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85748 Garching

TOPAS (under construction)

Thermal time-of-flight spectrometer with polarization analysis

TOPAS Scheme TOPAS Scheme


TOPAS will be a thermal time of flight spectrometer featuring polarization analysis. It is optimized for high flux and good energy resolution.

It features a 45 m elliptic neutron guide, which focuses the neutrons onto the sample in the new eastern neutron guide hall. The guide is designed to provide an intense neutron beam at the sample, and to keep the background low, which can arrive in the experimental area.
A beam stop at the centre of the ellipse blocks the direct line of sight to the reactor and a small 15 × 50 mm2 aperture at the reactor side feeds the guide only with neutrons, which can be used at the sample.

The chopper system consists of two Fermi choppers, which provide very short neutron pulses even for large neutron windows, which are a consequence of the elliptic neutron guide. We emphasize, that the Fermi choppers also provide high repetition rates up to 900 Hz. Such frequencies can be explored because of the high speed of thermal neutrons.
An additional disk chopper removes contamination pulses of unwanted neutron velocities.

The neutron beam can be polarized by a continuously operated spin exchange optical pumping 3He filter cell, which provides a constant polarization efficiency.
The final choppers and the flight path volume haven’t any window to keep parasitic scattering low. The vacuum system is designed to reach cryogenic vacuum to allow low temperature applications without additional neutron windows. The volume around the sample can be separated from the rest of the flight path volume for the exchange of the sample or sample environment.
The sample can be rotated by 360º to map the full dispersion relation in single crystalline materials.
The TOPAS detector consists of 289 3He position sensitive tubes covering a solid angle of 3 sr. The resolution of 20 x 20 mm2 is well adapted to the divergence transported by the neutron guide.

Typical Applications

By the use of thermal neutrons, a large range in energy and momentum transfer can be probed. In the direct time-of-flight geometry, the measurement of the momentum transfer is decoupled from the measurement of the energy transfer.
Time-of-flight spectroscopy from single crystals explores simultaneously the Q, ω space and maps a variety of excitations by covering a large solid angle with detectors.
Accordingly, TOPAS can be used for a wide range of applications, particularly for novel materials:

  • Mapping of coherent excitations in correlated electron systems, particularly magnetic excitations.
  • Measurement of the density of phonon states to probe the molecular dynamics in molecular solids.
  • Investigation of limited size and reduced dimensionality effects on the dynamic in nanoparticles.
  • Investigation of proton dynamics hydride materials for energy applications.

For many of the intended instrument applications polarization analysis will provide unique opportunities e.g. for the study of magnetic excitations or the separation of the spin incoherent scattering in hydrogen containing materials.

Technical Data

Primary Beam

  • Beam Port: SR5m
  • Chopper system: 2 Fermi choppers, 1 Disc chopper
  • Chopper Frequency range: 200 – 600 Hz
  • Neutron guide cross section in-pile: 15 x 50 mm2
  • Maximum neutron guide cross section: 81 x 121 mm2
  • Neutron guide cross section in front of 2nd Fermi chopper: 23 x 44 mm2

Experimental parameters

  • Incident wavelength range: 0.8 – 3 Å
  • Energy transfer range: -120 – 50 meV
  • Elastic energy resolution: 4 – 6 %
  • Horizontal divergence: ±0.75º
  • Vertical divergence: ±0.75º
  • Detector coverage: -30º – 5º, 5º – 150º
  • Simulated monochromatic neutron flux: 5 × 105 n/cm2s

Instrument Scientist

Dr. Jörg Voigt
Phone: +49 2461 61-6020
E-Mail: j.voigt@fz-juelich.de

Operated by

JCNS

Publications

Find the latest publications regarding TOPAS in our publication database iMPULSE:

impulse.mlz-garching.de

MLZ is a cooperation between:

Technische Universität München> Technische Universität MünchenHelmholtz-Zentrum Geesthacht> Helmholtz-Zentrum GeesthachtForschungszentrum Jülich> Forschungszentrum Jülich