Reflectometry Glossary

A

Alignment: The process of finding the position of an axis where it is centred/perpendicular to the neutron beam by scanning over a range of positions. This position is then usually defined as 0.

Diagram of axis conventions available here

Axis, X: Translation across the Natural Beam.

Axis, Y: Height relative to the Natural Beam.

Axis, Z: Displacement along the Natural Beam.

Axis, Chi: Yaw angle of a component relative to the Natural Beam. (Rotation in XZ Plane)

Axis, Phi: Pitch angle of a component relative to Natural Beam. (Rotation in YZ Plane)

Axis, Psi: Roll angle of a component relative to Natural Beam. (Rotation in XY Plane)

Axis, Long: Z Translation axis of disused INTER detector (pre tank upgrade)

Axis, Seesaw: Rotation of a bench around its centre, achieved by applying an equal but inverse height offset to its front and back height jacks. Used for bench Alignment. Different to Bench Angle which rotates around the virtual sample point

B

Beam, Incoming: The source of the beam segment before interacting with a given a component as described by Y, Z and Angle coordinates. This is the Outgoing Beam of the last previous reflecting component the beam has interacted with, or the Natural Beam if no reflections apply.

Beam, Outgoing: The origin of the beam path segment after interacting with a given component as described by Y, Z and Angle coordinates. This becomes the Incoming Beam for the following component. If this component is non-reflecting, repeat the outgoing beam of the last previous reflecting component.

Beam, Natural: (aka Straight Through Beam) The neutron beam as it enters an instrument blockhouse without any additional reflections. This is a downward 1.5° for TS1 beamlines and a downward 2.3° for TS2 beamlines. The Natural Beam defines the Z axis of the Y/Z tracking plane in which the coordinate system used by the reflectometry server exists.

Beam, Reflected: The current beam path including any reflections from mirrors & sample (often contrasted with the theoretical Natural Beam)

Beamline Object: The top level object holding & coordinating the whole geometry model inside the reflectometry server.

Bench: A large movable bench that pivots on an arc around a point of reflection i.e. a mirror or sample. Benches can be found on OFFSPEC and POLREF. They are all identical in dimensions and are each driven by 3 physical axes: Front height jack, back height jack and slide (which is a linear translation towards/away from the pivot). From these, we derive the bench angle and height offset from the beam. Benches usually have other components statically mounted on top of them e.g. Slits, Detectors

C

Characteristic Value: The low level motor value which a given Beamline Parameter is derived from. Can be added as a readback to a Beamline Parameter for diagnostics purposes.

Component: A node in the geometry model that interacts with the beam in some way (tracking or modifying its path). Each usually represents one physical device on the beamline (with some exceptions, e.g. the Theta Component which is virtual)

Component, Passive: An item on the beamline which interacts with the beam but does not change the direction of the incoming beam e.g. slits

Component, Reflecting: An item on the beamline that can change the direction of the incoming beam for e.g. a super mirror

Configuration: Configuration in reflectometry defines the beamline and the components in the beamline. Reflectometry configuration is written in python.

Constants: (aka Beamline Constants) A set of constant values for this beamline defined in the reflectometry configuration and exposed via PVs to be read in other places e.g. the OPI & the reflectometry scripts. These include the Z coordinates of each component, the maximum achievable Theta angle or the name of the correct front panel OPI for this beamline.

Coordinates, Mantid: Coordinate system where everything moves perpendicular to the Natural Beam. The beamline geometry model in the IOC is described in this coordinate system. So named as this convention originated in the Mantid project.

Coordinates, Room: Coordinate system where everything moves perpendicular to the floor. Axis positions at the motor level are reported in this coordinate system, as are distances measured between components as part of beamline surveys i.e. these constants usually need to be transformed into their equivalent distances along the Natural Beam for the reflectometry config.

Correction: See Engineering Correction

D

Detector, Point (0D): A simple tube is used to integrate intensity, the detector itself has no position sensitivity, just the angle it is positioned at

Detector, Linear (1D): A stack of tubes/pixels is used to integrate intensity with either vertical or horizontal position sensitivity equal to the pixel size

Detector, Area (2D): Similar to the 1D but with both vertical and horizontal position sensitivity. Note pixels may not be uniform in horizontal/vertical size.

Downstream: Further from the source of the neutron beam relative to a given point

Driver: (aka Composite Driver, Ioc Driver) part of the reflectometry server that interacts with the motor PVs relating to a given component. Handles simple reads and writes via a PV Wrapper, as well as some more complex logic such as engineering corrections, move synchronization and translating parking toggles into concrete positions

E

Engineering Correction: A correction that is added to setpoints / removed for readback values for a given axis at the Composite Driver level. These corrections can be of fixed value, or calculated by a user function or an interpolation matrix. These exist to account for slight inaccuracies in engineering, e.g. a mirror that is not perfectly flat.

F

Flux: The number of neutrons passing through a given point (e.g. a monitor, a detector) at any one time.

Footprint: The surface area of the sample illuminated by the neutron beam. Depends on Theta and slit gap sizes.

G

Goniometer: A rotation stage with several degrees of freedom that lets you freely rotate a mounted component.

I

Incident Angle: The angle between an incident ray (of neutrons) and a reflective surface (eg. a mirror or sample)

L

Laser: Typically refers to an optical laser that sits at the start of a reflectometry beamline and can be used for rough visual alignment of devices before aligning them more precisely with neutrons. Some instruments also have a Laser known by its manufacturer ("Keyence") which measures the precision height of the sample and is used to automatically apply a height offset depending on varying sample thickness so that it stays aligned to the beam.

M

Mirror: Some samples, such as liquids, cannot be angled. Mirrors can be used to change the incident angle of the beam while keeping the sample level to gravity, essentially rotating the whole beamline around the sample rather than rotating the sample around the beam.

Mirror, Super: Non-polarising mirror designed specifically for reflecting neutrons.

Mirror, Polarising: A mirror that additionally polarises the neutrons that it reflects.

Mode: A set of defaults which can be used to easily configure the behaviour of the reflectometry server's beamline model at runtime. Modes can define which parameters track i.e. automatically move to stay aligned to the reflected beam, which components should be moved in/out of the beam, which corrections get applied, and they can define a set of parameter default values to apply when you enter the mode (and optionally, to re-apply on every beamline move).

• Neutron Reflection mode (NR) : The simplest case - no mirrors or polarisers, just reflecting the beam off the sample.
• Liquid mode: Reflect the beam off a mirror (or multiple) to change the incident angle while keeping the sample level
• Polarised Neutron Reflection mode (PNR): Basically the same as liquid mode from a motion perspective, but with an added polariser system.
• Neutron Reflection mode with Analyser (NA): NR but with an analyser component in the beam between sample and detector
• Polarised Neutron Reflection mode with Analyser (PA): PNR & NA modes combined
• Disabled Mode: Disables all tracking and stops the beam path from being able to change while in this mode. Can be used e.g. for aligning a super mirror which would otherwise move the detector while scanning.

Mode Init: A default value for a given mode you can configure for a parameter. This value is automatically applied as SP when changing to this mode.

Move, Beamline: Move the whole beamline model i.e. apply all currently un-applied parameter SP values, and reapply SP:RBV values for all parameters in the current mode and tracking the beam.

Move, Parameter: Move a single parameter only i.e. (re-)apply its current SP and re-apply the SP:RBV of all downstream parameters in the current mode.

Move, Synchronised: (aka Concurrent Move) When moving several axes at the same time, the reflectometry server will calculate the duration for the slowest axis and then modify the velocity of all other axes to finish at the same time (more or less). This is not truly synchronised and not good enough for e.g. continuous scanning, but generally good enough to avoid what would be clashes if one axis "overtakes" another. Parameters have a flag to be be excluded from synchronised moves in case they are very slow and could make other axes stall, and do not have clash conditions.

P

Parameter: (aka Beamline Parameter) A top-level user parameter, describing some value relative to the incoming beam for the related component.

Parameter, Axis: Numerical parameter that controls a single axis relative to the incoming beam.

Parameter, Direct: A Parameter that talks directly to a given PVWRapper without a Component or Driver in between. Does not provide any tracking functionality - used e.g. for Slit gap/centre parameters, or for the INTER Tank where the tracking is done inside the motor controller and the virtual axes are exposed as motor PVs.

Parameter, InBeam: A toggle parameter that lets you move a component in or out of the beam. See also Parking

Parking: The act of moving a component to a predefined position via an InBeam Parameter - conceptually the same as a motion setpoint. However parking a component may apply setpoints for multiple of its axes (e.g. to park supermirror, set height to -20 and angle to 0)

Parking Sequence: A parking sequence defines a series of preset positions to move through when parking a component. The reason for this is that for some devices there is something physically in the way on the beamline between the in and out positions. (e.g. park a monitor by moving it up, to the side, then down)

PV Wrapper: A thin wrapper around a motor or slit axis which monitors and caches a specific subset of fields that we care about. This layer provides a couple of performance benefits: a) because the values are cached we do not need to wait for channel access whenever we need to read one, and b) we can bundle and trigger updates periodically so we don't have to process every monitor event. These can take quite a while to process as they may trigger updates of the whole beamline model and so may overwhelm the reflectometry server in great volumes.

R

Readback Value (RBV): The current actual position of a parameter or axis.

Redefine: Beamline Parameters can be redefined via the reflectometry server, which abstracts the process of setting a motor axes from USE to SET mode, redefining the user offset and going back to USE mode into a single PV write. Redefining via a Beamline Parameter also takes any engineering corrections into account.

S

Setpoint (SP): The target position to apply to a given parameter for the next time it receives an explicit move instruction.

Setpoint Readback Value (SP:RBV): The target position a given parameter received for their last move. Parameters in the current mode track the beam by automatically re-applying this (relative) value every time the beam path changes.

Scanning: The process of moving a given axis over a range of positions around the beam at discrete steps, taking neutron data at every step. Plotting out the beam intensity over the range of positions gives you a graph with some kind of feature (e.g. a peak) that indicates where the axis is perfectly aligned to the beam.

T

Tank: Refers to the INTER detector tank. This is a large component that pivots on an arc around the virtual sample point, like the benches found on POLREF and OFFSPEC. It is however different, in that instead of two jacks it is driven by a linear height and rotation axis, and that its slide axis moves parallel to the floor rather than parallel to the current bench angle.

Theta: (aka Incident Angle at the Sample) The reflection Angle of the beam at the sample. Neutron data for a single sample is usually collected at a few different Theta angles and then stitched together to form a complete dataset. NB Theta does NOT drive the sample phi angle. Instead Theta just describes the theoretical path the beam WOULD take for a given value so that downstream components can track it. Similarly, the readback value does not come from the sample phi angle either but from a representative axis of a downstream component, i.e. detector height if moving along a linear height stage, or bench/tank angle for detectors mounted on either of those components moving on an arc. The rationale for this is that we never not want to tilt the sample angle implicitly as it may have severe consequences e.g. if a large liquid tank is mounted there. Instead this is done via a separate PHI parameter which is never in the mode. So to make sure that the detector gets neutrons, Phi needs to be set accordingly for a given Theta.

Tracking: Automatically moving in order to stay centred on the reflected beam as and when it changes.

Trans: Can mean one of two things

• Translation: See Axis, X
• Transmission: A data collection run with the sample out of beam, used for normalising neutron data.

U

Upstream: Closer to the source of the neutron beam relative to a given point

V

Virtual Sample Point: The theoretical intersection between the incoming beam at the sample and its movement axes.