Event Workspace Development

The following information will be useful to you if you want to write an Algorithm that is Event Workspace aware.

Individual Neutron Event Data (TofEvent)

The TofEvent class holds information for each neutron detection event data:

  • PulseTime: An absolute time of the pulse that generated this neutron. This is saved as an INT64 of the number of nanoseconds since Jan 1, 1990; this can be converted to other date and time formats as needed. Internall the PulseTime is represented as a Kernel::DateAndTime type.
  • tof: Time-of-flight of the neutron, in microseconds, as a double. Note that this field can be converted to other units, e.g. d-spacing.

Tip

There are in fact several variants of the Event type within Mantid. The common by far is the RAW TOF described above, but there are also Weighted events that offer better compression.

Lists of Events (EventList)

  • The EventList class consists of a list of TofEvent’s. The order of this list is not significant, since various algorithms will resort by time of flight or pulse time, as needed.
  • Also contained in the EventList is a std::set of detector ID’s. This tracks which detector(s) were hit by the events in the list. EventList is a subtype of ISpectrum, which provides the interface to many of the spectrum level access methods.
  • The histogram bins (X axis) are also stored in EventList. The Y and E histogram data are not, however, as they are calculated on demand by the MRU (below).

The += operator can be used to append two EventList’s together. The lists of TofEvent’s get appended, as is the list of detector ID’s. Don’t mess with the udetmap manually if you start appending event lists - just call EventWorkpspace->makeSpectraMap to generate the spectra map (map between spectrum # and detector IDs) by using the info in each EventList.

Most Recently Used List (MRUList)

An Event Workspace contains a list of the 100 most-recently used histograms, a MRUList. This MRU caches the last histogram data generated for fastest display.

A note about workspace index / spectrum number / detector ID

For event workspaces there is no benefit, and only a drawback to grouping detectors in hardware, therefore most of the loading algorithms for event data match the workspace index and spectrum number in the Event Workspace. Therefore, in an Event Workspace, the two numbers will often be the same, and your workspace’s Axis[1] is a simple 1:1 map. As mentioned above, the detectorID is saved in EventList, but the makeSpectraMap() method generates the usual SpectraDetectorMap object.

Workspace2D compatibility

Event Workspace is designed to be able to be read (but not written to) like a MatrixWorkspace. By default, if an algorithm performs an operation and outputs a new workspace, the WorkspaceFactory will create a Workspace2D copy of your Event Workspace’s histogram representation. If you attempt to change an Event Workspace’s Y or E data in place, you will get an NotImplementedError raised, since that is not possible.

A Note about Thread Safety

Thread safety can be surprising when using an Event Workspace:

If two threads read a Y histogram at the same time, this can cause problems. This is because the histogramming code will try to sort the event list. If two threads try to sort the same event list, you can get segfaults.

Remember that the PARALLEL\_FOR1(), PARALLEL\_FOR2() etc. macros will perform the check Workspace->threadSafe() on the input Event Workspace. This function will return false (thereby disabling parallelization) if any of the event lists are unsorted.

You can go around this by forcing the parallel loop with a plain PARALLEL\_FOR() macro. Make sure you do not read from the same spectrum in parallel!