focusing monochromators

The instrument HYSPEC at SNS uses crystal arrays to focus and direct the neutron beam onto the sample. Provision is made for arrays of Highly Orientated Pyrolytic Graphite (HOPG) and Heusler crystals to perform experiments with an unpolarized and a polarized neutron beam, respectively. Both monochromators have a variable vertical focus so that the optimum vertical focusing can be chosen for different neutron energies. In order to change between these two types of focusing crystal arrays a vertical translating mechanism is used to position the relevant monochromator in (or out) of the neutron beam. The complete system including the monochromator mechanism, goniometer stages per monochromator, the vertical translation mechanism and all cabling was designed, fabricated, assembled and tested by SwissNeutronics.

The monochromator is equipped with an arrangement of permanent magnets to fully magnetise the Heusler crystals. SwissNeutronics performed simulations of the magnetic field distribution for the optimization of the geometry of the magnetic yoke. The monochromator is composed from an array of crystals. Two are mounted on a common holder, 9 rows are individually pivot-mounted to allow adjustable vertical focusing. All crystals were characterized by SwissNeutronics to take into account the individual offsets between the crystal lattice plane and the physical surface for the alignment of the crystals onto the mechanism. Finally the polarization of the Heusler monochromator was determined from neutron tests to be > 94%.

SwissNeutronics developed a filigree mechanism for doubly focusing monochromators following the tradition of high precision Swiss watches. The mechanism is variable in terms of number of rows/columns covering all sizes of beam area relevant for neutron monochromators. Typical parameters are given below. Three such systems were delivered to HZB.

The variable focusing of this monochromator is performed by bending the Si blades. Therefore, an extremely precise mechanism is required, which was designed and manufactured by SwissNeutronics. Among other benefits the neutron beam hits only Si because the bending mechanism is located at the side of the monochromating surface.

The ILL refurbished the 3-face monochromator of the neutron spectrometer IN4. The monochromator system has provision for three different monochromators: HOPG, Cu (220) and Cu(111). The mechanism of all is identical with fixed vertical and a variable horizontal focusing. A fourth position is available for future upgrade. The highly precise mechanics enable for very accurate tilting of the crystals in the range of 0.03° (absolute) and a repeatability better than 0.001°.

The monochromator is composed of 13 rows of Cu crystals, which were supplied and prepared by SwissNeutronics. It was desired that the orientation of the crystals allows to utilize the (110) reflection in reflection geometry (incident and reflected beam in front of mechnism) and the (001) reflection in a kind of transmission geometry. For this purpose SwissNeutronics has designed the focusing crystal array such that the neutron beam hits only Cu and no structural material (like AI) from the mechanism of the focusing system. SwissNeutronics performed the characterization of the monochromator crystals and the proper cutting. SwissNeutronics supplied two identical monochromators.