GetEi-v2.rst

Description

Uses $E=\frac{1}{2}mv^2$ to calculate the energy of neutrons leaving the source. The velocity is calculated from the time it takes for the neutron pulse to travel between the two monitors whose spectra were specified. If no spectra are specified, the algorithm will use the defaults for the instrument.

An initial energy guess is required for the algorithm to find the correct peak. The analysis will be done on the highest peak that is within 8% of the estimated TOF given by the estimate. If no initial guess is given, the algorithm will try to get it from the workspace, from a sample log variable called EnergyRequest.

Not all neutrons arrive at the monitors at the same time because their kinetic energies, and therefore velocities, are all different. The time of arrival of the neutron pulse is taken to be the mean of the two half peak height locations. The half height points are found as follows:

1. the peak height is the largest number of counts above the background in any bin in the window
2. the half height is half the above number
3. examine bins to the left of the bin with the highest number of counts looking for a bin with less than half that number above background
4. interpolate between this point bin and the one immediately previous to find the first half height location
5. repeat the steps 3 and 4 looking to the right of the highest point to get the second half height point
6. the mean of the X-values of the two half height points is the TOF arrival time of the neutrons

The above process is illustrated on a peak is shown below in the image below.

The distances between the monitors are read from the instrument definition file. It is assumed that the source and the monitors all lie on one line and that the monitors have the same delay time.

Usage

Example: Calculating the Ei

calcEi

import numpy as np import math

#create a workspace wth two pixels to act as monitors ws = CreateSampleWorkspace(bankPixelWidth=1,binWidth=10)

#set the data values peakOneCentre = 8500.0 sigmaSqOne = 250.0*250.0 peakTwoCentre = 10800.0 sigmaSqTwo = 50*50 peakOneHeight = 3000.0 peakTwoHeight = 1000.0 xArray = [] yArray0 = [] yArray1 = [] for i in range (ws.blocksize()): xValue = 5.0 + 5.5*i xArray.append(xValue) yArray0.append(peakOneHeight * math.exp(-0.5*pow(xValue - peakOneCentre, 2.)/sigmaSqOne)) yArray1.append(peakTwoHeight * math.exp(-0.5*pow(xValue - peakTwoCentre, 2.)/sigmaSqTwo)) xArray.append(5.0 + 5.5*ws.blocksize())

ws.setX(0, np.array(xArray)) ws.setX(1, np.array(xArray)) ws.setY(0, np.array(yArray0)) ws.setY(1, np.array(yArray1))

(ei, firstMonitorPeak, FirstMonitorIndex, tzero) = GetEi(ws,Monitor1Spec=1,Monitor2Spec=2,EnergyEstimate=15.0)

print "ei: %.2f" % ei print "firstMonitorPeak: %.2f" % firstMonitorPeak print "FirstMonitorIndex: %i" % FirstMonitorIndex print "tzero: %.2f" % tzero

Output:

calcEi

ei: 24.99 firstMonitorPeak: 8516.03 FirstMonitorIndex: 0 tzero: 1655.69

Example: Fixing the Ei

fixEi

ws = CreateSampleWorkspace(bankPixelWidth=1,binWidth=10)

(ei, firstMonitorPeak, FirstMonitorIndex, tzero) = GetEi(ws,Monitor1Spec=1,Monitor2Spec=2,EnergyEstimate=15.0,FixEi=True)

print "ei: %.2f" % ei print "firstMonitorPeak: %.2f" % firstMonitorPeak print "FirstMonitorIndex: %i" % FirstMonitorIndex print "tzero: %.2f" % tzero

Output:

fixEi

ei: 15.00 firstMonitorPeak: 8854.69 FirstMonitorIndex: 0 tzero: 0.00