The Instrument

LARMOR is one of the four “phase 2” instruments planned to be built at the Second Target Station of the UK spallation source ISIS. The go ahead for these instruments came March 2011, with the allocation of 21M£. LARMOR will view a fully coupled solid-methane/hydrogen moderator, which is kept at 26 K to generate an intense, long-wavelength neutron flux. The pulses have a full width at half maximum of 300 microseconds and a repetition rate of 10 Hz. The distance from neutron target to detector will be approximately 28 metres, which in combination with the low repetition rate gives a wavelength band of Δλ = λmaxmin ~ 14 Å. The pulse width gives the wavelength uncertainty of Δλ ~ 0.04 Å, which at the lowest wavelength of 2 Å gives Δλ/λ ~ 2% FWHM and a Q-resolution already an order of magnitude better than the typical monochromatisation of reactor based Neutron Spin Echo instruments. The beam delivery system includes a polarising neutron guide, which will polarise the beam and at the same time guide it from the target/moderator assembly to the actual instrument. Half way this guide a chopper system will shape the neutron pulse by cutting down unwanted wavelengths avoiding frame overlap.

With the allocated funds ISIS will build LARMOR as a medium resolution high flux Small Angle Neutron Scattering (SANS) instrument, with a high brilliance neutron extraction system and a highly performing position sensitive detector. The detector system covers 600×600 mm2 and has a resolution of ~8×8 mm2. It consists of 75 position sensitive 3He tubes with a diameter of 8mm. Each tube is capable of a maximum count rate of 250 kHz and along the tube length the resolution may vary between 5 and 8 mm.

This NWO grant will realise all polarized neutron and Larmor labelling extensions. The goal is to have a most performing and multipurpose setup by fully exploiting the capabilities offered by polarized neutrons and Larmor labelling and this can be done without compromising the performance of the instrument.

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