HIGH-RESOLUTION HIGH-LUMINOSITY TECHNIQUES FOR NEUTRON SCATTERING BASED ON BENT CRYSTAL MONOCHROMATORS. Mihai Popovici, William B. Yelon and Ronald R. Berliner, Missouri University Research Reactor
Neutron scattering instruments measure scattering laws S(Q) or S(Q,hw), the variables being the wave-vector transfer Q=ki-kf and the energy transfer hw=Ei-Ef=h2(ki2-kf2)/(2m). Conventional instruments try hard to precisely define both the incident and scattered wavevectors ki and kf with the aid of Soller collimators. This leads to large losses and to the neutron scattering being intensity limited even at the highest flux sources available.
Unconventional instruments with focusing monochromators have been developed recently. Their basic idea is to precisely define not ki and kf separately, but the combinations Q and hw only. This is done by controlling the correlations between ki, kf and the sample coordinate r with bent crystal monochromators. The theory, expressed in matrix language, is based on considering the (ki,r,kf) space and then making the transformation to the Q or (Q,hw) space. The potential of focusing techniques is far from being fully explored and some necessary tools have yet to be developed.
The special features of focusing instruments are:
-resolution is set by the sharpness of the correlations, thus by the crystal reflectivity widths and the sample size (thickness, with plate samples);
-beams are fully open (no Soller collimators), leading to high luminosity;
-gains over conventional instruments in both intensity and resolution are possible (intensity may not be sacrificed for resolution and vice-versa);
-high resolution is obtained with bent perfect crystals of narrow reflectivity;
-resolution can be traded for intensity by using mosaic crystals;
-focusing in scattering with open beams can be combined with real space focusing of neutrons onto small samples (Liouville theorem still obeyed);
-focusing monochromators make a perfect match with position sensitive detectors; -cold sources are not mandatory for high resolution work, thermal sources may be OK.