Ferenc Mezei

AMATERAS: A Cold-Neutron Disk Chopper Spectrometer

AMATERAS is a new disk-chopper-type spectrometer installed at Materials and Life Science Experimental Facility (MLF) of J-PARC. AMATERAS is equipped with an extra chopper for pulse shaping at the upstream position, in addition to a monochromating chopper, which conventional chopper spectrometers at pulsed source have. Owing to the use of these choppers and the high peak intensity from a coupled moderator source at MLF, the AMATERAS design realizes high-intensity and high-energy-resolution measurements in quasielastic and inelastic neutron scattering experiments. The spectrometer had the first neutron beam in May 2009. During the course of commissioning, the performance of the spectrometer was confirmed by conducting test experiments. AMATERAS is now open to users and is producing scientific outputs.

Monte Carlo simulations of neutron scattering instruments by VITESS: Virtual instrumentation tool for ESS

Abstract Analytical approaches have a limited role to play in complex experimental situations, such as neutron scattering. Even if most individual instrument components can be precisely described by analytical models, the integration of a large number of such components in an instrument makes analytical processes highly inefficient and time consuming. Monte Carlo (MC) simulation offers an efficient solution for achieving this goal, both in terms of computational time and the time invested by researchers, given that powerful program packages, such as VITESS, are available [l]. The VITESS [2] sub-project was implemented at HMI within the framework of the German HGF strategy fund project “Research and Development for the European Spallation Source (ESS),” as part of the ESS-Instrumentation activity. This project aims to improve and extend the efficiency of instruments in all fields of research with neutron beams, especially those on spallation sources. For the design and construction of high performance instruments, an efficient computer program has been developed which allows for simulation of the neutron flight from the targevmoderator to the detector.

High-resolution focusing SANS with a toroidal neutron mirror

Presently, high-resolution instruments for small-angle neutron scattering (SANS) use the conventional pinhole cameras. For high resolution they have to be extremely long (80 m for D11). Much shorter instruments can be built with focusing mirrors [1–3]. So far, however, this principle was not successful because of the insufficient quality of conventional optical mirrors. Recently, we succeeded in building a 20 m long focusing instrument at the NSE spectrometer IN15 in the ILL Grenoble. It uses a 4 m long, high-quality mirror as developed for X-ray telescopes. The image has a sufficiently low parasitic halo. Test experiments on polymeric precipitates down to Q=4×10−4 A−1 were successfully carried out. We expect that the Q-range can be extended to ≈10−4 A−1.

Neutron resonance spin echo using spin echo correction coils

Abstract We report on a recent experiment which was performed in order to examine a new experimental realization of the neutron resonance spin echo (NRSE) technique. In neutron spin echo (NSE) neutrons accumulate spin phase in long magnetic fields before and after the sample. The measured quantity is the final scattered beam polarization, which contains information on the sample dynamics (the intermediate scattering function S(Q,t)). This method is limited by the inhomogeneity of the magnetic field, leading to differences of the spin phase of individual neutrons and thus influencing the scattered beam polarization even in the absence of sample dynamics. The NRSE technique can overcome this limitation by reducing the dimensions of the field. However, all previously built NRSE spectrometers use transversal magnetic fields, making it impossible, in practice, to correct for additional limiting effects, such as the beam divergence. For the first time, we have built a NRSE setup with longitudinal field geometry, which does not have this disadvantage. In order to test this new approach, we combined a longitudinal NRSE setup in one spectrometer arm of the IN11 instrument at ILL with a (conventional) NSE setup in the other arm. This experiment demonstrates, how NRSE can be realized in longitudinal field geometry, and sheds light on the differences to previous NRSE setups. As a main result, we show that the effect of beam divergence could be corrected by means of standard Fresnel coils. The proper operation of this hybrid spectrometer is demonstrated by measurements of diffusive dynamics in a well-known micellar sample.

Multiplexing chopper systems for pulsed neutron source instruments

The choice of pulse repetition rates and pulse lengths provided by a pulsed neutron source cannot ideally fit the requirements of all of the various neutron scattering experiments. Novel multiplexing chopper systems we propose can help to enhance instrument performance by tailoring the effective pulse parameters to better meet experimental needs. In particular schemes to multiply pulse repetition rates and wavelength bands and to enhance wavelength resolution will be discussed, together with an efficient chopper system design tool based on the new concept of wavelength filtering.

The multiwavelength cold neutron time-of-flight spectrometer project IN500 at LANSCE

Abstract The IN500 projects takes advantage of three novel techniques to achieve a more than an order of magnitude gain in beam intensity delivered onto the sample at equal resolution compared to the best existing cold neutron TOF spectrometers: (a) the newly installed high intensity, long pulse coupled moderator at the Lujan center short pulse spallation source facility, (b) the recently proposed repetition rate multiplication trick to improve data collection efficiency and (c) the reduction of beam losses in long neutron guides by using a novel, “ballistic” design.

The long-wavelength neutron spin-echo spectrometer IN15 at the Institut Laue-Langevin

Abstract A new ultra-high resolution neutron spin-echo spectrometer at the ILL (Grenoble) extends the time range for measurements of the intermediate scattering function S(Q,t) far beyond what has previously been possible. The spectrometer design is traditional, utilizing long neutron wavelengths (8–25 A) and large, homogeneous precession coils to reach Fourier times approaching the microsecond. The standard operational mode is currently functional, providing a Fourier time range of 0.03–180 ns, and a momentum transfer between 0.01 and 0.2 A−1. The other two operational modes (neutron optical focusing and time of flight) are also discussed.

Monte Carlo simulation of crystal monochromators/analysers – Applications for the crystal-analyser neutron spectrometer IRIS

Abstract It is shown that Monte Carlo simulations of a crystal monochromator/analyser by means of the Virtual Instrumentation Tool for ESS (VITESS) offer a correct basis for the computation of the resolution function of crystal analyser spectrometers. Relying on comparisons of MC computed and measured data of vanadium and superfluid helium, a bench marking of the programme codes of IRIS was performed practically limited only by the statistics of the experimental calibration data. Recent results from the study of the input time pulses from the liquid hydrogen moderator and pulse-shaping choppers are also mentioned.

Dynamic scaling in spin glasses

We present neutron spin echo (NSE) results and a revisited analysis of historical data on spin glasses, which reveal a pure power-law time decay of the spin autocorrelation function

Monte-Carlo simulations for instrumentation at pulsed and continuous sources

s(Q,t)=S(Q,t)/S(Q)