The HANARO neutron reflectometer with horizontal sample geometry; engineering designs and performance simulation

A new neutron reflectometer with a horizontal sample geometry was designed and is now under construction at the HANARO, 30 MW research reactor. It was originally built and operated at the H9-A beam port at BNL, and was relocated to HANARO in 2004. We performed simulations of neutron ray-tracing to evaluate the performance of all of the optical components of the instrument with a Monte Carlo technique using McStas code. The feasible wavelength of the incident neutron beam is 2.52 Å. It produces a q-range up to 0.126 Å^− 1 with a supermirror as a deflector. Our studies indicated possibilities to improve the performance of the guide tube. Although the performance is limited (limited q-range and flux due to multiple reflections prior to the deflector), it promises to be the first reflectometer in Korea for the study of free surfaces, which is currently in demand.     

Scatter rejection in quantitative thermal and cold neutron imaging

The accuracy of quantitative neutron transmission radiography can be substantially decreased if highly scattering materials, such as water or plastics, exist in the sample. There are currently two main solutions to this problem: either performing experiments at a large distance between the detector and the sample or employ some numerical correction techniques. In the former case, the spatial resolution is substantially reduced by the limited beam divergence, while the latter correction requires a priori information about the sample and is limited to distances of above ∼2 cm. We demonstrate the feasibility of another technique, namely the possibility to remove the scattered neutron component from the transmitted neutron beam by a very compact polycapillary collimator. These ∼1 mm thick devices can be placed between the sample and the detector and remove most of the neutrons scattered at angles larger than the acceptance angle of the collimator (typically 1°). No image distortions above ∼10 μm scales are introduced by these collimators. The neutron transmission of highly scattering samples (water and plexiglass) is measured in our experiments with and without scatter rejection. In the latter case, the accuracy of measured transmission coefficient was substantially improved by our collimators.     

Measurement of neutron flux and beam divergence at the cold neutron guide system of the new Munich research reactor FRM-II

A sophisticated neutron guide system has been installed at the new Munich neutron source FRM-II to transport neutrons from the D₂ cold neutron source to several instruments, which are situated in a separate neutron guide hall. The guide system takes advantage of supermirror coatings and includes a worldwide unique “twisted” guide for a desired phase space transformation of the neutron beam. During the initial reactor commissioning in summer 2004, the integral and differential neutron flux as well as the distribution of beam divergence at the exit of two representative and the twisted neutron guide were measured using time-of-flight spectroscopy and gold-foil activation. The experimental results can be compared to extensive simulation calculations based on MCNP and McStas. The investigated guides fulfill the expectations of providing high neutron fluxes and reveal good quality with respect to the reflective coatings and the installation precision.     

Test of He-based neutron polarizers at NIST

Neutron spin filters based on polarized ³He are useful over a wide neutron energy range and have a large angular acceptance among other advantages. Two optical pumping methods, spin-exchange and metastability-exchange, can produce the volume of highly polarized ³He gas required for such neutron spin filters. We report a test of polarizers based on each of these two methods on a new cold, monochromatic neutron beam line at the NIST Center for Neutron Research.     

Spin filters and supermirrors: a comparison study of two methods of high-precision neutron polarisation analysis

The precise knowledge of the neutron polarisation is needed in tests of the electroweak Standard Model using angular correlations in polarised neutron beta decay. We performed an experimental comparison study of two different methods of polarisation analysis of a cold neutron beam which are based on spin-dependent reflection, respectively, transmission. The compared devices are a supermirror analyser used in prior neutron decay studies and an opaque transmission spin filter of either polarised ³He or polarised protons. The results of the neutron polarisation measured with the supermirror analyser and with the spin filter coincided in three different experiments within 0.1–0.2%.     

Performance test of neutron resonance spin echo at a pulsed source

Neutron resonance spin echo (NRSE) spectroscopy enables us to measure neutron quasielastic scattering with high-energy resolution. It is desirable to apply NRSE spectroscopy to pulsed neutron sources because this application allows a very wide range of the spin echo time. We have already developed the neutron resonance spin flipper applicable to a polychromatic pulsed neutron beam, which is necessary for the TOF–NRSE method. Using this flipper, we have succeeded in observing spin echo signals with visibility higher than 0.65 for the pulsed neutron beam with wavelength from 0.3 to 0.9 nm. We discuss the prospect of the NRSE spectrometer with high-energy resolution on the basis of the present result.     

Basic features of the upgraded S18 neutron interferometer set-up at ILL

The perfect crystal interferometer instrument S18 at the Institute Laue-Langevin (ILL) in Grenoble has been upgraded to allow more advanced neutron optics experiments for fundamental, nuclear and condensed matter physics. The new supermirror guide together with the multipurpose monochromator provides considerably higher intensities in a wide wavelength region. The optimal use of neutrons is obtained by a nondispersive arrangement of the monochromator and the interferometer crystals. This also allows to obtain completely polarised beams using permanent magnetic prism deflection. An additional third analyzer axis permits novel postselection experiments concerning momentum distribution and polarisation analysis of the interfering beams. Several types of large perfect crystal interferometers are available for different applications. The system can be configured as an advanced high-resolution Bonse–Hart small angle scattering camera. The results of various test measurements concerning intensities, interference contrast, long-term stability, the accessible wavelength range and the basic features as a SANS camera will be presented. Various proposals for experiments will be discussed as well.     

T Violation in the weak scalar and tensor interaction: neutron and nuclei

The motivation and status of a search for time-reversal violation in nuclear and neutron beta decay are discussed. A new experiment for free neutron decay is proposed. A hitherto unmeasured amplitude, R, of the directional correlation J·( p×σ), between the neutron spin J, the electron momentum p and the electron spin σ, will be determined. An accuracy well below 0.01 can be achieved using an intense cold neutron beam and an electron tracking detector, where the spin sensitivity is provided by large angle Mott scattering. This study provides an unique access to the exotic scalar S and tensor T interaction. Finite values, or tight constraints on the time-reversal violating scalar components, can be deduced in a combined analysis of the proposed experiment and a precise determination of the tensor couplings from a recent study of ⁸Li decay. The great interest in weak scalar interaction is stimulated by a multitude of scalar bosons which are introduced in most extensions of the Standard Model.     

Systematic analysis of neutron yields from thick targets bombarded by heavy ions and protons with moving source model

A simple phenomenological analysis using the moving source model has been performed on the neutron energy spectra produced by bombarding thick targets with high energy heavy ions which have been systematically measured at the Heavy-Ion Medical Accelerator (HIMAC) facility (located in Chiba, Japan) of the National Institute of Radiological Sciences (NIRS). For the bombardment of both heavy ions and protons in the energy region of 100–500 MeV per nucleon, the moving source model incorporating the knock-on process could be generally successful in reproducing the measured neutron spectra within a factor of two margin of accuracy. This phenomenological analytical equation is expressed having several parameters as functions of atomic number Z_p, mass number A_p, energy per nucleon E_p for projectile, and atomic number Z_T, mass number A_T for target. By inputting these basic data for projectile and target into this equation we can easily estimate the secondary neutron energy spectra at an emission angle of 0–90° for bombardment with heavy ions and protons in the aforementioned energy region. This method will be quite useful to estimate the neutron source term in the neutron shielding design of high energy proton and heavy ion accelerators.     

Development of neutron radiography facility for boiling two-phase flow experiment in Kyoto University Research Reactor

To visualize boiling two-phase flow at high heat flux by using neutron radiography, a new neutron radiography facility was developed in the B-4 beam hole of KUR. The B-4 beam hole is equipped with a supermirror neutron guide tube with a characteristic wavelength of 1.2 Å, whose geometrical parameters of the guide tube are: 11.7 m total length and 10 mm wide ×74 mm high beam cross-section. The total neutron flux obtained from the KUR supermirror guide tube is about 5×10⁷ n/cm² s with a nominal thermal output of 5 MW of KUR, which is about 100 times what is obtainable with the conventional KUR neutron radiography facility (E-2 beam hole). In this study a new imaging device, an electric power supply (1200 A, 20 V), and a thermal hydraulic loop were installed. The neutron source, the beam tube, and the radiography rooms are described in detail and the preliminary images obtained at the developed facility are shown.     

Specular reflection of thermal neutrons from Gd-containing layers and optimization of antireflective underlayers for polarizing coatings

A detailed analysis of specular neutron reflection from an absorbing medium is given. The experimental studies revealed that oxidation and roughness of the surface are the main factors that determine the neutron reflectivity of Gd-containing layers. It is concluded that the empirical approach used at present does not guarantee optimization of underlayer parameters (composition and thickness) and technological regimes. An algorithm of optimization is proposed, in which account is taken of the substrate potential, the dependence of the underlayer potential on the thermal neutron wavelength, the polarizing coating imperfections that enhance reflection of neutrons with the undesired spin. The antireflective TiZrGd underlayer for CoFe/TiZr supermirrors produced at the magnetron facility DIOGEN (PNPI, Gatchina) is optimized.     

Neutron diffraction measurements of residual stresses in a 50 mm thick weld

Residual stresses were determined through the thickness of a 50 mm thick ferrite steel weld plate using neutron diffraction. Whereas the limiting penetration depth for iron-based alloys is about 25 mm in the most typical neutron diffractometers, we significantly enhanced the penetration depth up to 50 mm with 2 mm spatial resolution by using the neutron wavelength of 2.39 Å. The selected wavelength minimizes the total neutron cross-section and beam attenuation, thereby, maximizes the neutron fluxes at depth. Two-dimensional mapping of the residual stresses shows that significant amounts of the tensile longitudinal stresses (over 90% of yield strength) were developed along the heat-affected zone of the weld.     

The high-resolution time-of-flight spectrometer TOFTOF

The TOFTOF spectrometer is a multi-disc chopper time-of-flight spectrometer for cold neutrons at the research neutron source Heinz Maier-Leibnitz (FRM II). After five reactor cycles of routine operation the characteristics of the instrument are reported in this article. The spectrometer features an excellent signal to background ratio due to its remote position in the neutron guide hall, an elaborated shielding concept and an s-shaped curved primary neutron guide which acts i.a. as a neutron velocity filter. The spectrometer is fed with neutrons from the undermoderated cold neutron source of the FRM II leading to a total neutron flux of 10¹⁰n/cm²/s in the continuous white beam at the sample position distributed over a continuous and particularly broad wavelength spectrum. A high energy resolution is achieved by the use of high speed chopper discs made of carbon-fiber-reinforced plastic. In the combination of intensity, resolution and signal to background ratio the spectrometer offers new scientific prospects in the fields of inelastic and quasielastic neutron scattering.     

On coherence in neutron imaging

The variety of imaging signals in neutron radiography and tomography became quite large compared to the pure absorption and scattering contrast in neutron radiographies and topographies in the early sixties or seventies of the last century. The diversity of absorption based techniques for neutron radiography and tomography is comparable to coherence based imaging techniques such as phase contrast, differential phase contrast, dark field imaging, diffraction enhanced contrast, refraction contrast, ultra small angle scattering contrast, grating interferometry and crystal interferometry, also the spin of the neutron was successfully used for imaging [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11] and [12]. We show which effects (total reflection, diffraction, refraction) contribute to e.g. a step boundary or a phase boundary. Taking this simple object, one can learn to understand the imaging procedure and what is displayed in a radiograph.     

A new neutron monitor and extended conversion coefficients for HP(10)

A new personal dose equivalent monitor for neutrons, the 'HpSLAB', is introduced. The device consists of a 30x30x15 cm3 polymethyl-methacrylate slab hosting a superheated drop detector embedded at a depth of 10 mm. The personal dose equivalent monitor was characterised experimentally with fast neutron calibrations in the 0.144-14.8 MeV range and numerically with Monte Carlo simulations. In order to evaluate the performance of the device, its response was compared to the fluence-to-directional dose equivalent conversion coefficients, hp(10;alpha,E). Since published coefficients only cover neutron angles of incidence up to 75 degrees, a new extended set of coefficients was computed for angles of incidence up to 180 degrees. The method used in these calculations was the very same used in the generation of the dose equivalent coefficients recommended by International Commission on Radiological Protection publication 74. The response of the HpSLAB follows with good approximation the trend of the conversion coefficients for monoenergetic neutrons above approximately 0.5 MeV. The device was extensively tested in broad-spectrum workplace-fields encountered at nuclear installations and its response was on average within a factor 1.4 of the reference personal dose equivalent values, regardless of angle and energy distribution of the neutron fluence.     

A highly adaptive detector system for high resolution neutron imaging

An optimized detector system that allows high-resolution neutron imaging with desired flexibility is described. The presented system can be adapted and integrated with standard CCD-based neutron detectors. Novel neutron scintillating materials with good photon discrimination and optical lens components are tested and optimized for high-resolution neutron tomographic purposes. The presented detector system provides variable field of view and can be used in combination with different techniques, including dark-field, energy-selective, and neutron spin polarized imaging.     

Wavelength dependent neutron transmission and radiography investigations of the high temperature behaviour of materials applied in nuclear fuel and control rod claddings

Neutron radiography was used for the investigation of the nuclear fuel and control rod cladding behaviour during steam oxidation under severe nuclear accident conditions. In order to verify the hypothesis that the unexpectedly high neutron cross-section found after oxidation of Zircaloy-4 in wet air containing 10% steam is caused by a strong hydrogen uptake, the wavelength dependence of the total macroscopic neutron cross-section of the specimens was measured. The characteristic dependence for hydrogen was not found, which is a proof that hydrogen is not absorbed significantly. The data agree mostly with the behaviour expected for β-Zr.Examinations of control rod simulators annealed until the failure in single-rod tests were performed. In order to separate the effect of the neutron absorber and control rod structure materials, radiographs taken with different neutron spectra were combined. This procedure clearly showed that the local melting resulting from the eutectic reaction between the stainless steel control rod cladding and the Zircaloy-4 guide tube is the reason for the failure.     

Neutron response function for a detector with He counters for the 0.39–1.54 MeV neutron energy range

An experimental study was carried out on the neutron response functions of two neutron detector arrays consisting of 39 ³He proportional counters with a polyethylene moderator for monoenergetic neutrons within the 0.39–1.54 MeV neutron energy range. Experimental data on the sensitivity of neutron counting to a change in neutron energy and the influence of the thickness of polyethylene moderator were obtained. The experimental efficiency curves were compared with the calculated response functions generated by a neutron transport code.     

An acceptance diagram analysis of the contaminant pulse removal problem with direct geometry neutron chopper spectrometers

Phased choppers are used to produce pulsed beams of monochromatic neutrons at research reactors and spallation neutron sources. Depending on the design of the instrument, it is very possible that the choppers will transmit neutrons with wavelengths other than those within the desired band of wavelengths. One or more additional choppers are typically needed to remove these contaminant pulses. We describe a method of determining the wavelength- and time-dependent transmission of a system of choppers using acceptance diagrams. The method is illustrated with calculations for the Disk Chopper Spectrometer at the NIST Center for Neutron Research and the proposed Cold Neutron Chopper Spectrometer at the Spallation Neutron Source (Oak Ridge, TN).