Description

By a given angle θ_M and a given lattice plane distance d (via crystal material and orientation) a selected wavelength λ can be extracted from the spectrum of the incoming radiation following the Bragg law (this is true also for the λ/n multiples -> λ/2 contamination, which is usually filtered). This principle gives us the monochromatized neutron radiation for our diffraction experiments.

Hardware

Mechanic
• 2θ_M goniometer to select the monochromator angle,
• translation table to position the virtual reflection centers of the selected monochromator crystal on the cross point of primary and secondary beam channel,
• tilt table to correct tilt errors,
• goniometer as a selector of up to four monochromator crystals with a max width of the crystals of 120 mm,
• focussing optics for a variable focussing of the radiation on the sample (r_min=0.5 m), the max. crystal height is 100 mm (7 horizontal elements with 14 mm height each)

Electric
• 2Radiation resistant limit and reference switches, standard electro motors for the two goniometers and the two tables, radiation protection of the motors via absorber between selector and platform for focussing optics,
• radiation resistant electro motors and resolvers for the focussing mechanic,
• controlled by two Trinamic Quadpack stepper motor controller with microstepping.

Crystals

Crystal type

Diffraction angles and wavelengths [Å]

Available Q space, Q=sinθmax/λ (θmax=120°)

Remarks
Ge (311) monochromator crystals have no λ/2 contamination due to their structural properties. Yet, the perfect crystalline structure of Ge crystals is a problem. The small half widths (arc seconds!) of the reflection profiles yield too small intensities for diffraction analysis with neutrons. Therefore special and time consuming treatment to Ge crystals has to be done to get reasonable intensities. The manufacturing process of the Ge crystals for HEiDi is almost completed. We expect The Ge-(311) monochromator unit to be available at summer 2005. Cu monochromator cystals show typically a strong λ/2 contamination. Therefore, neutron filters have to be used to cut off the neutrons around the λ/2 area from the neutron spectrum.