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## 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.

Input

boolean

False

Load the monitors from the file (optional, default False).

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.

Input

boolean

False

If true, only the meta data and sample logs will be loaded.

Input

boolean

True

Load only the Sample/DAS logs from the file (default True).

Input

boolean

False

Load all the logs from the nxs, without checking or processing them; if checked, LoadLogs will be ignored; use with caution

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)’]

Input

boolean

True

Reads the embedded Instrument XML from the NeXus file (optional, default True).

NumberOfBins

Input

number

500

The number of bins intially defined. Use Rebin to change the binning later. If there is no data loaded, or you select meta data only you will only get 1 bin.

AllowList

Input

str list

If specified, only these logs will be loaded from the file (each separated by a space).

BlockList

Input

str list

If specified, these logs will NOT be loaded from the file (each separated by a space).

## Description¶

The LoadEventNeXus algorithm loads data from an EventNexus file into an EventWorkspace. The histogram bin boundaries depend on the setting of NumberOfBins, which by default will be the whole range of time of flight able to hold all events (in all pixels) split into NumberOfBins linear bins, 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.

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¶

If LoadAllLogs is checked, all the logs in the Nexus files will be loaded directly in the sample logs as they are. The LoadLogs flag will be ignored. If only LoadLogs is checked, only a subset of the logs will be processed and loaded, in the manner described afterward.

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

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

## Usage¶

Note

Example - Load SNS/ISIS event Nexus file:

# Load SNS HYS event dataset

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 1 and 2 minutes

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


Output:

The number of events: 29753


Categories: AlgorithmIndex | DataHandling\Nexus