Sunao Okamoto

Neutron Reflectivities of Ni-Mn and Ni-Ti Multilayers for Monochromators and Supermirrors

Multilayer films for neutron mirror were made by vacuum depositing thin films of Ni and Mn or Ni and Ti on a glass plate alternately. Their neutron reflectivities were measured as a function of neutron wavelength by the time-of-flight technique. For Ni-Mn multilayer monochromators, the numbers of layers necessary to give a nearly perfect reflection (≥90%) at the main Bragg peak, were found to be 7, 11 and 21 for layer thicknesses of 163, 112 and 83 A, respectively. The reflectivities obtained experimentally were compared with those calculated with the method applying the conventional optics, and their agreements were quantitatively reasonable. Moreover, for supermirrors, the numbers of layers and the distributions of layer thicknesses were examined in order to get a reflectivity of near unity down to the wavelength much shorter than the critical wavelength of a totally reflecting mirror of Ni. The reflectivity of a supermirror consisting of 160 layers of Ni and Ti was observed to behave like that of a tot...

Thermal neutron standard field with a Maxwellian distribution using the KUR heavy water facility

Abstract A heavy water facility attached to the KUR (Kyoto University Reactor, swimming pool type, 5 MW) yields pure themal neutrons with a Maxwellian distribution. The facility is placed next to the core of KUR and contains about 2 t of heavy water. The width of the heavy water layer is about 140 cm. The neutron spectrum was measured with the time-of-flight technique using a fast chopper. The measured spectrum was in good agreement with a Maxwellian distribution in the whole energy region for thermal neutrons. The neutron temperature was slightly higher than the heavy water temperature. The contamination of epithermal and fast neutrons caused by photo-neutrons from the γ-n reaction in heavy water is very small. The maximum intensity of thermal neutrons is 3 × 1011 n/cm·s. When a bismuth scatterer is attached, the gamma ray contamination is decreased to a ratio of 0.05 of gamma rays to neutrons in Rem. This standard neutron field has been used for such experiments as thermal neutron cross section measurement, diffusion length measurement, detector calibration, activation analysis and for biomedical purposes.

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.

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.

Design and performance of a wide band chopper for neutron small angle scattering

Abstract Characterization of a wide band neutron chopper, which can provide a wide range of wavelengths of slow neutrons in a pulse, was performed by computer calculations to obtain the optimum value of the coefficient of neutron utilization. This was done after detailed examinations of neutron burst shapes and transmission probabilities for various shapes of slits of the rotor with the shapes of the slits determined in accordance with the neutron trajectories in the rotating rotor. In the calculations, the divergence angles of incident neutrons are fully taken into account. A wide band chopper designed on the basis of the characterization to provide polychromatic pulsed neutrons in the range of 2–10 A for neutron small angle scattering (NSAS) has been installed at Kyoto University Research Reactor. The manufacture of the rotor and the experimental tests for the rotor have satisfactorily been performed. As an application of the chopper to NSAS, precipitation phenomena in the Ti-Nb alloy aged at 643 K were successfully examined as a function of aging time by means of the time-of-flight technique. In the NSAS, scattering patterns were simultaneously provided for a wide range of the scattering vector. It has been demonstrated that the coefficient of neutron utilization by the present chopper is improved by about 57 times at λ = 2 A and 12 times at 5 A, respectively, in comparison with the coefficient for a conventional chopper.

Numerical simulation of self-regulating characteristics of a cold neutron source with a closed-thermosiphon

Abstract A cold neutron source (CNS) having a closed-thermosiphon cooling loop shows a self-regulating characteristic regarding the heat load fluctuations if the moderator transfer tube fulfills certain conditions. A dynamic equation for the closed-thermosiphon type CNS having such a property has been presented on the basis of the non-equilibrium thermodynamics. Kyoto University Reactor (KUR) CNS was investigated by numerical simulation of this equation. The numerical predictions are in agreement with an experimental Bode diagram.

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.

Data acquisition system of a time-of-flight analyzer for use in a multi-detector assembly for neutron scattering experiments

Abstract A data acquisition system with various modern electronic devices was designed and tested for use in neutron time-of-flight (TOF) measurements with the aim to apply it to a neutron spectrometer with multiple neutron detectors. The system is principally composed of TOF logic units with a control unit and a conventional microcomputer. The memory cycle time of data into the main memory of the TOF logic unit is less than 500 ns. The TOF logic unit (main memory, 2048 ch, 24 bits/ch) demonstrates a higher counting efficiency (1.7 times per channel width) than a conventional TOF logic unit. The system is also designed to expand the number of the TOF logic units up to 128 units.

Slips in crystallites observed by neutron double Bragg reflections at small scattering angle

Abstract Crystallographic orientation dependency of slips in metallic crystallites due to low strain-amplitude loads has been examined by a new experimental method of neutron double Bragg reflection (D.B.R.) measurements with use of time-of-flight (TOF) method. The slips in single crystal of lead under static loads and also in polycrystals of lead and copper under cyclic loadings have been investigated. Coordination of the orientation of mosaic blocks within crystallites was clearly observed. It is shortly discussed that the results may be useful for studying strength of materials and also slip phenomena in aggregates of amorphous metals which have no the well-defined slip systems as in pure metals.

Nickel Mirror And Supermirror Neutron Guide Tubes At The Kyoto University Research Reactor

We installed the first nickel mirror neutron guide tube with a characteristic wavelength of 2.85 Å at Kyoto University research reactor(KUR, 5MW, cooled and moderated by light water) in 1973 and a supermirror guide tube with a characteristic wavelength of 1.17 Å in 1984, in order to get more intense thermal neutron beam. Four guide tubes are under construction at a cold neutron source installed in 1986. Two of them are supermirror type with a characteristic wavelength of 3 Å and the others are supermirror and Ni-mirror type with characteristic wavelengths of 6 Å and 23 Å, respectively. Supermirrors are made by automatically controlled vacuum deposition of nickel and titanium metal with electron gun. Their averaging reflectivity for the first supermirror guide tube are the following: The apparent critical wavelength, λ/θ, of reflection is 240 Å in term of wavelength(λ/θ) corresponding to the component of wave number perpendicular to the mirror surface. The reflectivity is 0.65 at the apparent critical wavelength and becomes higher with increasing neutron wavelength up to nearly unity for wavelength longer than 500 Å. Supermirror guide tubes are featured by more available neutrons with larger divergent angles and shorter length of the guide tubes. These features would bring us significant advantages depending on experimental requirements.