Detection of high frequency intensity oscillations at RESEDA using the CASCADE detector

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.

Monte-Carlo simulations for the optimisation of a TOF-MIEZE instrument

“…A phase shift of 2π over the whole thickness of the detector would completely destroy the contrast, whereas a phase shift of π/2 would not reduce it by more than 10 %. For example in the CASCADE detector [16,17,18] we used in our MIEZE experiments, where the thickness of the detection planes is ∆d = To counter the effect of contrast reduction due to path length differences within the plane of the detector especially at higher MIEZE times τ M an area of 1 mm 2 is used in the simulation. This is in the order of the spatial resolution of a CASCADE detector.…”

Neutron spin echo spectroscopy under 17 T magnetic field at RESEDA

“…The secondary spectrometer arm, which is the same for both setups, allows to perform measurements for sample-detector distances between 0.5 and 5 m. A position sensitive CASCADE detector [17,18] with an active area of 20 cm × 20 cm and a time resolution of t = 50 ns is used. In the CASCADE detector six boron foils (with thicknesses varying from 0.8 to 1.5 µm) convert the neutrons, allowing to record a MIEZE signal with shifted phase on each foil and pixel.…”

RESEDA: Resonance spin echo spectrometer

“…Next to 3 He detectors, a 2D CASCADE detector with an active area of 20 cm x 20 cm characterised by a spatial resolution of 2.6 mm 2 and a time dynamics of the order of a few MHz is available (Häußler et al, 2011;Schmidt et al, 2010).…”