T. Ebisawa

A neutron spin echo spectrometer with two optimal field shape coils for neutron spin precession

Abstract We have designed and have been constructing at the C2−2 cold neutron guide port of JRR-3M, JAERI, a neutron spin echo spectrometer (NSE) which is equipped with two optimal field shape (OFS) coils for neutron spin precession with the maximum field integral of 0.22 T m, an assembly of position sensitive detectors (PSD), a converging polarizer and a wide area analyzer. The dynamic range of scattering vector Q covers from 0.005 A−1 to 0.2 A−1 and that of energy ħω from 10 neV to 30 μeV. Performance tests of the OFS coils show that the inhomogeneity of the magnetic field integral in the OFS coils with the spiral coils is so small that the NSE signal amplitude decreases little even for the neutron cross section of 30 ⊘mm diameter as the Fourier time t increases up to 25 ns, though the precession coils are close to iron covers of the neighboring neutron guide. This verifies that the OFS precession coils are appropriate for this NSE spectrometer. Another test experiment shows that the homogeneity condition of the precession magnet is loosened by use of PSD.

The neutron reflectometer (C3-1-2) at the JRR-3M reactor at JAERI

Abstract A neutron reflectometer has been installed at the cold neutron guide tube (C3-1-2) of the JRR-3M reactor at JAERI. Incident neutrons for the reflectometer have a long wavelength of 12.6 A, with a wavelength resolution of 3.2%. The reflectometer has the advantage of a large reflection angle which is appropriate for studies of low- q and off-specular phenomena. The neutron intensity reduction due to the long wavelength is substantially compensated by the relatively coarse beam divergence and wavelength resolution. The reflectometer is mounted in vertical geometry to yield a beam of 3 × 40 mm 2 and is applicable to measurements of mirror systems formed on flat substrates. Applications of the reflectometer to neutron optics and polymer studies are demonstrated and discussed.

Facility for the development of the multilayer neutron mirror at KUR

The facility for the development of the multilayer mirror as a device for handling slow neutrons is described. The facility consists of a vacuum evaporation system, a neutron reflectometer and a reflectivity calculation program. The properties of the evaporation are discussed. The neutron reflectometer makes use of the time-of-flight method and can measure the reflectivity down to 10−4. In the reflectivity calculation program, we adopt a neutron optical method with a modified optical potential, in which the contrast between layers is reduced, and the potential around the layer boundary is smoothed. It is shown that with a suitable choice of calculation parameters, experimental reflectivity of a supermirror is well reproduced by the calculation.

Development of a solid-state detector with a 6Li/Ti multilayer converter for ultracold neutrons

A new type of solid-state detector for ultracold neutrons has been developed and confirmed to be applicable at 4.2 K. 6Li/Ti multilayers, in which the effective potential is compensated, were formed as a neutron converter by vacuum evaporation on the surface of a silicon-surface-barrier detector. The detector was tested using a cold-neutron beam, and the reaction products in 6+Li+n → α + t were clearly observed in the pulse-height spectrum with a detection efficiency of 0.28% for 4A neutrons. This is consistent with the estimated value, and implies that ultracold neutrons can be detected with an efficiency of greater than 60%. The detector enables us to study the characteristics of ultracold neutrons in liquid helium, which is necessary to discuss the feasibility of an intense ultracold neutron source based on the superthermal method.

Multilayer mirror interferometer for very cold neutrons

Abstract The feasibility of a very cold neutron interferometer using multilayer mirrors is discussed. This device is a kind of Mach-Zehnder interferometer known in classical optics, using multilayer neutron monochromator mirror as optical elements. These mirrors consist of thin layers deposited on a very flat silicon multilayer substrate. Spacings of the multilayer are available in the range from 80 to 500 A. We discuss the characteristics of the mirror, the accuracy with which the mirror positions must be adjusted to their position and the perfection of the mirror including the present status of our development. Applications to physics are listed.

Neutron spin-echo spectrometer at JRR-3M

Abstract We have designed and have been constructing at the C 2−2 cold neutron guide port of JRR-3M, JAERI, a neutron spin echo spectrometer (NSE). It is equipped with two optimal-field-shape coils for neutron spin precession, a position-sensitive detector, a converging polarizer and a wide-area analyzer. The dynamic range of scattering vector Q covers from 0.005 A −1 to 0.2 A −1 and that of energy E from 10 neV to 30 μeV. In a performance test of the precession coils, the NSE signal amplitude P NSE decreased slightly for the neutron beam cross-section of 30 φ mm as the Fourier time t increases up to 25 ns. We have carried out NSE experiments on complex fluid systems and obtained their diffusion coefficient D eff .

Self-regulating characteristics of a cold neutron source with a closed-thermosiphon

Abstract This report describes self-regulating properties of the Kyoto University Reactor (KUR) Cold Neutron Source (CNS), which is cooled by a closed-thermosiphon loop of hydrogen, as are most of the CNS constructed recently. The circular moderator transfer tube is in a plane inclined about 14° to the horizontal plane. The hydrogen-helium cryogenic system of the CNS shows a self-regulating characteristic under thermal disturbances, if they are smaller than about 30% of the practical allowable heat load, 300 W at 25 K in our case. This self-regulating characteristic has been confirmed from Bodes diagram of the hydrogen pressure response in the reservoir tank to the heat load modulation. Due to this property, the liquid level in the moderator cell is kept almost constant during long term operation of the reactor. Measurements of the vertical distribution of the cold neutron flux from the moderator cell showed that a sufficient amount of liquid was stored in the cell and the ratio of the fraction of cold neutrons in the cold moderator to that in the ambient moderator was measured to be about 20 where the wavelengths are longer than 6 A using liquid hydrogen as a moderator.

Development of Drabkin energy filters for JSNS project

Abstract We have a plan to install Drabkin energy filters for neutron spectrometers at the Japanese Spallation Neutron Source (JSNS) project. Combined with a high-intensity coupled moderator, a Drabkin energy filter could be very useful to obtain a high-resolution polarized beam with higher intensity than a beam from a decoupled moderator. Hence, it could bring about a breakthrough for more efficient use of neutrons at neutron spectrometers. We have developed a Drabkin energy filter which has 10 periods of spatially oscillating magnetic field and measured spin–flip probabilities as functions of field strength and of neutron wavelength. In this paper, results of the performance tests of Drabkin energy filters are presented and discussed.

Development of a modified neutron spin echo spectrometer using multilayer spin splitters

Abstract A multilayer spin splitter (MSS) is a device which gives the same effect as a magnetic field to the Larmor-precessing neutron. Since MSS is a single mirror, a neutron spin echo (NSE) can be much smaller (less than 1 m along the neutron beam) when MSSs successfully replace the Larmor precession magnetic field. The aim of our research is to replace the precession magnetic fields in NSE spectrometer by a set of MSSs and realize a very compact neutron spin echo spectrometer. In the present report, we will present the results of the preliminary experiment of the compact NSE spectrometer, and discuss its applicability to the pulsed neutron sources.

Very cold neutron facility in horizontal arrangement with deuterium cold source

Abstract A very cold neutron guide tube with the characteristic wavelength of 23.5 A was horizontally arranged in combination with the deuterium cold source. The in-pile portion of the guide tube consists of nickel evaporated aluminum mirrors, and the external portion is made of ordinary nickel coated glass mirrors as a curved guide. The VCN output intensity and the energy spectrum are reported. The output is used for ultracold neutron production with a supermirror turbine and VCN interferometer study.