Crystal LabFerrara · UniFe & INFN

Research › X-ray and γ-ray optics

X-ray spectroscopy

Bent crystal analysers

Crystals bonded to shaped substrates keep the exact curvature that sharp X-ray spectra need.

The physics.

A bent crystal analyser on the Rowland circleSample, curved crystal and detector lie on a circle, the Rowland circle. The crystal is bent to twice the circle radius, so X-rays from the sample reflected anywhere on its surface at the Bragg angle reach the detector. θB bent crystal analyser (radius 2r) sampledetector Rowland circle(radius r) incident X-raysJohann geometry
Sample, bent crystal and detector on the Rowland circle. The crystal is bent to twice the radius of the circle.

Res­o­nant in­elas­tic X-ray scatter­ing, X-ray Raman and emis­sion spec­tros­co­py measure small changes in the energy of photons scat­tered by a sample. The energy is se­lect­ed by Bragg diffrac­tion in a crystal, 2d sin θB = λ: only photons that meet the planes at the right angle reach the de­tec­tor. Differentiat­ing the Bragg law gives the resolu­tion

ΔE / E = cot θB · Δθ

which im­proves as θB ap­proach­es 90°, so analysers work close to backscatter­ing.

To collect photons over a large solid angle the crystal is bent. Sample, crystal and de­tec­tor sit on the Rowland circle; a crystal curved to twice the radius of the circle re­flects photons from the sample onto the de­tec­tor from its whole surface (Johann ge­om­e­try), and a spher­i­cal crystal focuses in both direc­tions.

The resolu­tion then depends on how faith­ful­ly the crystal follows its shape: shape errors and strain widen the band of re­flect­ed en­er­gies. Most analysers today are thin silicon wafers anodi­cally bonded to glass. Quartz, lithium niobate and ger­ma­nium offer other lattice spac­ings, angles closer to backscatter­ing and higher re­flec­tiv­i­ty, but are hard to work and are usually glued, which leaves larger shape errors.

In CRYSTAL BOND, the INFN project we lead with Legnaro, Trieste and Frascati, crys­tals are bonded without glue, by anodic and thermo-compres­sion bonding, to sub­strates figured with optical preci­sion, and take their shape. Besides spher­i­cal analysers for Bragg ge­om­e­try we make cylindri­cally bent crys­tals for Laue ge­om­e­try, used in spec­tral imaging: a poly­chro­mat­ic syn­chro­tron beam is spread by energy across a de­tec­tor, and images on either side of an absorp­tion edge sep­a­rate a con­trast agent from tissue.

What we have shown.

We bend silicon and ger­ma­nium onto spher­i­cal and cy­lin­dri­cal sub­strates without glue, so the crystal keeps its cur­va­ture and does not twist. With CRYSTAL BOND we build analysers and mono­chromators for syn­chro­tron beam­lines and lab­o­ra­to­ry spec­trom­e­ters, and a PhD on bent-crystal optics for X-ray spec­trom­e­try and imaging, to­geth­er with Elec­tron MEC, takes them toward compact instru­ments.

Quartz, LiNbO₃

First analyser prototypes in quartz and lithium niobate, bonded to shaped glass supports by an adhesive-assisted process while the dedicated bonding equipment was being installed.

≈ silicon

Preliminary tests of a quartz prototype at ESRF: performance comparable with conventional silicon analysers.

0.1–1 m

Radii of the spherical and cylindrical crystals in development, in germanium, quartz and lithium niobate, with an energy-resolution goal around 10 meV.