FAST NEUTRON SPECTROMETER AND DETECTOR

Abstract

A device for measuring the energy of neutrons incident in a first direction is provided. The device comprises a gas between a cathode and an anode, the anode comprising a matrix array of electron detectors, the first direction being orthogonal to the anode-cathode direction.

Claims

1. A device for measuring the energy of neutrons incident in a first direction, the device comprising a gas between a cathode and an anode, the anode comprising a matrix array of electron detectors, the first direction being orthogonal to the anode-cathode direction, and the device further comprising a circuit configured for: a) measuring the number, the positions, and the times of arrival of electrons; b) determining, from the positions and times of arrival, the angle between the first direction and the direction of an ionized trace left by a nucleus of said gas after collision with one of the neutrons; and c) determining the energy of the neutron from the number of electrons and the angle .

2. The device as claimed in claim 1, wherein said circuit is configured to, in step c): determine, from the number of electrons N.sub.e, the ionization energy of the trace E.sub.i, via the relationship E.sub.i=N.sub.e*E, where E is the energy required to create an electron-ion pair in the gas; and determine the energy E.sub.n of the neutron via the relationship: $E_n=A.Math..Math.\frac{E_P}{{\cos }^2.Math.{}^{}}$ wherein E.sub.p respects the relationship E.sub.p=E.sub.i/Q(E.sub.i), Q(E.sub.i) being the nuclear recoil quenching factor associated with the ionization energy of the trace E.sub.i, and A is a constant coefficient dependent on the ratio between the mass of the neutron and the mass of said nucleus.

3. The device as claimed in claim 1, further configured for measuring the energy of neutrons incident in a second direction, the second direction being parallel to the anode-cathode direction, wherein said circuit is configured to: a) measure the number, the positions, and the times of arrival of the electrons; b) determine, from the positions and times of arrival, the angle between the second direction and the direction of an ionized trace left by a nucleus of said gas struck by one of the neutrons; and c) determine the energy of the neutron from the number of electrons and the angle .

4. The device as claimed in claim 3, wherein said circuit is configured to: determine, from the number of electrons N.sub.e, the ionization energy of the trace E.sub.i, via the relationship E.sub.i=N.sub.e*E, where E is the energy required to create an electron-ion pair in the gas; and determine the energy E.sub.n of the neutron via the relationship: $E_n=A.Math..Math.\frac{E_P}{{\cos }^2.Math.}$ wherein E.sub.p respects the relationship E.sub.p=E.sub.i/Q(E.sub.i), Q(E.sub.i) being the nuclear recoil quenching factor associated with the ionization energy of the trace E.sub.i, and A is a constant coefficient dependent on the ratio between the mass of the neutron and the mass of said nucleus.

5. The device as claimed in claim 3, further comprising a rotary holder having an axis of rotation orthogonal to the first direction and to the second direction, a rotation of one quarter of a revolution about the axis of rotation making the device pass from an orientation in which the first direction is parallel to the direction of incidence of the neutrons to an orientation in which the second direction is parallel to the direction of incidence of the neutrons.

6. The device as claimed in claim 1, wherein the cathode and the anode are located on opposite sides of a cylindrical field cage arranged to produce a uniform electric field.

7. The device as claimed in claim 6, wherein the field cage has a diameter between 10 and 35 cm and a length between 15 and 35 cm.

8. The device as claimed in claim 1, wherein the gas is a mixture of helium 4 and of CO.sub.2 comprising between 4 and 6% CO.sub.2.

9. The device as claimed in claim 1, wherein the precision of the measurement of the times of arrival of the electrons is comprised between 16 and 40 ns.

10. A method for measuring the energy of neutrons incident in a direction, the method comprising: a) providing a gas between a cathode and an anode, the anode comprising a matrix array of electron detectors; b) orienting the anode-cathode direction orthogonal to the direction of incidence of the neutrons; c) measuring the number, the positions, and the times of arrival of electrons coming from gas on the matrix array; d) determining, from the positions and times of arrival, the angle between the direction of incidence of the neutrons and the direction of an ionized trace left by a nucleus of said gas after collision with one of the neutrons; and e) determining the energy of the neutron from the number of electrons and from the angle .

11. The method as claimed in claim 10, wherein step e) comprises: determining, from the number of electrons N.sub.e, the ionization energy of the trace E.sub.i, via the relationship E.sub.i=N.sub.e*E, where E is the energy required to create an electron-ion pair in the gas; and determining the energy E.sub.n of the neutron via the relationship: $E_n=A.Math..Math.\frac{E_P}{{\cos }^2.Math.{}^{}}$ wherein E.sub.p respects the relationship E.sub.p=E.sub.i/Q(E.sub.i), Q(E.sub.i) being the nuclear recoil quenching factor associated with the ionization energy of the trace E.sub.i, and A is a constant coefficient dependent on the ratio between the mass of the neutron and the mass of said nucleus.

12. The method as claimed in claim 10, further comprising successively: b) orienting the anode-cathode direction parallel to the direction of incidence of the neutrons; and f) implementing steps c), d) and e).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] These features and advantages, and others, will be described in detail in the following description of particular nonlimiting embodiments, which is given with reference to the appended figures, in which:

[0034] FIG. 1 is a schematic cross-sectional view of a device for measuring neutron energy;

[0035] FIG. 2 is a schematic cross-sectional view of one embodiment of a device for measuring the energy of fast neutrons; and

[0036] FIG. 3 is a schematic perspective view of one embodiment of a device for measuring neutron energy.

DETAILED DESCRIPTION

[0037] Elements that are the same have been referenced with the same references in the various figures and, in addition, the various figures have not been drawn to scale. For the sake of clarity, only elements useful to the comprehension of the described embodiments have been shown and are detailed. In particular the gas enclosure, the field cage, the electrodes, the matrix array of electron detectors, and the supply and processing circuit have not been described in detail, production thereof based on the functional indications of the following description being within the ability of those skilled in the art.

[0038] In the following description, unless otherwise specified, the expression of the order of means to within 10%, and preferably to within 5%.

[0039] It turns out that, with a device of the type illustrated in FIG. 1, it is not possible to correctly determine the energy E.sub.n when the incident neutrons are fast neutrons n.sub.h of energy for example higher than 5 MeV.

[0040] Specifically, the angle between the direction of incidence of the neutron and the trace 122 follows a probability law that depends on the nature of the gas and on the energy of the incident neutron. As is illustrated in the bottom left-hand portion of FIG. 1, for fast neutrons n.sub.h, the angle has a high probability of being close to 90. For example, for neutrons of energy higher than 100 MeV, the angle between the direction of incidence of the neutrons and the trace is almost never smaller than 80 when the nuclei of the gas have a mass higher than 1 GeV/c.sup.2. Furthermore, when the recoil nucleus departs at an angle close to 90, its initial energy E.sub.p is low and the trace is short, for example between 1 and 5 mm. As a result, when the angle is close to 90, the times of arrival of the electrons at the anode detectors 126 and 128 get closer together. In practice, the values measured at these arrival times merge because of the limit of the precision of the measurement, which is typically of the order of 25 ns. If the computed difference t0t1 between the travel times is zero, the determined value of the angle is equal to 90. Relationship (3) then corresponds to a division by zero, and the energy E.sub.n of the neutron cannot be determined.

[0041] To solve this problem, it could be sought to measure the times of arrival of the electrons at the matrix array with a high precision, for example one better than 10 ns, or even 1 ns, without decreasing the number of detectors of the matrix array. This would cause various technical problems.

[0042] FIG. 2 is a schematic cross-sectional view of one embodiment of a device for measuring the energy of fast neutrons.

[0043] The device of FIG. 2 comprises the same elements as those of the device of FIG. 1, said elements being arranged in the same way with respect to one another, with the exception of the circuit 110, which has been replaced by a supply and processing circuit 110. Elements that are the same and their arrangement will not be described again in detail. Only differences between the devices of FIGS. 1 and 2 will be highlighted.

[0044] Unlike the device of FIG. 1, the direction of incidence of the neutrons in the device of FIG. 2, the direction 200, is orthogonal to the anode-cathode direction. To determine the energy of an incident neutron n.sub.h, the circuit 110 of FIG. 2 proceeds in the same way as the circuit 110 of FIG. 1, with the exception that instead of the angle , the circuit 110 determines the angle between the direction 200 and the direction of the trace 120.

[0045] The circuit 110 determines the angle from the positions of arrival of the electrons and from the difference t0t1 , while taking into account the speed at which the electrons move from this trace 120 to the anode.

[0046] The circuit 110 then determines the energy of the neutron n.sub.h from the energy E.sub.p and from the angle using the relationship:

[00005]$\begin{array}{cc}{E}_n=A.Math..Math.\frac{E_p}{{\cos }^2.Math.{}^{}}& \left(4\right)\end{array}$

where A is the constant number mentioned above dependent on the ratio between the masses of the neutron and of the recoil nucleus.

[0047] Thus, a particularly precise measurement of the energy E.sub.n of the neutrons, and in particular of fast neutrons n.sub.h, is obtained even when the angle is close to 90, and for example comprised between 80 and 89. This may be explained in the following way.

[0048] If the trace 122 is in a plane orthogonal to the anode (case illustrated in FIG. 2) the orientation of the ionized trace 122 is close to the anode-cathode direction when the angle is close to 90. With this orientation of the trace, the difference t0t1 between the times of arrival at the anode of the electrons generated along the trace is higher than with the orientation of FIG. 1. For example, for a neutron of 50 MeV, the difference t0t1 typically exceeds 500 ns with the orientation of the trace in FIG. 2, whereas it is smaller than 50 ns with the orientation of the trace in FIG. 1. As a result, the computed difference t0t1 is precise. It is thus possible to determine the angle with precision. Specifically, the determined angle is not indistinguishable from 90 provided that the anode detectors 126 and 128 reached by the electrons generated at the ends of the trace 122 are separate from each other. The resolution of the matrix array is chosen so that the detectors 126 and 128 are separate even for short traces and for angles close to 90. By way of example, the matrix array of detectors has a resolution better than 0.5 mm. It is then possible to use relationship (4) to determine the energy E.sub.n of the neutrons with precision, while avoiding the division by zero that would result if the determined angle were indistinguishable from 90.

[0049] A device allowing a measurement of the energy of fast neutrons, the energy of the neutrons being able to be higher than 5 MeV, and for example higher than 200 MeV, or even higher than 600 MeV, is thus obtained. It will be noted that the trace may be located outside of a plane orthogonal to the anode. In this embodiment of the inventionand contrary to document EP 2708918 A1 mentioned with respect to the prior artthere is no solid converter, the conversion (i.e. the generation of electrons from the incident neutrons) being carried out directly in the gas contained in the cell. This therefore allows conversion efficiency to be improved and the trace left by the recoil nucleus to be reconstructed in 3D.

[0050] By way of example, the electric field, the nature and the pressure of the gas are chosen so that the speed of the electrons between the trace and the anode is comprised between 5 and 20 mm/s, and is for example of the order of 10 mm/s. By way of example, the gas 102 is a mixture of helium 4, .sup.4He, and of carbon dioxide, CO.sub.2, comprising between 4 and 6% CO.sub.2. The absolute pressure of the gas is for example comprised between 500 mbar 1 bar, and preferably between 650 and 750 mbar. The field cage for example has a diameter comprised between 10 and 35 cm, and preferably of 10 cm, and a length comprised between 15 and 35 cm, and preferably of 25 cm. By way of example, the number of anode detectors is higher than 256256, and for example equal to 960960. By way of example, the precision of the measurement of the times of arrival of the electrons at the detectors is comprised between 16 and 40 ns.

[0051] As was mentioned above, the circuit 110 determines the energy E.sub.p depending on the ionization energy E.sub.i of the trace, via the relationship E.sub.p=E.sub.i/Q, where Q is the nuclear recoil quenching factor. By way of example, the factor Q may be a value Q(E.sub.i) measured during prior trials and expressed as a function of the ionization energy E.sub.i of the trace. By way of example, the circuit 110 comprises a memory in which various measured values Q(E.sub.i) of the factor Q are stored, or in which a mathematical relationship allowing values close to and for example within 10% of the measured values to be computed is stored. The lower the initial energy E.sub.p of the recoil nuclei, the lower the factor Q tends to be. Using measured Q(E.sub.i) values allows a precise measurement of the energy of fast neutrons to be obtained.

[0052] A device allowing the energy of fast neutrons, for example neutrons of more than 5 MeV, has been described above. However, in order to establish an energy spectrum of a neutron source for example, it may furthermore be desired to measure the energy of neutrons that are not fast, for example neutrons of less than 5 MeV, without however providing two different devices for this purpose.

[0053] FIG. 3 is a schematic perspective view of one embodiment of a device for measuring the energy of neutrons, independently of whether they are fast or not.

[0054] The device comprises a device of the same type as that illustrated in FIGS. 1 and 2, comprising a gas enclosure 100 of axis 109 (anode-cathode direction). The direction 200 is also indicated. As was mentioned above, the device allows the energy of fast neutrons to be measured when the direction 200 is aligned with the direction of incidence of the neutrons.

[0055] The device furthermore comprises a rotary holder 300. The axis 302 of rotation of the holder 300 is orthogonal both to the direction 200 and to the anode-cathode direction. By way of example, the axis 302 is vertical, and the rotary holder 300 comprises a trolley 304 on pivoting wheels 306 and a linking unit 308 that securely joins the trolley and the enclosure 100. By way of variant, any other rotary holder of (vertical or non-vertical) axis parallel to the axis 302 may be used.

[0056] A rotation 310 of the device by one quarter of a revolution makes it possible to pass from the position (drawn with solid lines and designated by the reference 312) in which the direction 200 is aligned with the direction of incidence of the neutrons, to the position (drawn with dashed lines and designated by the reference 314) in which the anode-cathode direction is aligned with the direction of incidence of the neutrons.

[0057] In the position 312, the circuit 110 measures the energy of fast neutrons in the way described above with reference to FIG. 2, i.e. using the angle . In the position 314, the circuit 110 measures the energy of non-fast neutrons in the same way as the circuit 110 in FIG. 1, i.e. using the angle .

[0058] Because the circuit 110 measures the energy of incident neutrons in the direction 200 and the energy of incident neutrons in the anode-cathode direction, it is possible to measure, with a single device, the energy of neutrons independently of whether they are fast or not. The rotary holder 300 allows this measurement to be carried out in a particularly simple way.

[0059] Preferably, in the position 314, the circuit 110 uses measured values Q(E.sub.i) of the nuclear recoil quenching factor dependent on the ionization energy E.sub.i of the trace.

[0060] Particular embodiments have been described. Various variants and modifications will appear obvious to those skilled in the art. In particular, although a particular gas was mentioned, it is possible to use any gas suitable for detecting neutrons by collision with a nucleus of the gas and ionization of a trace in the gas, for example a mixture of C.sub.4H.sub.10 and of CHF.sub.3 comprising between 30 and 50% CHF.sub.3, or a mixture of 70% CF.sub.4 and of 30% CHF.sub.3.

[0061] Furthermore, any anode and cathode configuration comprising a gas between the anode and cathode is possible, the enclosure being able to be cylindrical or not.

[0062] Although the device in FIG. 3 comprises a rotary holder, provision may also be made for any other means for orienting the device. In particular, provision could be made for the device to be devoid of rotary holder, for example if it is a portable device able to be oriented manually.

[0063] Although, in the embodiments described above, the neutrons all had the same direction of incidence orthogonal to the anode-cathode direction, the neutrons may also originate from a source of neutrons located close to the device. The neutrons then do not all have exactly the same direction of incidence. It will be understood that, when the direction of incidence of the neutrons is said to be orthogonal to the anode-cathode direction, this means that the direction of incidence is orthogonal to the anode-cathode direction to within 10%, and preferably to within 5%.