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## Summary¶

Loads a file in ISIS NeXus format.

## 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

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)

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

Example - Load without any optional arguments:

# Load LOQ histogram dataset

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.

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

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


Output:

The number of histograms (spectra) is: 246


Categories: AlgorithmIndex | DataHandling\Nexus