4.2

Peaks Workspace

The PeaksWorkspace is a special Workspace that holds a list of single crystal Peak objects.

Creating a PeaksWorkspace

Viewing a PeaksWorkspace

  • Double-click a PeaksWorkspace to see the full list of data of each Peak object.
  • In MantidPlot, you can drag/drop a PeaksWorkspace from the list of workspaces onto the Instrument View . This will overlay the peaks onto the detector face.
  • Right-click a PeaksWorkspace and select the menu to show the peak positions in 3D in the VatesSimpleInterface
  • In paraview, you can load a .Peaks file loader plugin to view a PeaksWorkspace.
  • PeaksViewer in the SliceViewer

The Peak Object

Each peak object contains several pieces of information. Not all of them are necessary:

  • Detector position and wavelength
  • Q position (calculated from the detector position/wavelength)
  • H K L indices (optional)
  • Goniometer rotation matrix (for finding Q in the sample frame)
  • Integrated intensity and error (optional)
  • Row/column of detector (only for RectangularDetectors )
  • An integration shape (see below)

The Peak Shape

Each Peak object contains a PeakShape. Only the integration algorithms which act on, and return PeaksWorkspaces set the shape of the peaks. The PeakShape is owned by the Peak, not the PeaksWorkspace, so when PeaksWorkspaces are split, or concatinated, the integration shapes are unaltered. Aside from the Null Peak Shape, each peak shape contains at least the following information.

  • The algorithm used to perform the integration
  • The version of the algorithm used to perform the integration
  • The frame in which the integration has been performed

Subtypes of PeakShape will then provide additional information. For example PeakShapeSpherical provides the radius as well as background inner, and background outer radius.

Calculate Goniometer For Constant Wavelength

If you set the wavelength (in Å) or energy (in meV) property on a PeaksWorkspace, or if the instrument on the PeaksWorkspace has the wavelength parameter, the goniometer rotation will be calculated when the createPeak method is used. This allows you to use one instrument definition for multiple goniometer rotations, for example adding peaks in Slice Viewer from multiple combined MD workspaces. It only works for a constant wavelength source and only for Q sample workspaces. It also assumes the goniometer rotation is around the y-axis only. For details on the calculation see “Calculate Goniometer For Constant Wavelength” at FindPeaksMD.

pws = mtd['name_of_peaks_workspace']
pws.run().addProperty('wavelength', 1.54, True)
# or
pws.run().addProperty('energy', 34.48, True)

Using PeaksWorkspaces in Python

The PeaksWorkspace and Peak objects are exposed to python.

PeaksWorkspace Python Interface

pws = mtd['name_of_peaks_workspace']
pws.getNumberOfPeaks()
p = pws.getPeak(12)
pws.removePeak(34)

Peak Python Interface

You can get a handle to an existing peak with:

p = pws.getPeak(12)

Or you can create a new peak in this way:

qlab = V3D(1.23, 3.45, 2.22) # Q in the lab frame of the peak
detector_distance = 2.5 # sample-detector distance in meters. Detector distances are optional. Calculated in not provided.
p = pws.createPeak(qlab, detector_distance)
# The peak can later be added to the workspace
pws.addPeak(p)

Once you have a handle on a peak “p” you have several methods to query/modify its values:

hkl = p.getHKL()
p.setHKL(-5, 4, 3)

q = p.getQSampleFrame()
q = p.getQLabFrame()
detid = p.getDetectorID()

p.setIntensity(1000.0)
p.setSigmaIntensity(31.6)
counts = p.getIntensity()

wl = p.getWavelength()
tof = p.getTOF()
d = p.getDSpacing()
shape = p.getPeakShape()

Category: Concepts