GetAllEi v1¶

Summary¶

Analyze the chopper logs and the signal registered by the monitors to identify energies used as incident energies in an inelastic experiment.

Properties¶

Description¶

Algorithm finds the estimate for all incident energies allowed by chopper system of an inelastic instrument and returns a workspace, with the estimates for positions, heights and width of incident energies provided by the choppers and registered on monitors. These estimates can be used as guess value for GetEi v2 algorithm or as inputs for a peak fitting procedure.

Algorithm performs number of steps to identify the values requested:

1. It takes appropriate log names from instrument definition file (IDF), namely chopper-position component and calculates last chopper speed and delay as average of the filtered log values. Guess chopper opening times are calculated from chopper speed and delay time. The “chopper-position” component with appropriate properties has to be present in IDF for this algorithm to work. See ISIS MARI or MAPS instrument definition files for example of “chopper-position” component.
2. Algorithm uses estimate for the minimal energy resolution of an instrument and searches for real peaks around guess values obtained earlier within 4 sigma of this resolution interval.
3. If peaks are found, the algorithm performs running averages over signal in the appropriate time interval until first derivative of the signal has only one zero. The position of this zero is returned as the guess energy and the distance between closest to the guess energy zeros of the second derivative are returned as the guess values for the peak width. The peak amplitude is estimated from the total intensity of the signal within the search interval, assuming that the peak shape is Gaussian.
4. Similar procedure is performed for second monitor. The peak is accepted only if the peak width lies between the minimal and maximal instrument resolution values and the distance between peaks positions on two monitors (on energy scale) is smaller then two sigma.

Algorithm returns matrix workspace containing single spectrum, with x-values representing peak positions, y-values: peak heights and the error: peak width. X-values are sorted according to energy in peaks (peaks with maximal energy are returned first).

Used Subalgorithms¶

The algorithm uses Unit Factory and ConvertUnits v1 algorithm to convert units from TOF to energy.

Example: Find all incident energies for test workspace

# BUILD SAMPLE WORKSPACE
# Build sample workspace with chopper and in energy units to
# have defined peaks in defined energy positions
wsEn=CreateSampleWorkspace(Function='Multiple Peaks', NumBanks=1, BankPixelWidth=2, NumEvents=10000, XUnit='Energy', XMin=10, XMax=200, BinWidth=0.1)
# convert units to TOF to simulate real workspace obtained from experiment
ws = ConvertUnits(InputWorkspace=wsEn, Target='TOF')
# find chopper log values would be present in real workspace
l_chop = 7.5  # chopper position build into test workspace
l_mon1 = 15. # monitor 1 position (detector 1), build into test workspace
t_mon1 = 3100. # the time of flight defined by incident energy of the peak generated by CreateSampelpWorkspace algorithm.
t_chop = (l_chop/l_mon1)*t_mon1
# Add these log values to simulated workspace to represent real sample logs
AddTimeSeriesLog(ws, Name="fermi_delay", Time="2010-01-01T00:00:00", Value=t_chop ,DeleteExisting=True)
AddTimeSeriesLog(ws, Name="fermi_delay", Time="2010-01-01T00:30:00", Value=t_chop )
AddTimeSeriesLog(ws, Name="fermi_speed", Time="2010-01-01T00:00:00", Value=900 ,DeleteExisting=True)
AddTimeSeriesLog(ws, Name="fermi_speed", Time="2010-01-01T00:30:00", Value=900)
#-------------------------------------------------------------

# FIND GUESS PEAKS
allEiWs=GetAllEi(ws,Monitor1SpecID=1,Monitor2SpecID=2)
# Analyze results
allEi = allEiWs.readX(0);
peakHeight = allEiWs.readY(0);
peakWidth  = allEiWs.readE(0);

# Check if peaks positions are indeed correct:
#-------------------------------------------------------------
resEi=[]
for ei_guess in allEi:
   nop,t_peak,monIndex,tZero=GetEi(InputWorkspace=ws, Monitor1Spec=1, Monitor2Spec=2, EnergyEstimate=ei_guess)
   resEi.append((nop,t_peak));
print("! Guess Ei ! peak TOF ! peak height ! peak width !")
for ind,val in enumerate(resEi):
   print("!  {0: >6.1f}  !  {1: >6.2f} !   {2: >6.2f}    ! {3: >6.2f}     !".format(allEi[ind],val[1],peakHeight[ind],peakWidth[ind]))
#
# NOTE: incident energy of GetEi is calculated from distance between monitor 1 and 2, and this distance is not correct in
# the test workspace. The tested point is that getEi can find energies from guess values and TOF for peaks is correct.

Output:

! Guess Ei ! peak TOF ! peak height ! peak width !
!    67.0  !  4188.03 !    34.68    !   2.35     !
!   124.1  !  3079.09 !    14.01    !   4.35     !

Categories: Algorithms | Inelastic\Ei

Source¶

C++ source: GetAllEi.cpp (last modified: 2017-09-28)

C++ header: GetAllEi.h (last modified: 2016-07-26)