Three-axis spectrometer FLEXX
The name FLEX for the cold neutron three-axis spectrometer is originally derived from the flexibility of the instrument parameters which permits non-standard experiments and its use in the development of novel methods. The instrument has been recently upgraded and rechristened FLEXX. Its new position at the end of the NL1B guide will allow a larger coverage in wavevector and energy transfer, and the guide itself has been upgraded with m=3 neutron supermirrors to allow more neutrons to be transported from the source to the instrument along with enhanced transmission in the thermal spectral range. In addition, there is an elliptical guide which focuses neutrons onto a virtual source (a slit) which is subsequently imaged on the sample via a double focussing monochromator. Finally, a velocity selector is used to select a wavelength band, and so obviate the need for a second order filter.
FLEXX also provides the opportunity to combine triple axis spectroscopy and high resolution spin-echo spectroscopy. The neutron resonance spin echo method (NRSE), which was pioneered on FLEX, is particularly suited to the phonon focussing technique which needs the tiltable high frequency flippers used in NRSE. By this method the line widths of dispersive elementary excitations can be measured with an energy resolution in the μeV range.
