M. Klein

Design and performance of a highly efficient mass spectrometer for molecular beams

We present a very efficient 90° sector magnet mass spectrometer detector, based on electron impact ionization, especially adapted for the use with atomic and molecular beams. In this setup, a cylindrically symmetric magnetic field compresses electrons emitted from a large filament into a long, narrow volume containing primarily the molecular beam. This has the advantage of both producing as little as possible background from residual gases and increasing the interaction region between beam and ionizer. Experimental tests using thermal He atoms show that the detector’s efficiency and sensitivity are very high, amounting to η⩾0.70±0.04% and S=0.22±0.03 A mbar−1, respectively. The unconventionally long ionization volume, however, may restrict the use of this mass spectrometer to experiments with no, or only moderate timing requirements.

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

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.

Scientific Reviews: The CASCADE Neutron Detector: A System for 2D Position Sensitive Neutron Detection at Highest Intensities

The need to match detection capabilities on future instruments at spallation neutron sources to the thousand-fold higher instantaneous flux prospected, likewise the competitive drive at existing neutron sources to improve and match performance to these modern sources, has served to uncover an enormous innovative deficit, which had accumulated during the past decades and now finally comes into focus.

CASCADE with NRSE: Fast Intensity Modulation Techniques used in Quasielastic Neutron Scattering

Fast intensity modulation techniques require neutron detectors with a response time below 1μs, a demand which can not be fulfilled by standard neutron detector technology. Here we present the CASCADE neutron detector developed at the University of Heidelberg, which is based on stacked, thin 10B layers as neutron converters and particularly addresses these needs. After explaining the CASCADE concept in context with fast intensity modulation techniques we report on two test runs performed at the RESEDA spectrometer at FRM II, Munich, which indeed prove MHz time resolution in this spectrometer system with a reasonably high detection efficiency.

CASCADE - a multi-layer Boron-10 neutron detection system

The globally increased demand for helium-3 along with the limited availability of this gas calls for the development of alternative technologies for the large ESS instrumentation pool. We report on the CASCADE Project - a novel detection system, which has been developed for the purposes of neutron spin echo spectroscopy. It features 2D spatially resolved detection of thermal neutrons at high rates. The CASCADE detector is composed of a stack of solid boron-10 coated Gas Electron Multiplier foils, which serve both as a neutron converter and as an amplifier for the primary ionization deposited in the standard Argon-CO2 counting gas environment. This multi-layer setup efficiently increases the detection efficiency and serves as a helium-3 alternative. It has furthermore been possible to extract the signal of the charge traversing the stack to identify the very thin conversion layer of about 1 micrometer. This allows the precise determination of the time-of-flight, necessary for the application in MIEZE spin echo techniques.

Radio frequency-induced polarization of ultra-cold neutrons or how to pump a two-level system

Abstract The well-known technique of optical pumping can be used to polarize a neutron beam in a transient way. In our experiment we demonstrate that an unpolarized beam of ultra-cold neutrons (UCN), interacting solely with a set of magnetic fields can become polarized without losing the “wrong” spin component.