1 Use of gratings in neutron instrumentation F. Ott, A. Menelle, P. Humbert and C. Fermon Laboratoire Léon Brillouin CEA/CNRS Saclay

2 Objective n Study of the neutron diffraction on periodical gratings. (produced by lithographic techniques). n Theoretical calculation of the diffraction intensities: – Born / DWBA approximation (fails for large diffraction intensities) – matrix formalism : full dynamical calculation. – Comparison with simulations (!?, getting worse) n Application of gratings in neutron optics. – Example: energy analyser for time of flight neutron reflectometer – Fabrication and tests of small prototypes (20x20mm²) (choice of materials, periodicities, shape of the grating, optimisation in the resolution, useful q range) n Extension to large surfaces (100x50mm²) Integration on the EROS reflectometer for measurements on liquids. Data processing (deconvolution)

3 Outline n Some experiments on D17 n Commercial ruled gratings n Holographic gratings n Energy analysis in a magnetic field gradient

4 Modelisation of the grating n

5 Increase of the diffraction efficiencies n Increase of the contrast between the incidence medium and the diffraction grating. n Three possibilities : – grating made out of a high index material (Nickel) – incidence medium with an index >1 (Titanium) – use of materials with an «high artificial index» : supermirrors. n Results – under some conditions, efficiencies > 20% – increase of the “diffraction bandwidth”: - high efficiency for a wide wavelength spectrum - or for a large range of incidence angles.

6 Glass grating with and without a Ni coating

7 Titanium coating (1st order diffraction mode efficiencies)

8 Time of flight reflectivity n Cu (30nm) sur Si qq 5 µs pulse Spatial spread = nm

9 Application in neutron instrumentation: Energy analysis. The diffraction direction is a function of the wavelength n

10 Application on a time of flight spectrometer for energy analysis. n

11 Detector view Specular reflection Mode mm Mode nm 0.2 nm 1.5 nm 0.2 nm Sample horizon I

12 Intensity gain n Use of a white beam  a reflectivity curve in a single “shot”. n Study of the evolution of materials or liquids on a time scale of a few minutes n Examples: – liquid interfaces – diffusion, sticking, breaking – anything with a “smooth” reflectivity curve.

13 Experiments on the D17 reflectometer n Some test experiments on the new reflectometer D17 at the ILL on various types of gratings

14 Ni grating on glass (Bob Cubbit and AlainMenelle on D17) Specular line No broadening of the diffraction lines is observed

15 Ruled gratings (Edmund Scientific Corp.)

16 Holographic gratings (Edmund Scientific Corp.)

17 Holographic gratings efficiencies (Edmund Scientific Corp.)

18 Ruled and holographic gratings n Main providers: – Edmund Scientific Co. ( – Instrument SA Inc. ( n Blaze angles and available periodicities: – Holographic : from 200 nm to 5 µm – Ruled gratings : from 0.5 µm to 50 µm with blaze angles de blaze from 1° to 20° n Large surface available, cheap but on epoxy

19 Field gradient energy analysis: principle

20 Basic simulation n Angular beam deflexion at the output of the field gradient region as a function of the wavelength. Position on the PSD at 4m (EROS configuration) Hypothesis: length 400mm and dB/dz = 0.3T/mm

21 Field gradient creation n Halbach type quadrupôle based on permanent magnets (M r = 1.14T => dB/dz = 0.25T/mm)

22 State of the art prototype – Use of high remanent field permanent magnets (NdFeB); ‘ ’ ID=13mm (magnet only) OD=60mm ( magnet only) Height=400mm Weight: ~20kg (in can)

23 Example Gradient 80 mT/mm

24 Conclusion n Near future work – efficiencies of optical ruled and holographic gratings (experiments on EROS and PRISM at the LLB) – supermirror deposition on 20x20mm glass gratings (home-made) and efficiency tests n Field gradient device – assess the problem of magnetic field and field gradient inhomogeneity and the limited resolution effects – Larger bore device (?)