3.3.1.19. NXevent_data¶

Status:

base class, extends NXobject

Description:

NXevent_data is a special group for storing data from neutron detectors in event mode. In this mode, the detector electronics emits a stream of detectorID, timestamp pairs. With detectorID describing the detector element in which the neutron was detected and timestamp the timestamp at which the neutron event was detected. In NeXus detectorID maps to event_id, event_time_offset to the timestamp.

As this kind of data is common at pulsed neutron sources, the timestamp is almost always relative to the start of a neutron pulse. Thus the pulse timestamp is recorded too together with an index in the event_id, event_time_offset pair at which data for that pulse starts. At reactor source the same pulsed data effect may be achieved through the use of choppers or in stroboscopic measurement setups.

In order to make random access to timestamped data faster there is an optional array pair of cue_timestamp_zero and cue_index. The cue_timestamp_zero will contain courser timestamps then in the time array, say every five minutes. The cue_index will then contain the index into the event_id,event_time_offset pair of arrays for that courser cue_timestamp_zero.

Symbols:

No symbol table

Groups cited:

none

Structure:

event_time_offset[i]: (optional) NX_NUMBER {units=NX_TIME_OF_FLIGHT}

A list of timestamps for each event as it comes in.

event_id[i]: (optional) NX_INT {units=NX_DIMENSIONLESS}

There will be extra information in the NXdetector to convert event_id to detector_number.

event_time_zero[j]: (optional) NX_NUMBER {units=NX_TIME}

The time that each pulse started with respect to the offset

@offset: (optional) NX_DATE_TIME

ISO8601

event_index[j]: (optional) NX_INT {units=NX_DIMENSIONLESS}

The index into the event_time_offset, event_id pair for the pulse occurring at the matching entry in event_time_zero.

pulse_height[i, k]: (optional) NX_FLOAT {units=NX_DIMENSIONLESS}

If voltages from the ends of the detector are read out this is where they go. This list is for all events with information to attach to a particular pulse height. The information to attach to a particular pulse is located in events_per_pulse.

cue_timestamp_zero: (optional) NX_DATE_TIME {units=NX_TIME}

Timestamps matching the corresponding cue_index into the event_id, event_time_offset pair.

@start: (optional) NX_DATE_TIME

cue_index: (optional) NX_INT

Index into the event_id, event_time_offset pair matching the corresponding cue_timestamp.

NXDL Source:

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