Event Workspace¶

Accessing Workspaces¶

The methods for getting a variable to an EventWorkspace is the same as shown in the Workspace help page.

If you want to check if a variable points to something that is an Event Workspace you can use this:

from mantid.api import IEventWorkspace

eventWS = CreateSampleWorkspace(WorkspaceType="Event")

if isinstance(eventWS, IEventWorkspace):
    print eventWS.name() + " is an " + eventWS.id()

Output:

eventWS is an EventWorkspace

Event Workspace Properties¶

In addition to the Properties of the MatrixWorkspace, the Event Workspace also has the following:

eventWS = CreateSampleWorkspace(WorkspaceType="Event")

print "Number of events:", eventWS.getNumberEvents()
print "Maximum time of flight:", eventWS.getTofMax()

Event lists¶

Event Workspaces store their data in event lists, one per spectrum. You can access them using:

eventWS = CreateSampleWorkspace(WorkspaceType="Event")

# get the number of event lists
evListCount = eventWS.getNumberHistograms()

# Get the first event list
evList = eventWS.getSpectrum(0)

# Get some basic information
print "Number of events in event List 0:", evList.getNumberEvents()
print "Minimum time of flight in event List 0:", evList.getTofMax()
print "Maximum time of flight in event List 0:", evList.getTofMax()
print "Memory used:", evList.getMemorySize()
print "Type of Events:", evList.getEventType()

# Get a vector of the pulse times of the events
pulseTimes = evList.getPulseTimes()

# Get a vector of the TOFs of the events
tofs = evList.getTofs()

# Get a vector of the weights of the events
weights = evList.getWeights()

# Get a vector of the errors squared of the weights of the events
weightErrors = evList.getWeightErrors()

# Integrate the events between  a range of X values
print "Events between 1000 and 5000:", evList.integrate(1000,5000,False)

#Check if the list is sorted in TOF
print "Is sorted by TOF:", evList.isSortedByTof()

import math
eventWS = CreateSampleWorkspace(WorkspaceType="Event")
# Get the first event list
evList = eventWS.getSpectrum(0)

# Add an offset to the pulsetime (wall-clock time) of each event in the list.
print "First pulse time before addPulsetime:", evList.getPulseTimes()[0]
seconds = 200.0
evList.addPulsetime(seconds)
print "First pulse time after addPulsetime:", evList.getPulseTimes()[0]

# Add an offset to the TOF of each event in the list.
print "First tof before addTof:", evList.getTofs()[0]
microseconds = 2.7
evList.addTof(microseconds)
print "First tof after addTof:", evList.getTofs()[0]

# Convert the tof units by scaling by a multiplier.
print "First tof before scaleTof:", evList.getTofs()[0]
factor = 1.5
evList.scaleTof(factor)
print "First tof after scaleTof:", evList.getTofs()[0]

# Multiply the weights in this event list by a scalar with an error.
print "First event weight before multiply:", evList.getWeights()[0], \
      "+/-", math.sqrt(evList.getWeightErrors()[0])
factor = 10.0
error = 5.0
evList.multiply(factor,error)
print "First event weight after multiply:", evList.getWeights()[0], \
      "+/-", math.sqrt(evList.getWeightErrors()[0])

# Divide the weights in this event list by a scalar with an error.
print "First event weight before divide:", evList.getWeights()[0], \
      "+/-", math.sqrt(evList.getWeightErrors()[0])
factor = 1.5
error = 0.0
evList.divide(factor,error)
print "First event weight after divide:", evList.getWeights()[0], \
      "+/-", math.sqrt(evList.getWeightErrors()[0])

# Mask out events that have a tof between tofMin and tofMax (inclusively)
print "Number of events before masking:", evList.getNumberEvents()
evList.maskTof(1000,5000)
print "Number of events after masking:", evList.getNumberEvents()

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.
• 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.

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.
• The histogram bins (X axis) are also stored in EventList. The Y and E histogram data are not, however, as they are calculated 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 EventWorkspace 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 EventWorkspace. Therefore, in an EventWorkspace, the two numbers will 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¶

EventWorkspace 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 EventWorkspace’s histogram representation. If you attempt to change an EventWorkspace’s Y or E data in place, you will get an error message, since that is not possible.

A Note about Thread Safety¶

Thread safety can be surprising when using an EventWorkspace:

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 EventWorkspace. 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!