Neutron Optics

The Neutron Optics Group (NOG) takes care of the neutron guide systems at the FRM II. Our duties, beyond maintenance and repair of the existing neutron guides, are the development of new neutron supermirrors, the acquisition or production of new guide elements, the support for the acquisition of crystal monochromators and the supply of spin-polarised ³He for neutron-spin-filters. Within this scope we run a DC magnetron sputtering device for the production of supermirrors, the ³He spin polarising system HELIOS and in cooperation with our partner JCNS the neutron reflectometer TREFF.

With the aim to minimise the mechanical stress, thereby increasing the reflectivity in the coated supermirrors, the DC sputtering process has been optimised in recent years. That way the NOG is now able to provide Ni/Ti supermirrors up to a coating of m = 4. For cases were polarised neutron experiments are needed, the NOG group has developed over the last few years low-depolarising m = 2 Cu/Ti supermirrors, which have been prepared using the standard DC magnetron sputtering facility. Control of the roughness, growth and interdiffusion allowed us to achieve a very good maximum angle of total reflection and polarised neutron reflectivity. Recent depolarisation measurements performed at ILL excluded depolarisation of a fully polarised neutron beam after a reflection on the Cu/Ti supermirror in the range of 10^-4.

We have facilities for all production steps of neutron guide elements in house. Shaping of the raw float glass plates is done with a precision diamond saw. Plates are coated in our DC magnetron sputtering facility with either Ni/Ti, NiMo/Ti or Cu/Ti supermirrors. Typical substrate materials used are Borofloat³, Eurofloat and Silicon, being 220 × 1000 mm² the maximum allowed surface. Precision benches serve for the assembly of up to 2.5 m long guide elements, either bended or straight. The neutron optical properties of the individual mirror plates are verified with our neutron reflectometer TREFF. Using either our neutron guide elements or purchased ones from other manufactures, we build up the neutron guide system for the instruments. For the alignment of the guide elements, we use either total stations or a laser tracker system. Radiation shielding optimisation provide a low background environment inside the neutron guide halls.

HELIOS is a MEOP station for the production of polarised ³He gas, which is used by several instruments at the MLZ to perform neutron polarisation analysis. It uses the Meta Stability Exchange Optical Pumping method and is equipped with four high power lasers providing very good stability of the laser beam polarisation. This ensures a high production rate of 1 bar-litre per hour of polarised ³He gas at about 72% gas polarisation. In addition to the continuous work on improving and optimizing the HELIOS facility to reach the highest possible degree of polarisation, we support the MLZ instruments by developing the ³He glass vessels (the so-called ³He cells) that go inside the instrumental neutron beam-line (with relaxation times of the ³He polarised gas up to 160 hours at 1 bar pressure), and specific magnetic cages to maintain very homogeneous magnetic fields across the ³He volume. The improvement of spin-flipper electronics for the MEOP cell applications resulted in the building of a unique portable device that integrates both a radio-frequency adiabatic fast passage (AFP) spin-flipper circuit, which serves to flip ³He polarisation, and a power supply for the current generating the static holding magnetic field for the polarised ³He cell. The spin-flip efficiency of such device was measured to be larger than F = 0.9999. An open collaboration with the NOG of the ILL (Grenoble, France) is currently pushing this efficiency forward.

In collaboration with JCNS we operate the general-purpose neutron reflectometer TREFF at the neutron guide NL5-S. A PG(002) double monochromator with fixed scattering angles of 90° provides a monochromatic beam with a wavelength of λ = 4.74 Å (2.4 Å / PG 004). Equipped with a position sensitive detector and polarisation analysis, and thanks to the flexible setup and low background at the instrument, TREFF is a perfect playground for developing of new neutron optical and detection components at MLZ. In its standard operation mode (high-resolution reflectometry) TREFF is often used e.g. for pre-characterisation and measurements of magnetic thin films. Being equipped with a polarised beam option and wide-angle polarisation analysis, TREFF is an excellent opportunity for the PhD students at MLZ to gain hands on experience in specular and off-specular polarised neutron reflectometry.

References:

Heinz Maier-Leibnitz Zentrum et al., (2017). TREFF: Reflectometer and instrument component test beamline at MLZ. Journal of large-scale research facilities, 3, A121.

J. M. Gomez-Guzman et al., Nucl. Ins. Meth. Phys. Res. A 1059, 169005 (2024).