PPubMed25 Apr 2015

Surface-treated self-standing curved crystals as high-efficiency elements for X- and γ-ray optics: theory and experiment 1 Surface-treated self-standing curved crystals

Elisa Bonnini, Elisa Buffagni, Andrea Zappettini, Stephen Doyle, Claudio Ferrari

Abstract

'Claudio Ferrari'] It is demonstrated that crystals with curved diffracting planes made of relatively light elements, such as silicon, germanium and gallium arsenide, in the Laue diffraction geometry and in an energy range between 60 and 600 keV can reach a higher diffraction efficiency than more dense mosaic crystals such as copper, silver and gold. In particular, self-standing curved crystals can be used as elements in γ-ray lenses.

A figure from Surface-treated self-standing curved crystals as high-efficiency elements for X- and γ-ray optics: theory and experiment 1 Surface-treated self-standing curved crystals — fig. from the paper

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Self‑standing curved Si, Ge and GaAs crystals, alone or stacked, match or exceed heavy mosaic crystals’ diffraction efficiency for 60–600 keV γ‑rays.

What’s new

i.Theoretical study of integrated reflectivity of curved‑diffracting‑plane (CDP) crystals to select optimal materials and parameters.
ii.Demonstration that Si, Ge and GaAs CDP crystals can achieve diffraction efficiency comparable or superior to high‑Z mosaic crystals in the 60–600 keV range.
iii.Development and experimental validation of a stacking method for thicker crystal elements with alignment within a few arcseconds.
iv.Implementation of a surface‑damage lapping technique to produce self‑standing curved crystals with radii down to 1 m.
v.Verification that epoxy glue has negligible effect on crystal curvature during assembly.

How they did it

The authors performed theoretical calculations of integrated reflectivity using dynamical diffraction theory to identify optimal materials, curvature, and thickness, then fabricated surface‑damaged self‑standing curved crystals and experimentally tested single tiles and stacked Si crystals with X‑ray diffraction measurements.

What they found

CDP crystals with curvature radii of several tens of metres are in the strongly bent regime for 60–600 keV X‑ and γ‑rays, enabling high integrated reflectivity.
Optimal crystal thickness t_max = 1/μ maximizes integrated reflectivity, giving Si, Ge and GaAs efficiencies comparable to or better than Cu/Au mosaics.
A stack of two Si(001) CDP tiles (28 × 8 × 0.5 mm, R = 2 m) achieved alignment within a few arcseconds and produced a single well‑defined focal spot at 19 keV.
Glueing a GaAs tile (R = 30.2 m, t = 28 mm) with DEVCON epoxy showed negligible change in curvature, confirming the adhesive’s suitability.

Caveats

✕Single‑crystal thickness h is limited to a few millimetres by the surface‑damage bending technique.
✕Accurate alignment in stacked crystals is critical; misalignments of >1.5% in thickness can cause hundreds of arcseconds error.
✕The method has so far been demonstrated only for semiconductor crystals (Si, Ge, GaAs).
✕Predicted integrated reflectivity values may differ from real crystals due to imperfections and deviations from ideal mosaic behaviour.

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