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MICRO : a special diffractometer for high pressure studies

Since 1992, neutron diffraction experiments are being performed in LLB at pressures as high as 30 Gpa. The G6.1 diffractometer (wavelength 4.1 to 4.8 Å) is now operating half of the year in the high pressure mode, using special focusing devices to increase the neutron flux. It is especially adapted to study magnetic structures on powders at very high pressures, and temperatures down to 1.4 K. At the end of 1998, we expect to have a new multidetector covering a higher solid angle. The expected gain in counting rate is about 5. It should allow us to increase the available pressure up to 50 Gpa for systems with large magnetic moments (rare earth compounds), or to enlarge the studies of systems with smaller moments (about one or a few m B at pressure up to 10-20 GPa (mirebea@llb.saclay.cea.fr).

PAPOL : small angle scattering with polarised neutrons

In addition to the three regularly scheduled small angle cameras PACE, PAXY, PAXE, there is a special purpose small angle scattering spectrometer (G5.5, PAPOL) available. Its main feature is a polarised incident beam and it is therefore particularly suited to study correlations between nuclear and magnetic fluctuations in the Q-range from 0.05 to 5 nm-1. The characteristics of PAPOL are a fixed wavelength band 0.75<l <0.85 nm, a 7 m fixed collimation distance, a sample to detector distance variable in steps of 1 m from 1 to 4 m and a LETI area detector with 128 x 128 pixels of 5 mm size. Neutron intensity at the sample is 1.5 x 105 n.s-1 cm-2 in absence of entrance collimation (beam of 25 x 25 mm). Of the various possible sample environments, we mention a horizontal split-coil magnet which provides fields up to 3.8 T with an access of ± 20 deg parallel and ± 10 deg perpendicular to the field. It can be used with a variable temperature insert able to cool a sample in a He4 bath down to 0.2 K (glattli@amoco.saclay.cea.fr).

A new resonance spin echo spectrometer

A NRSE-spin echo spectrometer – which in contrast to standard spin echo relies on RF-coils rather than on static magnetic fields – had been installed at the guide G1bis within the last years. It was designed and built at the TU München as a multi-purpose instrument for a wavelength range from 3.5 10 Å, scattering angles up to 110° and a beam cross section of 4 x 4 cm2. Its effective magnetic field ranges from 17 G to » 1000 G with variable lengths between 0.5 to 2 m for each arm. With an additional static spin echo field option, the lower bound of this range was extended to » 1 G and a dynamic range of 7.103 in the spin echo time is possible without change of the wavelength.

Such characteristics gives access to a very wide range of time (~ 1.5 ps to 15 ns). Moreover, in its short configuration, its brightness allows to measure long characteristic times at large scattering angles (longevil@llb.saclay.cea.fr).

3T2 new monochromator

Since November’97, the " thermal " high resolution powder diffractometer 3T2 is equipped with a Ge (335) focusing monochromator, with five composite blades of 60´ 20´ 9.6mm3, each blade being made of 12 elements 0.8mm thick. We still have 2q M» 90°, l =1.225Å, but due to the properties of the crystals (size and mosaicity) the flux at the sample is multiplied about four times. Let us note however that the incident beam doesn’t illuminate the whole monochromator, due to the small vertical size of the first collimator ; the focusing feature is then not fully used.

As for FWHM (Full Width at Half Maximum), the curves below compares the old and new versions : at low 2q values, FWHM is increased (maximum D FWHM)/FWHM=30% at 2q =0), and this increase is mainly due to the increase of the monochromator mosaicity. However, this degradation of the resolution at low angles is not so important, as the maximum peak overlap occurs at higher 2q values. For higher 2q values, FWHM is slighly lower than before. At a price of lower intensity, a better resolution could be obtained by introducing a lower a 2 value (horizontal divergence of the collimator between the monochromator and the sample).



As a consequence of the existence of this focusing monochromator, the time requested for an experiment must be reevaluated (less time needed for one diagram) (bouree@llb.saclay.cea.fr).

LABORATOIRE LEON BRILLOUIN mise à jour : 27/07/99
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