Hirohiko M. Shimizu

Terahertz imaging with a direct detector based on superconducting tunnel junctions

We demonstrated terahertz imaging using a direct detector based on niobium superconducting tunnel junctions (STJs). The detector is composed of linearly distributed junctions placed on a superconducting microstrip line and is integrated on two wings of a log-periodic antenna. We succeeded nondestructive imaging for an integrated-circuit card and dry material using the detector around its sensitivity peak (∼0.66THz). The dynamic range was measured to be higher than 4×107 (76dB). Thus, the STJ detector is applicable to high-sensitivity and high-speed terahertz imaging for various nondestructive inspection applications.

Upgrade of the 32 m small-angle neutron scattering instrument SANS-U

The small-angle neutron scattering instrument, SANS-U, owned by the Institute for Solid State Physics, The University of Tokyo, has been upgraded. The SANS-U is a 32 m SANS instrument installed in 1991 at the guide hall of the JRR-3M research reactor of the Japan Atomic Energy Research Institute, and has been serving for inter-university cooperative research use since 1993. The major upgrades include (i) replacement of the two-dimensional area detector by a multi-wire type position-sensitive proportional counter, (ii) renewal of the operating system from a VAX and sequencers to an integrated PXI system controlled by LabVIEW-RT software, (iii) a focusing collimation system, and (iv) a variety of accessory equipment. These upgrades provide a wide dynamic range of neutron counting, user-friendly operation and real-time circular averaging of two-dimensional data.

Focusing and polarized neutron small-angle scattering spectrometer (SANS-J-II). The challenge of observation over length scales from an ångström to a micrometre

SANS-J (a pinhole small-angle neutron scattering spectrometer at research reactor JRR3, Tokai, Japan) was reconstructed as a focusing and polarized neutron small-angle scattering spectrometer (SANS-J-II). By employing focusing lenses of a biconcave MgF2 crystal or of a sextupole permanent magnet and a high-resolution photomultiplier, the minimum accessible magnitude of the scattering vector qmin was improved from 3 × 10−3 A−1 to an ultra-small-angle scattering (USAS) of 3 × 10−4 A−1. Compared with a Bonse–Hart double-crystal method, the advantages of focusing USAS are the efficient detection of anisotropic USAS with an area detector, an improvement in q resolution Δq/q at conventional magnitudes of the scattering vector q ~ 10−3 A−1 and a gain in neutron flux in the conventional q region of q ~ 10−3 A−1.

Measurement of cold neutron-beam focusing effect of a permanent sextupole magnet

Abstract The magnetic focus effect of cold neutron beam was measured using a permanent sextupole magnet and a gain of more than 35 was observed. The result was analyzed with a numerical simulation.

Development of a neutron detector based on a position-sensitive photomultiplier

A neutron scintillation detector based on a position-sensitive photomultiplier has been developed for neutron spin echo and small angle neutron scattering measurements. This photomultiplier has good spatial resolution, less than 1 mm2. The detection efficiency of gamma ray background is very low for using a thin ZnS/6LiF scintillator. The effective area of this detector is around 60 cm2.

Cold neutron beam focusing by a superconducting sextupole magnet

We have developed a superconducting sextupole magnet with about 50 mm in bore diameter to focus cold neutron beams with large cross-section and have investigated its focusing and polarizing effect. Large gain in neutron intensity and high neutron polarization have been obtained by numerical simulation for highly collimated neutron beams. We discuss applications of this magnet for small-angle neutron scattering experiments.

Development of scintillation imaging device for cold neutrons

Abstract As an application of the wavelength-shifting (WLS) fiber technique recently developed in the field of high-energy physics, a novel type of imaging device for neutrons has been successfully investigated; a space resolution of 1 mm FWHM with detection efficiency of 55% for 10 A neutrons has been experimentally confirmed with a prototype made of a 0.5-mm-thick ZnS(Ag)+6LiF scintillator plate optically coupled to WLS fiber arrays. In addition to promising results obtained in this study, its simple structure and reliable operation allow us to foresee a new generation of imaging devices to meet the increasing demand for large-area and high space-resolution imaging devices for several new projects on spallation neutron sources in the world.

Improved Fabrication Method for Nb/Al/AlOx/Al/Nb Superconducting Tunnel Junctions as X-Ray Detectors

Niobium-based superconducting tunnel junctions (STJs) with aluminum trapping layers as X-ray detectors were fabricated. We applied a fabrication technique for reducing the leakage current and improving the energy resolution for X-rays at around 6 keV. The edges of two Al layers sandwiching a tunneling barrier were oxidized by plasma discharge. The performance of these new STJs was compared with other types of STJs to which thermal oxidization in an O2 atmosphere had been applied. The oxidization process in O2 plasma significantly improved the energy resolution for X-rays, which reached values (FWHM) of 41 eV, 58 eV, 65 eV, and 129 eV for STJs with the size of 20×20 µm2, 100×100 µm2, 200×200 µm2, and 500×500 µm2, respectively.

A focusing-geometry small-angle neutron scattering instrument with a magnetic neutron lens

We have constructed a focusing-geometry small-angle neutron scattering (FSANS) instrument, SANS-J-II, with two kinds of neutron focusing device: a series of compound refractive lenses made of MgF2 and a magnetic neutron lens based on an extended Halbach-type sextupole magnet. In this study, we investigated the performance of the FSANS instrument with the magnetic neutron lens. The intensity distribution of a direct neutron beam focused on the detector plane by the magnetic neutron lens had a ratio of the peak height to the background level of \sim\!6\times 10^4 for a polarized neutron beam with a polarization degree of \sim \!0.99. It is found that a minimum value of the measurable q range [where q is the modulus of the scattering vector and is defined as q = (4\pi/\lambda)\sin(\theta/2), where \theta is the scattering angle and \lambda is the neutron wavelength], q_{\rm min}, of 6.5 \times 10^{-4} A−1 can be achieved by the FSANS instrument with the magnetic neutron lens using neutrons with \lambda = 6.6 A and \Delta \lambda/\lambda = 0.13 for the full width at half maximum.

Cold neutron imaging detection with a GSO scintillator

Abstract The pulse-height spectrum and two-dimensional image of a 0.5 mm thick GSO scintillator were investigated for a 6 A cold neutron beam. The 31 and 81 keV peaks resulting from neutron absorption by Gd nuclei were identified in the pulse-height spectrum by using a photomultiplier tube. Images of 1.5 and 2.1 mm (FWHM) in diameter were observed for 1 and 2 mm diameter incident beams with an image intensifier and viewed by a CCD camera, corresponding to a position resolution of 1.3 mm (FWHM). The result implies that a position resolution of better than 100 μm would be achievable by employing a GSO scintillator thinner than 20 μm .