Neutrons for Science and Industry

Crystal-Monochromator Spectrometer on Hot Neutron Source: IN1-LAGRANGE at ILL for Studies of Vibration Dynamics in Complex Materials

by Dr Alexandre Ivanov (Institut Laue-Langevin)

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The hot-neutron spectrometer IN1-LAGRANGE is dedicated to studies of high-energy excitations, such as molecular vibrations, in different classes of complex materials available as a rule in non-oriented forms (polycrystals, powders, glasses, composites, biological matter etc.). It is an indirect geometry instrument with incident neutron energies in the range of  - ~1000 meV produced by a multi-face double focusing monochromator what allows, in particular, access to the full range of proton vibrations in the hydrogen-containing materials. The design of the secondary spectrometer is based on the space focusing of neutrons scattered by the sample in a very large solid angle and then registered by a relatively small single counter. The analyzer is built from appropriately oriented pyrolytic graphite crystals set to reflect neutrons with a fixed central energy of 4.5 meV. The subtended solid angle is as high as ~2.5 Steradian with characteristic analyzer dimensions less than 1 meter. A cooled beryllium filter is put in the neutron scattering path in order to suppress higher-order reflections by the analyzer crystals with an absorber installed at the sample-filter-counter axis. The sensitivity of the instrument is such that vibration spectra of samples with sub-milligram hydrogen content can be measured on a time scale of several hours. This also permits measurements with small samples at high-pressures using dedicated pressure cells. It is important that the use of single crystal reflection in the analyzer in combination with the reduced collimation of the incident monochromatic beam will permit experiments with relative energy resolution down to ~1% what is the high-resolution edge of the existing neutron spectrometers of similar destination elsewhere. The resolution/intensity optimization can be comfortably performed in a broad range with typical time for changing over from one instrument setting to another less than an hour.

Organized by

Dr. Jitae Park
Dr. Dominic Hayward

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