RU154865U1 - POSITIVE SENSITIVE DETECTOR FOR SIMULTANEOUS RECORDING OF NEUTRON AND GAMMA OF RADIATIONS - Google Patents

Abstract

A position-sensitive detector for the simultaneous detection of neutron and gamma radiation, containing many scintillators separated by a reflective material placed between the scintillators, each scintillator is in optical contact with a photodetector, characterized in that the scintillator is made in the form of two coaxial cylindrical scintillator sets inserted into each other fibers, the outer and inner sets provide registration, respectively, of neutron and gamma radiation, scintillating waves the windows are equipped with reflective shells and opaque coatings, the opposite ends of the scintillating fibers are connected via optical connectors with two fiber optical fibers, which are on the opposite side in optical contact with two matrix photodetectors, the number of photosensitive elements in each of which is equal to or more than the total number of all scintillating fibers.

Description

The utility model relates to the field of registration of ionizing radiation and can be used in downhole tools used for logging oil and gas wells to determine the nature of the saturation of formations (oil, water), their filtration-capacitive properties and oil saturation coefficient.

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 probing radiation sources: 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. These neutrons propagate in the rock under study almost isotropically, while undergoing elastic and inelastic scattering on the atomic nuclei of the rock.

Propagating in a medium, fast 14 MeV neutrons undergo elastic and inelastic scattering by the atomic nuclei of the medium. As a result of elastic scattering, the fast neutrons of the generator slow down and gradually come into thermal equilibrium with the medium. The distance from the target of the generator at which thermal equilibrium sets in depends on the properties of the medium 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 the atoms of the medium, 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, allows one to determine the neutron porosity, density and composition of the rock. These characteristics are used to determine the nature of formation saturation (oil, water), their filtration-capacitive properties and oil saturation coefficient.

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.

The part of the logging equipment lowered into the well is called a downhole device. There is a wide variety of composition and designs of downhole devices.

So the main elements of a typical multifunctional downhole INC device are: a neutron source in the form of a neutron generator, neutron and gamma probes, a protective screen installed between the neutron generator and gamma radiation detectors, electronic devices.

The length of neutron generators used in neutron logging is usually not less than 150 cm. At the same time, the length of the measuring probes usually does not exceed 50-70 cm. Therefore, the location of neutron or gamma radiation detectors along the axis of the downhole device outside the neutron generator significantly reduces the intensity of the incident the gamma radiation on them and, thus, increases the measurement time, and also increases the length of the downhole device, which is undesirable to ensure free wiring Nogo downhole device.

The length of the detectors included in neutron or gamma probes is about 10 cm and determines the axial spatial resolution of the probes currently used.

The diameter of the neutron generators used in borehole devices designed for neutron logging is not more than 34 mm, and the inner diameter of the body of the borehole device is usually not less than 80 mm, which allows you to place detectors with a diameter of up to about 20 mm.

The problems of neutron logging currently boil down to the need to create a detector that provides separate registration of neutron and gamma radiation and has an axial (single-coordinate) spatial resolution, the design of which makes it possible to place it in the gap between the body of the downhole device and the neutron generator. The length of the 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.

Such a detector with a sufficiently high axial spatial resolution makes it possible to replace several probes consisting of several detectors with one device, removes the problem of choosing the number and length of probes, the number of detectors in each probe, reduces the length of the downhole device, improves the accuracy of data interpretation by simultaneously measuring neutron and gamma radiation at the same location in the well.

The well-known "borehole position-sensitive gamma-ray counter", consisting of a cathode body, along the axis of symmetry of which an anode is placed on the supporting insulators, made in the form of a filament with partitions 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 is the impossibility of simultaneously registering neutron and gamma radiation in the same place on the axis of the cathode body.

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 body and photomultipliers at opposite ends of the scintillator, each photomultiplier connected to a corresponding amplitude analyzer and through it to the controller, which serves to determine the axial position registered neutron in relation to the amplitudes of optical signals recorded by photomultipliers. Canadian Patent CA 2798070, IPC G01V 5/10. 2011. Analog.

The disadvantage of the analogue is the impossibility of simultaneously registering neutron and gamma radiation in one place on the axis of the well.

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. 201320033, IPC: G01V 5/10, 2012. Prototype.

The disadvantage of the prototype is the inability to simultaneously register neutron and gamma radiation in one place on the axis of the well.

The technical result of the utility model is the ability to simultaneously record neutron and gamma radiation in one place on the axis of the well.

The technical result is achieved by the fact that a position-sensitive detector for simultaneous detection of neutron and gamma radiation, containing many scintillators separated by reflective material placed between the scintillators, each scintillator is in optical contact with the photodetector, the scintillator is made in the form of two coaxial cylindrical sets inserted into each other scintillating fibers, the outer and inner sets provide registration of, respectively, neutron and gamma radiation scintillating fibers are provided with reflective claddings and opaque coatings, the opposite ends of the scintillating fibers are connected via optical connectors with two fiber optical fibers, which are on 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 total number of all scintillating fibers .

The essence of the utility model is illustrated in the drawing, where

1 - outer and inner cylindrical sets of scintillating fibers;

2 - fiber optical fibers;

3 - matrix photodetectors;

4 - optical connectors;

5, 6 - scintillating fibers from different materials for detecting neutrons and gamma radiation.

On the right, section AA shows a cross section of a positionally sensitive detector.

When implementing neutron logging using a neutron generator as a neutron source, the cylindrical shape of the device makes it possible to place it in the gap between the neutron generator and the borehole device, and a length of several tens of centimeters allows you to replace several probes with it, providing a high spatial resolution of about 1 cm along the axis of the borehole device (V.N. Dubinina, V.E. Kovtun, “The concept of a new generation radiation portal monitor”, Vestnik Kharkivskog University No. 845 (2009) 108-121; RF patent No. 2351954, IPC: G01T 3/06, 2009).

The position-sensitive detector contains two coaxial, nested into each other, cylindrical sets 1, each of which contains scintillating fibers of the same type and provides registration of one type of radiation. Outside is a cylindrical set 1, designed to detect neutrons. Internal kit 1 is used to register gamma radiation. It is known that the cross section for scattering and absorption of neutron radiation is usually much higher than the same cross sections for gamma radiation. Therefore, the indicated mutual arrangement is preferable from the point of view of preventing excessive neutron losses that are possible when they pass through the outer cylindrical set.

The ends of the scintillating fibers 5 and 6 are connected to the ends of two fiber optical fibers 2 with optical contact by means of two optical connectors 4.

The opposite ends of the optical fibers 2 are connected with an optical contact with photosensitive elements (not shown) of two matrix photodetectors 3.

In each cylindrical set 1, scintillating fibers 5 and 6 are parallel to the axis of the device at the same distance from it. Scintillating fibers 5 and 6 are made of materials that provide registration of different types of radiation: neutrons or gamma rays, for example: lithium glass or sodium iodide.

Scintillating fibers 5 and 6 can be of different cross sections: round, square and rectangular. The cross-sectional size usually does not exceed a few millimeters and determines the angular resolution of the device.

The angular resolution of the device is determined by the ratio of the cross section of the scintillating fiber to the radius of the cylindrical set in which this element is located. The diameter of the neutron generator used in the downhole device is usually not more than 34 mm, and the inner diameter of the body of the downhole device is not less than 80 mm. With a scintillating fiber diameter of 1 mm (the diameter of commonly used scintillators is about 1 cm) and a circle radius of, for example, 20 mm, the angular resolution is 1/20 radian or better than 3 °.

The maximum length of scintillating fibers 5 and 6 is determined by the length of attenuation of the light emitted during the scintillation burst in them, and can reach several meters.

To improve light collection and increase the proportion of light transferred to the ends of scintillating fibers 5 and 6, their surface is coated with a reflective coating (single or double layer) with a lower refractive index than the fiber material, 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 spatial, spectrometric and temporal resolution of radiation detectors. Proceedings of the Russian Academy of Sciences. Physical Series, 2009, Volume 73, No. 10, pp. 1451-1456; RF Patent No. 2411543, IPC: G01T 1/20, 2008).

To prevent light from the scintillation flash occurring in any of the scintillating fibers 5 and 6 from entering the adjacent fibers, its surface is additionally coated with an opaque thin coating, for example, aluminum, titanium dioxide, magnesium oxide. The coating thickness, providing complete absorption of light, is not more than 1 μm.

The ends of the scintillating fibers 5 and 6 are connected using optical connectors 4 with two optical fibers 2 with an optical contact. Optical connectors 4 provide mechanically optical coupling of the ends of the scintillating fibers 5 and 6 with the ends of the optical fibers. The cross section of the optical fibers 2 is usually equal to or greater than the cross section of the scintillating fibers 5 and 6 in order to reduce light loss at the junction of their ends. Fiber optic fibers are usually made of glass or plastic with reflective and light-absorbing coatings that play the same role as in the case of scintillating fibers.

Opposite torn fiber optic fibers 2 are connected to the photosensitive elements of the array photodetectors 3 with an optical contact.

The photosensitive elements of the matrix photodetectors 3 can be, for example, the so-called silicon photomultipliers or two-coordinate photomultipliers. The number of photosensitive elements in each of the matrix photodetectors 3 is equal to or greater than the total number of scintillating fibers 5 and 6.

The device operates as follows.

Neutron and gamma radiation coming out into the well have axial and angular distributions that are related to the composition and structure of the rock surrounding the well. Partially, these emissions fall on the outer and inner cylindrical sets 1 of scintillating fibers 5 and 6.

Neutron radiation is mainly absorbed in the fibers 5 of the outer set 1. The fraction of absorbed neutrons is determined by the material of the scintillating fibers 5 and their cross section. Gamma radiation passes through the fibers 5 with virtually no attenuation and is mainly absorbed in the scintillating fibers 6 of the inner set 1. The fraction of absorbed gamma radiation is also determined by the material of the scintillating fibers 6 and their cross section. Non-absorbed neutrons and gamma radiation go into the borehole walls.

Thrones and gamma radiation absorbed in scintillating fibers 5 and 6 of it cause scintillation bursts in them.

Photons from a scintillation flash arising in one of the scintillating fibers 5 and 6 are transported to its ends by means of a reflective shell.

A light-absorbing coating deposited on scintillating fibers 5 and 6 prevents the scintillation photons arising in one of the scintillating fibers from passing into neighboring scintillating fibers, preventing a deterioration in angular spatial resolution associated with this passage.

Photons reaching the ends of one of the scintillating fibers 5 and 6 pass through optical connectors 4 and then through fiber optic fibers 2 to matrix photodetectors 3 located at opposite ends of the device, where they are recorded, causing an electrical signal in the corresponding photosensitive elements of matrix photodetectors 3.

Scintillating fibers 5 and 6, as well as the photosensitive elements of the matrix photodetectors 3 are pre-numbered. Also, the correspondence of the numbers of scintillating fibers 5 and 6 to the numbers of photosensitive elements is pre-established.

When registering neutron and gamma radiation, scintillation photons arising in one of the scintillating fibers 5 and 6 cause an electric signal in photosensitive elements with corresponding numbers located in the matrix photodetectors 3 at opposite ends of the device. The axial coordinate of the interaction of radiation with one of the scintillating fibers 5 and 6 is determined by the ratio of the amplitudes of the signals received almost simultaneously from opposite ends of this fiber. 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 intensity of the signals received from scintillating fibers located at different azimuthal angles with respect to the axis of the borehole device determines the azimuthal distribution of incoming radiation, which, assuming rock uniformity around the borehole, is used to determine the position of the borehole device relative to the borehole, as well as to correct the intensity signals (patent application US 2013/0187035, IPC: G01V 5/08, G01V 5/10, 2013) received from various scintillating fibers, taking into account the polo zheniya.

Claims (1)

A position-sensitive detector for the simultaneous detection of neutron and gamma radiation, containing many scintillators separated by a reflective material placed between the scintillators, each scintillator is in optical contact with a photodetector, characterized in that the scintillator is made in the form of two coaxial cylindrical scintillator sets inserted into each other fibers, the outer and inner sets provide registration, respectively, of neutron and gamma radiation, scintillating waves the windows are equipped with reflective shells and opaque coatings, the opposite ends of the scintillating fibers are connected via optical connectors with two fiber optical fibers, which are on the opposite side in optical contact with two matrix photodetectors, the number of photosensitive elements in each of which is equal to or more than the total number of all scintillating fibers.

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