K. Soyama

AMATERAS: A Cold-Neutron Disk Chopper Spectrometer

AMATERAS is a new disk-chopper-type spectrometer installed at Materials and Life Science Experimental Facility (MLF) of J-PARC. AMATERAS is equipped with an extra chopper for pulse shaping at the upstream position, in addition to a monochromating chopper, which conventional chopper spectrometers at pulsed source have. Owing to the use of these choppers and the high peak intensity from a coupled moderator source at MLF, the AMATERAS design realizes high-intensity and high-energy-resolution measurements in quasielastic and inelastic neutron scattering experiments. The spectrometer had the first neutron beam in May 2009. During the course of commissioning, the performance of the spectrometer was confirmed by conducting test experiments. AMATERAS is now open to users and is producing scientific outputs.

High Intensity Chopper Spectrometer 4Seasons At J-parc

Design and performance of a new chopper spectrometer 4SEASONS, which is under construction for the spallation neutron source at Japan Proton Accelerator Research Complex (J-PARC), are described. This spectrometer has middle energy resolution and two orders higher performance than current state-of-the-art chopper spectrometers by utilizing a unique beam transport design and the repetition rate multiplication (RRM) method. 4SEASONS has a specification optimized for the study of the high critical temperature (T c) superconductivity, and should be suitable for collecting weak inelastic signals from a single crystal over a wide four-dimensional (4D) Q-ω space.

Figuring of plano-elliptical neutron focusing mirror by local wet etching

Local wet etching technique was proposed to fabricate high-performance aspherical mirrors. In this process, only the limited area facing to the small nozzle is removed by etching on objective surface. The desired objective shape is deterministically fabricated by performing the numerically controlled scanning of the nozzle head. Using the technique, a plano-elliptical mirror to focus the neutron beam was successfully fabricated with the figure accuracy of less than 0.5 microm and the focusing gain of 6. The strong and thin focused neutron beam is expected to be a useful tool for the analyses of various material properties.

Effect of interfacial roughness correlation on diffuse scattering intensity in a neutron supermirror

Neutron supermirrors are increasingly important devices for transporting, bending, and focusing neutron beams. Reflected neutrons from a supermirror are divided into specular and off-specular (diffuse) components. Suppression of the diffuse component is important since it reduces the signal-to-noise ratio, a serious problem when a supermirror is used in a focusing system for purposes such as small angle scattering measurements. The diffuse intensity can be decreased by more than one order of magnitude by adopting NiC/Ti multilayers instead of conventional Ni/Ti multilayers. In order to obtain insight into the mechanism that controls the diffuse intensity from a supermirror, the crystal structure of Ni and NiC monolayers and the interface structure of Ni/Ti and NiC/Ti multilayers were investigated. The crystallite size in the NiC monolayer was found to be smaller than that in the Ni monolayer by a factor of 4.1 by x-ray diffraction measurement. The interface structure of the Ni/Ti and NiC/Ti multilayers wa...

Development and demonstration of in-situ SEOP 3He spin filter system for neutron spin analyzer on the SHARAKU polarized neutron reflectometer at J-PARC

A new neutron reflectometer, SHARAKU, with a vertical sample-plane geometry was installed at beam line 17 at J-PARC Materials and Life Science Facility. Although a polarizing supermirror was previously installed as a neutron spin analyzer on SHARAKU, a 3He spin filter is advantageous because it can cover a large solid angle. An in-situ SEOP 3He spin filter system using a new compact laser unit has been developed for the analyzer. In this paper, we report a successful off-specular measurement with the new compact in-situ SEOP analyzer at SHARAKU.

One-dimensional neutron focusing with large beam divergence by 400mm-long elliptical supermirror

Reflective optics is one of the most useful techniques for focusing a neutron beam with a wide wavelength range since there is no chromatic aberration. Neutrons can be focused within a small area of less than 1 mm2 by high-performance aspherical supermirrors with high figure accuracy and a low smooth substrate surface and a multilayer interface. Increasing the mirror size is essential for increasing the focusing gain. We have developed a fabrication process that combines conventional precision grinding, HF dip etching, numerically controlled local wet etching (NC-LWE) figuring, low-pressure polishing and ion beam sputtering deposition of the supermirror coating to fabricate a large aspherical supermirror. We designed and fabricated an piano-elliptical mirror with large clear aperture size using the developed fabrication process. We obtained a figure error of 0.43 μm p-v and an rms roughness of less than 0.2 nm within an effective reflective length of 370 mm. A NiC/Ti supermirror with m = 4 was deposited on the substrate using ion beam sputtering equipment. The results of focusing experiments show that a focusing gain of 52 at the peak intensity was achieved compared with the case without focusing. Furthermore, the result of imaging plate measurements indicated that the FWHM focusing width of the fabricated mirror is 0.128 mm.

Detection of Fast Neutron by Storage Phosphors With Low Gamma-Ray Sensitivity

Storage phosphors such as SrBPO5:Eu2+ with low gamma-ray sensitivity were applied to fast neutron detection using polyethylene (PE) sheets as a proton generator that were set in front of the phosphors. The photostimulated luminescence (PSL) yields were measured by changing PE thickness when neutron energy was 5 MeV. The experimental results were compared with those by Monte Carlo simulation using PHITS code. The PSL yields by gamma-ray associated with fast neutron fields were estimated for both SrBPO5:Eu2+ sheets and commercial available imaging plates. The results showed that the imaging methods using such storage phosphors with low gamma-ray sensitivity had negligible gamma-ray influences and will be used for fast neutron imaging.

High-precision figured thin supermirror substrates for multiple neutron focusing device

An aspherical supermirror is one of the most useful neutron-focusing optics. We aim to develop multiple aspherical supermirror devices using high-precision figured aspherical focusing supermirrors to focus neutron beams with high intensities, because multiple mirrors collect a very large beam divergence. Thin mirrors with a millimeter thickness are required to minimize the absorption loss of incident neutron beams since the thickness of a mirror shadows the reflective area of the other mirrors. However, it is difficult to fabricate thin mirror substrates with a form accuracy of sub-micrometer level by conventional machining. Conventional machining deforms a substrate by machining force and spring back after machining causes figure error. Furthermore the deposition of supermirrors deforms the mirror substrate by film stress. Thus, we developed a new process of fabricating a precise millimeter-thick elliptical supermirror. This process consists of noncontact figuring by the numerically controlled local wet etching technique and the ion beam sputter deposition of NiC/Ti multilayers on both sides of the mirror substrate to compensate for film stress. In this paper, we report on the fabrication results and focusing performance of elliptical supermirrors with a thickness of 1.5 mm.

Simulation program for multiwire-type two-dimensional neutron detector with individual readout

We are currently developing a multiwire-type two-dimensional neutron detector system for use at the Materials and Life Science Experimental Facility at J-PARC. This system can attain a high response time and a high spatial resolution using the individual line readout method, and the performance parameters of the system strongly depend on a gas composition and pressure of a fill gas that is a gas mixture of He-3 and CF4. A simulation program for our system was developed to determine this gas condition. In addition, a small detector prototype was fabricated to evaluate the simulation program. The program involves the following calculations for each gas condition: the probability of a reaction between a neutron and He-3, the ranges of secondary particles generated by the nuclear reaction, the ejection angle of the particles, the wall effect caused by the conversion gap, and the pitch of the multiwire element. The simulation results agreed well with the experimental results obtained using the small detector prototype. Thus, it was confirmed that the simulation program can be effectively used to determine the gas composition and pressure when the required spatial resolution and detection efficiency are known.

Observation of neutron-induced signals using two-dimensional micro-pixel gas chamber

A gas-based neutron detection system which reads out individual channels with a micro-pixel detector element was constructed, and preliminary neutron irradiation tests were conducted. The detection system consisted of a micro-pixel detector element, a gas chamber that had feedthroughs for lines of 541 channels, amplifier-shaper-discriminator boards, position encoders with field programmable gate arrays, and a device capable of fast data acquisition. The micro-pixel detector element pulses had a short duration, the full width at half maximum of the pulse being 160 ns. Its gas gain was about 50 with an anode-cathode voltage of 560 V and a gas pressure of 0.5 MPa (0.45 MPa for He and 0.05 MPa for CF4). Signal pulse peaks of neutrons could be identified. The neutron detection system could easily distinguish neutron signals from other signals arising from electronic noise, gamma events, etc. The gas gain of the micro-pixel detector element did not change for 240 min in our irradiation test.