RU2579157C1 - Multispectral one-dimensional x-ray and gamma-radiation detector - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: invention relates to measurement equipment, namely to devices for recording directed x-ray or gamma radiation. Multispectral one-dimensional x-ray and gamma-radiation detector has a layer of scintillator, opaque along the direction of radiation propagation and transparent in the perpendicular direction, the layer of scintillator consists of parallel to each other and optically separated assemblies plates scintillators, opaque along the direction of propagation of radiation and transparent in the direction perpendicular to the surface of scintillator located close to each other in ascending order of average atomic number of scintillators in direction of propagation of radiation; the length of scintillator plates l is selected accordingly condition:

where µ(E_f-k)-coefficient of attenuation of radiation with energy Ef-k, at which photoabsorption section and section of Compton scattering in the material of scintillator plate is measured surface of the scintillator is in optical contact with two-coordinate position-sensitive photodetector.

EFFECT: technical result consists in reduction of the spectrum of x-ray and gamma-radiation in the presence of incident radiation of x-ray or gamma-quanta with energy near the K-edge of photoelectric absorption material of the scintillator.

1 cl, 2 dwg, 2 tbl

Description

The invention relates to measuring technique, namely, devices for recording directional x-ray or gamma radiation and can be used in x-ray examination systems, medical tomographs, as well as in devices for analyzing the spectral composition of x-ray and gamma radiation.

The detectors used to record the spectral distribution of x-ray or gamma radiation (hereinafter “radiation”) can be conditionally divided into two groups. The action of the detectors belonging to the first group is based on filtering radiation using a set of metal foils or individual detector elements located closer to the source (RG Waggener, M.M. Blough, JA Terry, et al., “X-ray spectra estimation using attenuation measurements from 25 kVp to 18 MV ", Med. Phys. 26 (1999) 1269; C. Avila, J. Lopez, JC Sanabria, G. Baldazzi, et al.," Contrast cancellation technique applied to digital x-ray imaging using silicon strip detectors ", Med. Phys. 32 (2005) 3755).

Detectors of the second group record the amplitude distribution of the signal (T. Yasuhiro, S. Yuji, M. Shinjiro, et cet., "Energy discrimination type photon counting radiation line sensor (X-ray color scanner)", Ionizing Radiation, Vol. 32, No . 1 (2006) 39-47; K. Ogawa, T. Kobayashi, F. Kaibuki, et al., “Development of an energy-binned photon-counting detector for X-ray and gamma-ray imaging”, NIM A 664 (2012) 29-37).

In both cases, the restoration of the spectrum of the radiation that came to the detector is carried out in the energy windows (zones) by the appropriate mathematical procedure, taking into account the calibration data obtained by calculation and / or experimentally.

In medical tomographs and X-ray inspection systems, for obtaining radiographic images, mainly single-coordinate recording devices are used, consisting of a set of identical scintillation detectors located along one straight line or circle and equipped with single-coordinate photodetector devices, usually in the form of photodiode arrays. In these devices, the restoration of the radiation spectrum is usually carried out in two energy windows. For this, the recording device usually contains two sets of scintillation detectors of different thicknesses from the same scintillator, for example CsI, located along the radiation flux one after another and separated by a copper foil filter.

The invention relates to the field of x-ray and gamma radiography, namely the registration of x-ray and gamma radiation using single-axis detectors with the ability to restore the radiation spectrum in several energy windows.

The known "X-ray analyzer" made of flat elements containing scintillator layers located along the direction of radiation propagation, opaque in this direction and transparent in the perpendicular direction, and substrates in the form of a honeycomb structure, characterized in that the scintillator layers are made in the form of other plates made of polystyrene scintillators length not less than 3 mm, CaF ₂ length of at least 2 mm, ZnO length of at least 2 mm, CsI length of at least 8 mm, BGO protyazhennos Strongly not less than 15 mm. RF patent No. 2504756, IPC: G01N 23/223, 2014. Analog.

The disadvantage of the analogue is the lack of linear coordinate resolution of the device required for x-ray inspection systems.

The well-known "X-ray analyzer" made of flat elements containing a scintillator layer deposited on a substrate or introduced into its composition, and fiber-optic elements at the ends of which photodetectors are installed, characterized in that the scintillator layer is located along the radiation propagation direction, is opaque this direction and is transparent in the perpendicular direction, and the substrate is made in the form of a honeycomb structure. Patent of the Russian Federation No. 2388015, IPC: G01T 1/00, 2009. Prototype.

The disadvantage of the prototype is the inability to restore the spectrum of x-ray and gamma radiation in the presence of incident radiation of x-ray or gamma rays with energy near the K-edge of the photoelectric absorption of the scintillator material.

The technical result of the invention is the ability to restore the spectrum of x-ray and gamma radiation in the presence in the spectrum of incident radiation of x-ray or gamma rays with energy near the K-edge of the photoelectric absorption of the scintillator material.

The technical result is achieved in that in a single-axis spectrozonal x-ray and gamma-ray detector containing a scintillator layer that is opaque along the radiation propagation direction and transparent in the perpendicular direction, the scintillator layer consists of scintillator plates parallel to each other and optically separated, opaque along the radiation propagation direction and transparent in the direction perpendicular to the surface of the scintillator, located close to each other in in order of increasing average atomic number of scintillators in the direction of radiation propagation, the length of the plates of scintillators l is selected from the condition:

where μ (Е _f-k ) is the coefficient of linear attenuation of radiation with energy Е _f-k , at which the photoabsorption cross section and the Compton scattering cross section in the material of the scintillator plate are compared, the surface of the scintillator is in optical contact with a two-coordinate position-sensitive photodetector.

The invention is illustrated in FIG. 1, 2 and Tables 1, 2.

In FIG. 1 schematically shows a device for a spectrozonal single-axis detector of x-ray and gamma radiation, consisting of, where:

1 - assembly of plates of scintillators located close to each other;

2 - scintillator plates made of various scintillating substances, for example, listed in Table 1, and included in the assemblies;

3 - lightproof partitions that optically separate the assembly;

4 - two-coordinate position-sensitive photodetector;

5 - flow direction of x-ray or gamma radiation;

6 - lightproof partitions that can be installed inside the plates of scintillators 2 when using optically transparent scintillators.

In FIG. 2 as an example, shows the spatial distribution of energy release in plates 2 along direction 5, calculated for monoenergetic radiation quanta with energy E _x :

7 - 22.5 keV;

8 - 36 keV;

9 - 55 keV;

10 - 90 keV;

11 - 135 keV;

12 - 225 keV;

13 - 360 keV.

Table 1 for the polystyrene, CaF ₂ , ZnO, CsI and BGO scintillators shows the energies characterizing the scintillator material:

E _K-edge is the energy of the K-edge of photoabsorption;

E _fk - energy at which the cross section of the photoelectric effect and Compton scattering are compared;

E _e-e is the energy at which the production of electron-positron pairs begins to make a significant contribution to the scintillation signal.

Table 2 for scintillators made of polystyrene, CaF ₂ , ZnO, CsI and BGO shows the attenuation lengths of 1 / μ (E) radiation at energies: E ⁱ _K-edge , E ^{i + 1} _K-edge , E ^i-1 _{f- k} and E ⁱ _f-k (i is the number in the order of the scintillator in tables 1 and 2).

A single-axis spectrozone x-ray and gamma-ray detector contains a scintillator layer consisting of a set of assemblies 1 parallel to each other and optically separated by opaque baffles 3, containing plates 2 made of scintillating materials of different atomic number and located along direction 5 in increasing order the average atomic number of the material of the plates 2.

A two-coordinate positionally sensitive photodetector device 4 is located on the surface of the scintillator layer with optical contact 4. To prevent light propagation in the scintillator layer along direction 5, when 2 optically transparent scintillating substances are used in the plates, lightproof partitions are installed in such plates (light-reflecting or light-absorbing) 6, which are not required when used in the plate 2 powder scintillator.

The single-coordinate sensitivity of the device is ensured by the use of a two-coordinate position-sensitive photodetector 4 and the use of opaque baffles 3, which provide spatial resolution in the plane of the scintillator layer in a direction perpendicular to direction 5.

The spatial resolution along direction 5 provides a measure of the spatial distribution of the signal in the plates 2 used to reconstruct the radiation spectrum.

To correctly measure the spatial distribution of energy release in the plates 2, the device 4 should mainly receive only scintillation photons that propagate perpendicular to the surface of the scintillator layer. This can be achieved in various ways using:

- powder scintillator (In Kyung Cha, Jong Yul Kim, Gyuseong Cho b, Chang-Woo Seo, Sungchae Jeon, Young Huh, Quasi-pixel structured nanocrystalline Gd ₂ O ₃ (Eu) scintillation screens and imaging performance for indirect X-ray imaging sensors, Nuclear Instruments and Methods in Physics Research A 648 (2011) S12-S15);

- scintillator, consisting of a set of plates of scintillators, separated by opaque baffles 6;

- matrix scintillator, for example fiber;

- an optical collimator installed between the scintillator layer and the photodetector.

As a two-coordinate position-sensitive photodetector 4 can be used:

- CCD arrays (Sol M. Gruner, Mark W. Tate, Eric F. Eikenberry, Charge-coupled device area x-ray detectors, Review of Scientific Instruments, Vol. 73, # 8 (2002) 2815-2842);

- two-axis PMTs;

- positionally sensitive detectors based on image panels of amorphous silicon (David L. Gilblom. Device and method for obtaining x-ray images using a flat image panel of amorphous silicon. RF Patent No. 2181491. IPC: G01N, G01T, H05G. 2000).

FIG. 2 shows the spatial dependence of energy release in plates 2 made of the following scintillators: polystyrene 3 mm long, CaF ₂ (2 mm), ZnO (2 mm), CsI (8 mm), BGO (15 mm) located along the direction of radiation flux 5 in the indicated order (ascending average atomic number).

In order to unambiguously reconstruct the emission spectrum, it is necessary that radiation pass through each subsequent plate 2, for which the energy dependence of the linear attenuation coefficient µ (E) is uniquely determined, i.e. did not contain extremes. It is also important that the dependence of the linear attenuation coefficient µ (E) in the plates 2 in the region of energies of the radiation incident on them is characterized by the maximum possible derivative. To fulfill these conditions, it is necessary that a radiation spectrum is incident on each of the subsequent plates 2 mainly in the energy range of the photoelectric effect, which does not contain energies, firstly, to the left of the K-edge of photoabsorption and, secondly, in the production region of electron-positron pairs.

The jumps in the dependence μ (E) in the photoabsorption region take place at energy values determined by the binding energy of the electrons of the atoms that make up the scintillator. It is necessary that the left edge of the spectrum of the radiation incident on the scintillator E _min lie above the energy of the K-edge of the absorption band E _K-edge (E _min > E _K-edge ) of the atom with the largest atomic number of the atoms that make up the scintillator, with the exception of atoms activator, the concentration of which is usually quite low.

The right edge of the spectrum of the radiation incident on the scintillator E _max should be limited from above by the energy E _ee , at which the production of electron-positron pairs begins to make a significant contribution to the scintillation signal. The value of E _e (Table 1) is several MeV in the case of scintillators containing atoms with a large charge of the electron shell (large atomic number) and> 10 MeV in the case of a small charge.

In the thus defined energy range (E _min , E _max ), the main types of interaction of X-rays with matter are photoabsorption and Compton scattering, and the photoabsorption cross-section is characterized by a more pronounced energy dependence than the Compton scattering cross-section. Therefore, in the energy region at which radiation attenuation occurs predominantly due to photoabsorption, the spectrum can be restored more accurately than with Compton scattering. As the right boundary of this region, we can arbitrarily take the energy E _fk , at which the photoabsorption cross section is compared with the Compton scattering cross section. For efficient absorption of quanta with an energy E _fk characteristic of the scintillator material used in plate 2, the following conditions must be met:

where μ (E _f-k ) is the coefficient of linear attenuation of radiation with an energy E _f-k , at which the photoabsorption cross section and the Compton scattering cross section in the scintillator material are compared.

In the transition region (E _f-k , E _e-e ) µ (E) changes relatively weakly and therefore, when dividing the spectrum into energy groups, it may be appropriate to represent this energy region as one group.

Table 1 shows the above-defined values of E _K-edge , E _fk , as well as E _e-e for polystyrene, CaF ₂ , ZnO, CsI and BGO. From table 1 it is seen that the values of the E _K-edge for polystyrene, CaF ₂ and ZnO are in the region of practically little energy used (E <10 keV). For CsI and BGO E, the _K-edge is 37 keV and 91 keV, respectively, and falls into the energy range used in examination and medical devices.

From the values of E _K-edge and E _e- given in table 1, the range of working energies of the device when using the scintillators listed in it lies in the energy range from 0.3 keV to 3 MeV. It is also seen that the value of E ⁱ _f-k for the i-th scintillator is significantly greater than the value E ^{i + 1} _K-edge for the (i + 1) -th scintillator (i is the number in the order of the scintillator in table 1).

The coefficient of linear attenuation of radiation in the scintillator increases with decreasing radiation energy, therefore, for radiation incident on the i-th scintillator, the fulfillment of relation (1) at an energy E _x = E ⁱ _f-k for the i-th scintillator means its unconditional fulfillment at an energy equal to E _x = E ^{i + 1} _K-edge , characteristic of the material of the (i + 1) -th scintillator.

Table 2 shows the attenuation lengths (1 / µ (E)) of the radiation in the scintillators listed in Table 1 at energies E _K-edge and E _f-k .

Table 2 shows that the attenuation length in the ith scintillator for quanta with energy E ^{i + 1 the} _K-edge is 440 μm for polystyrene, 100 μm for CaF ₂ , 260 μm for ZnO, and 1 mm for CsI, respectively. This, for example, means that to reduce the number of quanta with energy E _x = 91 keV (E _K-edge for BGO) incident on the scintillator plate from BGO, a CsI plate less than 1 cm long is enough by 3 orders of magnitude.

The radiation spectrum is reconstructed by solving an overdetermined system of linear equations.

Where:

C is the restored spectrum, a column vector with elements with _i equal to the number of photons in the i-th energy group of the restored spectrum,

A is a calibration matrix with elements a _ij equal to the average signal caused by a photon of the i-th group in the j-th element (pixel) of a position-sensitive photodetector, which is determined by calibration measurements and (or) calculation,

B is the signal in the j-th element (row vector).

To evaluate the accuracy of reconstructing the spectrum of the radiation incident on the device, we chose a design with a cross section of assemblies 1 equal to 0.8 × 0.8 mm composed of polystyrene, CaF ₂ , ZnO, CsI, and BGO scintillator plates 2 arranged one after another with a length of 3 mm, 2 mm, 2 mm, 8 mm and 15 mm, respectively.

In the calculations, it was assumed that the radiation flux propagates along direction 5, and the spatial resolution of the two-coordinate position-sensitive photodetector 4 and plates 2 along this direction is 100 μm. The estimates were carried out in the case when radiation is incident on the device in the range (10 ÷ 250) keV of the uniform spectrum. Recovery was carried out in 5 energy groups. The maximum standard deviation for the number of quanta in the energy windows of the reconstructed spectrum was <4% at 1% of the statistical accuracy of the signal summed over all elements of the device 4.

In addition to the scintillators listed in Table 1, others can be used (N.V. Klassen, V.N. Kurlov, S.N. Rossolenko, O.A. Krivko, A.D. Orlov, S.Z. Shmurak. Scintillation fibers and nanoscintillators for improving the spatial, spectrometric and temporal resolution of radiation detectors. Izvestiya RAN. Physical Series, 2009, Volume 73, No. 10, pp. 1451-1456; RF Patent No. 2411543, IPC: G01T 1/20, 2008).

The device operates as follows.

The power of device 4 is turned on. X-ray or gamma radiation enters the end surface of the scintillator layer along direction 5. When x-ray or gamma-quanta interact with the substance of one of the plates 2, electrons are formed in it, which excite a scintillation flash in the plate 2. Photons from a scintillation flash partially fall on one or more photosensitive elements of device 4, partially exit in the opposite direction from it, and are absorbed (or reflected) when propagating in other directions in partitions 3 and 6 (if any). The number of photons in a scintillation flash and the magnitude of the electric signal caused by photons in the photosensitive elements of device 4 are proportional to the energy released by the electron in the plate 2.

The signal that occurs in the photosensitive elements of the device 4, located perpendicular to the direction of propagation of radiation 5 (along the direction Y in Fig. 1), forms a single-coordinate image of the flow of radiation incident on the device. This signal, if necessary, is integrated over the width of the assemblies 1 (along the direction Y in Fig. 1) and along the direction of propagation of radiation 5.

The signal that occurs in the photosensitive elements of the device 4 located along the direction of propagation of radiation 5 (along the X direction in Fig. 1), if necessary, is integrated over the width of the assemblies 1 (along the Y direction in Fig. 1). The spatial distribution of this signal along the X direction (Fig. 1) is used in expression (1)) to reconstruct the radiation spectrum, which is attributed to the Y values corresponding to the position of the axial lines of the assemblies 1.

The correct reconstruction of the spectrum of the radiation incident on assembly 1 using expression (1) in the presence of x-ray or gamma rays with energy near the K-edge of photoelectric absorption for scintillators in one or more plates 2 included in the assembly is ensured by the fact that such quanta effectively absorbed in previous plates 2, made of scintillators with a lower average atomic number and therefore characterized by a lower energy of the K-edge of photoelectric absorption, which goes beyond the detection limits radiation spectrum. Moreover, the use of a two-coordinate position-sensitive photodetector provides the determination of the radiation spectrum at each point of a single-coordinate x-ray or gamma image.

Claims (1)

A single-axis spectrozonal x-ray and gamma-ray detector containing a scintillator layer that is opaque along the radiation propagation direction and transparent in the perpendicular direction, characterized in that the scintillator layer consists of scintillator plates parallel to each other and optically separated, opaque along the radiation propagation direction and transparent to direction perpendicular to the surface of the scintillator, located close to each other in ascending order it atomic number scintillator in the direction of propagation of radiation, the scintillator plate length l is chosen from the condition:
μ (E _fc ) l> 1,
where μ (E _fc ) is the linear attenuation coefficient of radiation with an energy E _fc , at which the photoabsorption cross section and the Compton scattering cross section in the material of the scintillator plate are compared, the scintillator surface is in optical contact with a two-coordinate position-sensitive photodetector.

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