1. Slow Neutron Physics and Neutron Scattering

概要

INTRODUCTION: Neutron diffraction is a powerful tool to precisely determine the positions of light elements (H, Li, etc.) in solids. This is the main reason why neutron powder diffractometers are critical for structural investiga- tions of energy storage materials such as rechargeable lith- ium-ion batteries and hydrogen-absorbing materials. The B–3 beam port of Kyoto University Research Reactor (KUR) had long been used as a four-circle single-crystal neutron diffractometer (4CND). For the last decade, how- ever, the 4CND was so old that its research activity on neu- tron science was quite low. Nowadays, the versatile com- pact neutron diffractometer (VCND) has been installed in- stead of the 4CND, as shown in Fig. 1 [1]. The neutron wavelength, λ, which is monochromatized by the (220) plane of a Cu single crystal (i.e., Cu monochromator), is 1.0 Å. To cover the detector area of 6 º ≤ 2θ ≤ 130 º, twenty-five 3He tube detectors (1/2 inch in diameter) are used, where 2θ is the scattering angle. A detector bank in- cluding twenty-five 3He tube detectors is placed on the arm of the HUBER-440 goniometer. The distance from the Cu monochromator to the sample is approximately 2 m, and the distance from the sample to the detector is 1.2 m.

INTRODUCTION: Progress of neutron optical devic- es is very significant. We have established fabrication method for aspherical focusing supermirror with metal substrate [1-3]. The metallic substrate is robust and duc- tile, to which able to fabricate steeply curved surface with high form accuracy. It is also applicable to use under high radiation irradiation and high-temperature filed, even at a place close to the neutron target and moderator. Further- more, it is possible to fabricate a large focusing mirror by combining multiple segmented mirrors with mechanical fastening entailing the usage of screw holes and fixture tabs. We have solved the problem of required surface roughness for neutron mirror. The roughness should be smaller than 0.3 nm for high-m supermirror coating. Here m is the maximum critical angle of the mirror in units of critical angle of natural nickel. By using electroless nick- el-phosphorus (Ni-P) plating, we overcame the problem and are establishing fabrication process for aspherical focusing supermirror. There is still a problem of peeling off for high-m supermirror coating on metal substrate with steep curvature. It is also important to improve re- flectivity of the supermirror. In this study, we report a status of mass production for high-m neutron focusing supermirrors.

INTRODUCTION: Neutron interferometry is a pow- erful technique for studying fundamental physics. Nu- merous interesting experiments [1] have been performed since the first successful test of a single-crystal neutron interferometer [2]. However, the single-crystal interfer- ometer is inherently not able to deal with a neutron that has a wavelength longer than twice its lattice constant. In order to investigate problems of fundamental physics, including tests of quantum measurement theories and searches for non-Newtonian effects of gravitation, the interferometry of cold neutrons is extremely important, since the sensitivity of interferometer for small interac- tion increases with the neutron wavelength. A large scale of interferometer also has the advantage to increase the sensitivity to small interactions.
One of the solutions is an interferometer using neutron multilayer mirrors [3]. We succeeded in developing a multilayer interferometer for cold neutrons in which two paths are completely separated for the first time using wide-gap etalons [4]. We can easily control parameters such as Bragg angle, reflectivity, and Bragg peak width by selecting the deposited material and tuning the bilayer thickness and the number of layers.
We have started the development of multilayer interfer- ometer at the beamline 05 NOP in MLF. From 2019, we are continuing the experiments with etalons with mono- chromatic mirrors in order to demonstrate the perfor- mance of the interferometer. Figure 1 shows the interfer- ence fringes with etalons according to time-of-flight. The phase of interferogram depends on the wavelength of neutrons. We are testing the practical application of the interferometer. Neutron coherence scattering length of the material can be measured by inserting the sample into a path of the interferometer. The results of the trial meas- urements were consistent with the literature values.
Because the mirrors have narrow bandwidth of the neu- tron reflectivity, the number of neutrons contributing to the interference is limited. When the neutron supermir- rors whose lattice constants vary gradually are utilized in the interferometer, the effective range of neutron wave- length can be broadened to be applicable to a pulsed source. In addition, the wavelength dependence of the interactions can be measured simultaneously by using pulsed neutrons.

INTRODUCTION: Nuclear emulsion is a 3D-tracking detector for charged particles with a submicron resolution. It consists of silver halide crystals with diameter of sev- eral ten – several hundred nm. It works as a slow neutron detector by combining it with nuclides which absorb neu- trons and emit charged particles. A cold/ultracold neutron detector with spatial resolution less than 100 nm has been developed by combining nuclear emulsion with a thin converter layer including 10B[1]. An experiment (Exper- iment 1) was conducted at PF2, Institut Laue-Langevin (ILL) in order to obtain a spatial distribution of UCNs in the Earth’s gravitational field using a detector fabricated by coating a nuclear emulsion on a converter layer sput- tered by an ion beam sputtering system (KUR-IBS) in KURRI[2]. Nuclear emulsion can also be used for fun- damental studies of radiology such as that of proton bo- ron capture therapy (PBCT)[3]. A related experiment (Experiment 2) was conducted at CN-3 beam line. Stud- ies for applications of emulsion detectors to neutron im- aging are going on[4,5]. Also applications of fluorescent nuclear track detectors (FNTD) [9] to neutron imaging has been studied in parallel. An experiment with the latter detector (Experiment 3) was conducted at CN-3.

INTRODUCTION: Existence of non-zero permanent electric dipole moments (EDM) of the fundamental parti- cles violates time reversal symmetry. Under CPT conser- vation, T violation implies CP violation. Thus, a precise measurement of an EDM may reveal the origin of matter dominant universe. The TUCAN (TRIUMF Ultra-Cold Advanced Neutron source) collaboration aims to measure a neutron EDM with a sensitivity of 10-27 ecm, which is more than one order better sensitivity than the current best measurement.
The neutron EDM is measured by the precise measure- ment of spin precession frequency of neutrons. Ultra-Cold Neutrons, whose kinetic energy is less than a few 100 neV, is used for the measurement. One of the key components of the measurement is a spin analyzer of the UCN. Since the kinetic energy of an UCN is so low that magnetic po- tential can is used as a spin filter. When iron, which has a large saturation magnetization of 2.2 T, is used for the spin filter, the effective potential 𝑉𝑒𝑓𝑓 is 𝑉𝑒𝑓𝑓 = 𝑉𝐹𝑒 ∓ |𝜇| ∙ |𝐵| = 90 neV, or 330 neV Where 𝑉𝐹𝑒 = 210 neV is the Ferimi potential of the iron, μ = 60 neV/T is the magnetic moment of the neutron, and B = 2.2 T. Only one spin state of UCNs with kinetic energies between 90 neV to 330 neV can transmit the iron magnetic potential. Therefore, magnetized iron functions as an UCN spin filter. In order to reduce UCN absorption, the iron should be as thin as an order of 100 nm.

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