnitude of the energy E. charged particle lost in the scintillator,
where - The average number of photons coming out - The average energy of photons. Each scintillator emits not mono-energy quanta, but a solid spectrum characteristic of this scintillator.

It is very important that the spectrum of photons emerging from the scintillator coincides or at least partially overlapped with the SPEU spectral characteristic.

The degree of overlapping of the outer spectrum of scintillation with the spectral characteristic. This FEU is determined by the coefficient of coordination where is the outer spectrum of the scintillator or the spectrum of photons that are out of the scintillator. In practice, when comparing scintillators, combined with TEU data, the concept of scintillation efficiency is introduced, which is determined by the following expression:

where I. 0 - the maximum value of the intensity of scintillation; t. 0 - The time constant of the attenuation, determined as time during which the intensity of scintillation decreases in e. time.

Number of photons of light n. , emitted during the time t. After entering the recorded particle, it is expressed by the formula

where is the total number of photons emitted during the scintillation.

The fluorescence (highlighting) of phosphorus is divided into two types: fluorescence and phosphorescence. If the flashing occurs directly during the excitation or during the time interval of the order of 10 -8 sec The process is called fluorescence. Interval 10 -8. sec It is selected because it is in order of magnitude equal to the lifetime of an atom in an excited state for the so-called allowed transitions.

Although the spectra and the duration of fluorescence do not depend on the type of excitation, the release of fluorescence significantly depends on it. So, when the crystal is excited by a particles, the fluorescence yield is almost an order of magnitude less than when photovating.

Under phosphorescence, they understand the luminescence, which continues considerable time after the cessation of excitement. But the main difference between fluorescence and phosphorescence is not the duration of the afterglow. The phosphorescence of crystal phosphors occurs during the recombination of electrons and holes that occurred during excitation. In some crystals, it is possible to tighten the afterglow due to the fact that electrons and holes are captured by "traps", of which they can free themselves, only receiving additional energy required. Hence the dependence of the duration of phosphorescence from temperature is obvious. In the case of complex organic molecules, phosphorescence is associated with their stay in a metastable state, the probability of transition from which can be small to the ground state. And in this case, the dependence of the damping rate of phosphorescence from temperature will be observed.

Scintillation counters

In a scintillation counter, the registration of the charged particle is associated with the excitation of atoms and molecules along its trajectory. Excited atoms living a short time are moving to the ground state, the emitting electromagnetic radiation. A number of transparent substances called phosphoria or phosphoras, a part of the spectrum of this radiation occurs on the light area. Passage of a charged particle through such a substance causes a flash of light. To increase the release of light and reduce its absorption in phosphorus, the so-called activators add to the latter. The type of activator is indicated in brackets after the symbol of phosphorus. For example, the NAI crystal activated by Tallium is denoted by NAI (TL).

The hitting of a fast charged particle in phosphorus causes a light outbreak - scintillation. The latter is converted to an electrical pulse and is intensified at 10,5 -10 6 times with a photoelectric multiplier (FEU). Similar combination of two elements - phosphorus and FEU-use in scintillation counters(Fig. 5.7).

Fig. 5.7. Schematic diagram of a scintillation counter.

1 - NAI crystal; 2 - photocathod; 3 - focusing electronic lens;

4 - Emitters (dinododody); 5 - anode

Registration of γ-quanta in a scintillation counter occurs due to secondary electrons and positrons, generated by phosphorus γ-quanta. Since phosphors have good optical transparency, ensuring the collection of light on the FEU photocatode with a significant volume of phosphorus, large thickness phosphors can be applied to register γ-quanta. This ensures the high efficiency of registration of γ-quanta with a scintillation counter, an order of magnitude and more exceeding the efficiency of gas-filled counters.

Photoelectronic multipliersconsist from a photocathode, multiplying electrodes of ionic (see Fig. 5.7). The potential of each subsequent electrode for some value (about 10 V) exceeds the potential of the previous one, which ensures the acceleration of electrons between them. Photons coming from phosphorus onto a photocatode, several dozen or hundreds of electrons are made of it, which focus and accelerate the electric field and bombard the first dyna. When braking in the remode, each accelerated electron knives up to 5-10 secondary electrons. Such a process, repeating on each subsequent disting, provides multiplication of electrons to many millions times.

Scintillation counters in nuclear geology and geophysics are used to register γ-quanta , less often neutrons and β-particles. When registering severe charged particles, it is difficult to ensure their input to phosphorus. Therefore, ionization chambers or end meters are most often used to register α-particles. Only for registration of α-activity of emanation is widely used with a scintillation chamber, the inner walls of which are covered with ZNS (AG) .

Because of the thermoelectronic emission of the photocathode and the first directs at the exit even completely darkened FEU, some dark current occurs, creating small background pulses. For their cut-off, discriminators are introduced into the registration scheme.

Features of using scintillation counters for γ-radiation spectrometry.When registering γ-quanta, the pulse amplitude as a scintillation counter at its output is proportional to the energy of the electron and the positron, which formed during the interaction of the quantum with the scintillator. If the photoelectron energy is equal to the energy of a quantum (minus a small magnitude - communication energy TO-Electron), then an electron with compaton scattering and a pair of an electron-positron in the effect of vapor formation is transmitted only part of the quantum energy. With a compute effect, depending on the angle of scattering an γ-quantum, the electron energy may vary in wide limits (Fig. 5.8.), And with the effect of the formation of steam - the kinetic energity of the pair of 1.02 MeV is less than the quantum energy.

Fig. 5.8. Simplified secondary energy distribution scheme

electrons in phosphore at: a - photoeffect, b - Compton scattering,

in - the formation of steam; N is the number of pulses, e - the energy of secondary electrons.

As a result, the spectrum of the energy of secondary particles formed in the scintillator with a monochromatic bunch of γ-quanta has a complex view. The appearance of additional lines e v \u003d 0.51 MeV and E U.with the effect of the formation of steam due to the fact that in some cases one or even both γ-quantum with an energy of 0.51 MeV generated during the annihilation of the positron, absorbed in the scintillator as a result of the photoelectric and flash from these photoelectrodes merges with a flash from the primary pair of electron positron . Maximum Energy of the Compton Electron

. (5.17)

The actual amplitude distribution of pulses at the FEU output is more vague than the electron spectrum in Fig. 5.8 Due to the statistical nature of the processes in Phosphorus and FEU. It is not discrete, but continuous. A typical equipment of the isotope 24 Na (E \u003d 1.38 and 2.76 MeV) is shown in Fig.5.9.

For the line of 1.38 MeV, the contribution of the effect of the formation of steam is insignificant and the corresponding peaks are almost invisible, only peak 1.38 MeV is formed, due to the photo effect, as well as the less clear Compton peak with an energy of 1.17 MeV. For the 2.76 MeV line, there are three peaks with energies of 1.74, 2.25 and 2.76 MeV. The first two peaks are obliged to the effect of the formation of pairs, and the last peak (2.76 MeV) to three processes: photo effect, the effect of vapor education, accompanied by the absorption of both annihilation quanta; Compton effect when the scattered quantum is also absorbed by phosphorus as a result of a photo effect. In all three processes, all the energy of the quantum turns into light energy. Therefore, this peak is called pick of full absorption.

The form of a full absorption peak is close to a Gaussian curve. Attitude μ \u003d ΔЕ / ehalf widths of the peak ΔE at half of its height to medium energy E.call amplitude resolutioncounter. Than less μ, the better the spectrometer. Value μ usually grows with a decrease in energy and for good scintillation spectrometers at e V. \u003d L, 33 MeV (60 Co) is 6%.

Scintillation counters ensure much greater efficiency of registration of γ-quanta (up to 30-50% or more) than gas-discharge, and make it possible to study the spectral composition of radiation. The advantages of scintillation counters also include the lower level of their own and cosmic background.

Fig. 5.9. The apparatus γ-radiation spectrum containing lines

with energy 1.38 and 2.76 MeV.

However, scintillation counters are more complex and require more qualified maintenance than discharge. This is due to the greatest influence of the temperature on the phosphor light impact, incomparably higher demands on the stabilization of the power supply, as well as a stronger change in the characteristics of scintillation counters in time.

The scintillation counter is a combination of a scintillator (phosphorus) and a photoelectron multiplier (FEU). The set of the counter also includes the source of the electric power supply of the FEU and radio equipment, which ensures the gain and registration of the pulses of the FEU. Sometimes the combination of phosphorus with FEU is performed through a special optical system (Sveta). The principle of the scintillation meter is as follows: a charged particle, passing through a scintillator, along with the ionization of atoms and molecules excites them. Returning to the unexcited (basic) state, atoms empty photons . The radiated light is assembled - in the spectral range of the scintillator - on the photodetector. As the latter often serves a photoelectron multiplier (FEU). The photoelectron multiplier is a glass cylinder, powered up to residual pressure not higher than 10-6 mm Hg. Art., in the end of which there is a transparent flat window, on the surface of which the thin layer of the substance with a small operation of the electron exit (photocathod) is applied on the surface of the evacuated volume (photocathode), usually based on antimony and cesium. Next, in the evacuated space, there is a series of remodes electrodes, for which, using a voltage divider from the power supply source, a sequentially increasing potential difference is supplied. FEU dinododes are also made from the substance also with the low operation of the electron output. They are capable of bombarding their electrons to emit secondary electrons in quantities exceeding the number of primary several times. The last Dyna is an anode FEU. The main parameter of the FEU is the gain coefficient at a certain power mode. Usually, the FEU contains nine and more distorts and the gain of the primary current reaches for various multipliers of 10 5 - 10 times, which makes it possible to obtain electrical signals amplitude from volts to dozens volts.

Fig. 1.9. Block-diagram of a scintillation counter

Photons, getting on the photocatode of the FEU, as a result of the photoeffect electrons knock out , as a result, an electrical impulse arises on the anode to the FEU, which further enhances the reconnect system due to the mechanism of secondary electronic emission. The anode current signal FEU - through the amplifier or directly - is fed to the input of the measuring instrument - the pulse counter, oscilloscope, analog-digital converter, etc. The amplitude and pulse duration at the outlet are determined by the properties of both the scintillator and the FEU.

In some cases, at the outlet of the amplifier, there is a large number of pulses (usually small in amplitude) that are not related to the registration of nuclear particles, namely, the pulses of their own noise of the FEU and the accelerator. To eliminate the noise between the amplifier and the pulse counter, an integral amplitude discriminator is included, which transmits only those impulses whose amplitudes are greater than some threshold voltage. Detection of neutral particles (neutrons, γ-quanta) occurs according to secondary charged particles generated by the interaction of neutrons and γ-quanta with a scintillator atoms.

The advantages of the scintillation counter: high efficiency of registration of various particles; speed; The ability to manufacture scintillators of different sizes and configurations; High reliability and relatively low cost. Thanks to these qualities, scintillation counters are widely used in nuclear physics (for example, to measure the lifetime of the excited states of the nuclei, measurement of the division section, registration of fragments of division by gas scintillation meters), physics of elementary particles and cosmic rays (for example, experimental neutrino detection) , in industry (gamma defectoscopy, radiation control), dosimetry (measurement of γ-radiation fluxes emitted by man and other alive organisms), radiometry , geology, medicine, etc. Disadvantages of a scintillation counter: low sensitivity to low-energies particles (1 keV), low resolution energy . For registration of charged particles with a scintillation counter, almost all phosphors are suitable. More comfortable solid phosphors such as organic single crystals or plastics. The main difficulty arising from the register of charged particles and especially hard, ensuring the input of particles in phosphorus.

Phosphorus, as a rule, is packaged into a metal container, through the walls of which particles may not pass. Therefore, heavy particles typically register with simpler detectors - ionization chamber or proportional meter. Electrons register with scintillation meters in cases where a good allowing time is required. The main phosphories are usually organic monocrystals of anthracene, stybean or plastic. The effectiveness of registration of charged particles with a scintillation counter is close to 100%. Scintillation counters are used especially widespread to register γ-radiation. In addition to good allowing time, such a detector has much greater than the gamer-muller counter, efficiency to γ-quanta. In some cases, it is possible to provide almost 100% registering γ-radiation. The efficiency of the scintillation counter to γ-quantum depends on the material and thickness of phosphorus. The interaction of γ-quanta with phosphorus substance is determined by the density of electrons and the energy of γ-quanta. Therefore, the most efficient γ-radiation is recorded by scintillation meters with phosphorus, having a large density and high medium sequence number Z. These phosphoruses include nonorganic NAI single crystals (TL), CSI (TL), Ki (TL). With less efficiency, γ-radiation is registered with liquid phosphories and plastics as a result of neutron interaction with atomic nuclei. To register slow, nuclear reactions of splitting of lung nuclei under the action of neutrons [ 10V (N,α ) 7li., 6Li (N,α ) 3h.and 3He (N, P) 1H ] with the registration of A-particles and protons; dividing heavy nuclei with the registration of division fragments; Radiation grip of neutron nuclei (N, γ) with registration of γ-quanta, as well as excitation of artificial radioactivity. For registration of A-particles, protons and fragments of division, ionization chambers and proportional counters are used, which are filled with gaseous BF3 and other gases containing in or 3H, or cover their walls with a thin layer of solid in, Li or dividing substances. The design and dimensions of such chambers and counters are diverse. Proportional counters can reach 50 mm in diameter and 2 m lengths. Neutron detectors containing 10B or 3He have the greatest efficacy of thermal neutron. To register slow neutrons, scintillation counters (on LII crystals with an EU admixture, on scinting lithium glasses, or a mixture of boron-containing substances and a ZNS scintillator) are used. The effectiveness of registration of rapid neutrons listed detectors is hundreds of times less, so fast neutrons pre-slow down in the paraffin block surrounding the neutron detector. Specially selected shape and sizes of blocks make it possible to obtain the almost constant effectiveness of neutron registration in the energy range from several keV to 20 MeV (Movie-meter). With direct detection of neutrons with energies of ~ 100 keV, elastic neutron scattering in hydrogen or helium is usually used or recoil kernels are recorded. Since the latter energy depends on neutron energy, such neutron detectors make it possible to measure the energy spectrum of neutrons. Scintillation neutron detectors can also register rapid neutrons along the protons of returns in organic and hydrogen-containing liquid scintillators. Some heavy kernels, such as 238U and 232th, are divided only under the action of rapid neutrons. This allows you to create threshold detectors that serve to register fast neutrons against the background of thermal. This registration of nuclear reaction products of neutrons with nuclei B and Li, nuclear photographic emulsions are also used protons and fragments of division. This method is particularly convenient in dosimetry, as it allows to determine the total number of neutrons for the time of exposure. When dividing nuclei, the energy of fragments is so great that they produce noticeable mechanical destruction. This is based on this one of the ways of their detection: fragments of division slow down in glass, which is then poisoned by a platform acid; As a result, traces of fragments can be observed under a microscope. The excitation of artificial radioactivity under the action of neutrons is used to register neutrons, especially when measuring the density of the neutron flux, since the number of decays (activity) is proportional to the neutron flow through the substance (measurement of activity can be performed after the neutron irradiation). There are a large number of different isotopes used as radioactive neutron indicators of different energies. E.. In the thermal energy of the energies, 55mn, 107Ag, 197AU have the greatest distribution: 55mn is used to register resonant neutrons ( E.\u003d 300 eV), 59CO ( E.\u003d 100 eV), 103rh, 115in ( E.\u003d 1.5 eV), 127i ( E.\u003d 35 eV), 107Ag, 197AU ( E.\u003d 5 eV). In the region of large energies, 12C threshold detectors are used ( E.\u003d 20 MeV), 32S ( E.\u003d 0.9 MeV) and 63CU ( E.\u003d 10 MeV) ._

Equipmentfor various radioactive research methods (except Yamm) has a lot in common. Her basic function- Measuring the intensity of neutrons or gamma quanta, and therefore it contains electronic circuits for various research methods based in general on the same principles.

The main differences of the equipment for various methods are associated with the design of probes, source, filters and radiation detectors. Given the overall function of all types of radiometric equipment - measurement of the radiation intensity, this equipment is called called well radiometers. Structurally, all radiometers consist of a borehole device and a ground console connected by a geophysical cable. The simplified flowchart of the measuring part of the radiometric instrument is shown in Figure 54. Consistently consider the purpose and the device of individual blocks:

Radiation detectors- the most important elements of radiometers. As radiation detectors in, well appliances use gas discharge or scintillatingcounters. Gas discharge countersconstructively represent a cylindrical cylinder, along the axis of which a metal thread is stretched, which serves an anode (Fig. 55). The metal side surface of the cylinder serves as a cathode. A constant voltage is supplied between the cathode and anode, equal to different types of meters from 300 - 400 V to 2 - z square meters.

Counters for registration of gamma quanta are filled with a mixture of inert gas with high-molecular particulate pairs or halogens. When the gamma-radiation interacts with the cathode, an electron is knocked out from it. The electron, which falls into the gas filled volume of the meter, carries out gas ionization, i.e., in turn, erects electrons from gas atoms, turning them into positively charged ions.

These electrons called primary, accelerated by electric field, on the way to the anode cause secondary ionization, etc. As a result, the number of electrons increases avalanche-like, exceeding the number of primary electrons in thousands and hundreds of thousands of times - a discharge occurs in the meter. With a relatively low voltage, the total number of electrons is proportional to the number of primary electrons, and consequently, the energy of a nuclear particle, registered by the meter - such counters are called proportional. With a large voltage between the anode and cathode, the total number of electrons ceases to depend on the number of primary electrons and on the energy of the registered particle - such is called Geiger Muller meters.

For registration of gamma quantain borehole radiometers use Geiger counters. Their advantage is greater than in proportional counters, output signal (up to several volts), which simplifies the gain and transmission of signals to the surface.

Neutrons do not ionize gasin the meter. Therefore, counters intended for registration of neutrons are filled with gas, in the molecule of which includes a substance, with the interaction of neutrons with which rapid charged particles arise that produce ionization. Such a substance is gas fluoride BB 3 or one of Helium isotopes 3 not. When absorbing slow neutrons, an alpha particle is formed by the yad of the isotope. Therefore, when the thermal and laid-heat neutrons enter the counter filled with the boron compound, alpha particles arise, which cause a discharge in the gas volume of the meter and the voltage pulse at its output. When typing neutrons, the kernel 3 does not arise a quick proton.

Neutron countersthey work in proportional mode, which makes it possible to eliminate pulses from gamma quanta, which have a much smaller value than pulses from alpha particles, or protons.

Scintillation counter It consists of a scintillator associated with a photoelectron multiplier (FEU). When the gamma quantum falls into the scintillator, the initiation of the latter atoms occurs. Excited atoms emit em radiation, part of which lies in the light area. Light quanta from the scintillator falls on the FEU photocathode and electrons are knocked out from it.

The photoelectron multiplier except the photocathode contains an anode and a system of electrodes (distilles), placed between the anode and the cathode (Figure - the scheme of the scintillation counter: 1 - scintillator, 2 - housing, 3 - reflector, 4 - photon, 5 - FEU body, 6 - photocathode, 7 - focusing electrode, 8 - dinododa, 9 - collecting electrode (anode), R 1 -RN - voltage divider). A positive (relative to the cathode) is supplied to dinodes (relative to the cathode) voltage from the voltage divider R l -r n, while the farther anode from the cathode, the potential above. As a result, the electrons emitted by the photocathode when light hit it, accelerate, bombard the first of the distances and knock out secondary electrons from it. In the future, these electrons are accelerated under the action of the potential difference applied between the first and second dinodes, bombard the second dyna and knock out "tertiary" electrons from it. This happens on each of the distances, as a result of which the total number of electrons increases in geometric progression. The overall strengthening of the FEU flow can reach 106 times or more. Thus, when the flash is hit on the photocatode at the entrance of the FEU, a voltage pulse is formed, through the container FROM Submitted to the input of the amplifier.