RU156517U1 - Scintillation Neutron Detector Sensor - Google Patents

Abstract

A scintillation neutron detector sensor containing a photodetector, a cylinder-shaped scintillator, the outer cylindrical surface of the scintillator is covered with a reflective film, a hollow cylindrical channel with an optical waveguide located in it is located parallel to the scintillator axis, characterized in that it contains two photodetector devices connected optically to opposite ends of the scintillator , the fiber is made in the form of a cylinder with a diameter comparable to the diameter of the scintillator, and is located in the optical contact with the surface of the hollow channel inside the scintillator, the fiber is made of a material that is not scintillating under the action of ionizing radiation, transparent to light from scintillation bursts arising in the scintillator, and having a refractive index that is as close as possible to the refractive index of the scintillator material.

Description

The utility model relates to the field of registration of ionizing radiation and can be used to create neutron detectors used, in particular, in portal monitors, scientific research, and in geophysical neutron logging equipment.

For registration of thermal neutrons, proportional counters are mainly used based on the nuclear reactions ³ He (n, p) ³ H, ¹⁰ B (n, α) ⁷ Li. Scintillation detectors with a scintillator are also used, for example, in the form of a LiI single crystal or lithium-containing glass, the main disadvantage of which, in comparison with gas proportional counters, is a higher gamma-ray detection efficiency due to the higher mass load of the solid-state scintillator compared to the gas medium in the proportional counter .

To register fast neutrons, scintillation detectors with a scintillator based on hydrogen-containing substances are mainly used: stilbene or anthracene crystals, as well as plastics.

When neutrons are detected by a scintillation detector in the presence of background radiation, such as gamma, the scintillator is usually made in the form of a thin flat washer or, as suggested below, a thin-walled cylinder. This is because an increase in the scintillator volume leads to an increase in the contribution to the background radiation detector.

Radiation detectors include a radiation sensor. A radiation element is usually considered a sensitive element (receiver) that converts the radiation into an electrical signal suitable for technical use.

In the case of a scintillation detector, a scintillator and a photodetector are included in the sensor. In the scintillator, the radiation causes a scintillation flash, the photons of which, getting into the photodetector, are converted into an electrical signal.

The well-known "Position-sensitive tubular scintillation detector"("A position-sensitive tubular scintillator-based detector as an alternative to ³ He-gas-based detector for neutron scattering instruments". NIM A. Vol. 741 (2014) 42-46) containing a ZnS / ⁶ LiF tubular scintillator surrounded by a spectroscopic fiber helix. The analogue.

A disadvantage of the analogue is the low efficiency of collecting scintillation photons on a photodetector, due to the fact that the spectroscopic fiber captures with its cladding and transports to each of its ends no more than 5% of the total number of photons formed in it.

Known "Scintillation detector" containing at least one sensor, consisting of a plastic scintillator based on an organic hydrogen-containing material that is sensitive to neutrons and neutrinos, a light-collecting fiber, a photodetector and an electronic signal processing unit, characterized in that the plastic scintillator is made in the form of a cylinder coated with a retroreflective film, in the center of which there is a hollow channel with a light-collecting optical fiber placed in it. Patent RU 2308056, IPC: G01T 3/06. 2007 Prototype.

The disadvantage of the prototype is the low efficiency of the fiber, due to a significant difference in the diameters of the fiber and the scintillator, as well as the lack of optical contact between the surface of the hollow channel inside the scintillator and the fiber, the result of which is the presence of total internal reflection of scintillation photons on this surface.

The technical result of the utility model is to increase the efficiency of the fiber through the use of a fiber whose diameter is comparable to the diameter of the scintillator and the light refractive index is as close as possible to the refractive index of the scintillator, as well as by providing optical contact between the surface of the hollow channel inside the scintillator and the fiber. The consequence of this is the elimination of the total internal reflection of scintillation photons on the surface of the hollow channel inside the scintillator and a corresponding decrease in the number of photon reflections during their transportation to the photodetector.

The technical result is achieved in that the scintillation neutron detector sensor containing a photodetector, a scintillator in the shape of a cylinder, the outer cylindrical surface of the scintillator is covered with a reflective film, a hollow cylindrical channel with an optical fiber placed in it is located parallel to the scintillator, it contains two photodetector devices connected optically to opposite the ends of the scintillator, the fiber is made in the form of a cylinder with a diameter comparable to the diameter of the scintillator and oditsya in optical contact with the surface of the hollow channel within the scintillator, the light guide is made of a material not scintillating under ionizing radiation, is transparent to scintillation light flashes from occurring in the scintillator, and having a maximum refractive index close to the refractive index of scintillator material.

In FIG. 1 shows a sensor device with two photodetectors located on opposite ends of the scintillator, in the case of a coaxial arrangement of the scintillator and the optical fiber.

In FIG. 2 shows the possible trajectories of scintillation photons propagating in the direction of the ends of the scintillator in the absence of a fiber,

Where:

1 - scintillator,

2 - reflective film,

3 - the surface of the hollow channel inside the scintillator 1,

4 - photodetector devices,

5 - light guide

6 - scintillation flash,

7 - trajectory of photons propagating to the ends of the scintillator 1 in the absence of a fiber 5 due to reflection from the reflective film 2 and from the surface 3.

8 - the trajectory of photons propagating to the ends of the scintillator 1 in the absence of a fiber 5 due to reflection from the reflective film 2.

In FIG. 1, the power supply unit of photodetectors 4 is not shown.

The scintillation neutron detector sensor includes:

a scintillator 1 in the form of a cylinder with an internal channel bounded by a surface 3;

a reflective film 2 deposited on the outer surface of the scintillator 1;

a cylinder-shaped optical fiber 5 located in the hollow channel of the scintillator 1 and in optical contact with the surface 3;

photodetectors 4 connected optically to the ends of the scintillator 1.

The device is operable if there is only one photodetector 4. The use of two photodetectors 4 provides the ability to determine the coordinates of the interaction of radiation with the scintillator 1 along its axis by measuring the amplitudes of the electrical signals coming from both photodetectors. The ratio of the amplitudes of the signals is determined by the ratio of the distances from the place of interaction to the ends of the scintillator 1.

The thickness of the cylindrical wall of the scintillator d is selected based on ensuring the necessary neutron detection efficiency, an acceptable contribution to the background radiation signal, and reducing the loss of scintillation photons during their transportation to the photodetector.

When registering thermal neutrons, the wall thickness d of the scintillator 1, which provides effective absorption of thermal neutrons incident on the scintillator 1 from the outside, is determined by the ratio:

where n, σ are, respectively, the concentration and capture cross section of the thermal neutron for atoms that are part of scintillator 1 and emit charged particles.

In the case of a lithium glass scintillator 1 with a lithium weight concentration of about 6%, enriched with ⁶ Li isotope up to 95%, the thickness d, defined by relation (1), is less than 1.5 mm. The light refractive index for such a glass manufactured by Saint-Gobain Crystals is 1.55.

The outer diameter of the scintillator 1 is usually a few centimeters. In this case, the tubular shape of the scintillator 1, on the one hand, provides effective absorption of thermal neutrons, and, on the other hand, reduces the possible contribution of background radiation detected by the scintillator in the case of a scintillator without an internal cavity.

The length of the scintillator 1 is determined by the absorption of scintillation photons in the scintillator material and their losses upon reflection from the interfaces, i.e. the number of reflections N, which scintillation photons experience on their way from the place of origin to the photodetector.

The collection efficiency k of scintillation photons going in the direction of one of the ends of the scintillator 1 is:

where R is the reflection coefficient from the interface between the scintillator 1 and the reflective surface,

L is the distance along the axis of the scintillator 1 traveled by scintillation photons from the point of occurrence of the scintillation flash 6 to the photodetector 4,

l is the average distance along the axis of the scintillator 1 between two successive reflections of scintillation photons.

The efficiency of collecting k scintillation photons going in the direction of the end face of the scintillator 1 is maximum in the case of a coaxial arrangement of the scintillator 1 and the fiber 5 and decreases with increasing distance between their axes, but the device does not lose operability.

In the absence of optical fiber 5, photons from scintillation flashes 6 arising in scintillator 1 are transported to its ends along two types of trajectories (Fig. 2):

1) Photons falling along trajectories 7 are incident on the interface between the scintillator 1 and air on surface 3 at a fairly small angle. They propagate throughout the scintillator volume, experiencing reflections, on the one hand, from the reflective film 2, and on the other hand, due to total internal reflection from the interface between the scintillator 1 and air on surface 3.

2) Photons falling along trajectories 8 are incident on the interface between the scintillator 1 and the air on surface 3 at a sufficiently large angle. These photons exit into the hollow channel inside the scintillator 1, experiencing refraction on the surface 3, and are reflected only from the reflective film 2.

Calculations show that in the case of scintillator 1 made of lithium glass with a refractive index n = 1.55, scintillation photons emitted at an angle in the range from 0 ° to ≈50 experience total internal reflection at the interface between scintillator 1 and air on surface 3 ° in relation to this surface. The fraction of such photons from the total number of emitted scintillation photons is about 36%.

In the case of scintillator 1 with a wall thickness d = 1.5 mm and an external diameter of 30 mm, the average distance between two successive reflections l ₁ for photons traveling along trajectories 7 is approximately 2.3 mm. At the same time, as for photons traveling along trajectories of 8, l ₂ ≈32 mm.

When evaluating the efficiency of collecting k scintillation photons going in the direction of the end face of scintillator 1 along trajectories 7 and 8, in the absence of optical fiber 5, it was assumed that:

- the scintillator 1 is made of lithium glass with a refractive index of n = 1.55;

- the reflection coefficients from the retroreflective film 2 and with total internal reflection at the interface between the scintillator 1 and the air are equal and are R = 0.98;

- the distance from the place of occurrence of the scintillation flash 6 to the photodetector 4 is L = 100 mm;

- the absorption of scintillation photons in the scintillator is negligible;

- scintillator 1 and fiber 5 are aligned.

In this case, the collection efficiency for scintillation photons traveling in the direction of the end face of scintillator 1 along trajectories 7, in accordance with expression (2), is k≈41%, and for scintillation photons traveling along trajectories 8, k≈0.92%, those. about 2.2 times higher.

From the expression (2) it follows that the ratio of the collection efficiency F for two types of trajectories is determined by the ratio:

, т.е. отношение эффективностей сбора для двух типов траекторий, т.е. эффективность световода тем выше, чем больше R, L и меньше l₁.From (3) it is clear that for

, i.e. ratio of collection efficiencies for two types of trajectories, i.e. the fiber efficiency is higher, the more R, L and less than l ₁ .

The introduction into the sensor device of the optical fiber 5 in the form of a cylinder, which is in optical contact with the scintillator 1 and made of a material with as close as possible refractive index, leads to the fact that the photons traveling in the absence of the optical fiber 5 along the paths 7 cease to experience total internal reflection on the surface 3 and are transported to the ends of the scintillator 1 along trajectories similar to trajectories 8.

Since the fraction of photons traveling along trajectories 7 in the case of scintillator 1 made of lithium glass is ≈36%, the maximum possible gain from the use of optical fiber 5 among the photons that reached photodetector 4 is subject to a negligible absorption of photons in the scintillator and optical fiber can also reach ≈36%.

The gain from the use of a fiber will be even higher when the absorption of photons in the scintillator cannot be neglected, i.e. with a sufficiently large scintillator length 1 or a relatively small value of d.

If instead of a cylindrical waveguide 5, a fiber waveguide is used, the probability that the scintillation photon will fall into the fiber waveguide and not experience reflection from the reflective film 2 is extremely small. This probability is determined by the ratio of the surface area of the fiber to the area of the outer surface of the scintillator 1, which are related as corresponding diameters. It should be borne in mind that the diameter of the fiber is usually not more than 100 microns (DB Shumkova, AE Levchenko. Special fiber optic fibers. Publishing house of the Perm National Research Polytechnic, University. 2011. http: // dic. academic.ru). While the diameter of the scintillator 1 is usually about two centimeters (Scintillators and containers of ionizing radiation detectors. GOST 27841-88. Http: //standartgost.ru).

The absence of optical contact between the surface of the optical fiber and the surface 3 of the hollow channel inside the scintillator 1 further reduces the efficiency of the optical fiber due to the fact that scintillation photons that experience total internal reflection on the surface 3 do not fall into it.

Even if there is optical contact between the surface of the optical fiber and surface 3, scintillation photons can be reflected from the surface of the optical fiber if the refractive index of the cladding of the optical fiber differs from the refractive index of scintillator 1.

In addition to the above, a fiber light guide is quite effective only in the case of the use of the so-called spectroscopic fiber ("'A position-sensitive tubular scintillator-based detector as an alternative to ³ He-gas-based detector for neutron scattering instruments". NIM A. Vol 741 (2014) 42-46).

To ensure maximum proximity of the refractive indices of the scintillator and the fiber, both of them can be made of the same material, which differs in the presence of a scintillator and, in the case of a fiber, by the absence of an active additive, which ensures the appearance of a luminescent flash.

In the case of lithium scintillating glass, such an additive may be the ⁶ Li isotope, which traps thermal neutrons, leading to the creation of charged particles in the scintillator. In a fiber, the ⁶ Li isotope can be replaced by the ⁷ Li isotope, which does not absorb neutrons. The same result can be obtained due to the presence or absence of atoms of the optical luminescence activator, in this case cerium atoms.

In the case of organic scintillators, the active additive is dyes that are introduced into the scintillator in very small (less than 1%) quantities, which may not be introduced into the fiber.

In order to reduce the contribution of the background radiation to the sensor signal, the fiber material 5 should not scintillate under the influence of these radiation. In addition, to reduce the loss of scintillation photons inside the fiber 5, its material should be sufficiently transparent for the radiation spectrum of scintillator 1.

All these conditions are fulfilled, for example, in the case of a scintillator made of lithium glass activated by cerium and a fiber made of the same glass but containing instead of the ⁶ Li isotope ⁷ isotope and not containing cerium.

The scintillation neutron detector sensor operates as follows.

Neutron radiation is incident on the scintillator 1 of the sensor. Neutrons entering the scintillator 1 give rise to charged particles in it, which excite the scintillator and cause scintillation bursts in it 6.

Scintillation photons from scintillation flashes 6 go in all directions and are transported to both ends of the scintillator 1 due to reflection from the reflective film 2. In this case, the scintillation photons freely cross surface 3 and pass through the optical fiber 5.

Scintillation photons reaching one of the ends of the scintillator 1 or the optical fiber 5 enter the photodetector 4, causing an electric signal (pulse) to appear at its output, which then goes to the signal processing circuit (the circuit is not shown in Fig. 1).

Claims (1)

A scintillation neutron detector sensor containing a photodetector, a cylinder-shaped scintillator, the outer cylindrical surface of the scintillator is covered with a reflective film, a hollow cylindrical channel with an optical waveguide located in it is located parallel to the scintillator axis, characterized in that it contains two photodetector devices connected optically to opposite ends of the scintillator , the fiber is made in the form of a cylinder with a diameter comparable to the diameter of the scintillator, and is located in the optical contact with the surface of the hollow channel inside the scintillator, the fiber is made of a material that is not scintillating under the action of ionizing radiation, transparent to light from scintillation bursts arising in the scintillator, and having a refractive index as close as possible to the refractive index of the scintillator material.

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