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MLZ (eng)

Lichtenbergstr.1
85748 Garching

KOMPASS (in commissioning)

Cold three axes spectrometer with polarisation analysis

This instrument is focussed on cold neutrons. Therefore, please carefully check the “Technical data WITHOUT cold source” section. Deviating parameters are in bold. The instrument team is happy to answer any further questions!

2024 Instrument Scheme

KOMPASS — the new cold neutron three axes spectrometer is fully designed to work exclusively with polarised neutrons and to provide a zero-field 3D polarisation analysis, complementary to the other three axes spectrometers at the MLZ. Typical applications are the investigations of complex magnetic structures and excitations.

The instrument is located at the end position of the curved, cold neutron guide NL1. Serial polarising V-cavities in the upstream part of the guide system provide a permanent high-polarised incident neutron beam. The parabolic focussing guide design before the monochromator further includes different exchangeable guide front ends for optimised energy- and Q-resolution, respectively. The focussing guide, together with the variable double-focussing monochromator (and analyser), provides a high neutron flux over a large dynamic range at a small sample volume combined with a superior energy resolution at the expense of a slightly reduced transverse Q-resolution.

For measurements with high Q-resolution or the investigation of steep dispersion relations, the parabolic front ends can be exchanged for the straight elements. Higher-order wavelength contaminations can be suppressed by an optional velocity selector installed before the monochromator. For sample positioning, the sample table is equipped with motorised xy-stages and cradles. The spin state of the scattered neutrons is analysed with a multichannel polarising V-cavity installed between the analyser and the detector. Optionally, for higher divergence, the Heusler analyser from Instrument PANDA is available.

Apart from a standard closed-cycle cryostat, the instrument is equipped with a set of Helmholtz coils for longitudinal polarisation analysis and the 3^rd generation ILL system CryoPAD for neutron polarimetry.

Construction and commissioning of KOMPASS are supported by the BMBF through project 05K19PK1.

Typical applications

A selection of predestinated systems for the study of their spin dynamics and structures with polarised neutrons and neutron polarimetry are:

Complex (chiral) magnetic structures (e.g. rare earths)
All types of weak magnetic orders
Quantum effects associated with longitudinal magnetic excitations
Quantum critical fluctuations
Multiferroic and magneto-electric materials
Unconventional and high-T_C-superconductors
Itinerant magnetic systems
Systems of reduced dimensions (e.g. thin films and multilayer structures)

Technical data WITHOUT cold source

With thermal neutrons, KOMPASS will be mainly operated in polarised neutron diffraction mode with longitudinal polarisation analysis.

Neutron Guide System and Design

End position of the neutron guide NL1, dimensions: 60 × 120 mm² (W × H)
Permanently installed triple polarising V-cavity providing high incident polarisation of P > 98 %
Parabolic focussing in the scattering plane
Exchangeable straight and parabolic focusing guide front ends for optimised (E,Q)-resolution
Compact design with M-S and S-A distances of only 120 cm, A-D distance of 100 cm

Velocity Selector

Δλ/λ = 31 %, effective down to λ_min = 2.35 Å wavelength
Effectively suppresses higher-order contaminations and lowers the background
Selector can be exchanged with ‘virtual source’ slit-system located in the focus point of the parabolic polarising guide

Monochromator

HOPG (d = 3.355 Å), dimensions: 273 × 188 mm² (W × H)
- 28° < 2Θ_M < 135°
- 1.04 Å^-1 < k_i < 3.87 Å^-1
- Doubly variable focussing

Analyser

HOPG (d = 3.355 Å), dimensions: 220 × 225 mm² (W × H)
- -130° < 2Θ_A < 130°
- Doubly variable focussing
Optional Heusler Analyser with variable horizontal focussing (from PANDA)

Polarisation Analyser

Compact polarising V-cavity
- 5 channels with double-sided Fe/Si coated silicon wafers, m = 4.2
- P_f > 96 % for neutrons with 1.5 ≤ E_n ≤ 15 meV
- Optional 10’ and 30’ horizontal collimators
- Integrated Hallbach Array

Detectors

2” ³He tube (standard)
1” PSD ³He tube with active length of 250 mm (optional)

Collimation

α0: 20’, 40’
α1: 10’, 20’, 40’, 80’
α2: 10’, 20’, 40’, 80’
α3: 10’, 20’, 40’, 80’
30’ and 60’ radial collimators between focussed analyser and detector for suppression of HOPG diffuse scattering

Main parameters

Incident energy range: 2.2 meV < E_i < 25 meV
Scattering angle at the sample: 2Θ_S up to 140°
Analyser scattering angle: -130°< 2Θ_A < 130°
Energy transfer: up to 20 meV
Momentum transfers: up to Q = 4.5 Å^-1

Technical data WITH cold source

With the cold source, the full spectrum of inelastic measurements and measurements with CryoPAD are feasible.