TOPAS (under construction)

Thermal time-of-flight spectrometer with polarisation analysis

TOPAS Scheme

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

It features a 45 m elliptic neutron guide, which transports the neutrons to the experimental area. The guide is designed to provide an intense neutron beam at the sample position while keeping the background low. A beam stop at the guide’s centre blocks the direct line of sight to the reactor, and a small 15 × 50 mm² aperture at the reactor side feeds the guide with neutrons that are transported to the experimental area without parasitic effects.

The chopper system consists of two Fermi choppers, which provide very short neutron pulses even for a broad range of neutron wavelengths/ velocities due to the elliptic neutron guide. It’s worth noting that the Fermi choppers also provide high repetition rates of up to 900 Hz. Those are required to ensure monochromatic pulses of thermal, high-speed neutrons. An additional disk chopper removes neutrons contaminating the pulses with neutrons of unwanted velocities.

The neutron beam can be polarised by a continuously operated spin-exchange optical pumping ³He filter cell (SEOP), providing constant polarisation efficiency. The sample space and detector tubes reside in the common vacuum of the TOPAS main chamber (76 m³), without additional windows separating those elements. The main chamber is designed to reach a cryogenic vacuum to allow low-temperature applications and minimise parasitic scattering at the flight path of neutrons. The volume around the sample, the sample chamber of 0.3 m³, can be separated from the rest of the main chamber to exchange the sample or sample environment. The sample can be rotated by 360º to map the dispersion relations in single crystals.

The TOPAS detector bank consists of 288 ³He position-sensitive detector tubes in the main chamber, covering a solid angle of 2.33 sr. The resolution of 20 × 20 mm² for a single ³He tube is well adapted to the divergence transported by the neutron guide.

Typical applications

Using thermal neutrons, a large range in energy and momentum transfer can be investigated. In the direct time-of-flight geometry, the measurement of the momentum transfer is decoupled from the energy transfer measurement. Time-of-flight spectroscopy from single crystals explores the Q, ω space simultaneously and maps a variety of excitations by covering a large solid angle with detector bank.

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 dynamics in nanoparticles
Investigation of proton dynamics in hydride materials for energy applications

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

Technical data

Primary beam