There are a number of different methods to validate simulated data against experi- mental data, some examples of methods used can be found in [52, 53]. Due to the dead time used for the experimental data being an estimate the simulation used will be validated against the final image resolution rather than the Compton efficiency. The factors influencing the image resolution are the detectors energy resolution, position resolution and the geometry of the setup [53]. If the images appear to be similar in
(a) (b)
Figure 7.1: Schematic diagram of the full circular scatterer (a) showing the circle as the red line and pixel set as green boxes. (b) shows the schematic diagram of the scatterer when it was simulated to match the experimental detector.
resolution these factors should be correctly set within the simulation.
The simulation will use a137Cs source positioned at 4.6cm from the scatterer crystal,
with an activity of 300kBq the image produced is shown in Figure 7.2a. The image produced experimentally from a source at the same distance from the detector can be seen in Figure 7.2b. The images were produced using the same number of cones to provide a direct comparison.
The image resolutions seen at 662keV are very similar. The results are displayed in Table 7.1.
Simulated Experimental
Energy FWHM X ± FWHM Y ± FWHM X ± FWHM Y ±
(keV) (mm) (mm) (mm) (mm)
662 13.4 0.7 13.0 0.7 14.2 0.7 13.2 0.8
Table 7.1: Simulated and experimental FWHM measurements for comparison.
7.3.1 Efficiency
The efficiency of a detector system will be influenced by the materials around the detector crystals, additional materials in any holding structures, the dead time of the system including the detector reset time, low energy threshold settings and the energy of the incident gamma-ray [53].
To produce a simulated efficiency plot, a gamma ray source was positioned 11.6cm away from the scatterer crystal in GAMOS. This matches the geometry of the 152Eu
(a) (b)
(c) (d)
Figure 7.2: Simulated ((a) and (c)) and experimental ((b) and (d)) images produced with a 137Cs source at a distance of 4.6cm from the scatterer crystal. All the images
have a 1mm compression.
amount of surrounding material, energy thresholds and the dead time of the detectors can be altered to allow the simulated efficiency to match that found experimentally. Each energy from152Eu was run individually and the results are combined to produce an absolute Compton efficiency plot which can be compared to the one found experi- mentally. The simulated points are found to be significantly higher than those of the experimental data, by a factor of three as shown in Figure 7.3. The shape of the curve seen is as expected and follows that found experimentally indicating that there is a systematic error in the experimental data causing the discrepancy between the simu- lated and experimental data. The GAMOS data has negligible errors (square root of the number of counts measured in each peak, due to the low numbers run this will be a small value) while the experimental result has an estimated 10% error as shown on the experimental points.
By applying a coincidence time delay of 7µs to the detectors allows the simulation to be made to match the experimental data, as seen in Figure 7.4.
0.00E+00 1.00E-03 2.00E-03 3.00E-03 4.00E-03 5.00E-03 6.00E-03 0 200 400 600 800 1000 1200 1400 1600 Ef fi ci e n cy (%) Energy (keV)
Absolute Compton Efficiency
Fold 1 Exp GAMOS
Figure 7.3: Plot of the absolute Compton efficiency found using the GAMOS simula- tion compared to the experimental plot. This plot indicates a systematic error in the experimental data resulting in a loss of data.
0.00E+00 5.00E-04 1.00E-03 1.50E-03 2.00E-03 2.50E-03 0 200 400 600 800 1000 1200 1400 1600 Ef fi ci e n cy (%) Energy (keV)
Absolute Compton Efficiency
Fold 1 Exp GAMOS2
Figure 7.4: Plot of the absolute Compton efficiency found using the GAMOS simulation compared to the experimental plot after adding an additional non physical time factor.
This way of matching the data sets is not physical and just provides a method of removing data points from the simulation until it matches the experimental data. The difference between the data sets at low energies following this matching process is due to the harsh energy threshold being set by GAMOS, the experimental data has threshold of 15 and 30keV as set in GAMOS however there is a± 10keV error on the experimental data. At higher energies the possibility of the charge cloud being created by an interaction drifting across a strip boundary and thus the event being recorded as a fold 2 event is increased experimentally. This leads to the simulated data being higher than that seen from experimental. These issues have previously been seen [53] so were expected and taken into consideration when applying the coincidence time delay to the simulated data.
The experimental data collected had a coincidence window of 250ns, this removed
∼12% of the data at 511keV. This value will vary with gamma ray energy. In addition to this the dead time of the system is not fully understood with the new digital electronics, and incorrect settings may of contributed to the data loss causing additional dead time or event pile up. In the system pile up events, events which occur while the detector system is processing another event, are deleted. Dead time will be introduced from the system writing out traces and also the shaping time used on the moving window deconvolution filter which is used to provide the energies deposited in the
detectors. To investigate this fully it is recommended that a comparison between an analogue electronics setup and the new digital system be carried out, this should provide information on the dead time and any other possible data loss in the system.