ABSTRACT
We describe the upgrade of the neutron resonance spin-echo setup at the cold neutron triple-axis spectrometer FLEXX at the BER II neutron source at the Helmholtz-Zentrum Berlin. The parameters of redesigned key components are discussed, including the radio frequency (RF) spin-flip coils, the magnetic shield, and the zero field coupling coils. The RF-flippers with larger beam windows allow for an improved neutron flux transfer from the source to the sample and further to the analyzer. The larger beam cross sections permit higher coil inclination angles and enable measurements on dispersive excitations with a larger slope of the dispersion. Due to the compact design of the spin-echo units in combination with the increased coil tilt angles, the accessible momentum-range in the Larmor diffraction mode is substantially enlarged. In combination with the redesigned components of the FLEXX spectrometer, including the guide, the S-bender polarizer, the double focusing monochromator, and a Heusler crystal analyzer, the count rate increased by a factor of 15.5, and the neutron beam polarization is enhanced. The improved performance extends the range of feasible experiments, both for inelastic scattering on excitation lifetimes in single crystals, and for high-resolution Larmor diffraction. The experimental characterization of the instrument components demonstrates the reliable performance of the new neutron resonance spin-echo option, now available for the scientific community at FLEXX.
ABSTRACT
We have explored the technological potential of combining neutron resonance spin echo (NRSE) with the time-of-flight method in quasielastic neutron scattering (QENS) experiments. For these test measurements at the new NRSE instrument RESEDA (FRM II, Munich), we have employed CASCADE, one of the fastest neutron detectors in the world, developed at the University of Heidelberg. Conventionally, scintillation detectors are used, in order to detect neutron intensities with high time resolution. In contrast, we used the new CASCADE detector converting neutrons in thin (10)B layers being capable of resolving neutron intensity modulations up to the megahertz regime. This fast detector allows us to abandon the last resonance flip coil of a standard NRSE setup. The classical spin echo signal is replaced by a time-modulated signal. In this setup, fast intensity modulations are present at the detector position. In order to demonstrate, that NRSE-CASCADE operates well up to detector frequencies of 10 MHz, we performed elastic polarization test measurements on a standard sample. The CASCADE detector is a multidetector accumulating counts in 128 × 128 pixels on a surface of 200 mm × 200 mm. We have analyzed the signal in 600 pixels, providing information about the spin phase reaching the detector and about the resolution function of this new variant tested at RESEDA.
