Speaker
Description
TOPAS was conceived to increase the capability to study high energy excitations at the MLZ.
While the thermal three axis spectrometer PUMA is ideal to study coherent excitations in condensed matter tuning the instrument to the respective reciprocal space region, TOPAS aims for mapping large regions in reciprocal space or the study of localized excitations, where one can benefit from the massive coverage of the approx. 16 m2 position sensitive detector.
While TOPAS is still waiting for commissioning, the development of the T-REX spectrometer for ESS has opened a route to optimize the instrument even more for the study of very high energy exchange, i.e. very large neutron energy loss.
This optimization will allow the study of high energy excitations with improved flux/resolution conditions important e.g. for small single crystal specimen.
One of the unique features of TOPAS, the provision of neutron polarization analysis at large energy transfer even beyond 100 meV will benefit in particular from the increased intensity in the large neutron energy loss dynamic range.
TOPAS was built to explore the dynamics in the thermal neutron energy range. In particular it aimed to complement and increase the capabilities of the MLZ instrument suite at high energies. The position sensitive detector is perfectly suited to map out the coherent excitation landscape in the energy range up to 100 meV, while localized excitations with relaxed requirements on momentum resolution can be explored even at higher energy. The optimization towards neutron energy loss and hence low final neutron velocity resulted in a comparably compact secondary spectrometer with a sample to detector distance L${_SD}$ = 2.5 m. The chopper system was matched to this requirement as the pulse length ratio of the resolution defining choppers was matched according to Lechners formula for λ' / λ=2.
$$ \frac{\tau_1}{\tau_2} = \frac{L_{12}}{L_{2s} + (\lambda\prime/\lambda)^3 L_{SD} +1} $$
Here L${12}$ denotes the distance between the two choppers, L${2S}$ is the distance between the sample and the last chopper and $\tau_i$ are the respective pulse lengths of chopper 1 and 2.
On the other hand, a direct geometry chopper spectrometer is characterized the secondary bandwidth Δλ=h/m_n (νL${SD})^{-1}$ with the repetition rate ν and the sample-to-detector distance L${SD}$, e.g. TOPAS features Δλ' = 3.956Å, for a repetition rate of 400 Hz.
These rather broad bandwidth implies, that primary and secondary energy resolution are matched only for a small fraction of the available dynamic range.
In the course of the conceptualisation of the spectrometer T-REX it became clear, that repetition rate multiplication is not just a tool to improve the duty cycle of the instrument.
RRM implies, that the initial neutron wavelength λ is increased step-like during one period of the neutron source as indicated by the different color of the initial wavelength λ in Fig. 1.
Hence the dynamic range of each pulse has already been probed by the previous pulsed, but with relaxed energy resolution.
By optimizing the very high relative neutron energy loss λ'/λ > 4 with the chopper settings we provide ideal conditions to study high energy excitations, but still cover a large dynamic range employing matched initial wavelength λ.
The optimization of large neutron energy loss requires very strict λ resolution constraints, requesting a larger distance L$_{12}$ between the choppers, while the secondary resolution can be relaxed, allowing a larger pulse length $\tau_2$, which in the present design is the limiting factor of the reachable energy resolution.
For the technical realization it is necessary to adapt the chopper system of TOPAS by replacing the Fermi Chopper FC1 by a pair of disc choppers moving upstream significantly, probably into the experimental hall.
The higher order removal chopper has most likely to be equipped with a different slit pattern to provide the appropriate band selection.
