Reflectometry at ISIS

work in progress

For an introduction to Reflectometry, see this document. It was written for the Mantid team in the context of data reduction but contains a good, brief introduction to the fundamentals of the technique as well as information on some of the differences between the various ISIS reflectometers.

Reflectometry is a technique where you shoot a beam at a flat sample at an angle and study the reflected neutron beam to gather insights about the properties of the surface or boundary between media. There are currently 5 reflectometry instruments at ISIS (in order of increasing complexity): CRISP, INTER, SURF, POLREF and OFFSPEC. The basic setup of a reflectometry experiment is as follows:

[diagram] With an incident angle of θ on the sample, detectors need to be at 2θ relative to the incident beam. Detectors on the beamlines move around to coincide with the angle of the sample. At this time, CRISP, INTER and SURF use point detectors while POLREF and OFFSPEC use 1D (linear) detectors. 2D (area) detectors are planned for the future.

Some of the instruments contain additional equipment:

• Supermirrors: SURF and INTER are optimized for liquid samples, which cannot be angled. Instead, these instruments are equipped with supermirrors to change the angle of the incident beam.
• Polarizers: POLREF and OFFSPEC are equipped with polarizers that align all neutrons to one of two spin directions and allow scientists to study magnetic properties of the sample.

POLREF Overview

(diagram from slides or make own) POLREF can be run in polarized or non-polarized mode. Furthermore, it can be run in horizontal or vertical mode. This refers to the sample orientation, i.e. the detector would be in vertical / horizontal mode respectively. If in horizontal mode, the detector moves up and down to coincide with the angle of the sample. If in vertical mode, the bench the detector is mounted on slides around the sample area on a circular path.

Most of the time, the instrument is run in polarized and horizontal mode. Mounting a flat sample vertically requires extensive and laborious changes to the setup of the sample stack. When it is run in vertical mode, then usually with a block sample the beam passes through (rather than a flat one) which does not require these changes.

Details on the setup:

• The main area of interest is inside the blockhouse (beyond monitor 2)
• The beam enters the blockhouse at a natural (downward) angle of 2.3°
• All jaws are adjustable in both height and angle. They want to be centered and perpendicular to the beam path as this allows the beam to be shaped more sharply. Each pair of jaws is slightly offset to avoid collisions.
• The first set of polarizers consists of:
  • a frame overlap mirror (or FOM) designed to filter out noise in the form of slow neutrons from previous pulses
  • a current sheet which aligns the spin of all neutrons along the same axis
  • a reflection polarizer which filters out all spin-down neutrons. Important to note: the reflection polarizer introduces a slight angle to the beam that passes through, which all subsequent beamline equipment has to adhere to!
• This is followed by a flipper which can be turned off or on depending on whether the user wants spin-up or spin-down neutrons at the sample point.
• Everything past the sample point sits on a movable bench which is driven by 2 pairs of motors on the front/back corners of the bench in order to lift and angle it. Additionally the bench slides forwards and backwards as the detectors have to maintain a fixed distance to the sample. The pivot point of the bench is roughly 2/3rds of the way to the back of the bench. As such, motion calculations are fairly complex.
• There is a laser mounted on the beamline to assist with alignment of components, however the final fine tuning is done with neutrons as the laser will not be 100% accurately tuned to the beam path.

The alignment process on POLREF

The most important task by far while aligning the beamline is scanning, i.e. taking measurements of beam intensity at regular intervals within a range of values along an axis. The This is performed multiple times for each beamline component and takes up the bulk of the time needed for alignment of the instrument. This is done to within a 5-10 micron level of precision.

Apart from the scan range / step size, each scan requires 3 elements: the axis to be scanned (e.g. slit 2 gap) and two monitors, one before and after the component in question, in order to scale the results.

• Scanning, scanning, scanning
• "rifle sight"
• a lot of judgement involved

Requirements 1: Must-have

• Top level views with high information density (POLREF dashboard, Motors table Level of detail, setpoints)
• Ability to scan blocks + visualise scan

Requirements 2: Improvements

• generic scan command with multi axis support
• live view (raw pixel data fine)