The complete detector has been simulated for 50 GeV and 180 GeV pion (π−) and 180 GeV muon µ− events [88]. The experimental setup has been simulated using GEANT4 and the CALICE Mokka framework [93]. The simulated detector was composed of a 39 layer struc-ture (38 layers for the DHCAL, plus one additional AHCAL layer), 1 m wide and 1 m high, each layer formed by tungsten absorber and sensitive material. The exact gas mixture filling the RPCs has not been simulated, and the corresponding sensitive layers have been replaced with air for simplicity. Plastic scintillator was used as sensitive material for the final AHCAL layer. The TMCT following the AHCAL has not been simulated. It is expected that this simplification underestimates the back scattered neutrons (Albedo) and thus late components of hadron showers. Late components in the shower are mostly caused by late neutron evap-oration, see chapter 3.1.5. Tungsten as absorber material provides a rich time structure by producing more slow neutrons than the iron absorber. The difference between simulation packages from Geant4 become more apparent for this absorber. In Geant4, physics simula-tions are driven by look up tables for processes and different simulation packages in different energy regimes, see figure 12.3 above. Most packages split the simulation process and cross section table use depending on the energy of the simulated process. Explanation of the differ-ent physics lists can be found in [94]. The QGSP BERT package has been proven to be the most successful package in describing the shower behavior, as seen in [81]. The QGSP BERT and QGSP BERT HP package differ in the treatment of neutrons in material. Figure 12.3 below shows the difference between the two packages in simulations for the time of hit in the simulated calorimeter compared to the QBBC list and data from the T3B experiment [31].
This work focuses on the differences between QGSP BERT and QGSP BERT HP that uses a high precision model for thermal neutron capture below 20 MeV. In order to simulate the triggering behavior of the Spiroc2b, a sliding window technique was applied. First, the hits in a cell are ordered in time. The energy is summed up in a sliding window of 15 ns, imitating the fast shaper in the Spiroc2b. Once trigger threshold is passed within that sliding window, the chip is triggered and a 150 ns window is integrated. This simulated the slow shaper used for the energy signal production in the Spiroc2b. A schematic description of the algorithm can be seen in figure 12.4. The sub-hitstag1...4 are ordered in time. tag1 does not yet pass the threshold but the sum of tag1+tag2 occurring within a 15 ns window passes the threshold.
tag2 is the time of hit saved. Esum in blue in figure 12.4 is the 150 ns sliding window that
12.2 Geant4 Simulations of the Test Beam 115
Figure 12.3:
Above: Schematic representation of different physics lists used by the Geant4 simulation package. The choice of physics lists depends on the energy of the process, in the overlap regions a list is chosen randomly [94].
Below: Comparison of the time of first hit in the T3B experiment and in the acording simulations with different physics lists as shown in [81].
is saved, including consequent sub-hitstag3 and tag4. Disabled channels in the detector are also disabled in the simulations. Figure 12.17 shows the detector with all disabled channels, summing up to 7 % of all channels. The channels are either excluded from the analysis be-cause of the faulty behavior during calibration in the DESY II test beam (see chapter 11.2.2), or disabled during data taking because their high noise would overwhelm the beam rate at the SPS. The simulation is convoluted with the time resolution according to the amount of triggered channels in the real data to compare simulations to data. Figure 11.17 on the right in chapter 11.2.4 shows the resolution according to the number of triggered channels used to distort the simulations. The noise can be estimated by taking the number of hits in a specific time interval before the main trigger arrives. This has been used as a constant offset c for the simulation and was determined for each data set.
116 12. Hit time measurement in a Hadron Test Beam
Figure 12.4: Determination of the time of hit for simulations. In the Spiroc2b the SiPM signal is fed through a 15 ns shaper to a threshold discriminator. The time of hit is saved once the threshold is overstepped. The actual energy signal is integrated through a 150 ns shaper. In the simulation hit energies are added within a sliding 15 ns window. The time stamp of the hit is saved when the summed up energy reaches the threshold.
The energy information is summed over 150 ns starting withTHit-15 ns. [95]
12.2.1 Energy calibration
MIP MPV DESY [ADC]
MIP MPV CERN [ADC]
Figure 12.5: MPV found for the MIP calibration in the SPS versus the calibration at the DESY II test beam.
The eight chips with sufficient entries are shown channel-wise. The difference can be explained due to the temperature difference and mismatching voltages, see chapter 8
A total of 99700 usable muon events unevenly distributed over all channels do not provide
12.2 Geant4 Simulations of the Test Beam 117
enough entries for a complete energy calibration. A MIP calibration was carried out at the DESY II test beam facility prior to the SPS test beam. The temperature at the DESY test beam was around 30◦C while the SPS operational temperature was around 25◦C with variations of 3◦C, see figure 12.2. Due to the wide range of operating voltages (table 11.1) and the sensitivity of the CPTA SiPMs of over 50mV◦C a precise ADC calibration proved difficult.
The temperature dependency of the MPV is not linear and the DESY II test beam data cannot be used to transfer the calibration to the SPS test beam since the tile SiPM system has not been characterized for temperature changes. Figure 12.5 shows the comparison of the calibration point for a MIP of all channels separated by chips. A precise energy calibration is not necessary for the primary goal of this timing analysis so a chip-wise rough energy calibration has been applied. For each chip all triggered hits from all channels are collected in one histogram. Figure 12.6 shows the resulting histograms for the six chips with the most
Pedestal Energy
-ADC
ADC
0 200 400 600 800 1000
Peak
N / N
0 1 2 3
Chip 133 Chip 135 Chip 138 Chip 140 Chip 141 Chip 143
Figure 12.6: MIP calibration and off-line threshold calibration. Data from muon runs at the SPS. 6 ASICs out of the 16 with the most entries are shown for this calibration the pedestal subtracted energy hits from all channels are collected for each chips. The light red line indicates the common threshold applied for the 0.5 MIP cut.
Amount of entries for the different chips 133:737 135:20331 138:24246 140:9676 141:24083 143:6153
entries. For comparison, they have been rescaled to the MIP MPV.
Chips with noisy channels show a peak in the amplitude range close to the threshold cut (see for example chip 133 in red), due to non completely suppressed noise contribution.
The second peak is the MPV of the energy deposition of muons. The MIP position for all chips was assumed to be around 300 ADC and the 0.5 MIP threshold was set to be 150 ADC for all channels in the detector. The impact of a wrong threshold setting for the analysis of the time of hits is shown in figure 12.7 on simulations. To estimate the impact on the analysis if the off line threshold is not correctly set at 0.5 MIP the threshold of every single channel in
118 12. Hit time measurement in a Hadron Test Beam
the simulation has been varied randomly by 10 %, 20 %, and 50 %. No impact on the result could be seen and the rough energy calibration is justified for this analysis.
0[ns]
Hit-t 10 T
− 0 10 20 30 40 50
0 0.2 0.4 0.6 0.8 1
0[ns]
Hit-T
10 102 T
5
10− 4
10− 3
10− 2
10− 1
10−
1
Figure 12.7: 180 GeV π− have been simulated with the described setup. The threshold on the individual channels have been varied randomly by 10 % (red),20 % (green),and 50 % (black).