Neutron measuring device

Abstract

The invention relates to a neutron measuring device (1). This device (1) comprises a housing (90), a high-density polyethylene block (10) within the housing (90), a neutron-absorbing material envelope (20) integrated into the block (10), a first scintillation neutron detector (30) located inside the envelope (20) and capable of emitting a signal proportional to the number of neutrons detected per unit time, and a module (40) capable of converting the signal into a neutron count rate. Figure 2

Description

Title of the invention: Neutron measuring device

[0001] The present invention relates to a neutron measuring device. Three types of neutrons can be distinguished that one might wish to detect: - Slow neutrons: these neutrons are called thermal neutrons. Their kinetic energy is less than 0.025 eV and their speed is less than 2200 m / s. - Fast neutrons: their kinetic energy is greater than 0.9 MeV and their speed is greater than 13 km / s. - Intermediate neutrons: these neutrons are called epithermal neutrons. Their kinetic energy and speed are between those of slow and fast neutrons.

[0002] Two categories of neutron measuring devices are currently known. The first category includes dose rate measuring devices that provide a direct measurement of the dose absorbed per unit time by an organism, the dose being defined as the energy received per unit mass (expressed in sieverts (with 1 Sv = 1 J / kg)) or in rem (with 100 rem = 1 Sv)). These devices are suitable for detecting neutrons across the entire energy range (thermal, epithermal, and fast). Thus, their dose rate response is relatively independent of possible variations in the neutron spectrum and the corresponding thermalization level. This category includes devices that incorporate helium-3 detectors. These devices are heavy and unwieldy, their mass exceeding 8 kg due to the thermalization material required for the detection of fast neutrons by helium-3.This category also includes devices whose measurement principle is based on the elastic scattering of fast neutrons on hydrogen. These devices are more manageable because they have a lower mass (around 3 kg) than helium-3 detectors, but they are incompatible with safety requirements because they contain an explosive gas (methane).

[0003] A second category of neutron measuring device includes neutron radiometers whose primary function is the detection of neutron sources (or hot spots) and not necessarily a dose rate measurement, and which provide a neutron count rate expressed in c / s (counts per second). This count rate can then be converted into a dose rate. These devices have a lower mass (approximately 1.5 kg) than dose rate measuring devices. However, their dose rate response is generally very sensitive to variations in the neutron spectrum and their thermalization level. Indeed, neutron radiometers contain a thermalization block which consists of polyethylene (PE), which can lead to saturation of the detector response and loss of linearity between the measurement signal and the amount of plutoniferous material in areas where the proportion of thermal or epithermal neutrons is high.

[0004] In certain cases, particularly in the context of dismantling operations for structures and buildings in the nuclear industry, the aim is to produce fine-mesh neutron dose rate maps in various radioactive environments. The aim is also to produce neutron dose rate maps of various objects such as glove boxes (BAGs) used for handling MOX or plutonium. To produce these maps, it is necessary to use a neutron measuring device that is easy to handle, has the shortest possible measurement time to minimize the total measurement time for different parts of an environment, and offers good measurement reliability regardless of variations in the neutron spectrum. Furthermore, the neutron measuring device must meet safety requirements. Description of the invention

[0005] The invention aims to provide a neutron measuring device which has the lowest possible mass so as to be used by a user for a certain period of time and without danger, which has a dose rate response that is relatively insensitive to variations in the neutron spectrum, and a short stabilization time for measurements.

[0006] This goal is achieved thanks to the fact that the neutron measuring device comprises a housing, a high-density polyethylene block in the housing, a neutrophilizing material envelope integrated into the block, a first scintillation neutron detector located inside the envelope and capable of emitting a signal proportional to the number of neutrons detected per unit of time, and a module capable of converting the signal into a neutron counting rate.

[0007] Thanks to these features, the neutron measuring device can be used safely because it does not contain any explosive gas, and for extended periods without excessive user fatigue, particularly for inspecting confined environments. Tests carried out by the inventors have shown that the time required to obtain a reliable measurement with this device is less than 10 seconds in a low dose rate environment (less than 10 qSv / h) and less than 5 seconds in a higher dose rate environment (on the order of a few tens of qSv / h), thus enabling its efficient and practical use in any neutron environment.

[0008] For example, the neutron measuring device has a weight of less than 3 kg.

[0009] For example, the first neutron detector includes a silicon photomultiplier.

[0010] Advantageously, the neutron measuring device further comprises a second neutron detector which is located in the housing and outside the block.

[0011] For example, the first neutron detector and the second neutron detector include a silicon photomultiplier.

[0012] For example, the neutrophilizing material envelope is flush with at least part of the surface of the block.

[0013] For example, the neutrophing material has a boron charge.

[0014] For example, the neutrophing material is a boron elastomer.

[0015] For example, the neutrophil material comprises about 5% boron by mass.

[0016] For example, the neutron measuring device further includes a handle which allows a user to enter it.

[0017] The invention will be better understood and its advantages will become more apparent upon reading the following detailed description of embodiments shown by way of non-limiting examples. The description refers to the accompanying drawings in which:

[0018] [Fig-1] Fig. 1 is a perspective view of a neutron measuring device according to the invention.

[0019] [Fig.2] Fig.2 is a cross-sectional view of the neutron measuring device of the [Fig.l] according to plan ILII.

[0020] [Fig. 3] Fig. 3 is a cross-sectional view of the neutron measuring device of the [Fig.l] according to plan 111-111.

[0021] [Fig.4] Fig.4 is a graph that illustrates the result of calibration tests of the neutron measuring device according to the invention.

[0022] [Fig. 5] [Fig. 5] is a perspective view of a neutron measuring device according to another embodiment of the invention.

[0023] [Fig. 6] Fig. 6 is a cross-sectional view of the neutron measuring device of the [Fig.5] according to plan VLVI.

[0024] [Fig.7] Fig.7 is a cross-sectional view of the neutron measuring device of the [Fig.5] according to plan VILVII. Detailed description of the invention

[0025] Figure 1 shows a portable neutron measuring device 1 according to the invention. The device 1 is rectangular in shape, however, the device 1 can have other shapes. The device 1 extends in a principal plane XY where the length is along the X direction and the width is along the Y direction. The thickness is along the Z direction perpendicular to the principal plane XY. Figure 2 is a cross-sectional view in the principal plane XY along the ILII direction of Figure 1. Figure 3 is a cross-sectional view in a transverse plane YZ along the III-III direction of Figure 1. The device 1 comprises a housing 90, a high-density polyethylene (HDPE) block 10 located within the housing 90, a neutrophil-absorbing material envelope 20 integrated into the block 10, and a first scintillation neutron detector 30 embedded in block 10 is located inside the casing 20. In the example shown, block 10 is rectangular. The casing 20 is a box in the shape of a parallelepiped whose walls are parallel to the faces of block 10. A first embodiment of the invention corresponds to the case where the neutron measuring device 1 comprises a single neutron detector (first neutron detector 30).

[0026] The HDPE of block 10 slows down fast neutrons in order to transform them into thermal and epithermal neutrons, which allows their detection by the first neutron detector 30. The density of HDPE is for example between 940 and 965 kg / m3.

[0027] The first neutron detector 30 is a scintillation neutron detector, which includes, for example, a silicon photomultiplier tube comprising lithium 6Li and ZnS. Slow neutrons interact with the lithium and emit alpha particles that are captured by the ZnS, which then emits photons by scintillation. These photons subsequently generate an electrical signal. Thus, the neutron detector 30 is capable of emitting a signal that is proportional to the number of neutrons detected per unit of time. The first neutron detector 30 is, for example, a TN15 detector from the Kromek® brand.

[0028] The neutrophil-absorbing material envelope 20 enables linearity between the above signal and the amount of plutonium-bearing material for the detection of fast neutrons using block 10, and also for the detection of slower neutrons using the neutrophil-absorbing material envelope 20. Prior calibration of the measurements is necessary by comparison with a previously qualified dose rate measuring device. Figure 4 illustrates a curve obtained after such calibration, which confirms the linearity between the neutron count rate Tcn in counts per second and the dose rate DD in pSv / h (microsieverts per hour). This linearity is necessary for the reliable operation of the neutron measuring device 1. Several calibrations may be necessary depending on the level of thermalization of the spectrum in the first embodiment.

[0029] The neutrophil-absorbing material casing 20 is flush with at least part of the surface of the block 10, the remainder of this casing 20 being located within the block 10. For example, as illustrated in Figures 2 and 3, the casing 20 is flush with the two faces of the block 10 perpendicular to the X direction and with the two faces of the block 10 perpendicular to the Y direction, and is separate from the two faces of the block 10 perpendicular to the Z direction. For example, in one embodiment, the casing 20 is flush with at least two of the opposite faces of the block 10. Tests carried out by the inventor show that the performance of the neutron measuring device 1 is improved in this embodiment.

[0030] For example, the neutrophilizing material of the casing 20 contains a boron filler. For example, this neutrophilizing material is a boron elastomer. The boron filler is, for example, boron carbide (B4C), and is present, for example, at a content of at least 30% by mass of the total mass of the elastomer, preferably at least 50% by mass of the total mass of the elastomer. This elastomer is, for example, natural rubber. Tests carried out by the inventor show that detection by the neutron measuring device 1 is optimized when the neutrophilizing material comprises approximately 5% boron by mass, and optionally when the thickness of the casing 20 is on the order of 5 mm.

[0031] The neutron measuring device 1 includes a module 40 capable of converting the signal emitted by the first neutron detector 30 into a neutron count rate. Furthermore, if the measuring device 1 includes a display screen for the measurement results, the module 40 manages the interface with this display screen and, where applicable, the wireless data exchange with external equipment.

[0032] For example, the neutron measuring device 1 further includes a handle 50 which allows a user to grip it conveniently. This handle 50 is located outside the housing 90.

[0033] Unlike dose rate measuring devices, the neutron measuring device 1 is free of any material toxic to humans or explosive during normal operation. Thus, the neutron measuring device 1 can be used safely, especially in a confined space such as a room in a building.

[0034] A second embodiment is now described in which the neutron measuring device 1 further comprises a second neutron detector 60. This second neutron detector 60 is located in the housing 90 and outside the block 10. For example, this second neutron detector 60 is identical to the first detector 30. Since it is located outside the block 10, this second neutron detector 60 is capable of directly detecting thermal neutrons and a portion of the epithermal neutrons. The neutron measuring device 1 is therefore capable of directly and independently detecting both thermal and epithermal neutrons and, thanks to the first neutron detector 30, faster and higher-energy neutrons. The neutron measuring device 1 is thus capable of utilizing the results of the counts provided by the two detectors (30, 60).The neutron measuring device 1 according to the second embodiment is therefore more precise and less sensitive to spectral variations than the neutron measuring device 1 according to the first embodiment. Consequently, only one calibration curve needs to be configured in the neutron measuring device 1 for it to be able to automatically calculate the dose rate regardless of the energy spectrum of the detected neutrons. The neutron measuring device 1 is therefore capable. to adapt to any type of neutron spectrum (distribution between fast, epithermal and thermal neutrons).

[0035] The neutron measuring device 1, thanks to its structure, has a short stabilization time and a sensitivity which allows a measurement acquisition time of less than 10 seconds, for example less than 5 seconds.

[0036] The neutron measuring device 1, thanks to its structure, is portable and has a sufficiently low mass so that it can be handled by a user for extended periods. By "handled," it is meant that the device can be manipulated at arm's length without fatigue to perform a plurality of measurements over a long period, for example, several dozen measurements over at least half an hour. For example, the neutron measuring device 1 has a mass of less than 3 kg. For example, the neutron measuring device 1 has a mass of less than 2.6 kg in the second embodiment, and a mass of less than 1.7 kg in the first embodiment. For example, the approximate dimensions of the casing 90 (without the handle 50) are, in the second embodiment: 25 cm (along the X-axis) x 15 cm (along the Y-axis) x 13 cm (along the Z-axis); and in the first embodiment: 18 cm (along the X axis) x 10 cm (along the Y axis) x 7 cm (along the Z axis).

Claims

Demands

1. Neutron measuring device (1) characterized in that it comprises a housing (90), a high-density polyethylene block (10) in said housing (90), a neutrophilizing material envelope (20) integrated in said block (10), a first scintillation neutron detector (30) located inside said envelope (20) and capable of emitting a signal proportional to the number of neutrons detected per unit time, and a module (40) capable of converting said signal into a neutron count rate.

2. Neutron measuring device (1) according to claim 1 such that it has a weight of less than 3 kg.

3. Neutron measuring apparatus (1) according to claim 1 or 2 wherein said first neutron detector (30) comprises a silicon photomultiplier.

4. Neutron measuring device (1) according to claim 1 or 2 further comprising a second neutron detector (60) which is located in said housing (90) and outside said block (10).

5. Neutron measuring apparatus (1) according to claim 4 such that said first neutron detector (30) and said second neutron detector (60) comprise a silicon photomultiplier.

6. Neutron measuring device (1) according to any one of claims 1 to 5 wherein said neutrophilizing material envelope (20) is flush with at least a portion of the surface of said block (10).

7. Neutron measuring apparatus (1) according to any one of claims 1 to 6 wherein said neutrophil material (20) comprises a boron charge.

8. Neutron measuring apparatus (1) according to claim 7 wherein said neutrophilizing material (20) is a boron elastomer.

9. Neutron measuring apparatus (1) according to claim 7 or 8 such that said neutrophil material (20) comprises about 5% boron by mass.

10. Neutron measuring device (1) according to any one of claims 1 to 9 further comprising a handle (50) which enables a user to grasp it.

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