RU2574322C1 - Spectrometric position-sensitive detector - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: spectrometric position-sensitive detector comprises a scintillator consisting of three sets of scintillating elements embedded in each other and arranged in parallel to the axis of the device; the outer and middle sets are formed by scintillating fibres made of material which enables to detect slow neutrons, and scintillating elements of the inner set form a cylinder and are in the form of angular sectors of the same size and enable detection of gamma radiation; the number of angular sectors is two or more; each angular sector is provided with a spectrum-shifting fibre which passes through the centre of the angular sector in parallel to the axis of the device; scintillating elements of the middle set are placed inside a tubular neutron retarder which fills the space between the outer and inner sets; a shield which absorbs slow neutrons is placed on the outer surface of the neutron retarder; scintillating elements of all sets and spectrum-shifting fibres of the inner set are provided with light-reflecting covers; the surface of the scintillating elements is coated with a light-permeable coating; opposite ends of each scintillating element of the outer and middle sets, as well as opposite ends of each spectrum-shifting fibre of the inner set are connected by optical connectors to two fibre-optic guides, situated at the opposite side in optical contact with two photodetector arrays, the number of photosensitive elements in each of which is equal to or greater than the number of scintillating elements.

EFFECT: simultaneous detection of slow neutrons, epithermal neutrons and gamma radiation at one point on the axis of a downhole device.

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Description

The invention relates to the field of registration of ionizing radiation and can be used to create radiation detectors used, for example, in the geophysical apparatus of neutron and gamma-ray logging for registration and determination of the spatial distribution of neutron and gamma radiation.

Currently, nuclear physics methods are widely used for detailed geological studies in wells. These include, in particular, neutron logging methods based on the use of neutron sources as ampoule radiation probes: ampoule or neutron generators emitting fast neutrons. In this case, neutron generators can be continuous or pulsed.

The most informative methods of neutron logging include the method of pulsed neutron logging (INC), the essence of which is as follows.

A neutron generator is lowered into the well, which periodically for short (several microseconds) time intervals irradiates the rock around the well with a stream of fast neutrons with an energy of 14 MeV.

Propagating in the rock, fast 14 MeV neutrons undergo elastic and inelastic scattering on the atomic nuclei of the rock. As a result of elastic scattering, the fast neutrons of the generator slow down and gradually come into thermal equilibrium with the rock. The distance from the target of the generator at which thermal equilibrium occurs depends on the properties of the rock and, to a large extent, on the amount of hydrogen-containing substances contained in it. Thermal neutrons diffuse in all directions and are gradually absorbed by rock atoms, emitting gamma rays of radiation capture.

Inelastic scattering of fast neutrons leads to the formation of gamma rays of inelastic scattering emitted during neutron pulses. The energy of these gamma rays is characteristic of each element. So, as a result of inelastic scattering, gamma rays with energies of 4.43 MeV are formed on carbon (C) nuclei, and 6.13 MeV on oxygen nuclei. The number of gamma rays recorded in certain energy regions is proportional to the concentration of elements emitting these gamma rays.

The registration of thermal and / or epithermal neutrons, as well as gamma rays of inelastic scattering and radiation capture at a specific location in the well, makes it possible to determine the neutron porosity, density and composition of the rock at this location. These characteristics are used to determine the nature of formation saturation (oil, water), their filtration-capacitive properties and oil saturation coefficient.

The part of the logging equipment lowered into the well is called a downhole device. The main elements of a typical INC multifunctional downhole device are: a neutron source in the form of a neutron generator, neutron and gamma detectors, a protective screen installed between the neutron generator target and gamma radiation detectors, electronic devices.

The distance between the target of the neutron generator and the detector (probe length) affects the size of the investigated area around the well (sounding depth) and the size of the measured effect associated with the nuclear physical characteristics of the rock.

Due to the fact that the rock around the well has a variable composition and density as it moves away from the axis of the well, it is necessary to use several probes of different lengths to determine the radial distribution of its properties.

Due to the difference in diameters of the borehole device and the borehole, there is a cavity between their walls, the size of which is different in different azimuthal directions and varies randomly during logging. This leads to a change in the probe detector count, which is not related to the characteristics of the rock around the well. To account for the influence of the cavity, probes are used that contain several detectors located uniformly around the circumference around the axis of the borehole device (patent application US 2013/0187035, IPC: G01V 5/08, G01V 5/10, 2013).

The use of a large number of multi-detector probes of various types in a downhole device is practically impracticable. The way out in this case is the use of a positionally sensitive detector that simultaneously records several types of radiation with axial (single-coordinate) and angular spatial resolution.

The length of such a detector should be of the order of the distance between commonly used probes consisting of several identical detectors, for example, proportional counters or scintillation detectors. This distance is usually several tens of centimeters.

The use of a detector extended along the axis of a borehole device with an axial spatial resolution and also with an angular resolution that allows recording several types of radiation in one place allows replacing several probes consisting of several detectors with one device, and reduces the length of the downhole device. With sufficient spatial resolution, such a detector removes the problem of choosing the number and length of probes, the number of detectors in the probes, improves the accuracy of measurements by: measuring neutron and gamma radiation at the same location in the well, providing axial and angular resolution, providing correction of the position of the downhole device relative to the axis of the well (application US 2013/0187035, IPC: G01V 5/08, G01V 5/10, 2013).

The well-known "downhole position-sensitive gamma-ray counter", consisting of a cathode casing, along the axis of symmetry of which an anode is placed on the supporting insulators, made in the form of a filament with baffles rigidly fixed on it in the form of glass beads with a diameter of at least 1 mm, which divide anode thread into sections sections. Patent RU 2152105, IPC G01T 1/18, G01V 5/06. 2000, Analog.

The disadvantages of the analogue are the inability to determine the direction under which the radiation arrives at the detector in a plane perpendicular to the axis of the cathode body (lack of azimuthal angular resolution), the inability to simultaneously register several types of radiation.

The well-known "Method and apparatus for neutron logging using a position-sensitive neutron detector", which contains a scintillator with an axis parallel to the axis of the instrument body, and photomultipliers at opposite ends of the scintillator, each photomultiplier is connected to a corresponding amplitude analyzer and through it to the controller, which serves to for determining the axial position of a detected neutron in relation to the amplitudes of optical signals recorded by photomultipliers Patent CA 2798070, IPC G01V 5/10 2011 A tax.

The disadvantage of the analogue is the impossibility of determining the direction under which the radiation arrives at the detector in a plane perpendicular to the axis of the detector (lack of azimuthal angular resolution), as well as the impossibility of simultaneously registering several types of radiation.

Known "Azimuthally sensitive gamma detectors", including a scintillator whose shape provides azimuthal sensitivity relative to the axis of the well, or a plurality of scintillators separated by reflective material placed between the scintillators, each scintillator is in optical contact with the photodetector. Application of Norway NO 20120033, IPC: G01V 5/10, 2012. Prototype.

The disadvantage of the prototype is the inability to simultaneously register thermal and epithermal neutrons, as well as gamma radiation in one place along the axis of the downhole device.

The technical result of the invention is the ability to simultaneously register thermal and epithermal neutrons, as well as gamma radiation in one place on the axis of the downhole device.

The technical result is achieved in that in a spectrometric position-sensitive detector containing a scintillator in optical contact with the photodetector, the scintillator consists of three sets of scintillating elements nested in parallel to each other, the outer and middle sets are made up of scintillating fibers of material providing registration of thermal neutrons, and the scintillating elements of the internal set form a cylinder and are made in the form of identical size ru of angular sectors and ensure registration of gamma radiation, the number of angular sectors is two or more, each angular sector is equipped with a spectroscopic fiber passing through the center of the angular sector parallel to the axis of the device, scintillating elements of the middle set are placed inside a neutron moderator of a tubular shape filling the space between the external and internal screens, on the outer surface of the neutron moderator is a screen that absorbs thermal neutrons, scintillating elements of all s and the spectroscopic fibers of the inner set are provided with reflective sheaths, a lightproof coating is applied to the surface of the scintillating elements, the opposite ends of each scintillating element of the outer and middle sets, as well as the opposite ends of each spectroscopic fibers of the inner set are connected via optical connectors with two fiber optical fibers located on the opposite side in optical contact with two matrix photodetectors, the number of photosensitive nyh elements each of which is equal to or greater than the number of scintillator elements.

The drawing schematically shows a detector device for the simultaneous registration of thermal and epithermal neutrons, as well as gamma radiation, where:

1, 5 - external and average sets of scintillating elements for detecting thermal neutrons;

2 - fiber optical fibers;

3 - matrix photodetectors;

4 - optical connectors;

6 - an internal set of scintillating elements for detecting gamma radiation;

7 - spectroscopic fibers;

8 - neutron moderator;

9 is a screen for absorbing thermal neutrons.

The right side of the drawing shows a cross section of the device.

The device contains: sets of scintillating elements 1, 5 and 6, spectroscopic fibers 7 passing through the centers of scintillating elements of set 6, fiber optical fibers 2, optical connectors 4, photodetector arrays 3, each of which consists of a set of photosensitive elements (not shown in the drawing) , neutron moderator 8 and screen 9 for absorption of thermal neutrons.

Scintillating elements in sets 1, 5 and 6 are parallel to the axis of the device.

The scintillating elements in sets 1 and 5 are made of scintillating fiber, which provides registration of thermal neutrons, for example, lithium glass. The number of scintillating elements in sets 1 and 5 is determined by the ratio of the circumference on which they are to the cross section of the scintillating fiber used.

Currently, fiber scintillating elements of various cross sections are manufactured: round, square and rectangular. The cross-sectional size usually does not exceed a few millimeters.

The possible length of the device is determined by the minimum length of attenuation of light in fiber scintillating elements or spectroscopic fibers, which reaches about 100 cm.

The scintillating elements of the inner set 6 form a cylinder, are made in the form of equal-sized angular sectors of this cylinder, are made of a scintillator that provides registration of gamma radiation, for example, NaI, BGO, etc.

In the case when the diameter of the cylinder composed of scintillating elements included in set 6 is at least 2.5 cm, it is possible to determine the energy of gamma rays by summing the signals received from all the scintillating elements of the inner set 6 almost at the same time (from the same gamma ray).

The minimum number of angular sectors providing information about the azimuthal distribution of gamma radiation coming from the walls of the well to the device is two. The use of more scintillating elements increases the accuracy of determining the azimuthal distribution.

The scintillating elements of the inner set 6 are equipped with spectroscopic fibers 7 passing in each corner sector through its center parallel to the axis of the device, which is required for a more complete collection of photons from the scintillation burst that arose with the scintillating element.

The angular resolution of the device for a particular radiation (thermal or epithermal neutrons, gamma radiation) is determined by the ratio of the cross section of the scintillating element of the corresponding set to the radius of the circle on which the scintillating elements are located. With a cross section of scintillating elements of set 1 or set 5 of 1 mm and the radius of the circle on which these elements are located, for example, 20 mm, the angular resolution for detecting thermal or epithermal neutrons is 1/20 radian or better than 3 °.

To improve light collection and increase the proportion of light transferred to the ends of the scintillating elements of sets 1 and 5, as well as to the ends of the spectroscopic fibers 7, the scintillating elements of sets 1 and 5, as well as the spectroscopic fibers 7, are equipped with a reflective sheath (single and double layer), made of material with a lower refractive index than theirs, or fibers with a given radial composition gradient are grown (N.V. Klassen, V.N. Kurlov, S.N. Rossolenko, O.A. Krivko, A.D. Orlov, S .Z. Shmurak. Scintillation fibers and nanoscintillators to improve .. Rostranstvennogo, spectrometric and temporal resolution radiation detectors Izvestiya RAN: Physics, 2009, Volume 73, №10, from 1451-1456, the Russian Patent №2411543, MPK:. G01T 1/20, 2008).

To prevent light from the scintillation burst that occurred in the scintillating element of kits 1, 5 and 6 from entering the neighboring scintillating elements, their surface is additionally coated with a light-proof thin coating, for example, aluminum, titanium dioxide, magnesium oxide.

The opposite ends of each scintillating element of sets 1 and 5, as well as the opposite ends of each spectroscopic fiber 7 of the inner set 6 are connected using optical connectors 4 with two optical fibers 2 with an optical contact. Optical connectors 4 provide a mechanical connection of the ends of the scintillating elements, as well as the ends of the spectroscopic fibers with the ends of the optical fibers.

Fiber optical fibers 2 are usually made of glass or plastic with reflective and light-absorbing coatings that fulfill the same role as in the case of fiber scintillating elements or spectroscopic fibers. The ends of each of the optical fibers 2 are connected to the matrix photodetectors 3 with an optical contact.

Matrix photodetectors 3 contain photosensitive elements, which can be used photodiodes, such as silicon photomultipliers or elements of two-coordinate photomultipliers. The total number of photosensitive elements in each matrix photodetector 3 should be not less than the number of scintillating elements.

The photosensitive elements of the matrix photodetectors 3 and the scintillating elements included in sets 1, 5, 6 are numbered in advance. It is also predetermined which two photosensitive elements of two oppositely mounted matrix photodetectors receive photons from one or another scintillating element.

The scintillating elements included in the middle set 5 are placed in a tube-shaped neutron moderator 8 filling the space between the external set 1 and the internal set 6. A screen 9 is absorbed by the thermal neutrons on the outer surface of the neutron moderator. The thickness of the layer of neutron moderator 8 is selected from the condition of deceleration of epithermal neutrons to the energy of thermal neutrons and in the case of polyethylene is about 1 cm

The device operates as follows.

Thermal and epithermal neutrons are incident on the detector, as well as gamma radiation coming out of the borehole walls. The intensity of these emissions has an axial and azimuthal distribution. The axial distribution is associated with the layer structure of the rock surrounding the well. The azimuthal distribution is caused mainly by the asymmetric position of the downhole device with respect to the well.

Thermal neutrons trapped in the scintillating elements of external set 1 are absorbed in them, causing scintillation bursts. Those thermal neutrons that passed through the scintillating elements of the external set 1 into the detector without being absorbed enter the screen 9 and are absorbed in it.

Epithermal neutrons mainly pass through scintillating outer set 1 and screen 9 without being absorbed due to the fact that their energy is significantly higher than the thermal neutron energy, and the capture cross section is correspondingly smaller. Epithermal neutrons enter the moderator 8, where they dissipate, lose energy (slow down) and become thermal, propagate in all directions and partially fall on the scintillating elements of the middle set 5, where they are recorded.

Photons from scintillation flares that have arisen in the scintillating elements of sets 1 and 5 of the sets are transported by means of a reflective shell to their ends.

Gamma radiation is slightly attenuated in the scintillating elements of sets 1 and 5, as well as in the screen 9 and moderator 8, and passes to the scintillating elements of the internal set 6, designed to detect gamma rays. Photons from scintillation flashes arising in the scintillating elements of the inner set 6 under the action of gamma quanta propagate in all directions, are reflected from the walls of the scintillating elements of the inner set 6 and partially fall on the spectroscopic fibers 7, where they are absorbed. When absorbing light quanta from a scintillation flash, the spectroscopic fiber 7 emits light quanta of slightly lower energy, which are transported by means of a reflective sheath to the ends of the spectroscopic fiber 7.

The light-absorbing coating applied to the scintillating elements of sets 1 and 5 and the light-reflecting coating applied to the scintillating elements of the inner set 6 prevent the scintillation photons from passing from one scintillating element to another, preventing the spatial resolution of the device associated with this passage.

Photons reaching the ends of the scintillating elements of sets 1 and 5 and the ends of the spectroscopic fibers 7, through optical connectors 4 connected to the optical contact with the optical fibers 2, are transferred through them to the photosensitive elements of the matrix photodetectors 3, where they are recorded, causing an electrical signal.

When registering, the electrical signal from the opposite ends of the scintillating elements of sets 1 and 5 and the spectroscopic fibers 7 is analyzed and then summed. The ratio of the amplitudes of the signals received from opposite ends determines the axial coordinate of the radiation interaction. The accuracy of determining the axial coordinate is about 1 cm (V.N. Dubinina, V.E. Kovtun, “The concept of a new generation radiation portal monitor”, Bulletin of Kharkov University No. 845 (2009) 108-121; RF patent No. 2351954, IPC: G01T 3/06, 2009).

The azimuthal distribution of the total signal from various scintillating elements of sets 1, 5 and 6 determines the azimuthal distribution of the corresponding radiation, which is used to determine the position of the downhole device relative to the well, and then to correct the intensity of the signals from the scintillating elements (US patent application 2013/0187035, IPC: G01V 5/08, G01V 5/10, 2013).

The signal obtained by summing the signals received from all the scintillating elements of internal set 6 from one gamma quantum, i.e., almost simultaneously, is used to estimate the energy of the gamma quantum and determine the spectrum of gamma radiation.

Claims (1)

A spectrometric position-sensitive detector containing a scintillator in optical contact with a photodetector, characterized in that the scintillator consists of three sets of scintillating elements nested in each other parallel to the axis of the device, the outer and middle sets are made up of scintillating fibers made of a material that records thermal neutrons, and the scintillating elements of the internal set form a cylinder and are made in the form of equal-sized angular sectors and provide registration of gamma radiation, the number of angular sectors is two or more, each angular sector is equipped with a spectroscopic fiber passing through the center of the angular sector parallel to the axis of the device, scintillating elements of the middle set are placed inside the tube-shaped neutron moderator, filling the space between the external and internal sets, on the outer surface of the neutron moderator is a screen that absorbs thermal neutrons, scintillating elements of all sets and spectroscopic the loca of the inner set are provided with reflective shells, a lightproof coating is applied to the surface of the scintillating elements, the opposite ends of each scintillating element of the outer and middle sets, as well as the opposite ends of each spectroscopic fiber of the inner set, are connected via optical connectors to two optical fibers located on the opposite side in optical contact with two matrix photodetectors, the number of photosensitive elements in each of which is equal to or greater than the number of scintillating elements.