To compare the simulated spectra to the measured data it is necessary to take into account the natural background measured in the setup in the low level laboratory (LLL). There are two ways to do this. One is to compare the simulated spectra to measured spectra from which the background has been subtracted. These background subtracted measured spectra have been used to calculate the suppression efficiencies (see chapter 6) and this method is useful to compare the predicted and measured suppression factors. However in regions of low statistics, where the source signal is of comparable intensity as the background, this subtraction can lead to a vanishing signal. To compare the spectral shape and the peak to Compton ratios the background is therefore added to the simulated spectrum and this ’background added MC spectrum’ is compared to the measured spectra.
Figure 7.1 shows the comparison for a137Cs-source. The left plot is the comparison for the
unsuppressed spectrum, the right plot is the comparison for the anti-Compton spectrum. The plots show the raw Monte Carlo spectrum for a simulated 137Cs-source (light blue and ochre respectively), the measured background (grey), the sum of both which is the background added Monte Carlo spectrum (cyan / brown) and the full measured data from the 137Cs source (dark
blue / red). It is visible that the overall spectral shape for both spectra is very well reproduced. To fit the intensity of the simulated unsuppressed spectrum to that of the measured spectrum, however, a correction factor of 1.08 was necessary. This correction is larger than the uncertainty of the activity of the source, which was cited with a 3% error. An error in the source distance to account for the necessity of this correction factor could be excluded by comparing the peak- to-Compton ratio of the simulated spectrum to that of the measured spectrum. The peak to Compton ratio is the number of counts in a peak divided by the number of counts in an arbitrary chosen integral over the associated Compton plateau. It’s absolute value has no physical meaning since it depends on the integration range over the Compton plateau chosen, but for a fixed integration range it depends on the size of the diode and on the distance of the source. Therefore it is a good tool for the comparison of the simulated and measured spectra to test whether the geometry of the diode and source was implemented correctly. In the measured data, the peak to Compton ratio is 0.236±0.002 and in the simulated spectrum it is 0.240±0.003. Within the errors these values agree, so it is assumed that the necessity of the correction factor of 1.08 is due to an error in source strength or to an error in the run-time of the measurement.
Using the same correction factor for the simulated suppressed spectrum, its intensity is slightly lower than that of the real anti-Compton spectrum.
Figure 7.2 shows the same comparison for an internal 60Co source using the same colour scheme. Figure 7.3 shows the comparison of the simulated and real spectra for the internal
232Th-source and figure 7.4 shows the comparison of the simulated and real spectra for the 226Ra-source. A common feature is that the spectral shape is correctly reproduced, but that the
suppression efficiency is systematically over-estimated. This over-estimation is strongest in the case of60Co.
In the232Th spectrum the source strength had to be corrected as well to fit it to the measured
data. Here the correction factor was 0.74, but given that the source was home-made from232Th wire and the activity was only estimated and that the wire is far from the point-like source that has been used in the MC code, the correction factor is not surprising. In the comparison of the corrected spectrum it is still noticeable that the measured intensity in the low energy part is higher than the intensity predicted by MC. A possible reason for this is again that the source is homemade and was assembled just before the measurements. This means that the 232Th progeny 228Ac, which contributes the two lines at 911 keV and 969 keV and their associated
Energy [keV] 0 100 200 300 400 500 600 700 800 -4 10 -3 10 -2 10 -1 10 rate [hz] Ge signal mc + bkgd mc bkgd Energy [keV] 0 100 200 300 400 500 600 700 800 -4 10 -3 10 -2 10 -1 10
rate [hz] Ge signal with LAr veto
mc + bkgd with LAr veto mc with LAr veto bkgd with LAr veto
Figure 7.1: The comparison of the simulated full spectrum (left) and anti-Compton spectrum (right) with the measured real data for a137Cs-source.
Energy [keV] 0 500 1000 1500 2000 2500 -5 10 -4 10 -3 10 -2 10 -1 10 rate [hz] Ge signal mc + bkgd mc bkgd Energy [keV] 0 500 1000 1500 2000 2500 -5 10 -4 10 -3 10 -2 10 -1 10
rate [hz] Ge signal with LAr veto
mc + bkgd with LAr veto mc with LAr veto bkgd with LAr veto
Figure 7.2: The comparison of the simulated full spectrum (left) and anti-Compton spectrum (right) with the measured real data for an internal60Co-source.
Compton continuum, might not be in secular equilibrium (the decay scheme of228Ac is included in the appendix).
For 226Ra for simplicity only the 214Bi decay, which is the only γ emitter in the high energy
range and dominates the entire γ spectrum from the226Ra chain, was simulated. At low energies 4 lines are visible in the real data, that do not appear in the MC-spectra. These are the contribution from214Pb, the progenitor of214Bi, which was not simulated.
7.2.1 Quantitative comparison
For a quantitative comparison between the MC simulation data and the measured data the number of counts in the peaks, the RoI and selected Compton continua are compared. As a measure of the deviation between the simulated and measured data the ratios Rr/mc = Nreal/Nmc
and Rveto
r/mc = Nrealveto/Nmcveto are defined, where N is the number of counts in the selected region.
Table 7.1 lists the determined ratios.
Energy [keV] 0 500 1000 1500 2000 2500 -5 10 -4 10 -3 10 -2 10 -1 10 rate [hz] Ge signal mc + bkgd mc bkgd Energy [keV] 0 500 1000 1500 2000 2500 -5 10 -4 10 -3 10 -2 10 -1 10
rate [hz] Ge signal with LAr veto
mc + bkgd with LAr veto mc with LAr veto bkgd with LAr veto
Figure 7.3: The comparison of the simulated full spectrum (left) and anti-Compton spectrum (right) with the measured real data for an internal232Th-source.
Energy [keV] 0 500 1000 1500 2000 2500 -5 10 -4 10 -3 10 -2 10 -1 10 rate [hz] Ge signal mc + bkgd mc bkgd Energy [keV] 0 500 1000 1500 2000 2500 -5 10 -4 10 -3 10 -2 10 -1 10
rate [hz] Ge signal with LAr veto
mc + bkgd with LAr veto mc with LAr veto bkgd with LAr veto
Figure 7.4: The comparison of the simulated full spectrum (left) and anti-Compton spectrum (right) with the measured real data for a226Ra-source.
most sources and energy regions. This means that the MC simulation predicts less events than measured or in other words that the simulated suppression efficiency is larger than the measured one.