Development of Neutron Interferometer Using Multilayer Mirrors and Measurements of Neutron-Nuclear Scattering Length with Pulsed Neutron Source.

This study entailed the successful deployment of a novel neutron interferometer that utilizes multilayer mirrors. The apparatus facilitates a precise evaluation of the wavelength dependence of interference fringes utilizing a pulsed neutron source. Our interferometer achieved an impressive precision of 0.02 rad within a 20-min recording time. Compared to systems using silicon crystals, the measurement sensitivity was maintained even when using a simplified disturbance suppressor. By segregating beam paths entirely, we achieved successful measurements of neutron-nuclear scattering lengths across various samples. The values measured for Si, Al, and Ti were in agreement with those found in the literature, while V showed a disparity of 45%. This discrepancy may be attributable to impurities encountered in previous investigations. The accuracy of measurements can be enhanced further by mitigating systematic uncertainties that are associated with neutron wavelength, sample impurity, and thickness. This novel neutron interferometer enables us to measure fundamental parameters, such as the neutron-nuclear scattering length of materials, with a precision that surpasses that of conventional interferometers.

Single-shot optical imaging with spectrum circuit bridging timescales in high-speed photography.

Single-shot optical imaging based on ultrashort lasers has revealed nonrepetitive processes in subnanosecond timescales beyond the recording range of conventional high-speed cameras. However, nanosecond photography without sacrificing short exposure time and image quality is still missing because of the gap in recordable timescales between ultrafast optical imaging and high-speed electronic cameras. Here, we demonstrate nanosecond photography and ultrawide time-range high-speed photography using a spectrum circuit that produces interval-tunable pulse trains while keeping short pulse durations. We capture a shock wave propagating through a biological cell with a 1.5-ns frame interval and 44-ps exposure time while suppressing image blur. Furthermore, we observe femtosecond laser processing over multiple timescales (25-ps, 2.0-ns, and 1-ms frame intervals), showing that the plasma generated at the picosecond timescale affects subsequent shock wave formation at the nanosecond timescale. Our technique contributes to accumulating data of various fast processes for analysis and to analyzing multi-timescale phenomena as a series of physical processes.

Pendellösung interferometry probes the neutron charge radius, lattice dynamics, and fifth forces.

Structure factors describe how incident radiation is scattered from materials such as silicon and germanium and characterize the physical interaction between the material and scattered particles. We used neutron Pendellösung interferometry to make precision measurements of the (220) and (400) neutron-silicon structure factors and achieved a factor-of-four improvement in the (111) structure factor uncertainty. These data provide measurements of the silicon Debye-Waller factor at room temperature and the mean square neutron charge radius rn2=−0.1101±0.0089 square femtometers. Combined with existing measurements of the Debye-Waller factor and charge radius, the measured structure factors also improve constraints on the strength of a Yukawa modification to gravity by an order of magnitude over the 20 picometer­to­10 nanometer length scale range.

Application of precise neutron focusing mirrors for neutron reflectometry: latest results and future prospects.

Neutron reflectometry (NR) is a powerful tool for providing insight into the evolution of interfacial structures, for example via operando measurements for electrode-electrolyte interfaces, with a spatial resolution of nanometres. The time resolution of NR, which ranges from seconds to minutes depending on the reflection intensity, unfortunately remains low, particularly for small samples made of state-of-the-art materials even with the latest neutron reflectometers. To overcome this problem, a large-area focusing supermirror manufactured with ultra-precision machining has been employed to enhance the neutron flux at the sample, and a gain of approximately 100% in the neutron flux was achieved. Using this mirror, a reflectivity measurement was performed on a thin cathode film on an SrTiO3 substrate in contact with an electrolyte with a small area of 15 × 15 mm. The reflectivity data obtained with the focusing mirror were consistent with those without the mirror, but the acquisition time was shortened to half that of the original, which is an important milestone for rapid measurements with a limited reciprocal space. Furthermore, a method for further upgrades that will reveal the structural evolution with a wide reciprocal space is proposed, by applying this mirror for multi-incident-angle neutron reflectometry.

Sequentially timed all-optical mapping photography boosted by a branched 4f system with a slicing mirror.

We present sequentially timed all-optical mapping photography (STAMP) with a slicing mirror in a branched 4f system for an increased number of frames without sacrificing pixel resolution. The branched 4f system spectrally separates the laser light path into multiple paths by the slicing mirror placed in the Fourier plane. Fabricated by an ultra-precision end milling process, the slicing mirror has 18 mirror facets of differing mirror angles. We used the boosted STAMP to observe dynamics of laser ablation with two image sensors which captured 18 subsequent frames at a frame rate of 126 billion frames per second, demonstrating this technique's potential for imaging unexplored ultrafast non-repetitive phenomena.

Elliptic neutron-focusing supermirror for illuminating small samples in neutron reflectometry.

This paper details the development of a precise assembly of two supermirrors for neutron-focusing, designed for installation in neutron reflectometer SOFIA at BL16 in J-PARC MLF to intensify the illumination for small samples. The supermirrors are sputtered on two metal substrates, whose surfaces are coated with amorphous Ni-P plating, and are figured by diamond cutting and polished to subnanometer roughness. Special care is taken while polishing the substrates to reduce waviness and surface roughness for achieving a sharp focusing spot and uniform neutron reflectivity. The supermirror could converge the neutrons into a focal spot with a width of 0.13 mm in the full width at half maximum.

Measurement and alleviation of subsurface damage in a thick-crystal neutron interferometer.

The construction is described of a monolithic thick-crystal perfect silicon neutron interferometer using an ultra-high-precision grinding technique and a combination of annealing and chemical etching that differs from the construction of prior neutron interferometers. The interferometer is the second to have been annealed after machining and the first to be annealed prior to chemical etching. Monitoring the interference signal at each post-fabrication step provides a measurement of subsurface damage and its alleviation. In this case, the strain caused by subsurface damage manifests itself as a spatially varying angular misalignment between the two relevant volumes of the crystal and is reduced from ∼10-5 rad to ∼10-9 rad by way of annealing and chemical etching.

Characterization of all second-order nonlinear-optical coefficients of organic N-benzyl-2-methyl-4-nitroaniline crystal.

Full elements of second-order nonlinear optical (NLO) tensor can be completely characterized for an organic NLO crystal for the first time. As-grown bulk N-benzyl-2-methyl-4-nitroaniline (BNA) crystal was processed to expose (100) and (010) crystal orientations with fine optical surfaces by using precision lathe and diamond blade. Then, every five nonvanishing second-order NLO coefficient of BNA can be determined quantitatively using the precisely processed crystals based on 1st-kind Maker fringe measurements. Our method makes it possible to clarify uncertain NLO property of any organic materials and to accelerate application study via precise device fabrications even for fragile organic materials.

Development of precision elliptic neutron-focusing supermirror.

This paper details methods for the precision design and fabrication of neutron-focusing supermirrors, based on electroless nickel plating. We fabricated an elliptic mirror for neutron reflectometry, which is our second mirror improved from the first. The mirror is a 550-millimeter-long segmented mirror assembled using kinematic couplings, with each segment figured by diamond cutting, polished using colloidal silica, and supermirror coated through ion-beam sputtering. The mirror was evaluated with neutron beams, and the reflectivity was found to be 68-90% at a critical angle. The focusing width was 0.17 mm at the full width at half maximum.

Development of a large plano-elliptical neutron-focusing supermirror with metallic substrates.

Results of this study demonstrated that electroless nickel-phosphorus (NiP) plated metal substrate is an excellent material for producing large aspherical neutron-focusing supermirrors. A large plano-elliptical neutron-focusing supermirror comprising two metallic segments was fabricated using single-point diamond cutting, precision polishing and supermirror coating. The average surface roughness of the metallic substrates was approximately 0.3 nm rms. For evaluation, the focusing supermirror was installed at the SOFIA neutron reflectometer, showing high neutron reflectivity and giving minimal beam width of 0.34 mm in FWHM. Because of the large beam divergence accepted by the mirror, the count rate with the focusing mirror was 3.3 times higher than that obtained using conventional two-slit collimation.