Problem of λ/2 Contamination

During the monochromatization of the spetrum of the neutron source using monochromator crystals not only the wanted wavelength λ but also the higher orders of λ/2, λ/3, etc. are reflected to the direction of the sample. The reflectivity decreases significantly to higher orders. Therefore, only λ/2 contamination has to be taken care of.

Because of

sinθ=λ/d_hkl = (λ/2)/d_{2h 2k 2l}

λ/2 contamination gives measurable reflex intensities

• at forbidden reflex positions and
• in addition to reflex intensities at allowed reflex positions.

The generation of additional reflections and the change of the intensities of existing reflextions can yield wrong determination of the space group or of the atomic positions in the unit cell.

Solution

To minimize the λ/2 contamination one can use resonance filters - completely analogous to the supression of the K_β line in x-ray diffractometers.
HEiDi uses nine seperate wavelengths. Three of them have no λ/2 contamination (because of the Ge (311) space group). For the other six wavelengths the following filters were selected from the knowledge of the neutron spectrum of HEiDi:

(*) Not available due to manufacturing problems (State at February 2005).
(**) No suitable filter known.

Construction

The filter unit for the assembly of the neutron filters consists of a cube like PE tube. The tube is mounted directly behind the secondary beam tube of the biological shielding. In the filter unit itself there are eight flat pressure cylinders. With them, the filter sheets (60*50mm²) and as an option attenuators can moved into the beam. The cylinders are controlled remotely by a pressure controller with a serial port.