LoadEventNexus v1

../_images/LoadEventNexus-v1_dlg.png

LoadEventNexus dialog.

Summary

Loads an Event NeXus file and stores as an EventWorkspace. Optionally, you can filter out events falling outside a range of times-of-flight and/or a time interval.

Properties

Name Direction Type Default Description
Filename Input string Mandatory The name of the Event NeXus file to read, including its full or relative path. The file name is typically of the form INST_####_event.nxs (N.B. case sensitive if running on Linux). Allowed extensions: [‘.nxs.h5’, ‘.nxs’, ‘_event.nxs’]
OutputWorkspace Output Workspace Mandatory The name of the output EventWorkspace or WorkspaceGroup in which to load the EventNexus file.
NXentryName Input string   Optional: Name of the NXentry to load if it’s not the default.
FilterByTofMin Input number Optional Optional: To exclude events that do not fall within a range of times-of-flight. This is the minimum accepted value in microseconds. Keep blank to load all events.
FilterByTofMax Input number Optional Optional: To exclude events that do not fall within a range of times-of-flight. This is the maximum accepted value in microseconds. Keep blank to load all events.
FilterByTimeStart Input number Optional Optional: To only include events after the provided start time, in seconds (relative to the start of the run).
FilterByTimeStop Input number Optional Optional: To only include events before the provided stop time, in seconds (relative to the start of the run).
BankName Input str list   Optional: To only include events from one bank. Any bank whose name does not match the given string will have no events.
SingleBankPixelsOnly Input boolean True Optional: Only applies if you specified a single bank to load with BankName. Only pixels in the specified bank will be created if true; all of the instrument’s pixels will be created otherwise.
Precount Input boolean True Pre-count the number of events in each pixel before allocating memory (optional, default True). This can significantly reduce memory use and memory fragmentation; it may also speed up loading.
CompressTolerance Input number -1 Run CompressEvents while loading (optional, leave blank or negative to not do). This specified the tolerance to use (in microseconds) when compressing.
ChunkNumber Input number Optional If loading the file by sections (‘chunks’), this is the section number of this execution of the algorithm.
TotalChunks Input number Optional If loading the file by sections (‘chunks’), this is the total number of sections.
LoadMonitors Input boolean False Load the monitors from the file (optional, default False).
MonitorsLoadOnly Input string   If multiple repesentations exist, which one to load. Default is to load the one that is present. Allowed values: [‘’, ‘Events’, ‘Histogram’]
FilterMonByTofMin Input number Optional Optional: To exclude events from monitors that do not fall within a range of times-of-flight. This is the minimum accepted value in microseconds.
FilterMonByTofMax Input number Optional Optional: To exclude events from monitors that do not fall within a range of times-of-flight. This is the maximum accepted value in microseconds.
FilterMonByTimeStart Input number Optional Optional: To only include events from monitors after the provided start time, in seconds (relative to the start of the run).
FilterMonByTimeStop Input number Optional Optional: To only include events from monitors before the provided stop time, in seconds (relative to the start of the run).
SpectrumMin Input number Optional The number of the first spectrum to read.
SpectrumMax Input number Optional The number of the last spectrum to read.
SpectrumList Input int list   A comma-separated list of individual spectra to read.
MetaDataOnly Input boolean False If true, only the meta data and sample logs will be loaded.
LoadLogs Input boolean True Load the Sample/DAS logs from the file (default True).
LoadType Input string Default Set type of loader. 2 options {Default, Multiproceess},’Multiprocess’ should work faster for big files and it is experimental, available only in Linux. Allowed values: [‘Default’, ‘Multiprocess (experimental)’]
LoadNexusInstrumentXML Input boolean True Reads the embedded Instrument XML from the NeXus file (optional, default True).

Description

The LoadEventNeXus algorithm loads data from an EventNexus file into an EventWorkspace. The default histogram bin boundaries consist of a single bin able to hold all events (in all pixels), and will have their units set to time-of-flight. Since it is an EventWorkspace, it can be rebinned to finer bins with no loss of data.

Child algorithms used

Sample logs, such as motor positions or e.g. temperature vs time, are also loaded using LoadNexusLogs.

Monitors are loaded using LoadNexusMonitors.

Instrument geometry

There are a series of approaches for extracting the instrument geometry. These follow the escalation path as follows:

  • Tries to load embedded instrument_xml from the NXinstrument if present using LoadIDFFromNexus.
  • Else tries to load embedded nexus geometry from the NXinstrument if present
  • Else tries to load the instrument using the name extracted from NXinstrument

The latter two possibilities are achieved via LoadInstrument

Optional properties

If desired, you can filter out the events at the time of loading, by specifying minimum and maximum time-of-flight values. This can speed up loading and reduce memory requirements if you are only interested in a narrow range of the times-of-flight of your data.

You can specify to load only certain spectra within the file, using the SpectraMax, SpectraMin and SpectraList properties. This will load data only matching those restrictions. At facilities that do not group detectors in hardware such as the SNS, then this will also equate to the detector IDs.

You may also filter out events by providing the start and stop times, in seconds, relative to the first pulse (the start of the run).

If you wish to load only a single bank, you may enter its name and no events from other banks will be loaded.

The Precount option will count the number of events in each pixel before allocating the memory for each event list. Without this option, because of the way vectors grow and are re-allocated, it is possible for up to 2x too much memory to be allocated for a given event list, meaning that your EventWorkspace may occupy nearly twice as much memory as needed. The pre-counting step takes some time but that is normally compensated by the speed-up in avoid re-allocating, so the net result is smaller memory footprint and approximately the same loading time.

Veto Pulses

Veto pulses can be filtered out in a separate step using FilterByLogValue v1:

FilterByLogValue(InputWorkspace="ws", OutputWorkspace="ws", LogName="veto_pulse_time", PulseFilter="1")

Data Loaded from Nexus File

The nexus file must have /raw_data_1 or /entry as its main group and that group be of type NXentry. It also needs a group of type NXevent_data.

The data is read from each group of type NXevent_data.

If the file has an isis_vms_compat then it is taken to be an ISIS file and the data will be modified according to the information obtained from this group.

Here are some tables that show it in more detail:

Description of Data Found in Nexus file (within ‘raw_data_1’) Placed in Workspace (Workspace2D)
Monitor Data groups of Class NXMonitor (one monitor per group) Monitor Data
Detector Data groups of Class NXevent_data (one bank per group) Event data
Instrument group Instrument Workspace instrument if not overridden
Spectrum of each detector ID If isis_vms_compat exists, NSP1, UDET and SPEC within it, else one spectrum per detector assumed Spectra-Detector mapping
Run mainly as loaded from LoadNexusLogs v1 Run Object
Sample If isis_vms_compat exists, SPB and RSPB within isis_vms_compat else sample not loaded Sample Object

Invalid Period Logs Error

There is an issue specific to ISIS, particularly on long runs, where noise on data collection instruments can change the period of the data. This can cause the period associated with certain data points to exceed the total number of periods we expect.

LoadEventNeXus compares the total number of periods we expect, which can be found in the periods/number attribute of the run, and the greatest period number found in framelog/period_log/value attribute of the data. If they do not match, an error message is raised which explains this problem, and loading of the data will be unsuccessful.

In this case, the data is fundamentally corrupted, so Mantid does not know how to correct this automatically. If you only expect 1 period, there is a script in the script repository, user/TomTitcombe/correct_period_logs.py, which will change all the periods in framelog/period_log/value to 1.

If you expect more than 1 periods, there is currently no solution script available through Mantid. You should contact the Mantid team to discuss the problem and possible solutions.

Sample Object

If isis_vms_compat exists, then the following sample properties are read from it:

Nexus Workspace sample object
SPB[2] Geometry flag
RSPB[3] Thickness
RSPB[4] Height
RSPB[5] Width

This is the same as read by LoadISISNexus v2.

Usage

Note

To run these usage examples please first download the usage data, and add these to your path. In MantidPlot this is done using Manage User Directories.

Example - Load SNS/ISIS event Nexus file:

# Load SNS HYS event dataset
ws = LoadEventNexus('HYS_11092_event.nxs')

print("The number of histograms (spectra) is: {}".format(ws.getNumberHistograms()))

Output:

The number of histograms (spectra) is: 20480

Example - Load event nexus file with time filtering:

# Load SNS CNCS event dataset between 10 and 20 minutes
ws = LoadEventNexus('CNCS_7860_event.nxs', FilterByTimeStart=600, FilterByTimeStop=1200)

print("The number of events: {}".format(ws.getNumberEvents()))

Output:

The number of events: 112266

Categories: AlgorithmIndex | DataHandling\Nexus

Source

C++ source: LoadEventNexus.cpp (last modified: 2019-08-27)

C++ header: LoadEventNexus.h (last modified: 2019-06-07)