Neutron Guides Research Articles

Transmission measurements of guides for ultra-cold neutrons using UCN capture activation analysis of vanadium

The efficient transport of ultra-cold neutrons (UCN) from their source to the experimental site is a major issue for various kinds of precision experiments. Neutron guides often have to transport the UCN several tens of meters with acceptable losses. In order to qualify the guides, their UCN transmission properties have to be determined with high precision. For this purpose we have developed a novel method. The transmitted UCN were absorbed at the end of the guide in a vanadium disc producing the β-emitter 52 V (half life 3.74 min). The intensity of the 1434 keV γ-ray following the β-decay was measured. UCN guides of non-magnetic nickel alloys made by the replication technique were studied. They show a high Fermi pseudopotential V F for UCN and a low surface roughness. For these guides the transmission per meter was determined with a relative error of ±0.6%, resulting in values above 0.95/m. By an absolute calibration of the γ-ray detection system we also deduced the absolute value of the UCN current absorbed in the vanadium plate. Possible applications of this method are discussed.

Wombat and Echidna: The Powder Diffractometers

The OPAL reactor has two thermal neutron guides that feed the guide hall. The first of these, TG1, supplies thermal neutrons to Wombat and Echidna, the two powder diffractometers, shown in Figure 1. TG1 is a 300 mm high × 50 mm wide guide with coatings of m = 2, 2.5, or 3 on the side plates of various sections, providing excellent transport of the neutrons from the reactor face to the instruments (at the Wombat monochromator position the flux is 2.83 × 109ncm2s−1).

Accurate simulation of neutrons in less than one minute Pt. 1: Acceptance diagram shading

We describe an algorithm for modelling neutron guides based on an extension of acceptance diagram methods: neutron acceptance diagram shading (NADS), using a computer graphical approach. It has a similar black-box, modular strategy to Monte-Carlo simulations, but often outperforms Monte-Carlo by orders of magnitude in terms of calculation speed without sacrificing significant detail. NADS is expected to increase the productivity in the design stage of neutron instrument development, and facilitate the widespread application of advanced, modern artificial-intelligence-based optimisation algorithms for the design of neutron instrumentation.

Recent developments of prompt gamma activation analysis at Budapest

The PGAA facility at the Budapest Research Reactor has been continually upgraded and developed since its start-up in 1996, as a result of which its performance has improved considerably. The installation of the cold neutron source, the partial change to supermirror neutron guides and their realignment increased the flux by almost two orders of magnitude. The data acquisition has been modernized as well; digital spectrometers were tested and implemented in novel forms of gamma-ray spectrum collection. This year a higher-efficiency HPGe detector and a new data acquisition module were put into operation. Most recently all the neutron guides were changed to supermirror-coated ones to further increase the neutron flux. The improved evaluation software makes possible a more reliable elemental analysis of the samples. In this progress report these developments are critically reviewed. The characteristics of the latest system are also described. It is the first time that a set of new partial gamma-ray production cross sections are presented, which are based on the new intensity values of 14N(n,γ)15N calibration standard.

Focusing neutron guides for VULCAN—Design aspects, estimated performance, and detector deployment

The neutron optics design and performance for VULCAN—the SNS engineering diffractometer—is presented. A gradually tapered focusing neutron guide is used to deliver desired beam divergence at sample position. Selection of instrument resolution is achieved with an interchangeable guide–collimator system in the last 3 m of the neutron guide system. The effect of focusing on instrument resolution was evaluated and the strategy for detector deployment is discussed.

Novel neutron guides

For almost three decades neutron guides have been built as multipurpose installations: essentially straight and with a rectangular cross section. At that time instruments on guides were planned and designed from the guide exit to the detectors. With

POWTEX – the high-intensity time-of-flight diffractometer at FRM II for structure analysis of polycrystalline materials

In order to provide the large chemistry and materials science as well as the geosciences communities with a powerful tool for rapid data acquisition, a time-of-flight powder diffractometer to be installed at the new Munich reactor has been designed. The time-of-flight technique is expected to outperform a monochromator instrument by at least an order of magnitude in data acquisition time, particularly on small samples of less than a cubic centimetre. The construction of this innovative type of diffractometer utilizes modern components such as focusing super-mirror neutron guides, a four-unit high-speed disk chopper system and linear position-sensitive detectors covering a solid angle of about 2π steradian. The diffractometer design enables an easy enlargement of the focal spot size and is therefore equally well suited for the texture analysis of large geological and archaeological samples.

Monte Carlo simulations of the SANS instrument in Petten

Abstract The small-angle neutron-scattering facility at the 45 MW high-flux reactor in Petten, The Netherlands, was constructed in the late 1980s. It has a q -range of 5×10 −3 to 0.4 A −1 , operating at a fixed wavelength of 4.75 A, which is realized by six pairs of double pyrolytic graphite monochromators. In this paper, we study the flux gain for the instrument installed at a neutron guide by Monte Carlo simulations using the program packages McStas [L. Lefmann, K. Nielsen, Neutron News 10 (1999) 320; P. Willendrup, E. Farhi and K. Lefmann, Physica B 350 (2004) 735] and Vitess [G. Zsigmond et al., Nucl. Instrum. Methods A 529 (2004) 218; http://www.hmi.de/projects/ess/vitess/ ]. In doing so, the instrument is relocated from its current position to the HB10 radial beam tube, the double monochromator is replaced by a velocity selector and neutron guides are used for transporting the neutrons.

Elliptic neutron guides—focusing on tiny samples

We have investigated the focusing properties of the prototype of a transportable elliptic neutron guide. We demonstrate that the elliptic guide allows to map a point-like neutron source in one focal point on a tiny spot in the other focal point. We also show that an elliptically tapered neutron guide increases the neutron flux density in the focal point by at least one order of magnitude compared to the flux density of the original neutron beam. This allows to use neutron beams more efficiently in experiments with small samples. We used the elliptic neutron guide to focus a several centimeters wide neutron beam on a tiny NiS 2 sample with the dimensions 0.1 mm 3 and easily detected the ordered moment of this antiferromagnetic insulator. This suggests that many more small samples, in particular in high pressure experiments, can be studied with elliptic neutron guides.

Neutron guide system for small-angle neutron scattering instruments of the Jülich Centre for Neutron Science at the FRM-II

Following the shut-down of the FRJ-2 research reactor in Jülich a large part of the neutron scattering instrumentation operating there is currently being moved to the FRM-II research reactor in Garching-München. The installation of these instruments requires the design and set-up of new neutron guides with geometrical and optical features imposed by the positioning of the instruments in the neutron guide hall and by the foreseen significant improvement of the instrument performance. Particularly three SANS diffractometers require a special approach due to on one hand, their pre-determined size and on the other hand, the demanded neutron wavelength range. Expected characteristics of three neutron guides (currently under construction) optimized using VITESS and McStas simulation packages, namely the vertically “S-shaped” guides serving the KWS2 and KWS1 conventional SANS instruments and the horizontally “S-shaped” guide serving the focusing KWS3 instrument, will be reported on.

Non-magnetic supermirrors produced at Mirrotron Ltd.

Non-magnetic neutron guides are needed where polarized neutrons are to be transmitted. Mirrotron Ltd. is able to produce guides of standard length and width coated with non-magnetic Ni(Mo)/Ti supermirrors. Hot-pressed and vacuum-melted Ni(Mo) targets were tried out for the sputtering process. High-quality Ni(Mo)/Ti supermirrors with various critical angles were produced. The reflectivity at the critical angle was 0.97, 0.95, 0.92, 0.9 and 0.85 for m 1.2, 1.5, 2, 2.5 and 3, respectively. The non-magnetic properties of the mirrors were proved at HMI using polarized neutrons.

First production of ultracold neutrons with a solid deuterium source at the pulsed reactor TRIGA Mainz⋆

The production rates of ultracold neutrons (UCN) with a solid deuterium converter have been measured at the pulsed reactor TRIGA Mainz. Exposed to a thermal neutron fluence of $\ensuremath \sim 1\cdot 10^{13}$ n·cm^-2·pulse^-1, the number of detected very cold and ultracold neutrons ranges up to 200 000 at 7mol of solid deuterium (sD2) in combination with a pre-moderator (mesitylene). About 50% of the measured neutrons can be assigned to UCN with energies E of $\ensuremath V_{\rm F}({\rm sD}_2)\leq E \leq V_{\rm F}{\rm (guide)}$ where V F(sD 2) = 105 neV and V F(guide) = 190 neV are the Fermi potentials of the sD2 converter and our stainless steel neutron guides, respectively. Thermal cycling of solid deuterium, which was frozen out from the gas phase, considerably improved the UCN yield, in particular at higher amounts of sD2.

Replika mirrors—Nearly loss-free guides for ultracold neutrons—Measurement technique and first preliminary results

The reflectivity of ultracold neutron (UCN) guides produced with a dedicated technique called Replika has been studied. The guides are made of nickel, where the surface quality was copied from a glass layer. This results in a surface roughness smaller than 10 Å. The reflectivity was measured to be 99.9 (1)%. Those guides can be used at present or future UCN sources to transport UCN over long distances to the respective experiments without significant losses.

Diamond-like carbon coated ultracold neutron guides

It has been shown recently that diamond-like carbon (DLC) with a sp 3 fraction above 60% is a better wall coating material for ultracold neutron applications than beryllium. We report on results of Raman spectroscopic and XPS measurements obtained for diamond-like carbon coated neutron guides produced in a new facility, which is based on pulsed laser deposition at 193 nm. For diamond-like carbon coatings on small stainless steel substrates we find sp 3 fractions in the range from 60 to 70% and showing slightly increasing values with laser pulse energy and pulse repetition rate.

Design and Fabrication of Ni/Ti Multilayer for NeutronSupermirror

In the applications of neutron guides and focusing devices, by usingthe Ni/Ti multilayer supermirrors (SM), the neutron flux issignificantly enhanced, because the critical reflective angle ofsupermirrors increases m times compared to the one of naturalbulk Ni. We design and fabricate the Ni/Ti multilayer supermirrors byconsidering the effect of the interfacial imperfection, such asinterface roughness and diffusion, and by using the direct currentmagnetron sputtering technology. The reflective performances of thesesupermirrors are measured on a V14 neutron beam line at the BerlinNeutron Scattering Centre (BENSC), Germany. The measurement datasuggest that the critical angles of the supermirrors are 1.5 and 2.2times that of bulk Ni, respectively.

Construction of the neutron beam facility at Australia's OPAL research reactor

Australia's new research reactor, OPAL, has been designed principally for neutron beam science and radioisotope production. It has a capacity for 18 neutron beam instruments, located at the reactor face and in a neutron guide hall. The neutron beam facility features a 20 l liquid deuterium cold neutron source and cold and thermal supermirror neutron guides. Nine neutron beam instruments are under development, of which seven are scheduled for completion in early 2007. The project is approaching the hot-commissioning stage, when criticality will be demonstrated. Installation of the neutron beam transport system and neutron beam instruments in the neutron guide hall and at the reactor face is underway, and the path to completion of this project is relatively clear. This paper will outline the key features of the OPAL reactor, and will describe the neutron beam facility in particular. The status of the construction and a forecast of the program to completion, including commissioning and commencement of routine operation in 2007 will also be discussed.

Performance of an elliptically tapered neutron guide

Supermirror coated neutron guides are used at all modern neutron sources for transporting neutrons over large distances. In order to reduce the transmission losses due to multiple internal reflection of neutrons, ballistic neutron guides with linear tapering have been proposed and realized. However, these systems suffer from an inhomogeneous illumination of the sample. Moreover, the flux decreases significantly with increasing distance from the exit of the neutron guide. We propose using elliptically tapered guides that provide a more homogeneous phase space at the sample position as well as a focusing at the sample. Moreover, the design of the guide system is simplified because ellipses are simply defined by their long and short axes. In order to prove the concept we have manufactured a doubly focusing guide and investigated its properties with neutrons. The experiments show that the predicted gains using the program package McStas are realized. We discuss several applications of elliptic guides in various fields of neutron physics.

Radiation shielding for neutron guides

Models of the neutron guide shielding for the out of bunker guides on the thermal and cold neutron beam lines of the OPAL Reactor (ANSTO) were constructed using the Monte Carlo code MCNP 4B. The neutrons that were not reflected inside the guides but were absorbed by the supermirror (SM) layers were noted to be a significant source of gammas. Gammas also arise from neutrons absorbed by the B, Si, Na and K contained in the glass. The proposed shielding design has produced compact shielding assemblies. These arrangements are consistent with safety requirements, floor load limits, and cost constraints. To verify the design a prototype was assembled consisting of 120 mm thick Pb(96%)Sb(4%) walls resting on a concrete block. There was good agreement between experimental measurements and calculated dose rates for bulk shield regions.

MC ray-tracing optimization of lobster-eye focusing devices with RESTRAX

The enhanced functionalities of the latest version of the RESTRAX software, providing a high-speed Monte Carlo (MC) ray-tracing code to represent a virtual three-axis neutron spectrometer, include representation of parabolic and elliptic guide profiles and facilities for numerical optimization of parameter values, characterizing the instrument components. As examples, we present simulations of a doubly focusing monochromator in combination with cold neutron guides and lobster-eye supermirror devices, concentrating a monochromatic beam to small sample volumes. A Levenberg–Marquardt minimization algorithm is used to optimize simultaneously several parameters of the monochromator and lobster-eye guides. We compare the performance of optimized configurations in terms of monochromatic neutron flux and energy spread and demonstrate the effect of lobster-eye optics on beam transformations in real and momentum subspaces.

Study of converging neutron guides for the cold neutron double-chopper spectrometer at J-PARC

Performance of converging neutron guides was studied for the cold neutron double-chopper spectrometer proposed for the spallation neutron source at J-PARC. Intensity, spatial distribution, and angular distribution of a neutron beam at a sample are compared for linearly tapered and elliptic geometries by Monte Carlo simulation. For low-energy neutrons, the flux on the sample is much increased by an elliptic geometry whose focal points are on the moderator and on the sample in compensation for the angular resolution. To keep a substantial gain for a wide energy range, an elliptic guide with one of the focal points far behind the moderator is better. It also gives good spatial and angular intensity profiles at the sample.