# 3.3.1.16. NXdisk_chopper¶

Status:

base class, extends NXobject

Description:

A device blocking the beam in a temporal periodic pattern.

A disk which blocks the beam but has one or more slits to periodically let neutrons through as the disk rotates. Often used in pairs, one NXdisk_chopper should be defined for each disk.

The rotation of the disk is commonly monitored by recording a timestamp for each full rotation of disk, by having a sensor in the stationary disk housing sensing when it is aligned with a feature (such as a magnet) on the disk. We refer to this below as the “top-dead-center signal”.

Angles and positive rotation speeds are measured in an anticlockwise direction when facing away from the source.

Symbols:

This symbol will be used below to coordinate datasets with the same shape.

n: Number of slits in the disk

Groups cited:

NXgeometry

Structure:

type: (optional) NX_CHAR

Type of the disk-chopper: only one from the enumerated list (match text exactly)

Any of these values:

• Chopper type single

• contra_rotating_pair

• synchro_pair

rotation_speed: (optional) NX_FLOAT {units=NX_FREQUENCY}

Chopper rotation speed. Positive for anticlockwise rotation when facing away from the source, negative otherwise.

slits: (optional) NX_INT

Number of slits

slit_angle: (optional) NX_FLOAT {units=NX_ANGLE}

Angular opening

pair_separation: (optional) NX_FLOAT {units=NX_LENGTH}

Disk spacing in direction of beam

slit_edges[2n]: (optional) NX_FLOAT {units=NX_ANGLE}

Angle of each edge of every slit from the position of the top-dead-center timestamp sensor, anticlockwise when facing away from the source. The first edge must be the opening edge of a slit, thus the last edge may have an angle greater than 360 degrees.

Timestamps of the top-dead-center signal. The times are relative to the “start” attribute and in the units specified in the “units” attribute. Please note that absolute timestamps under unix are relative to 1970-01-01T:00:00.

@start: (optional) NX_DATE_TIME

beam_position: (optional) NX_FLOAT {units=NX_ANGLE}

Angular separation of the center of the beam and the top-dead-center timestamp sensor, anticlockwise when facing away from the source.

slit_height: (optional) NX_FLOAT {units=NX_LENGTH}

Total slit height

phase: (optional) NX_FLOAT {units=NX_ANGLE}

Chopper phase angle

delay: (optional) NX_NUMBER {units=NX_TIME}

Time difference between timing system t0 and chopper driving clock signal

ratio: (optional) NX_INT

Pulse reduction factor of this chopper in relation to other choppers/fastest pulse in the instrument

distance: (optional) NX_FLOAT {units=NX_LENGTH}

Effective distance to the origin

wavelength_range[2]: (optional) NX_FLOAT {units=NX_WAVELENGTH}

Low and high values of wavelength range transmitted

GEOMETRY: (optional) NXgeometry

NXDL Source:

https://github.com/nexusformat/definitions/blob/master/base_classes/NXdisk_chopper.nxdl.xml