Scintillation counter physical principle of operation. Studying the principle of operation of the scintillation detector

The basic device of a scintillation counter is quite simple. A radioactive particle hits the scintillator, as a result of which its molecules pass into an excited state. Following this, their return to the main energy state accompanied by the emission of a photon, which is registered by the detector. Thus, the number of flashes (scintillations) is proportional to the number of absorbed radioactive particles. Measured intensity photon radiation then converted into the intensity of radiation of radioactive particles.

Scintillation counters are an alternative to devices with Geiger-Muller counters, while they have a number of significant advantages over the latter. The efficiency of registration of gamma radiation with their help reaches 100%. However, this is not the most important thing. The main thing is that with their help you can register beta and even alpha radiation. As is known, alpha particles, expressed in terms of nuclear physics, are heavy, their range even in the air is only centimeters, and a sheet of plain paper placed in their path will completely absorb them. Of course, the registration of such particles with the help of a gas discharge tube is out of the question; these particles simply cannot penetrate through its walls. Liquid scintillation counters, liquid scintillator devices, come to the rescue. The radioactive sample is introduced into the cuvette with the scintillator solution and then installed in the counter. In such a situation, a radioactive particle, leaving the molecule of the sample under study, immediately collides with the scintillator molecules surrounding it, and then everything that is described above.

Scintillation counters are widely used in medicine and radiobiology. The most popular all over the world are devices from American manufacturers Beckman Coulter and Perkin Elmer.

On our portal you can find scintillation counters by favorable price. If you do not find the desired ad among the "Offers from individuals" in the category, then refer to the same category in the section "Proposals of companies" or start the search with .

- How a scintillation counter works

- Scintillators

- Photomultipliers

- Designs of scintillation counters

- Properties of scintillation counters

- Examples of using scintillation counters

- List of used literature

SCINTILLATION COUNTERS

The method of detecting charged particles by counting flashes of light that occur when these particles hit a zinc sulfide (ZnS) screen is one of the first methods for detecting nuclear radiation.

As early as 1903, Crookes and others showed that if a zinc sulfide screen irradiated with a-particles is viewed through a magnifying glass in a dark room, then one can notice the appearance of individual short-term flashes of light - scintillations. It was found that each of these scintillations is created by a separate a-particle hitting the screen. Crookes built a simple device called the Crookes spinthariscope, designed to count a-particles.

The visual scintillation method was subsequently used mainly for detecting a-particles and protons with an energy of several million electron volts. It was not possible to register individual fast electrons, since they cause very weak scintillations. Sometimes, when a zinc sulfide screen was irradiated with electrons, it was possible to observe flashes, but this happened only when enough big number electrons.

Gamma rays do not cause any flashes on the screen, creating only a general glow. This makes it possible to detect a-particles in the presence of strong g-radiation.

The visual scintillation method makes it possible to register a very small number of particles per unit time. Best conditions for counting scintillations are obtained when their number lies between 20 and 40 per minute. Of course, the scintillation method is subjective, and the results depend to some extent on individual qualities experimenter.

Despite its shortcomings, the visual scintillation method played a role huge role in the development of nuclear and atomic physics. Rutherford used it to register a-particles as they were scattered by atoms. It was these experiments that led Rutherford to the discovery of the nucleus. For the first time, the visual method made it possible to detect fast protons knocked out from nitrogen nuclei when bombarded with a-particles, i.e. first artificial fission of the nucleus.

The visual scintillation method had great importance right up to the thirties, when the emergence of new methods for recording nuclear radiation made him forget for some time. The scintillation registration method was revived in the late forties of the XX century on new basis. By this time, photomultiplier tubes (PMTs) had been developed that made it possible to register very weak flashes of light. Scintillation counters have been created, with the help of which it is possible to increase the counting rate by 108 or even more times compared to visual method, and it is also possible to register and analyze in terms of energy both charged particles and neutrons and g-rays.

§ 1. The principle of operation of the scintillation counter

A scintillation counter is a combination of a scintillator (phosphorus) and a photomultiplier tube (PMT). The counter kit also includes a PMT power supply and radio equipment that provides amplification and registration of PMT pulses. Sometimes the combination of phosphorus with PMT is made through a special optical system(light guide).

The principle of operation of the scintillation counter is as follows. A charged particle entering the scintillator produces ionization and excitation of its molecules, which, after a very a short time (10-6- 10-9 sec ) go into a stable state by emitting photons. There is a flash of light (scintillation). Some of the photons hit the PMT photocathode and knock out photoelectrons from it. The latter, under the action of the voltage applied to the PMT, are focused and directed to the first electrode (dynode) of the electron multiplier. Further, as a result of secondary electron emission, the number of electrons increases like an avalanche, and a voltage pulse appears at the PMT output, which is then amplified and recorded by radio equipment.

The amplitude and duration of the output pulse are determined by the properties of both the scintillator and the PMT.

As phosphorus are used:

organic crystals,

Liquid organic scintillators,

hard plastic scintillators,

gas scintillators.

The main characteristics of scintillators are: light output, spectral composition radiation and duration of scintillations.

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

The light output or conversion efficiency of the scintillator c is the ratio of the light flash energy , going outside, to the amount of energy E charged particle lost in the scintillator

where - the average number of photons going out, - average energy photons. Each scintillator emits not monoenergetic quanta, but a continuous spectrum characteristic of this scintillator.

It is very important that the spectrum of photons emerging from the scintillator coincide or at least partially overlap with the spectral characteristic of the photomultiplier.

The degree of overlap of the outer scintillation spectrum with the spectral response. of this PMT is determined by the matching coefficient

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

where I 0 - maximum value of scintillation intensity; t - decay time constant, defined as the time during which the scintillation intensity decreases in e once.

Number of light photons n , emitted over time t after the hit of the detected particle, is expressed by the formula

where - total number photons emitted during the scintillation process.

The processes of luminescence (glow) of phosphorus are divided into two types: fluorescence and phosphorescence. If the flashing occurs directly during excitation or during a period of time of the order of 10-8 sec, the process is called fluorescence. Interval 10-8 sec chosen because it is equal in order of magnitude to the lifetime of an atom in an excited state for the so-called allowed transitions.

Although the spectra and duration of fluorescence do not depend on the type of excitation, the yield of fluorescence essentially depends on it. Thus, when a crystal is excited by a-particles, the fluorescence yield is almost an order of magnitude lower than when it is photoexcited.

Phosphorescence is understood as luminescence, which continues for a considerable time after the termination of excitation. But the main difference between fluorescence and phosphorescence is not the duration of the afterglow. Phosphorescence of crystal phosphors arises from the recombination of electrons and holes that have arisen during excitation. In some crystals, the afterglow can be prolonged due to the fact that electrons and holes are captured by "traps", from which they can be released only after receiving additional energy. the necessary energy. Hence, the dependence of the duration of phosphorescence on temperature is obvious. In case of complex organic molecules phosphorescence is associated with their presence in a metastable state, the probability of transition from which to the ground state may be small. And in this case, the dependence of the decay rate of phosphorescence on temperature will be observed.

§ 2. Scintillators

Inorganic scintillators . Inorganic scintillators are crystals inorganic salts. Practical use in scintillation technique have mainly halogen compounds of some alkali metals.

The process of scintillation formation can be represented using zone theory solid body. In a separate atom that does not interact with others, electrons are located on well-defined discrete energy levels. In a solid, the atoms are at close distances, and their interaction is quite strong. Thanks to this interaction, the levels of external electron shells split and form zones separated from each other by band gaps. The outermost allowed band filled with electrons is the valence band. Above it is a free zone - the conduction band. Between the valence band and the conduction band there is a band gap, the energy width of which is several electron volts.

If the crystal contains any defects, lattice disturbances, or impurity atoms, then in this case, the appearance of energy electronic levels located in the band gap is possible. Under external action, for example, when a fast charged particle passes through a crystal, electrons can pass from the valence band to the conduction band. Will remain in the valence band vacancies, which have the properties of positively charged particles with a unit charge and are called holes.

The described process is the process of excitation of the crystal. The excitation is removed by the reverse transition of electrons from the conduction band to the valence band, and the recommendation of electrons and holes occurs. In many crystals, the transition of an electron from the conduction to the valence band occurs through intermediate luminescent centers, the levels of which are in the band gap. These centers are due to the presence of defects or impurity atoms in the crystal. During the transition of electrons in two stages, photons are emitted with an energy smaller than the band gap. For such photons, the probability of absorption in the crystal itself is small, and therefore the light output for it is much greater than for a pure, undoped crystal.

In practice, to increase the light output of inorganic scintillators, special impurities of other elements, called activators, are introduced. For example, thallium is introduced as an activator into a sodium iodide crystal. The scintillator based on the NaJ(Tl) crystal has a high light output. The NaJ(Tl) scintillator has significant advantages over gas-filled counters:

greater efficiency registration of g-rays (with large crystals, the registration efficiency can reach tens of percent);

short duration of scintillation (2.5 10-7 sec);

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

The last property needs some explanation. The light output of the scintillator has some dependence on the specific energy loss of a charged particle.

At very large quantities significant violations are possible. crystal lattice scintillator, which lead to the appearance of local quenching centers. This circumstance can lead to a relative decrease in the light output. Indeed, the experimental facts indicate that for heavy particles the yield is nonlinear, and linear dependence begins to manifest itself only with an energy of several million electron volts. Figure 1 shows the dependence curves E: curve 1 for electrons, curve 2 for a particles.

In addition to the indicated alkali halide scintillators, other inorganic crystals are sometimes used: ZnS (Tl), CsJ (Tl), CdS (Ag), CaWO4, CdWO4, etc.

Organic crystalline scintillators. Molecular bonding forces in organic crystals are small compared to the forces acting in inorganic crystals. Therefore, the interacting molecules practically do not perturb the energy electronic levels each other and the process of luminescence of an organic crystal is a process characteristic of individual molecules. In the ground electronic state, the molecule has several vibrational levels. Under the influence of the detected radiation, the molecule passes into an excited electronic state, which also corresponds to several vibrational levels. Ionization and dissociation of molecules are also possible. As a result of recombination of an ionized molecule, it is usually formed in an excited state. Initially excited molecule may be on high levels excitement and after a short time (~10-11 sec) emits a high energy photon. This photon is absorbed by another molecule, and part of the excitation energy of this molecule can be spent on thermal motion and the subsequently emitted photon will have a lower energy than the previous one. After several cycles of emission and absorption, molecules are formed that are at the first excited level; they emit photons, the energy of which may already be insufficient to excite other molecules, and thus the crystal will be transparent to the emerging radiation.

Rice. 2. Dependence of light output

anthracene from energy to various particles.

Thanks to most of the excitation energy is spent on thermal motion, the light output (conversion efficiency) of the crystal is relatively small and amounts to a few percent.

For the registration of nuclear radiation, the following organic crystals are most widely used: anthracene, stilbene, naphthalene. Anthracene has a fairly high light output (~4%) and a short glow time (3 10-8 sec). But when registering heavy charged particles, a linear dependence of the scintillation intensity is observed only at a fairly high energies particles.

On fig. Figure 2 shows the graphs of the dependence of the light output c (in arbitrary units) on the energy of electrons 1, protons 2 , deuterons 3 and a-particles 4 .

Stilbene, although it has a slightly lower light output than anthracene, but the duration of scintillation is much shorter (7 10-9 sec), than that of anthracene, which makes it possible to use it in those experiments where registration of very intense radiation is required.

plastic scintillators. Plastic scintillators are solid solutions of fluorescent organic compounds in a suitable transparent substance. For example, solutions of anthracene or stilbene in polystyrene or plexiglass. The concentrations of the dissolved fluorescent substance are usually low, a few tenths of a percent or a few percent.

Since there is much more solvent than the dissolved scintillator, then, of course, the registered particle produces mainly the excitation of the solvent molecules. The excitation energy is subsequently transferred to the scintillator molecules. Obviously, the emission spectrum of the solvent must be harder than the absorption spectrum of the solute, or at least match with him. Experimental facts show that the excitation energy of the solvent is transferred to the scintillator molecules due to the photon mechanism, i.e., the solvent molecules emit photons, which are then absorbed by the solute molecules. Another mechanism for energy transfer is also possible. Since the concentration of the scintillator is low, the solution is practically transparent to the resulting scintillator radiation.

Plastic scintillators have significant advantages over organic crystalline scintillators:

The ability to manufacture scintillators is very large sizes;

The possibility of introducing spectrum mixers into the scintillator to achieve better matching of its luminescence spectrum with the spectral characteristic of the photocathode;

Possibility of introduction into the scintillator various substances required in special experiments (for example, in the study of neutrons);

Possibility of using plastic scintillators in vacuum;

short glow time (~3 10-9 sec). Plastic scintillators prepared by dissolving anthracene in polystyrene have the highest light output. A solution of stilbene in polystyrene also has good properties.

Liquid organic scintillators. Liquid organic scintillators are solutions of organic scintillators in certain liquid organic solvents.

The mechanism of fluorescence in liquid scintillators is similar to the mechanism that occurs in solid solutions-scintillators.

Xylene, toluene, and phenylcyclohexane turned out to be the most suitable solvents, while p-terphenyl, diphenyloxazole, and tetraphenylbutadiene turned out to be the most suitable solvents. The scintillator made by dissolving

p-terphenyl in xylene at a solute concentration of 5 g/l.

The main advantages of liquid scintillators:

Possibility of manufacturing large volumes;

Possibility of introduction into the scintillator of the substances necessary in special experiments;

Short flash duration ( ~3 10-9sec).

gas scintillators. When charged particles pass through various gases, the appearance of scintillations was observed in them. The heavy noble gases (xenon and krypton) have the highest light output. A mixture of xenon and helium also has a high light output. The presence of 10% xenon in helium provides a light output that is even greater than that of pure xenon (Fig. 3). Negligibly small impurities of other gases sharply reduce the intensity of scintillations in noble gases.

Rice. 3. Dependence of the light output of the gas

scintillator on the ratio of the mixture of helium and xenon.

It was experimentally shown that the duration of flashes in noble gases is short (10-9 -10-8 sec), and the intensity of flashes in wide range is proportional to the lost energy of registered 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 far ultraviolet region, so light converters are used to match the spectral sensitivity of the photomultiplier. The latter should have a high conversion rate, optical transparency in thin layers, low elasticity saturated vapors as well as mechanical and chemical resistance. As materials for light converters, various organic compounds, For example:

diphenylstilbene (conversion efficiency about 1);

P1p'-quaterphenyl (~1);

anthracene (0.34), etc.

The light converter is deposited in a thin layer on the photomultiplier photocathode. An important parameter of a light converter is its flash time. In this regard, organic converters are quite satisfactory (10-9 sec or several units for 10-9 sec). To increase light collection, the inner walls of the scintillator chamber are usually coated with light reflectors (MgO, enamel based on titanium oxide, fluoroplast, aluminum oxide, etc.).

§ 3. Photoelectronic multipliers

The main elements of the PMT are: photocathode, focusing system, multiplier system (dynodes), anode (collector). All these elements are located in a glass container evacuated to a high vacuum (10-6 mmHg.).

For the purposes of nuclear radiation spectrometry, the photocathode is usually located on inner surface flat end part of the PMT container. As the material of the photocathode, a substance is chosen that is sufficiently sensitive to the light emitted by the scintillators. The most widespread are antimony-cesium photocathodes, the maximum spectral sensitivity of which lies at l = 3900¸4200 A, which corresponds to the maxima of the luminescence spectra of many scintillators.

Rice. 4. Schematic diagram of the PMT.

One of the characteristics of a photocathode is its quantum yield, i.e., the probability of a photoelectron being ejected by a photon that hits the photocathode. The value of e can reach 10-20%. The properties of the photocathode are also characterized by the integral sensitivity, which is the ratio of the photocurrent (mka) to incident on the photocathode luminous flux (lm).

The photocathode is applied to glass as a thin translucent layer. The thickness of this layer is significant. On the one hand, for a large absorption of light, it must be significant, on the other hand, the emerging photoelectrons, having a very low energy, will not be able to leave the thick layer and the effective quantum yield may turn out to be small. Therefore, the optimal thickness of the photocathode is selected. It is also essential to ensure a uniform thickness of the photocathode so that its sensitivity is the same over the entire area. In scintillation g-spectrometry, it is often necessary to use large solid scintillators, both in thickness and in diameter. Therefore, it becomes necessary to manufacture photomultipliers with large photocathode diameters. In domestic photomultipliers, photocathodes are made with a diameter from several centimeters to 15¸20 cm. photoelectrons knocked out of the photocathode must be focused on the first multiplier electrode. For this purpose, an electrostatic lens system is used, which is a series of focusing diaphragms. To obtain good temporal characteristics of the PMT, it is important to create such a focusing system that the electrons hit the first dynode with a minimum time spread. Figure 4 shows a schematic arrangement of a photomultiplier. The high voltage supplying the PMT is connected to the cathode with a negative pole and distributed between all electrodes. The potential difference between the cathode and the diaphragm ensures the focusing of photoelectrons on the first multiplying electrode. Multiplying electrodes are called dynodes. Dynodes are made from materials whose secondary emission coefficient is greater than unity (s>1). In domestic PMTs, dynodes are made either in the form of a trough-shaped form (Fig. 4) or in the form of blinds. In both cases, the dynodes are arranged in a line. An annular arrangement of dynodes is also possible. PMTs with a ring-shaped dynode system have the best time characteristics. The emitting layer of dynodes is a layer of antimony and cesium or a layer of special alloys. Maximum value s for antimony-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= 12¸14, in the second s=7¸10. In PMT operating modes, the value of s is somewhat smaller. A fairly good re-emission factor is s= 5.

Photoelectrons focused on the first dynode knock out secondary electrons from it. The number of electrons leaving the first dynode is several times more number photoelectrons. All of them are sent to the second dynode, where secondary electrons are also knocked out, etc., from dynode to dynode, the number of electrons increases by s times.

When passing through the entire system of dynodes, the electron flux increases by 5-7 orders of magnitude and enters the anode - the collecting electrode of the PMT. If the PMT operates in the current mode, then the anode circuit includes devices that amplify and measure the current. When registering nuclear radiation, it is usually necessary to measure the number of pulses that arise under the influence of ionizing particles, as well as the amplitude of these pulses. In these cases, a resistance is included in the anode circuit, at which a voltage pulse occurs.

An important characteristic PMT is the multiplication factor M. If the value of s for all dynodes is the same (with full collection of electrons on the dynodes), and the number of dynodes is equal to n , then

A and B are constants, u is the electron energy. multiplication factor M not equal to the coefficient amplification M", which characterizes the ratio of the current at the PMT output to the current leaving the cathode

M" =CM,

where With<1 - electron collection coefficient characterizing the efficiency of photoelectron collection on the first dynode.

It is very important that the gain is constant. M" PMT both in time and with a change in the number of electrons emerging from the photo cathode. The latter circumstance makes it possible to use scintillation counters as nuclear radiation spectrometers.

On interference in photomultipliers. In scintillation counters, even in the absence of external irradiation, a large number of pulses can appear at the PMT output. These pulses usually have small amplitudes and are called noise pulses. The largest number of noise pulses is due to the appearance of thermoelectrons from the photocathode or even from the first dynodes. Cooling is often used to reduce PMT noise. When registering radiation that creates large-amplitude pulses, a discriminator is included in the recording circuit that does not transmit noise pulses.

Rice. 5. Scheme for PMT noise suppression.

1. When registering pulses whose amplitude is comparable to noise, it is rational to use one scintillator with two PMTs included in the coincidence circuit (Fig. 5). In this case, a temporal selection of pulses arising from the detected particle occurs. In fact, a flash of light that arose in the scintillator from a registered particle will simultaneously hit the fluorocathodes of both PMTs, and pulses will simultaneously appear at their output, forcing the coincidence circuit to work. The particle will be registered. Noise pulses in each of the PMTs appear independently of each other and most often will not be registered by the coincidence circuit. This method makes it possible to reduce the PMT intrinsic background by 2–3 orders of magnitude.

The number of noise pulses increases with the applied voltage, at first rather slowly, then the increase sharply increases. The reason for this sharp increase in the background is the field emission from the sharp edges of the electrodes and the appearance of a feedback ionic connection between the last dynodes and the PMT photocathode.

In the region of the anode, where the current density is highest, the glow of both residual gas and structural materials may occur. The resulting weak glow, as well as the ionic feedback, cause the appearance of the so-called accompanying pulses, which are 10-8 ¸10-7 apart in time from the main ones. sec.

§ 4. Designs of scintillation counters

The following requirements are imposed on the designs of scintillation counters:

Best collection of scintillation light on the photocathode;

Uniform distribution of light over the photocathode;

Darkening from the light of extraneous sources;

No influence of magnetic fields;

The stability of the PMT gain.

When working with scintillation counters, it is always necessary to achieve the highest ratio of the amplitude of the signal pulses to the amplitude of the noise pulses, which forces the optimal use of the flash intensities arising in the scintillator. Typically, the scintillator is packed in a metal container closed at one end with flat glass. Between the container and the scintillator is placed a layer of material that reflects light and contributes to its most complete exit. Magnesium oxide (0.96), titanium dioxide (0.95), gypsum (0.85-0.90) have the highest reflectivity, aluminum is also used (0.55-0.85).

Particular attention should be paid to the careful packaging of hygroscopic scintillators. So, for example, the most commonly used phosphorus NaJ (Tl) is very hygroscopic and when moisture penetrates into it, it turns yellow and loses its scintillation properties.

Plastic scintillators do not need to be packed in airtight containers, but a reflector can be placed around the scintillator to increase light collection. All solid scintillators must have an output window at one end, which is connected to the photomultiplier photocathode. There may be significant loss of scintillation light intensity at the junction. To avoid these losses, Canadian balsam, mineral or silicone oils are introduced between the scintillator and PMT, and optical contact is created.

In some experiments, for example, measurements in vacuum, in magnetic fields, in strong fields of ionizing radiation, the scintillator cannot be placed directly on the PMT photocathode. In such cases, a light guide is used to transmit light from the scintillator to the photocathode. As light guides, polished rods made of transparent materials are used - such as lucite, plexiglass, polystyrene, as well as metal or plexiglass tubes filled with a transparent liquid. The loss of light in a light guide depends on its geometric dimensions and on the material. In some experiments it is necessary to use curved light guides.

It is better to use light guides with a large radius of curvature. Light guides also make it possible to articulate scintillators and PMTs of different diameters. In this case, cone-shaped light guides are used. The PMT is coupled to the liquid scintillator either through a light guide or by direct contact with the liquid. Figure 6 shows an example of a PMT joint with a liquid scintillator. In various operating modes, the PMT is supplied with a voltage from 1000 to 2500 in. Since the gain of the PMT depends very sharply on the voltage, the supply current source must be well stabilized. In addition, self-stabilization is possible.

The PMT is powered by a voltage divider, which allows each electrode to be supplied with the appropriate potential. The negative pole of the power source is connected to the photocathode and to one of the ends of the divider. The positive pole and the other end of the divider are grounded. The resistors of the divider are selected in such a way that the optimal mode of operation of the PMT is implemented. For greater stability, the current through the divider should be an order of magnitude higher than the electron currents flowing through the PMT.

Rice. 6. PMT coupling with a liquid scintillator.

1-liquid scintillator;

2- PMT;

3- light shield.

When the scintillation counter operates in a pulsed mode, short (~10-8 sec) impulses, the amplitude of which can be several units or several tens of volts. In this case, the potentials on the last dynodes may experience sharp changes, since the current through the divider does not have time to replenish the charge carried away from the cascade by electrons. To avoid such potential fluctuations, the last few resistances of the divider are shunted with capacitances. Due to the selection of potentials on the dynodes, favorable conditions are created for the collection of electrons on these dynodes, i.e. a certain electron-optical system corresponding to the optimal regime is implemented.

In an electron-optical system, the electron trajectory does not depend on the proportional change in potentials at all electrodes that form this electron-optical system. Similarly, in a multiplier, when the supply voltage changes, only its gain changes, but the electron-optical properties remain unchanged.

With a disproportionate change in potentials on the PMT dynodes, the conditions for focusing electrons in the area where the proportionality is violated change. This circumstance is used for self-stabilization of the PMT gain. For this purpose, the potential

Rice. 7. Part of the divider circuit.

of one of the dynodes with respect to the potential of the previous dynode is set constant, either with the help of an additional battery, or with the help of an additionally stabilized divider. Figure 7 shows a part of the divider circuit, where an additional battery is connected between dynodes D5 and D6 ( Ub = 90 in). To obtain the best self-stabilization effect, it is necessary to select the resistance value R". Usually R" more R 3-4 times.

§ 5. Properties of scintillation counters

Scintillation counters have the following advantages.

High time resolution. The pulse duration, depending on the scintillators used, ranges from 10-6 to 10-9 sec, those. by several orders of magnitude less than counters with self-discharge, which allows for much higher counting rates. Another important temporal characteristic of scintillation counters is the small value of the pulse delay after the passage of the registered particle through the phosphor (10-9 -10-8 sec). This allows the use of coincidence schemes with low resolution time (<10-8sec) and, consequently, to measure coincidences at many large loads on individual channels with a small number of random coincidences.

High Registration Efficiency g -rays and neutrons. To register a g-quantum or a neutron, it is necessary that they react with the substance of the detector; in this case, the resulting secondary charged particle must be registered by the detector. It is obvious that the more substances are in the path of g-rays or neutrons, the greater will be the probability of their absorption, the greater will be the efficiency of their registration. At present, when large scintillators are used, g-ray detection efficiency of several tens of percent is achieved. The efficiency of neutron detection by scintillators with specially introduced substances (10 V, 6 Li, etc.) also far exceeds the efficiency of neutron detection by gas-discharge counters.

Possibility of energy analysis of registered radiation. Indeed, for light charged particles (electrons), the flash intensity in a scintillator is proportional to the energy lost by the particle in this scintillator.

Using scintillation counters attached to amplitude analyzers, one can study the spectra of electrons and g-rays. The situation is somewhat worse with the study of the spectra of heavy charged particles (a-particles, etc.), which create a large specific ionization in the scintillator. In these cases, the proportionality of the intensity of the burst of the lost energy is observed not at all particle energies and manifests itself only at energies greater than a certain value. The nonlinear relationship between the pulse amplitudes and the particle energy is different for different phosphors and for different types of particles. This is illustrated by the graphs in Figures 1 and 2.

The possibility of manufacturing scintillators of very large geometric dimensions. This means that it is possible to detect and analyze energy particles of very high energies (cosmic rays), as well as particles that weakly interact with matter (neutrinos).

Possibility of introducing into the composition of scintillators substances with which neutrons interact with a large cross section. Phosphors LiJ(Tl), LiF, LiBr are used to detect slow neutrons. When slow neutrons interact with 6 Li, the reaction 6 Li(n,a)3 H takes place, in which an energy of 4.8 Mev.

§ 6. Examples of the use of scintillation counters

Measurement of the lifetimes of excited states of nuclei. During radioactive decay or in various nuclear reactions, the resulting nuclei often end up in an excited state. The study of the quantum characteristics of excited states of nuclei is one of the main tasks of nuclear physics. A very important characteristic of the excited state of the nucleus is its lifetime t. Knowing this value allows one to obtain many information about the structure of the nucleus.

Atomic nuclei can be in an excited state for various times. There are various methods for measuring these times. Scintillation counters have proved to be very convenient for measuring the lifetimes of nuclear levels from a few seconds to very small fractions of a second. As an example of the use of scintillation counters, we will consider the delayed coincidence method. Let the nucleus A (see Fig. 10) by b-decay turns into a nucleus AT in an excited state, which gives off an excess of its energy for the successive emission of two g-quanta (g1, g2). It is required to determine the lifetime of the excited state I. The preparation containing isotope A is installed between two counters with NaJ(Tl) crystals (Fig. 8). The pulses generated at the output of the PMT are fed to the fast coincidence circuit with a resolution time of ~10-8 -10-7 sec. In addition, pulses are fed to linear amplifiers and then to amplitude analyzers. The latter are configured in such a way that they pass pulses of a certain amplitude. For our purpose, i.e. for the purpose of measuring the level lifetime I(see fig. 10), amplitude analyzer AAI must pass only pulses corresponding to the photon energy g1, and the analyzer AAII - g2 .

Fig.8. Schematic diagram to define

lifetime of excited states of nuclei.

Further, pulses from the analyzers, as well as from the fast coincidence circuit, are fed to the slow one (t ~ 10-6 sec) triple match pattern. In the experiment, the dependence of the number of triple coincidences on the value of the time delay of the pulse included in the first channel of the fast coincidence circuit is studied. Typically, the pulse delay is carried out using the so-called variable delay line LZ (Fig. 8).

The delay line must be connected exactly to the channel in which quantum g1 is registered, since it is emitted before quantum g2. As a result of the experiment, a semi-logarithmic graph of the dependence of the number of triple coincidences on the delay time is constructed (Fig. 9), and the lifetime of the excited level is determined from it I(in the same way as it is done when determining the half-life using 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 lifetimes 10-7 -10-9 sec. If faster organic scintillators are used, then shorter lifetimes of excited states can be measured (up to 10–11 sec).

Fig.9. The dependence of the number of coincidences on the magnitude of the delay.

Gamma flaw detection. Nuclear radiation, which has a high penetrating power, is increasingly used in technology to detect defects in pipes, rails and other large metal blocks. For these purposes, a g-radiation source and a g-ray detector are used. The best detector in this case is a scintillation counter, which has a high detection efficiency. The radiation source is placed in a lead container, from which a narrow beam of g-rays emerges through a collimator hole, illuminating the tube. A scintillation counter is installed on the opposite side of the tube. The source and counter are placed on a movable mechanism that allows them to be moved along the pipe and rotated about its axis. Passing through the pipe material, the g-ray beam will be partially absorbed; if the tube 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 time, but if there is a sink in some place of the tube, then the g-rays will be absorbed in this place less, the counting speed will increase. The location of the sink will be revealed. There are many examples of such use of scintillation counters.

Experimental detection of neutrinos. Neutrino is the most mysterious of elementary particles. Almost all properties of neutrinos are obtained from indirect data. The modern theory of b-decay assumes that the neutrino mass mn is equal to zero. Some experiments allow us to state that. Neutrino spin 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 распадеиспускается электрон (частица) и антинейтрино (`n-античастйца).

The creation of nuclear reactors, in which a very large number of nuclei with an excess of neutrons, gave hope for the detection of antineutrinos. All neutron-rich nuclei decay with the emission of electrons and, consequently, antineutrinos. Near a nuclear reactor with a capacity of several hundred thousand kilowatts, the antineutrino flux is 1013 cm -2 · sec-1 - a stream of enormous density, and with the choice of a suitable antineutrino detector, one could try to detect them. Such an attempt was made by Reines and Cowen in 1954. The authors used the following reaction:

n + p ® n+e+ (1)

In this reaction, the product particles are the positron and the neutron, which can be registered.

A liquid scintillator with a volume of ~1 m3, with a high hydrogen content, saturated with cadmium. The positrons produced in reaction (1) annihilated into two g-quanta with an energy of 511 kev each and caused the appearance of the first flash of the scintillator. The neutron was slowed down for several microseconds and captured by cadmium. In this capture by cadmium, several g-quanta were emitted with a total energy of about 9 Mev. As a result, a second flash appeared in the scintillator. Delayed coincidences of two pulses were measured. To register flashes, the liquid scintillator was surrounded by a large number of photomultipliers.

The count rate of delayed coincidences was three counts per hour. From these data, it was obtained that the reaction cross section (Fig. 1) s = (1.1 ± 0.4) 10 -43 cm2, which is close to the calculated value.

At present, very large liquid scintillation counters are used in many experiments, in particular, in experiments to measure g-radiation fluxes emitted by humans and other living organisms.

Registration of fission fragments. For registration of fission fragments, gas scintillation counters proved to be convenient.

Usually, an experiment to study the fission cross section is set up as follows: a layer of the element under study is deposited on some kind of substrate and irradiated with a neutron flux. Of course, the more fissile material is used, the more fission events will occur. But since usually fissile substances (for example, transuranium elements) are a-emitters, their use in significant quantities becomes difficult due to the large background from a-particles. And if fission events are studied with the help of pulsed ionization chambers, then it is possible to superimpose pulses from a-particles on pulses arising from fission fragments. Only an instrument with better temporal resolution will make it possible to use large quantities of fissile material without imposing pulses on each other. In this regard, gas scintillation counters have a significant advantage over pulsed ionization chambers, since the pulse duration of the latter is 2–3 orders of magnitude longer than that of gas scintillation counters. The pulse amplitudes from fission fragments are much larger than those from a-particles, and therefore can be easily separated using an amplitude analyzer.

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

Luminous camera. In 1952, Soviet physicists Zavoisky and others for the first time photographed the traces of ionizing particles in luminescent substances using sensitive electron-optical converters (EOCs). This particle detection method, called the fluorescent camera, has a high time resolution. The first experiments were made using a CsJ (Tl) crystal.

Later, plastic scintillators in the form of long thin rods (threads) began to be used to manufacture the luminescent chamber. The threads are stacked in rows so that the threads in two adjacent rows are at right angles to each other. This provides the possibility of stereoscopic observation to recreate the spatial trajectory of the particles. Images from each of the two groups of mutually perpendicular filaments are directed to separate electron-optical converters. The threads also play the role of light guides. Light is given only by those threads that the particle crosses. This light exits through the ends of the respective threads, which are photographed. Systems are produced with a diameter of individual threads from 0.5 to 1.0 mm.

Literature :

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

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

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

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

Publishing house of the Leningrad University, 1970.

5 G.S. Landsberg. Elementary textbook of physics (volume 3). M., Nauka, 1971

Scintillation counter

Principle of operation and scope

In a scintillation counter, ionizing radiation causes a flash of light in the corresponding scintillator, which can be either solid or liquid. This flash is transmitted to a photomultiplier tube, which turns it into a pulse of electric current. The current pulse is amplified in subsequent PMT stages due to their high secondary emission coefficient.

Despite the fact that, in general, more complex electronic equipment is required when working with scintillation counters, these counters have significant advantages over Geiger-Muller counters.

1. Efficiency for counting X-ray and gamma radiation is much greater; under favorable circumstances, it reaches 100%.

2. The light output in some scintillators is proportional to the energy of the exciting particle or quantum.

3. Temporal resolution is higher.

The scintillation counter is thus a suitable detector for detecting low intensity radiation, for energy distribution analysis with not too high resolution requirements, and for coincidence measurements at high radiation intensity.

B) Scintillators

1) Protons and other highly ionizing particles. If we are talking only about the registration of these particles, then all types of scintillators are equally suitable, and, due to their high stopping power, layers with a thickness of the order of a millimeter and even less are sufficient. It must, however, be borne in mind that the light output of protons and β particles in organic scintillators is only about 1/10 of the light output of electrons of the same energy, while in ZnS and NaJ inorganic scintillators they are both of the same order.

The relationship between the energy of light flashes and the magnitude of the pulses associated with it, as well as the energy of particles transferred to the scintillator, for organic substances is, generally speaking, non-linear. For ZnS 1 NaJ and CsJ, however, this dependence is close to linear. Because of their good transparency to their own fluorescent radiation, NaJ and CsJ crystals provide excellent energy resolution; care must, however, be taken to ensure that the surface through which the particles enter the crystal is very clean.

2) Neutrons. Slow neutrons can be detected using the reactions Li6Hs, B10Li" or CdlisCd114. As scintillators for this purpose, single crystals of LiJ, powder mixtures, for example, 1 weight part B 2 O 3 and 5 weight parts ZnS, are deposited directly on the PMT window; can also be applied

Block diagram of a scintillation spectrometer. 1 - scintillator, 2 - PMT, h - high voltage source, 4 - cathode follower, e - linear amplifier, 6 - amplitude pulse analyzer, 7 - recording device.

ZnS suspended in molten B 2 O 3 , corresponding boron compounds in synthetic scintillators, and mixtures of cadmium methyl borate or propionate with liquid scintillators. If it is necessary to exclude the effect of z-radiation in neutron measurements, then in those reactions that cause the emission of heavy particles, the above relation for the light output of various scintillators, depending on the type of particles, must be taken into account.

Fast neutrons are detected using recoil protons produced in hydrogen-containing substances. Since a high hydrogen content occurs only in organic scintillators, it is difficult to reduce the effect of γ-radiation due to the above reasons. The best results are achieved if the process of formation of recoil protons is separated from the excitation of the scintillator by r-rays. In this case, the layer of the latter must be thin, its thickness being determined by the range of the recoil protons, so that the probability of detecting z-radiation is substantially reduced. In this case, it is preferable to use ZnS as a scintillator. It is also possible to suspend powdered ZnS in a transparent artificial substance containing hydrogen.

It is almost impossible to study the energy spectrum of fast neutrons using scintillators. This is explained by the fact that the energy of recoil protons can take on all sorts of values, up to the total energy of neutrons, depending on how the collision occurs.

3) Electrons, p-particles. As for other types of radiation, the energy resolution of the scintillator for electrons depends on the ratio between the light energy and the energy transferred to the scintillator by the ionizing particle. This is due to the fact that the half-width of the distribution curve of the magnitudes of pulses caused by monoenergetic incident particles, due to statistical fluctuations, in the first approximation, is inversely proportional to the square root of the number of photoelectrons knocked out from the PMT photocathode. Of the currently used scintillators, NaJ 1 gives the largest pulse amplitudes, and for organic scintillators, anthracene, which, other things being equal, gives pulses of approximately two times smaller amplitude than NaJ.

Since the effective electron scattering cross sections increase strongly with increasing atomic number, when NaJ is used, 80-90% of all incident electrons are again scattered from the crystal; when using anthracene, this effect reaches approximately 10%. Scattered electrons cause impulses, the magnitude of which is less than the value corresponding to the total energy of the electrons. As a result, it is very difficult to quantify the β spectra obtained with NaJ crystals. Therefore, for β-spectroscopy it is often more expedient to use organic scintillators, which consist of elements with low atomic numbers.

Backscattering can also be weakened by the following methods. The substance whose β-radiation is to be investigated is either admixed with the scintillator if it does not suppress fluorescent radiation, or placed between two surfaces of scintillators whose fluorescent Iryny 1 Ienne acts on the photocathode, or, finally, a scintillator is used with an internal channel into which it passes in-radiation.

The dependence between the light energy and the energy transferred to the scintillator by radiation is linear for NaJ. For all organic scintillators, this ratio decreases at low electron energies. This nonlinearity must be taken into account when quantifying the spectra.

4) X-ray and gamma radiation. The process of interaction of electromagnetic radiation with a scintillator mainly consists of three elementary processes.

In the photoelectric effect, the energy of a quantum is converted almost completely into the kinetic energy of a photoelectron, and due to the short range of the photoelectron, it is in most cases absorbed in the scintillator. The secondary quantum corresponding to the binding energy of the electron is either absorbed by the scintillator or leaves it.

In the Compton effect, only part of the quantum energy is transferred to the electron. This part is absorbed with a high probability in the scintillator. The scattered photon, whose energy has decreased by an amount equal to the energy of the Compton electron, is also either absorbed by the scintillator or leaves it.

During the formation of pairs, the energy of the primary quantum, minus the energy of pair formation, passes into the kinetic energy of this pair and is mainly absorbed by the scintillator. The radiation generated during the annihilation of an electron and a positron is absorbed in the scintillator or leaves it.

The energy dependence of the effective cross sections for these processes is such that, at low photon energies, the photoelectric effect mainly takes place; Beginning with an energy of 1.02 Mae, the formation of pairs can be observed, but the probability of this process reaches an appreciable value only at significantly higher energies. In the intermediate region, the main role is played by the Compton effect.

With an increase in the atomic number Z, the effective cross sections for the photoelectric effect and for the formation of pairs increase much more strongly than with the Compton effect. However, in this case, the electron is transferred:

1) with the photoelectric effect, - in addition to the energy of the quantum, which turns into the energy of the electron already during the primary effect, there is still only the binding energy of the photoelectron, which corresponds to secondary radiation, soft and easily absorbed;

2) in the formation of pairs - only annihilation radiation with a discrete known energy. With the Compton effect, the energy of secondary electrons and scattered quanta has a wide range of possible values. Since, as already mentioned, the secondary quanta may not experience absorption and leave the scintillator, to facilitate the interpretation of the spectra, it is expedient to narrow as far as possible the region in which the Komhtohj effect predominates by choosing scintillators with large H, for example, NaJ. In addition, the ratio energy of light to the energy transferred to the scintillator for NaJ is practically independent of the energy of electrons, therefore, in all complex processes in which quanta are absorbed, the same amount of light is released.Such complex processes occur with the greater probability, the larger the size of the scintillator.

The attenuation of gamma rays in anthracene, μ is the attenuation coefficient; f is the photoabsorption coefficient, a is the Compton scattering coefficient, p is the pair formation coefficient.

scintillation counter, a device for detecting nuclear radiation and elementary particles (protons, neutrons, electrons, g-quanta, mesons, etc.), the main elements of which are a substance that luminesces under the action of charged particles (scintillator), and photomultiplier (FEU). Visual observations of light flashes (scintillations) under the action of ionizing particles (α-particles, nuclear fission fragments) were the main method of nuclear physics in the early 20th century. (cm. Spinthariscope ). Later S. with. was completely ousted ionization chambers and proportional counters. His return to nuclear physics occurred at the end of the 1940s, when multistage PMTs with a high gain were used to detect scintillations, capable of detecting extremely weak light flashes.

S.'s principle of action with. consists in the following: a charged particle passing through a scintillator, along with the ionization of atoms and molecules, excites them. Returning to the unexcited (ground) state, atoms emit photons (see Fig. Luminescence ). Photons hitting the PMT cathode knock out electrons (see Fig. Photoelectronic emission ), as a result, an electric pulse appears on the PMT anode, which is further amplified and recorded (see Fig. rice. ). Detection of neutral particles (neutrons, g -quanta) occurs by secondary charged particles formed during the interaction of neutrons and g -quanta with scintillator atoms.

Various substances (solid, liquid, gaseous) are used as scintillators. Plastics are widely used, which are easily manufactured, machined and give an intense glow. An important characteristic of a scintillator is the fraction of the energy of the detected particle that is converted into light energy (the conversion efficiency h). Crystalline scintillators have the highest h values: NaI, activated Tl, anthracene, and ZnS. Dr. an important characteristic is the glow time t, which is determined by the lifetime at the excited levels. The intensity of the glow after the passage of the particle changes exponentially: , where I 0 - initial intensity. For most scintillators, t lies in the range 10–9 - 10–5 sec. Plastics have short glow times (Table 1). The smaller t, the faster S. can be made with.

In order for a light flash to be registered by a PMT, it is necessary that the emission spectrum of the scintillator coincide with the spectral region of sensitivity of the PMT photocathode, and the scintillator material must be transparent to its own radiation. For registration slow neutrons Li or B is added to the scintillator. Fast neutrons are detected using hydrogen-containing scintillators (see Fig. Neutron detectors ). For spectrometry of g-quanta and high-energy electrons, Nal (Tl) is used, which has a high density and a high effective atomic number (see Fig. Gamma radiation ).

S. s. are made with scintillators of different sizes - from 1-2 mm 3 to 1-2 m 3 . In order not to "lose" the emitted light, good contact between the PMT and the scintillator is necessary. In S. with. a small scintillator is directly glued to the PMT photocathode. All other sides are covered with a layer of reflective material (for example, MgO, TiO 2). In S. with. large size use light guides (usually polished organic glass).

PMTs intended for S. s. must have a high photocathode efficiency (up to 2.5%), high gain (10 8 -10 8), short electron collection time (10 -8 sec) at high stability of this time. The latter makes it possible to achieve resolution in time S. s. £10 -9 sec. The high gain of the PMT, along with a low level of intrinsic noise, makes it possible to detect individual electrons knocked out from the photocathode. The signal at the PMT anode can reach 100 in.

Tab. 1. - Characteristics of some solid and liquid scintillators,

used in scintillation counters

Substance	Density, g/cm 3	Glow time, t , 10 -9 sec.	Conversion efficiency h, % (for electrons)
crystals
Anthracene C 14 H 10
Stilbene C 14 H 12



Liquids
Solution R-terphenyl in xylene (5 g/l) with the addition of POPOP 1 (0.1 g/l)
Solution R-terphenyl in toluene (4 g/l) with the addition of POPOP (0.1 g/l)
Plastics
Polystyrene with addition R-terphenyl (0.9%) and a-NPO 2 (0.05% by weight)
Polyvinyltoluene with the addition of 3.4% R-terphenyl and 0.1 wt% POPOP

1 POPOP - 1,4-di--benzene. 2 NPO - 2-(1-naphthyl)-5-phenyloxazole.

Advantages of S. with.: high efficiency of registration of various particles (practically 100%); speed; the possibility of manufacturing scintillators of different sizes and configurations; high reliability and relatively low cost. Thanks to these qualities S. with. widely used in nuclear physics, elementary particle physics and cosmic rays, in industry (radiation control), dosimetry, radiometry, geology, medicine, etc. Disadvantages of S. S.: low sensitivity to low-energy particles (£ 1 kev), low energy resolution (see Fig. Scintillation spectrometer ).

To study charged particles of low energies (< 0,1 mev) and nuclear fission fragments, gases are used as scintillators (Table 2). Gases have a linear dependence of the signal magnitude on the energy of the particle in a wide range of energies, fast response and the ability to change the stopping power by changing the pressure. In addition, the source can be introduced into the volume of the gas scintillator. However, gas scintillators require high gas purity and a special PMT with quartz windows (a significant part of the emitted light lies in the ultraviolet region).

Tab. 2. - Characteristics of some gases used as

scintillators in scintillation counters (at a pressure of 740 mm

rt. Art., for a-particles with energy 4.7 mev)

Illumination time t,	Wavelength at the maximum of the spectrum,	Conversion efficiency n, %



3×10 -9

Lit.: Birke J., Scintillation counters, trans. from English, M., 1955; Kalashnikova V. I., Kozodaev M. S., Detectors of elementary particles, in the book: Experimental methods of nuclear physics, M., 1966; Ritson D., Experimental methods in high energy physics, trans. from English, M., 1964.

Great Soviet Encyclopedia M.: "Soviet Encyclopedia", 1969-1978

- How a scintillation counter works

- Scintillators

- Photomultipliers

- Designs of scintillation counters

- Properties of scintillation counters

- Examples of using scintillation counters

- List of used literature

SCINTILLATION COUNTERS

The method of detecting charged particles by counting flashes of light that occur when these particles hit a zinc sulfide (ZnS) screen is one of the first methods for detecting nuclear radiation.

Gamma rays do not cause any flashes on the screen, creating only a general glow. This makes it possible to detect a-particles in the presence of strong g-radiation.

The visual scintillation method makes it possible to register a very small number of particles per unit time. The best conditions for counting scintillations are obtained when their number lies between 20 and 40 per minute. Of course, the scintillation method is subjective, and the results to some extent depend on the individual qualities of the experimenter.

Despite its shortcomings, the visual scintillation method played a huge role in the development of nuclear and atomic physics. Rutherford used it to register a-particles as they were scattered by atoms. It was these experiments that led Rutherford to the discovery of the nucleus. For the first time, the visual method made it possible to detect fast protons knocked out from nitrogen nuclei when bombarded with a-particles, i.e. first artificial fission of the nucleus.

The visual scintillation method was of great importance until the 1930s, when the appearance of new methods for detecting nuclear radiation made it forgotten for some time. The scintillation registration method was revived at the end of the 1940s on a new basis. By this time, photomultiplier tubes (PMTs) had been developed that made it possible to register very weak flashes of light. Scintillation counters were created, with the help of which it is possible to increase the counting rate by a factor of 108 and even more in comparison with the visual method, and it is also possible to register and analyze in terms of energy both charged particles and neutrons and g-rays.

§ 1. The principle of operation of the scintillation counter

A scintillation counter is a combination of a scintillator (phosphorus) and a photomultiplier tube (PMT). The counter kit also includes a PMT power supply and radio equipment that provides amplification and registration of PMT pulses. Sometimes the combination of phosphorus with a photomultiplier is produced through a special optical system (light guide).

The principle of operation of the scintillation counter is as follows. A charged particle entering the scintillator produces ionization and excitation of its molecules, which after a very short time (10 -6 - 10 -9 sec ) go into a stable state by emitting photons. There is a flash of light (scintillation). Some of the photons hit the PMT photocathode and knock out photoelectrons from it. The latter, under the action of the voltage applied to the PMT, are focused and directed to the first electrode (dynode) of the electron multiplier. Further, as a result of secondary electron emission, the number of electrons increases like an avalanche, and a voltage pulse appears at the PMT output, which is then amplified and recorded by radio equipment.

The amplitude and duration of the output pulse are determined by the properties of both the scintillator and the PMT.

As phosphorus are used:

organic crystals,

Liquid organic scintillators,

hard plastic scintillators,

gas scintillators.

The main characteristics of scintillators are: light output, spectral composition of radiation and duration of scintillations.

The light output or conversion efficiency of the scintillator c is the ratio of the light flash energy , going outside, to the amount of energy E charged particle lost in the scintillator

where - the average number of photons going out, - average photon energy. Each scintillator emits not monoenergetic quanta, but a continuous spectrum characteristic of this scintillator.

It is very important that the spectrum of photons emerging from the scintillator coincide or at least partially overlap with the spectral characteristic of the photomultiplier.

The degree of overlap of the outer scintillation spectrum with the spectral response. of this PMT is determined by the matching coefficient

where I 0 - maximum value of scintillation intensity; t 0 - decay time constant, defined as the time during which the scintillation intensity decreases in e once.

Number of light photons n , emitted over time t after the hit of the detected particle, is expressed by the formula

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

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

Phosphorescence is understood as luminescence, which continues for a considerable time after the termination of excitation. But the main difference between fluorescence and phosphorescence is not the duration of the afterglow. Phosphorescence of crystal phosphors arises from the recombination of electrons and holes that have arisen during excitation. In some crystals, the afterglow can be prolonged due to the fact that electrons and holes are captured by "traps", from which they can be released only after receiving additional necessary energy. Hence, the dependence of the duration of phosphorescence on temperature is obvious. In the case of complex organic molecules, phosphorescence is associated with their presence in a metastable state, the probability of transition from which to the ground state may be small. And in this case, the dependence of the decay rate of phosphorescence on temperature will be observed.

Portal for the student. Self-training

Principle of operation and scope

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