Use of gratings in neutron instrumentation

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Use of gratings in neutron instrumentation

• F. Ott, A. Menelle, P. Humbert and C. Fermon
  Laboratoire Léon Brillouin CEA/CNRS Saclay

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Objective

• Study of the neutron diffraction on periodical
  gratings.(produced by lithographic techniques).
• Theoretical calculation of the diffraction
  intensities
• Born / DWBA approximation (fails for large
  diffraction intensities)
• matrix formalism full dynamical calculation.
• Comparison with simulations (!?, getting worse)
• 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)
• Extension to large surfaces (100x50mm²)Integratio
  n on the EROS reflectometer for measurements on
  liquids.Data processing (deconvolution)

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Outline

• Some experiments on D17
• Commercial ruled gratings
• Holographic gratings
• Energy analysis in a magnetic field gradient

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Modelisation of the grating

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Increase of the diffraction efficiencies

• Increase of the contrast between the incidence
  medium and the diffraction grating.
• Three possibilities
• grating made out of a high index material
  (Nickel)
• incidence medium with an index gt1 (Titanium)
• use of materials with an high artificial index
  supermirrors.
• Results
• under some conditions, efficiencies gt 20
• increase of the diffraction bandwidth - high
  efficiency for a wide wavelength spectrum- or
  for a large range of incidence angles.

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Glass grating with and without a Ni coating
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Titanium coating(1st order diffraction mode
efficiencies)
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Time of flight reflectivity

Cu (30nm) sur Si
Dq
l 2 - 0.2 nm
5 µs pulse
Spatial spread
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Application in neutron instrumentation Energy
analysis.

The diffraction direction is a function of the
wavelength
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Application on a time of flight spectrometer for
energy analysis.

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Detector view
l

0.2 nm
Mode 1
I
1.5 nm
200 mm
Specular reflection
1.5 nm
Mode -1
0.2 nm
Sample horizon
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Intensity gain

• Use of a white beam
• a reflectivity curve in a single shot.
• Study of the evolution of materials or liquids on
  a time scale of a few minutes
• Examples
• liquid interfaces
• diffusion, sticking, breaking
• anything with a smooth reflectivity curve.

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Experiments on the D17 reflectometer

• Some test experiments on the new reflectometer
  D17 at the ILL on various types of gratings

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Ni grating on glass (Bob Cubbit and
Alain Menelle on D17)
Specular line
No broadening of the diffraction lines is observed
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Ruled gratings
(Edmund Scientific Corp.)
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Holographic gratings
(Edmund Scientific Corp.)
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Holographic gratings efficiencies
(Edmund Scientific Corp.)
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Ruled and holographic gratings

• Main providers
• Edmund Scientific Co. (www.edsci.com)
• Instrument SA Inc. (www.isainc.com)
• 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
• Large surface available, cheap but on epoxy

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Field gradient energy analysis principle
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Basic simulation
Hypothesis length 400mm and dB/dz 0.3T/mm

• 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)
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Field gradient creation

• Halbach type quadrupôle based on permanent
  magnets(Mr 1.14T gt dB/dz 0.25T/mm)

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State of the art prototype

• Use of high remanent field permanent magnets
  (NdFeB)  www.magnetic-solutions.com 

ID13mm (magnet only) OD60mm (magnet
only) Height400mm Weight 20kg (in can)
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Example
Gradient 80 mT/mm
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Conclusion

• 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
• Field gradient device
• assess the problem of magnetic field and field
  gradient inhomogeneity and the limited resolution
  effects
• Larger bore device (?)