2T1 a new polarised neutron triple axis spectrometer

The thermal beam triple-axis spectrometer 2T1 is renewed. The beam tube size has been increased to 50 x 120 mm2 (H x V) before the monochromator. Using the same monochromator, an increase of flux of ~ 35 % is already observed. The monochromators are planned to be changed during summer 1999 to adapt their sizes to the new beam tube size. After increasing of the monochromators size, a gain of about a factor 2 on the neutron flux at the monitor position is expected. Polarised neutrons beam option will be installed on this triple-axis. The polarization is obtained using high quality Heusler crystals recently grown at ILL. The size of both Heusler monochromator and analyzer have been chosen to fully cover the beam size. The monochromator (analyzer) will be equipped with a vertical (horizontal) curvature. The flux of the polarized beam on the detector is then expected to be ~ 5 times better than IN20 at ILL (best existing polarized neutrons triple-axis on thermal beam) with incident energy up to 75 meV.

A neutron Spin Echo spectrometer for high Q measurement (MUSES)

The neutron Resonance Spin Echo spectrometer G1 bis is particularly adapted for measurements over wide momentum transfer (0.05–2.5 Å-1). It is thus complementary to the small angle Spin-Echo spectrometer MESS, already installed at the LLB.

The performances of the spectrometer : a Fourier time domain of more than 4 decades (1 ps-20 ns) and the very high flux delivered by the polarising guide G1bis (2.107 polarised n.cm-2.s-1 at the sample position for l =4.8Å with D l /l =0.15) allow to perform high quality measurements within very reasonable time (typical experiments duration of 1-2 weeks). Examples of measurements on intermediate scattering function are displayed on the following figure : temperature dependence in the a -relaxation regime of a ionic glass-forming system (CRN) on the figure 1, and wavevector dependence of water in confined geometry in figure 2.

Figure 1 Figure 2

The sample environment (10-1300 K) available on MUSES allows measurements on huge number of systems exhibiting slow relaxation processes in the mesoscopic to microscopic scale : biological systems, glasses, liquids, polymers ….

New reflectometer PRISM (G-2.4)

The new spectrometer PRISM (Polarised Reflectometer for the Investigation of Surface Magnetism) is an upgrade of the previous reflectometer PADA (G-2.2). This instrument has been optimised in terms of flux for the study of small magnetic samples. The incident flux has been multiplied by a factor 15 and is presently of 5x105 n/s.cm². This flux is available over a height of 15 mm and makes it possible to study small samples of surfaces ranging from 0.25 to 1 cm². The available reflectivity range is now typically 10-5 to 10-6 depending on the size of the sample. These improvements have been made possible by using a new multilayer monochromator which provides a larger wavelength spectrum width (dl /l = 5%) and also by a new vertical focalisation system based on neutron supermirrors. Polarisation analysis is available on this spectrometer. The resolution can be varied between 0.01 to 0.1°. Magnetic fields up to 2 T are available and the temperature range is typically 4K-300K. In the year 2000, a new coil will provide a 7 T field.

SANS instrument with polarized neutrons (PAPOL)

PAPOL is a small angle spectrometer with a polarized neutron beam (SAPNS).

SAPNS is of particular interest for the study of nanoscale magnetic fluctuations. If there are nuclear i.e. density (bn) and magnetic (bm) fluctuations present, the scattered intensity can be written
I± (Q)=c{bn2 . Sn(Q) + bm2 sin2a . Sm(Q) ± 2bm bn . sin2a . Smn(Q)} + bg.

The ± sign refers to neutrons parallel or antiparallel to the magnetisation M. The dependence of the scattering on a , the angle between Q and M, allows one to separate the nuclear (Sn) and magnetic (Sm) contributions even with unpolarized neutrons (I+ + I -). The interference term between nuclear and magnetic fluctuations (Smn) needs polarized neutrons. Note that the difference eliminates all other terms including the background bg. Very small magnetic (resp. nuclear) fluctuations in presence of large nuclear (resp. magnetic) fluctuations can be detected this way. As an example, magnetonuclear cross-correlations in Sm(Sr)MnO3 has been studied [V.V. Runov et al JETP Letters 69.353,(1999). Even in case of comparable size of bn and bm, more information can be obtained with a polarized beam. The scattered intensities I+ , I and (I+ - I) for polarized neutrons are compared below with unpolarized scattering parallel (In) and perpendicular (In + Im) to the magnetisation .