RU2663307C1 - Position-sensitive radiation detector - Google Patents

Abstract

FIELD: physics.SUBSTANCE: invention relates to the detection of ionizing radiation and can be used to create position-sensitive detectors. Essence of the invention consists in that the position-sensitive radiation detector comprises a scintillator, with that scintillator is made in the form of cylindrical coaxial layers separated by cylindrical coaxial interlayers of the same diameter from the substance with a light attenuation length of the scintillation flashes is much larger than the total thickness of the scintillator layers and the length of the detector, the scintillator layers are in optical contact with the photodetectors and with the interlayers, a reflective coating is applied to the cylindrical surface of the layers of the scintillator and the interlayers, the transverse dimension and thickness of any scintillator layer are approximately equal to or exceed the length of attenuation of the detected radiation in the scintillator, the number of layers of the scintillator is chosen from the condition that the total thickness of all layers of the scintillator Lis defined by the expression: L<2∙λ∙ln(σ∙N/λ), where σ– given value of the spatial resolution in the scintillator layers along the axis of the position-sensitive radiation detector, λ – effective length of light attenuation of the scintillation flare in the scintillator material taking into account the quality of the surface of the scintillator, N is the number of photoelectrons produced in any of the photodetectors at λ→∞.EFFECT: increase in the spatial resolution of the radiation detector at a length of the detector comparable to the light attenuation length of the scintillation flash in the scintillator.1 cl, 1 dwg

Description

The invention relates to the field of registration of ionizing radiation and can be used to create position-sensitive detectors used, for example, in geophysical equipment for neutron and gamma-ray logging, in inspection equipment or equipment for detecting radiation sources, as well as in medicine.

In the case of logging, a downhole tool typically contains from one to three detectors. The maximum distance from the detector to the radiation source is not more than 90 cm. The choice of the type of detectors and their number depends on many factors: the type of radiation used, the characteristics studied, the nuclear physical properties of the environment (casing, borehole, geological rock), a given depth, the diameter of the well, the distance from the probe to the wall of the well is a complex scientific and technical task, the subject of a compromise and far from always optimal in terms of ensuring maximum sensitivity of wells live instrument in specific measurement conditions.

In this case, the problem of the number of detectors used and the choice of the optimal distance to the source can be solved by creating an extended detector with axial spatial (coordinate) resolution. The use of such a detector allows you to replace multiple detectors with one device, reduces the length of the downhole device, simplifies its design.

In addition, the measurement of spatial distributions in the general case of several detected radiations can improve the accuracy of measurements, provides, in particular, a more accurate determination of the boundaries of various geological formations, allows you to choose probe lengths based on the logging speed and nuclear-physical characteristics of the rock.

When using radiation from a scintillation detector, the problem of creating an extended detector is mainly associated with the high cost of large (more than 100 cm ³ volume) scintillating crystals, often the inability to create them due to the occurrence of internal stresses in them, leading to destruction of the crystal, and less uniform scintillation properties absorption of light in a scintillator. For example, the creation of LaBr ₃ (Ce) crystals with a diameter of 51 mm and a length of 76 mm (volume ≈ 155 cm ³ ) [Peter R. Menge, G. Gauter, A. Iltis, C. Rozsa, V. Solovyev. Performance of large lanthanum bromide scintillators, NIM A 579 (2007) 6-10].

Spatial resolution when using an extended scintillation detector can be achieved in two ways: by comparing the number of photons and / or their time of arrival at photodetectors located at opposite ends of the scintillator.

The propagation speed of optical photons in a scintillator is equal to the speed of light divided by the refractive index of the scintillator material and amounts to tens of centimeters in 1 ns. Therefore, measuring the arrival time requires photodetectors and electronics with high time resolution. It is much easier to measure and compare the number of photons arriving at the photodetectors.

The length of the scintillator is limited by the intrinsic absorption of light in the scintillator, leading to a significant decrease in the number of photons reaching the photodetector at a relatively large distance between the photodetector and the site of scintillation.

Self-absorption of light in the scintillator is due to the partial overlap of the emission spectra and light absorption spectra in the scintillator. As a result, the short-wavelength part of the emission spectrum can again be absorbed in the scintillator. For each absorption of a photon, there is a probability of a non-radiation (non-radiative) transition. Therefore, with repeated absorption and emission, the intensity of the short-wave radiation will decrease, and the more, the greater the path the photons travel in the scintillator.

The absorption probability is greater the smaller the Stokes shift of the emission and absorption spectra, which is 0.15 eV for SrI ₂ : Eu and Ba ₂ CsI ₅ : Eu and 2.07 eV for CsI: Na. The small Stokes shift and the associated large self-absorption is one of the reasons for the rare use of the SrI ₂ : Eu scintillator, despite the large photon yield in the scintillation burst.

The scintillator emission spectrum and the self-absorption coefficient depend on the scintillator material and the scintillation activator used. As the latter, in particular, Tl and Na are used in the NaI: Tl (Na) scintillator or Ce in YAP: Ce and LYSO: Ce scintillators. The activator concentration is usually a few percent and does not exceed 10% due to an increase in the self-absorption coefficient.

A factor determining the length of the scintillator is the fraction of photons of the scintillation flash (light collection coefficient) reaching the photodetector. The light collection coefficient depends on many factors:

- the location of the outbreak,

- the ratio of the length of the crystal to its cross section,

- heterogeneity of the composition and density of the scintillator,

- activator concentration (if any),

- scintillator temperature,

- surface quality of the scintillator (photon reflection coefficient),

- crystalline structure (in the case of a crystalline scintillator),

- the presence and type of reflective coating [E.N. Okrushko, V.Yu. Pedash, A.S. Raevsky. The use of different types of reflectors in long detectors to improve position sensitivity. Uzhhorod University Scientific Herald. Series Physics. Issue 29. 2011],

- the quality of the optical contact between the scintillator and the photodetector,

- the degree of degradation of the surface topography [A.V. Shkoropatenko, A.M. Kudin, L.A. Andryushenko et al. Reasons for the instability of the spectrometric characteristics of CsI: Tl crystals with a matted surface. FIP FIP PSE, 2015, vol. 13, No. 2, vol. 13, No. 2; www.pse.scpt.org.ua].

Light collection coefficient can vary within wide limits:

- in a GSO scintillator measuring 20 × 20 × 150 mm over a length of 150 mm, it is approximately 83% [V. Kalinnikov, E. Velicheva. Research of long GSO and LYSO crystals used in the calorimeter developed for the COMET experiment. Fundamental Materials, 22, No. 1 (2015) 126-134];

- in the LSO scintillator measuring 5 × 5 × 20 mm at a length of 20 mm — less than 50% [Emilie Roncali and Simon R. Cherry. Simulation of light transport in scintillators based on 3D characterization of crystal surfaces. Phys Med Biol. 2013 April 7; 58 (7): 2185-2198].

The attenuation length for scintillation burst photons in a LuAP: Ce crystal is approximately 1.1 cm [M. Balcerzyk et al. Perspectives for high resolution and high light output LuAP: Ce crystals. IEEE Transactions on Nuclear Science Vol .: 52, Iss.//, 2005].

The BC-422 and BC-422Q plastic scintillators have an attenuation length of about 8 cm.

The volumetric (not related to the quality of the outer surface of the scintillator) length of light attenuation in the YAP: Ce crystal does not exceed 25 cm [I. Vilardy et al. Optimization of the effective light attenuation of YAP: Ce and LYSO: Ce crystals for the novel geometrical PET concept. MM A 564 (2206) 506-514].

The light attenuation length determines not only the permissible scintillator length, but also the possible spatial resolution σ. In the case of a detector with photodetectors at opposite ends of the scintillator, σ is determined by the expression (1) [I. Vilardy et al. Optimization of the effective light attenuation of YAP: Ce and LYSO: Ce crystals for the novel geometrical PET concept. NIMA 564 (2206) 506-514]:

where λ is the effective length of the attenuation of light taking into account the quality of the outer surface of the scintillator, N is the number of photoelectrons generated in any of their photodetectors as λ → ∞ (when the effective length of the attenuation of light taking into account the quality of the outer surface of the scintillator tends to infinity), L is the length of the scintillator, z is the coordinate along the scintillator axis, counted from one of its ends.

It follows from expression (1) that the worst spatial resolution occurs at z≈L / 2. The spatial resolution is:

It follows from expression (2) that the spatial resolution substantially decreases at L ~ λ.

The thickness of the scintillator L _c at which the spatial resolution at any point on the axis of the scintillator does not exceed a predetermined value of σ ₃ , in accordance with expression (2) is:

The use of two photodetectors at opposite ends of the scintillator not only improves the spatial resolution of the measurements, but also makes it weakly dependent on the site of the scintillation burst along the entire length of the scintillator.

The well-known "Cylindrical position-sensitive detector" containing a scintillator with an axis parallel to the axis of the device, and photodetectors connected to amplitude analyzers and through them to the controller, the scintillator consists of one or more sets of fiber scintillating elements inserted into each other, each set contains scintillating elements for detecting gamma quanta, thermal and / or fast neutrons, arranged alternately parallel to the axis of the device at the same distance from it, are provided with reflective shells and opaque coatings, the opposite ends of each scintillating element are connected to two photodetectors by means of optical fibers and optical connectors, the total number of photodetectors is equal to twice the number of scintillating elements. Patent RU 2574323, IPC G01V 5/10, 02/10/2016.

The disadvantage of the analogue is the design complexity due to the use of several types of fiber scintillators, a large number of scintillating fibers, and the inability to measure the spectrum of gamma radiation.

The well-known "Spectrometric position-sensitive detector" containing a scintillator with an axis parallel to the axis of the device, and photodetectors connected to amplitude analyzers and through them to the controller, which serves to determine the axial position of the detected radiation relative to the amplitudes of the optical signals recorded by photodetectors, the scintillator consists of three sets of scintillating elements embedded in each other, the scintillating elements of the outer and middle sets are made of material for thermal neutron scattering, and the scintillating elements of the central set of gamma-ray radiation recording material are equipped with spectroscopic fibers passing at an equal distance from the side walls of the scintillating elements of the central set, the scintillating elements of the middle set are inside the neutron-slowing material, in the form of a cylinder, on the surface of which a screen that absorbs thermal neutrons is located, in the outer and inner sets the scintillating elements are parallel to the axis of the device at the same distance from it, scintillating elements and spectroscopic fibers are provided with reflective shells, reflective coatings are applied to the reflecting shells of scintillating elements, the opposite ends of the scintillating elements of the outer and middle sets, as well as the opposite ends of the spectroscopic fibers are connected to the photodetectors by means of optical fibers and optical connectors, the total number of photodetectors is equal to twice the number of scintillating elements and spectroscopic fibers. Patent RU 2574322, IPC G01T 3/20, 02/10/2016.

A disadvantage of the analogue is the design complexity due to the use of two types of fiber scintillators, a large number of scintillating and spectroscopic fibers.

The well-known “Method and apparatus for neutron logging using a positionally sensitive neutron detector”, which contains a scintillator with an axis parallel to the axis of the instrument’s body, and photomultipliers at opposite ends of the scintillator, each photomultiplier connected to a corresponding amplitude analyzer and through it to the controller, used to determine the axial position of the detected neutron in relation to the amplitudes of the optical signals recorded by the photomultipliers. Patent CA 2798070, IPC G01V 5/10. 11/10/2011. This technical solution was made as a prototype.

The disadvantage of the prototype is the low spatial resolution of the radiation detector with a detector length comparable to the attenuation length of a scintillation flash in a scintillator.

The device eliminates the disadvantages of analogues and prototype.

The technical result of the invention is to increase the spatial resolution of the radiation detector with a detector length comparable to the attenuation length of the scintillation flash in the scintillator.

The technical result is achieved by the fact that in a positionally sensitive radiation detector containing a scintillator with an axis coinciding with the axis of the device and photodetectors at opposite ends of the scintillator, each photodetector is connected to a corresponding amplitude analyzer and through it to a controller that serves to determine the axial coordinate of the detected particle from the ratio of the amplitudes of the optical signals recorded by the photodetectors, according to the invention, the scintillator is made in the form of cylindrical coaxial layers separated by coaxial coaxial interlayers of the same diameter from a substance with a light attenuation length of scintillation flashes are much larger than the total thickness of the scintillator layers and the length of the detector, the scintillator layers are in optical contact with photodetectors and interlayers, a reflective coating is applied to the cylindrical surface of the scintillator layers and interlayers, the transverse size and thickness of any scintillator layer is approximately equal to or greater than the attenuation length of the detected radiation in the scintillator, h the scintillator layer number is selected from the condition that the total thickness of all scintillator layers L _c is determined by the expression:

where σ ₃ is the specified spatial resolution in the scintillator layers along the axis of the position-sensitive radiation detector, λ is the effective length of the attenuation of light of the scintillation flash in the scintillator material, taking into account the quality of the scintillator surface, N is the number of photoelectrons generated in any of the photodetectors as λ → ∞.

The drawing schematically shows the arrangement of a cylindrical positionally sensitive radiation detector, where:

1 - scintillator layers;

2 - interlayers of a substance with a light attenuation length of scintillation flares are much larger than the total thickness of layers 1 and the length of the detector;

3 - reflective coating;

4 - photodetectors;

5 - scintillation flash;

1 ₁ and 1 ₂ - the distance from the place of occurrence of the scintillation flash 5 to one and the other photodetectors 4.

Amplitude analyzers connected to photodetectors and the controller, as well as the controller are not shown in the drawing.

The device positionally sensitive radiation detector contains cylindrical and coaxial layers of the scintillator 1, separated by layers of 2 substances with a length of attenuation of light of scintillation flashes 5 is significantly greater than the total thickness of the layers 1 and the length of the detector l ₁ + l ₂ .

The layers 1 are in optical contact with the photodetectors 4 and with the interlayers 2. To improve the light collection and increase the fraction of light reaching the photodetectors 4, a reflective coating 3 is applied to the outer (cylindrical) surface of the layers 1 and the interlayers 2, for example, layers of MgO or TiO ₂ with a thickness usually not exceeding 1 mm.

The distances 1 ₁ and 1 ₂ from the place of occurrence of the scintillation burst 5 to one and the other photodetectors 4 vary from ≈0 cm to the total thickness of layers 1 and 2.

The material used in the scintillator layers 1 depends on the type of radiation detected and on its energy. Several scintillators can be used to register several types of radiation. Additional scintillators can be used instead of interlayers 2 when the length of light attenuation of scintillation flashes 5 in them substantially exceeds the total thickness of layers 1 and the length of detector 1 ₁ +1 ₂ .

As photodetectors 4 can be used, for example, photomultipliers.

To ensure effective registration of a particular type of radiation, the transverse size and / or thickness of any layer 1 of the scintillator must be at least the attenuation length of the detected radiation in the layer of scintillator 1. Since the flux density of the detected radiation decreases with distance from the radiation source (not shown in the drawing) , then to ensure approximately equal statistical error of the signal arriving at photodetectors 4 from each of the scintillator layers 1, the thickness of the scintillator layer 1 should increase as Alenia from the radiation source is inversely proportional to the radiation flux density on the scintillator layer 1.

Since the flux density of the detected radiation decreases with distance from the radiation source, in order to ensure an approximately equal count rate of the signal arriving at the photodetectors 4 from each of the scintillator layers 1, the thickness of the scintillator layer 1 should increase in proportion to the distance from the radiation source radiation on layer 1 of the scintillator.

The number of layers 1 of the scintillator is selected according to expression (3), based on a predetermined error in determining the coordinate of the scintillation burst σ ₃ taking into account the number of photons emitted during the scintillation burst 5, the length of attenuation of light λ of the scintillation burst in the scintillator, the quantum efficiency of the photodetectors 4 and the noise of the subsequent electronics.

The ratio of the number of photons from the scintillation flash 5 arriving at the photodetectors 4 will be approximately proportional to the ratio of the thicknesses of the layers 1 of the scintillator passed by the photons before reaching the photodetectors 4.

The device operates as follows.

The recorded radiation enters one of the layers 1 of the scintillator and causes a scintillation flash 5. The photons of the scintillation flash 5 propagate in all directions, being reflected from the reflective coating 3 and absorbed in the layers 1 of the scintillator, passing through the interlayers 2 with almost no attenuation. Some light attenuation in the interlayers 2 is caused by losses due to reflection from the reflective coating 3, but it is small and can be neglected in comparison with the attenuation of light in the layers 1. The remaining part of the photons reaches the photodetectors 4, which leads to the formation of photoelectrons and electrical signals proportional to the number reached photons. Electrical signals are then fed to the inputs of two amplitude analyzers (not shown in the drawing). In amplitude analyzers, the signals are digitized and digitally fed to the controller (not shown in the drawing), in which the signal ratio is calculated and the axial coordinate of the place of occurrence of z _in (distance z ₁ and z ₂ ) of the scintillation flash 5 is determined from the obtained ratio value in accordance with the expression [I. Vilardy et al. Optimization of the effective light attenuation of YAP: Ce and LYSO: Ce crystals for the novel geometrical PET concept. NIM A 564 (2206) 506-514]:

Thus, the claimed technical result: an increase in the spatial resolution of the radiation detector with a detector length comparable to the attenuation length of the scintillation flash in the scintillator is achieved due to the fact that the scintillator is made in the form of cylindrical coaxial layers 1 separated by cylindrical coaxial layers 2 of the same diameter from substance with a long light attenuation scintillation flashes 5 significantly greater total thickness and length of layers 1 1 ₁ 1 detector ₂ as layers 2 may be used tsintillyatory for registering other kinds of radiation, provided that the length of the light attenuation scintillation flashes 5 in which substantially exceeds the total thickness of the layers 1 and a length detector 1 ₁ 1 ₂ 1 scintillator layers are in optical contact with the photo detectors 4 and with layers 2, on the cylindrical the surface of the scintillator layers 1 and the interlayers 2 is coated with a reflective coating 3, the transverse size and thickness of any scintillator layer 1 is approximately equal to or greater than the attenuation length of the detected radiation in the scintillator layers 1, when the thickness of the scintillator layers 1 can increase with distance from the radiation source inversely with the radiation flux density per scintillator layer 1, providing approximately the same statistics of the recorded signal in all layers of the scintillator, the number of layers of the scintillator 1 is selected from the condition that the total thickness of all layers 1 scintillator does not exceed the length L _c determined in accordance with the expression:

where σ ₃ is the specified spatial resolution in the scintillator layers 1 along the axis of the positionally sensitive radiation detector, λ is the effective attenuation length of the scintillation flash 5 in the scintillator material, taking into account the quality of the scintillator surface, N is the number of photoelectrons generated in any of the photodetectors 4 at λ → ∞.

Claims (3)

A positionally sensitive radiation detector containing a scintillator with an axis coinciding with the axis of the device, and photodetectors at opposite ends of the scintillator, each photodetector is connected to a corresponding amplitude analyzer and through it to a controller, which serves to determine the axial coordinate of the detected particle from the amplitude of the optical signals recorded by the photodetectors characterized in that the scintillator is made in the form of cylindrical coaxial layers separated by cylindrical coaxial p interlayer of the same diameter from a substance with a light attenuation length of scintillation flares is much larger than the total thickness of the scintillator layers and the length of the detector, the scintillator layers are in optical contact with photodetectors and interlayers, a reflective coating is applied to the cylindrical surface of the scintillator layers and interlayers, the transverse size and thickness of any scintillator layers are approximately equal to or greater than the length of attenuation of the detected radiation in the scintillator; the number of scintillator layers is selected from the condition i, that the total thickness of all layers of the scintillator L _c is determined by the expression:

where σ ₃ is the specified spatial resolution in the scintillator layers along the axis of the position-sensitive radiation detector, λ is the effective length of the attenuation of light of the scintillation flash in the scintillator material, taking into account the quality of the scintillator surface, N is the number of photoelectrons generated in any of the photodetectors as λ → ∞.

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