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LoadISISNexus v2

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Summary

Loads a file in ISIS NeXus format.

See Also

LoadEventNexus, SaveISISNexus

Properties

Name	Direction	Type	Default	Description
Filename	Input	string	Mandatory	The name of the Nexus file to load. Allowed extensions: [‘.nxs’, ‘.n*’]
OutputWorkspace	Output	Workspace	Mandatory
SpectrumMin	Input	number	0
SpectrumMax	Input	number	Optional
SpectrumList	Input	int list
EntryNumber	Input	number	0	0 indicates that every entry is loaded, into a separate workspace within a group. A positive number identifies one entry to be loaded, into one worskspace
LoadMonitors	Input	string	Include	Option to control the loading of monitors. Allowed options are Include,Exclude, Separate. Include:The default is Include option would load monitors with the workspace if monitors spectra are within the range of loaded detectors. If the time binning for the monitors is different from the binning of the detectors this option is equivalent to the Separate option Exclude:Exclude option excludes monitors from the output workspace. Separate:Separate option loads monitors into a separate workspace called: OutputWorkspace_monitors. Defined aliases: 1: Equivalent to Separate. 0: Equivalent to Exclude. Allowed values: [‘Include’, ‘Exclude’, ‘Separate’]

Description

Loads a Nexus file created from an ISIS instrument.

Data loaded from Nexus File

Not all of the nexus file is loaded. This section tells you what is loaded and where it goes in the workspace.

The nexus file must have a raw_data_1 top-level entry to be loaded.

The workspace data is loaded from raw_data_1/Detector_1.

Instrument information is loaded from raw_data_1/Instrument if available in file, otherwise instrument information is read from a MantidInstall instrument directory.

If the main entry contains a /isis_vms_compat the following entries will be read by the Algorithm: NSP1, UDET, SPEC, HDR, IRPB, RRPB, SPB and RSPB. The contents of isis_vms_compat are a legacy from an older ISIS format. If the legacy vms_compat block is not present, an equivalent entry in the file is used.

In the tables below, descriptions of the relevant nexus entries are given. If applicable, the location within the vms_block and an alternative location to read the entry from is given. In these tables /group refers to a group under the main entry, i.e /group is equivalent to raw_data_1/group.

Description of entry	vms_compat entry	Alternative Location
Spectra-Detector mapping	NSP1, UDET and SPEC within isis_vms_compat	/detector_1/spectrum_index (assumes 1-1 mapping)
Sample	SPB and RSPB within isis_vms_compat	/sample group

Description of Data	Found in Nexus file (within ‘raw_data_1’)	Placed in Workspace (Workspace2D)
Monitor Data	within groups of Class NXMonitor (one monitor per group)	Monitor histogram data (loaded depending on prop. LoadMonitors)
Detector Data	group Detector_1 (all detectors in one group)	Histogram Data
Instrument	group Instrument	Workspace instrument
Run	various places as shown below	Run object

Run Object

LoadISISNexus executes LoadNexusLogs v1 to load run logs from the Nexus runlog or some other appropriate group. It also loads the Nexus raw_data_1/periods/proton_charge group into the proton_charge_by_period property of the workspace run object.

Properties of the workspace Run object are loaded as follows:

vms_compat	Alternative location	Workspace run object
title	/title	run_title
start_time	/start_time	run_start
end_time	/end_time	run_end
(data)	(data)	nspectra
(data)	(data)	nchannels
(data)	(data)	nperiods
IRPB[6]	/instrument/source/frequency	freq
IRPB[7]	/proton_charge	gd_prtn_chrg
RRPB[9]	good_frames	goodfrm
RRPB[10]	/raw_frames	rawfrm
RRPB[21]	/experiment_identifier	rb_proposal

The group (data) indicates that the number is obtained from the histogram data in an appropriate manner.

IRPB and RRPB point to the same data. In IRPB, the data is seen as 32-bit integer and in RRPB it is seen as 32-bit floating point (same 32 bits). In all cases, integers are passed. The 32-bit floating point numbers are used only to store larger integers.

The other indices of IRPB and RRPB are not read.

Sample Object

Properties of the workspace sample object are loaded as follows:

vms_compat	Alternative location	Workspace sample object
SPB[2]	/sample/id	Geometry flag
RSPB[3]	/sample/thickness	Thickness
RSPB[4]	/sample/height	Height
RSPB[5]	/sample/width	Width

Usage

Note

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

Example - Load without any optional arguments:

# Load LOQ histogram dataset
ws = LoadISISNexus('LOQ49886.nxs')

print("The 1st x-value of the first spectrum is: {}".format(ws.readX(0)[0]))

Output:

The 1st x-value of the first spectrum is: 5.0

Example - Using SpectrumMin and SpectrumMax:

# Load from LOQ data file spectrum 2 to 3.
ws = LoadISISNexus('LOQ49886.nxs',SpectrumMin=2,SpectrumMax=3)

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

Output:

The number of histograms (spectra) is: 2

Example - Using EntryNumber:

# Load first period of multiperiod POLREF data file
ws = LoadISISNexus('POLREF00004699.nxs', EntryNumber=1)

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

Output:

The number of histograms (spectra) is: 246

Categories: AlgorithmIndex | DataHandling\Nexus

Source

C++ header: LoadISISNexus2.h

C++ source: LoadISISNexus2.cpp