Method to produce beam of monoenergetic neutrons, device for production of beam of monoenergetic neutrons and method to calibrate detector of dark matter with using of beam of monoenergetic neutrons

FIELD: power engineering.

SUBSTANCE: method includes radiation with a beam of protons with energy exceeding 1.920 MeV, a neutron-generating target, at the same time the beam of monoenergetic neutrons is formed from neutrons, which spread in direction that is reverse to direction of spread of the beam of protons. By varying the energy of protons and the angle of neutrons release, they create a monoenergetic neutron beam with any required energy. To exclude neutrons with other energies, which randomly got into the beam, it is possible to place a filter on the way of the beam. The method to calibrate the dark matter detector with liquid Ar as a working substance consists in the fact, that it is radiated with a beam of monoenergentic neutrons with energy of 74-82 keV, generated during radiation of the target 7Li(p,n)7Be with a beam of protons with energy exceeding 1.920 MeV, and formed in accordance with the above method using a sulphur filter with subsequent registration of the completed ionisation of liquid argon.

EFFECT: possibility to produce a beam of monoenergetic neutrons designed for calibration of a dark matter detector, with different energies without beam scattering.

9 cl, 4 dwg

 

The invention relates to nuclear technology, in particular the production of monoenergetic neutrons of low energy.

Neutron emission occurs when exposed to a stream of charged particles that interact with a target to generate neutrons, which is a structure consisting of a cooled substrate, coated with a thin layer of the substance, the actual source of neutrons. To obtain monoenergetic neutrons property is used kinematic collimation and, additionally, filters can be applied that can pass the neutrons with a certain energy.

At the present time in the world for metrological purposes are monoenergetic neutron beams with energies of neutrons from 8 Kev up to 390 MeV. In the area of low energy use mainly two reactions7Li(p,n)7Be and45Sc(p,n)45Ti (Nagao, T.Matsumoto, et al. Monoenergetic and quasi-monoenergetic neutron reference fields in Japan. Radiation Measurements 45 (2010) 1076-1082. V.Lacoste. Review of radiation sources, calibration facilities and simulated workplace fields. Radiation Measurements 45 (2010) 1083-1089) and receive beams with energies of 2, 8, 24,27, 70 and 144 Kev.

For calibration of the detector dark matter required monoenergetic neutrons with energies from 10 to 100 Kev. The reaction7Li(p,n)7Be is the best for this purpose since the spectrum of the generated neutrons are relatively mild, the reaction cross section rises is is great, and that is actually the most important near threshold cross section is growing rapidly (Fig 1. - Cross section for the reaction7Li(p,n)7Be from the database ENDF7B-VII.1).

In this energy mainly receive beams of neutrons with an energy of 24 Kev. The main reason for the choice of this energy is the ability to use iron as an effective filter permeable to neutrons of this energy and the scattering from the other. Thus, in the minimum cross section for scattering of neutrons by nuclei56Fe is 5 10-4the barn, which is 4 orders of magnitude less than typical cross-section at other energies kiloelectronvolt range.

ISO 8529-1 provides two ways of production 24 Kev neutrons: (i) from a nuclear reactor with a filter made of iron and aluminum; (ii) an accelerator in the reaction45Sc(p,n)45Ti. In the first method, inevitably there are other energy components of the neutron, since the cross section for the scattering of neutrons by nuclei56Fe are failures not only at an energy of 24 Kev, but at energies 73 and 137 Kev, though not so deep.

The monochromaticity of the beam quantitatively described by the width of the energy distribution. Distribution width at half-height was measured at the neutron beam with an energy of 144 Kev and amounted to 14% (M. Yoshizawa, S. Shimizu, Y. Kajimotoet, al. Present Status of Calibration Facility of JAERI, Facility of Radiation Standads. Proceedings of Symposium - IRPA-11, Madrid, May 2004. 3b46 (2004)).

A method for production of 24 Kev neutrons from the accelerator in the reaction7Li(p,n)7Be using iron filter (T.Matsumoto, H.Harano, J. Nishiyama, et al. Novel generation method of the 24-keV monoenergetic neutrons using accelerators. Proc. of the 20thInternational Conference on the Application of Accelerator in Research and Industry, Fort Worth, Texas, USA, Aug. 10-15, 2008, AIP Conf. Proc. 1099 (2009), pp.924-927). When the energy of the proton beam 1,890 MeV neutrons are emitted forward within a 30° angle and have energy from 8 to 65 Kev. These neutrons injections in the set for the target iron filter thickness of 80 mm, which passes only the neurons with an energy of 24 Kev and effectively dissipates other neutrons. Compared to the reactor by way of the completion of 24 Kev neutrons, this method has the advantage that in the initial neutron beam no neutrons with energies 73 and 137 Kev, which can also skip the iron filter.

This method is the closest analogue is the prototype of the present invention.

The disadvantages of this method include the following:

1. The neutrons pass through the substrate of the target, which, though minimized in thickness, but inevitably leads to scattering and deformation of the neutron spectrum.

2. When injectioni neutrons with a wide energy range filter allows you to select neutrons certain energy, but does not completely eliminate neutrons with other energies.

3. Use filter limits the opportunity to get monoenergetic neutrons with different energy values.

The present invention is to provide a method ensuring receipt of monoenergetic neutron beam. The invention is based on the following.

When using monoenergetic proton beam assuming a thin target (the target is called thin if small changes in the proton energy when passing neuronogenesis layer) energy and emission angle neutron uniquely determined by the kinematics. Figure 2 presents the dependence of the energy of the neutron E from the angle Θ (in the laboratory coordinate system) at different proton energy (MeV shown in lines) in the reaction7Li(p,n)7Be. Angle 0° coincides with the direction of the proton beam (.Lee, X. Zhou. Thick target neutron yields for the7Li(p,n)7Be reaction near threshold. Nucl. Instr. Meth. 152 (1999) 1-11). It is seen that the energy of the proton beam above the reaction threshold 1,882 MeV, but below 1,920 MeV, neutrons are emitted only in the forward hemisphere and characterized by two monochromatic lines. When the proton energy above 1,920 MeV neutrons are emitted in all directions and are characterized by only one monoenergetic line.

This property makes is so improved method of manufacturing 24 Kev neutrons from the accelerator to the reaction 71 7Li(p,n)7Be using iron filter, namely to use flying back neutrons that allows you to go to really monoenergetic neutrons.

This solution is not limited to receiving only 24 Kev neutrons, and opens the possibility of obtaining monoenergetic neutrons with different energies.

In addition, this solution is more attractive because of emitted back from the lithium layer neutrons do not pass through the substrate, which inevitably leads to scattering and deformation of the neutron spectrum. This property does not impose restrictions on the thickness of the substrate of the target, which allows you to do it intensively cooled, to raise the power of the beam and, as a result, significantly increase the neutron flux density.

Thus, this object is achieved in that in a known way to obtain monoenergetic neutrons, including irradiation by protons neuronogenesis target, according to the invention, using a proton beam with energy greater than 1,920 MeV and a beam of monoenergetic neutrons form of neutrons propagating in the direction opposite to the direction of the

distribution of the proton beam, and varying the proton energy and emission angle, create a monochromatic neutron beam with any desired energy. When you do this:

not the thrones receive from the accelerator in the reaction 7Li(p,n)7Be;

- nitrogeneous layer of the target thin;

- in the path of the beam can be accommodated filter, scattering neutrons with other energies, accidentally caught in the beam. As filter materials can be used:56Fe for 24, 73 and 137 Kev,58Ni for 12 and 59 Kev;48Ti to 35 and 48 Kev;28Si for 54 and 145 Kev and32S to 74 Kev.

From Figure 2 it is seen that, by varying the proton energy and angle, you can create a monoenergetic neutron beams with any energy. Thus, when the observation angle of 110° neutrons with an energy of 24 Kev are obtained when the energy of the proton beam 1,977 MeV, and the neutron energy 77 Kev - when 2,070 MeV. You should pay attention that in this field is rather weak dependence of the energy on the angle and the energy of the protons, which will ensure high monochromaticity and stability.

The proposed method of producing monoenergetic beam of neutrons can be implemented on a device comprising a vacuum chamber, which is covered by the proton beam, and neuronogenesis target placed on the path of propagation of the proton beam, and a collimator that forms the desired beam.

Description of the method and operation of the device illustrated by Figure 3, where 1 is the proton beam, 2 - vacuum chamber, 3 - neuronogenesis target, 4 - beam monoenergetic neutrons, 5 - collima the PRS, 6 - window.

The method is as follows. Monoenergetic protons 1 energy higher than 1,920 MeV, extending in the vacuum chamber 2, fall on the target 3. The target consists of a substrate on which side of the proton beam napalan (caused) a thin layer of lithium. The interaction of protons with the nuclei of lithium leads to the generation of neutrons emitted in all directions. For beam shaping monoenergetic neutrons 4 collimator 5 is used neutrons emitted ago (relative to the direction of movement of protons). The energy of the neutrons in the beam is determined by the emission angle and energy of the proton beam. The monochromaticity of the beam is determined by the solid angle and thickness of the lithium layer.

The beam of protons with energies greater than 1,920 MeV with high monochromaticity and stability in vacuum can be obtained by using a particle accelerator (Kuznetsov A.S., Malyshkin G.N., Makarov, A.N., and other First experiments on neutrons at the accelerator source for boron neutron capture therapy. Technical physics letters, 2009, vol 35, issue 8, p.1-6).

Controlled application of a lithium layer thickness of several micrometers on a substrate of the target is carried out, for example, thermally (Brano, Avioral, and s.yu.taskaev. Measurement of the thickness of the lithium layer. Instruments and experimental techniques, 1 (2008), 160-162).

When energy ol the tones above 1,920 MeV neutrons are emitted in all directions. The neutron energy is determined by the emission angle and energy of the protons. So, with the energy of the proton beam 1,977 MeV at an angle of 110° emitted neutrons with an energy of 24 Kev, and at an energy 2,070 MeV - neutrons with energy 77 Kev.

The monochromaticity of the emitted neutrons is determined by the thickness of the lithium layer, as by passing layer, the protons are inhibited and the energy of the emitted neutrons is reduced. So, after the passage of 1 μm lithium energy of the proton is reduced, for example, with the initial 2,070 MeV 3.1 Kev Hydrogen stopping powers and ranges in all elements. Ed. by H. Andersen N.Y.: Pergamon Press Inc., 1977), and the energy emitted at an angle of 110° neutrons decreases by 1.5 Kev. Thus, the lithium layer thickness of 1 μm leads to 2% of the width of the energy distribution of neutrons.

Monochromaticity is also defined solid angle is at proton energy of 2,070 MeV and the angle of emission of 110° variation of the angle in 1° lead to changes in the neutron energy of 1.4 Kev, i.e. to 2% of the width of the energy distribution of neutrons.

The monochromaticity of the beam deteriorates with the passage through the wall of the vacuum chamber, but this effect is minimized by reducing the thickness of the camera in the passage of the neutron beam (box 6 in figure 3) or almost disappears for fixed energies due to the presence of dips in the cross-section of scattering. So, if all of the vacuum chamber or box from which otopleni of iron, they are transparent to neutrons with energies 24, 73 and 137 Kev.

The monochromaticity of the beam can be improved by setting a filter. When the transverse size equal to the size of the neutron beam, the filter will not prevent the migration of the required neutrons, but will dissipate all the others, randomly caught in the beam. Such filters can be made from iron, Nickel, titanium, silicon or sulfur.

For specific applications - calibration of the detector for dark matter with liquid argon as the working substance, can be implemented the following solution. The energy of protons is equal to 2,070 MeV emission angle of 110°. Emitted in this corner of the neutrons have energies 77 Kev. Neutrons with such energy have the largest cross-section of scattering on nuclei argon - 35 barn, in the range from 74 to 82 Kev the cross section exceeds 10 barn (Figure 4 - cross Section for scattering of a neutron by a nucleus argon-40 from the database ENDF/B-VII.1). The beam can be improved by putting on his way sulphur filter. Scattering of neutrons by nuclei of sulfur-32 is characterized by a deep and wide failure data the neutron energy - it is 3 order of magnitude smaller than typical values at 74 Kev and 2 orders in the range 71-77 Kev. In the scattering of a neutron by a nucleus of argon the last transmitted pulse, which leads to ionization of the substance. The transmitted pulse is uniquely determined by the tsya angle neutron scattering, which is by detecting the scattered neutron. Check recoil is an electroluminescent increased ionization signal enabling detection of extremely small magnitude of ionization up to one electron (Snapmap and A. Bernstein. Two-Phase Emission Detector for Measuring Coherent Neutrino-Nucleus Scattering. IEEE Transactions on Nuclear Science, Vol.51, No. 5, October 2004, 2151).

1. A method of producing a beam of monoenergetic neutrons, including irradiation by protons neuronogenesis target, characterized in that the use of proton beam with energy greater than 1,920 MeV and a beam of monoenergetic neutrons form of neutrons propagating in the direction opposite to the propagation direction of the proton beam, and varying the proton energy and angle of emission of neutrons, creating a monochromatic neutron beam with any desired energy.

2. The method according to claim 1, characterized in that the monochromaticity of the neutron beam regulate energy proton beam, choice solid angle of emission of neutrons and thickness neuronogenesis layer of the target.

Cab according to claim 1, characterized in that the neutron is obtained using particle accelerator.

4. The method according to claim 1, characterized in that the used target with a thin neuronogenesis layer.

5. The method according to claim 1, characterized in that d is I the exception of neutrons with other energies, accidentally caught in the beam, the beam can be accommodated filter.

6. The method according to claim 5, characterized in that the quality of the materials filters can be used:56Fe for 24, 73 and 137 Kev,58Ni for 12 and 59 Kev;48Ti to 35 and 48 Kev;28Si for 54 and 145 Kev and32S to 74 Kev.

7. Device for producing a beam of monoenergetic neutrons, comprising a vacuum chamber, which is covered by the proton beam, and neuronogenesis target placed on the path of propagation of the beam of protons, characterized in that in the path of the neutron beam propagating in the direction opposite to the direction of propagation of the beam of protons, it further comprises a collimator neutron beam.

8. The device according to claim 6, characterized in that the spread of the neutron beam in the wall of the chamber formed by the window, minimizing the scattering of neutrons with the required energy.

9. The method of calibration of the detector for dark matter with liquid Ar as the working substance by irradiation of a beam of monoenergetic neutrons with energies 74-82 Kev, obtained by irradiation of a target7Li(p,n)7Be a beam of protons with energy greater than 1,920 MeV, and is formed of neutrons propagating in the direction opposite to the direction of propagation of the beam of protons, using sulfur filter (32 S) with the consequences of the soup registration produced by ionization of liquid argon.



 

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