Beamline Optical Elements

I16 Si 111 channel cut monochromator

I16 uses a Si (111) monochromator providing an energy range between 3 and ~20 keV and an energy resolution of ΔE/E = 10^-4.

The channel cut monochromator gives I16 high flux and high stability, as well as the ability to change energy quickly without significant realignment. The fixed nature of the second reflection means that the beam height moves with energy, as all the beamline change height accordingly.

By performing a double-bounce on the monochromator it is possible to access lower energies below 3 keV.

Location and relative orientation of the phase plate crystals

I16 is equipped with a complex in-vacuum quarter-wave phase retarder to vary the incident polarization of the light from the linear horizontal provided from the insertion device to circular left or right from 2.8 to 10 keV and, linear with arbitrary orientation from zero (// to the synchrotron orbit) to 90 (perpendicular to the synchrotron orbit) from 3.5 to 9 keV.

Read more here: Polarisation Control and Analysis

The main mirrors are used to focus the beam at the sample position to ~200 microns horizontally and 30 microns vertically. The mirrors sit at the 2:3 position, giving low beam divergence. Both mirrors are horizontally deflecting.

The first (vertically focusing) mirror is a 96mm sagittal cylinder, meaning the pitch controls the posiiton of vertical focus.

The second (horizontally focusing) mirror has a polished silicon strip and a Rh coated strip. Rh gives high reflectivity to the highest photon energies (~15 keV at the usual incidence angle of 4 mrad), whereas the si strip gives high reflectivity (very slightly higher than Rh) up to 8 keV, and is used to provide harmonic rejection.

The beamline has a set of Al sheets that can be put into the beam in different arrangements to set different attenuation factors of the incident beam.

The attenuators are used to reduce the incident beam intensity to stop saturation of the detectors and protect sensitive samples from the heatload of the beam.

The attenuators are controlled via a GDA command and the associated transimission coefficient is stored in the metadata, allowing comparison of detector measurements at different attenuation values.

Harmonics arrising from the monochromator are suppressed initially using the main focussing mirrors. By selecting either the Rh strip or uncoated Silicon on these mirrors a reasonable degree of harmonic rejection is achievable.

For almost complete harmonic rejection, an additiona set of flat mirrors can be automatically installed near the sample position. These horizontally deflection "mini mirrors" are rotated such that silicon reflectivity is close to 1 for the desired energy and almost 0 at twice the energy or higher.

The main mirrors on the beamline focus the beam to ~200(h) x 30(v) microns, however further focussing is possible using a set of fixed alignment Kirkpatrick-Baez (KB) mirrors on a motorised tripod system.

This system allows us to achieve microfocussing at the sample position of a few microns.

Setting up the KB mirrors and aligning them is a time consuming process and must be requested prior to beam time.

The I16 Kappa-geometry 6 circle diffractometer

The I16 diffractometer is a 6-circle kappa geometry diffractometer, capable of aligning a sample in any orientation relative to the incident beam.

The large sample space provides space to fit a wide range of sample environments including cryostats, fields and various detectors.

We use a eulerian geometry to drive the sample and detector positions, however these are not always the true motors, for example Chi is a composite of real motors kappa and kphi.

In GDA it is possible to drive the diffractometer using standard Eulerian angles, with the naming convention shown below:

We typically operate the diffratometer in one of two geometries - Vertical or Horizontal, however alternative geometries are possible, for example grazing incidence.

We use DiffCalc to drive the diffractometer directly in reciprocal space, allowing a large number of movement modes including - fixed phi, bisecting, fixed alpha, fixed psi (azimuth).

Read more about the diffractometer: I16 6-axis Diffractometer

A polarisation analyser stage is fitted to the detector arm with 360 degrees of motion from -180 to 180 degrees about the scattered beam. A selection of different analyser crystals are availble depending on what energy and crystal mosaic are required.

Read more about the polarisation analysers here: Polarisation Control and Analysis

The Pilatus3 100K is our workhorse detector used for the majority of measurements. The detector is in-vacuum and has an ultra-high gain option for low energy measurements. The detector is mounted on the detector arm ~8 degrees above the analyser crystal, alowing us to easily switch between different detectors.

Pilatus Geometry

Quad Merlin detector inside PA vessel on I16. The detector is 1,310 mm from the sample and 60 mm from the analyser crystal.

Merlin Geometry

This large area detector sits on an extending table to control the detector distance. Horizontal and vertical alignment is possible on the table, and the table can be manually rotated around the gamma-axis of the diffractometer.

Additional options for the Pilatus 2M include a flight tube and beam stop.

Pilatus 2M Geometry

Detector image

Flight Tube