The Digital Tracking Calorimeter Prototype
3.3 Experimental Test in a Proton Beam
The measurements reported upon in this study were performed in December 2014 at KVI-CART in Groningen, the Netherlands. The cyclotron at the AGOR facility for Irra-diations of Materials (AGORFIRM) delivers proton beams with energies from 40 MeV and up to 190 MeV (Van der Graaf et al., 2009). The beam line is shown in Fig. 3.6.
3.3 Experimental Test in a Proton Beam 55
Hit # 1 2 3 4 5 6 · · · 418
Pixel X 308 -311 -313 -314 -320 -321 · · · -311 Pixel Y 412 -311 -32 -579 -575 514 · · · 362
Layer # 8 5 21 20 0 12 · · · 15
Table 3.2:Example of the preprocessed output from a single detector event from the 188 MeV proton beam. The full event contained 418 hits (activated pixels), many of which were noise (isolated pixels) and parts of charge diffused pixel clusters where the proton track passed through. The four physical sensor chips correspond to the four quadrants in the coordinate system (-640,-640)→(640, 640).
Energy [MeV] Number of spills Run numbers Chip threshold
120 104 16, 17, 18, 19, 29, 31 5·10−4
139 12 45 5·10−4
151 23 43 5·10−4
160 20 41 5·10−4
170 112 12, 13, 21, 22, 24 5·10−4
180 23 39 5·10−4
188 75 4, 6, 9, 26 5·10−4
188 33 34, 37 10−5
Table 3.3: Overview over the beam test experiments performed using the calorimeter prototype at KVI-CART in Groningen. The chip threshold is given in units of fake acti-vation probability per pixel.
3.3.1 Overview over the Experiments
The beam tests lasted a week, and in the end 46 different experiments (runs) were per-formed, each run consisting of 5–50beam spills. Of these, 21 consisted of usable data, the remainder being either pedestal (noise correction) data, inadequate proton beam qual-ity or failed data acquisition. The extracted beam energies ranges from 120–188 MeV.
An overview over the different runs at the different beam energies are given in Table 3.3.
3.3.2 Beam Specifications
The sensor layers have a surface area of approximately 4×4 cm2, and during the exper-iment, the proton beam was shaped to this same field size. The intensity of the beam was set for delivering at most one proton per readout frame, with a detector readout fre-quency of1/(642 µs)≃2 kHz. The proton intensity in terms of particle rate is estimated to have been approximately 1350 protons/s, this value is deduced from the finding that
56 3. The Digital Tracking Calorimeter Prototype
Figure 3.4:Left: Lateral positions of the incoming protons at the front face of the detec-tor.Right: Lateral positions of the protons at their stopping position inside the detector.
Both figures are constructed with experimental data at all the available proton beam energies.
about 67% of the readout frames contains proton tracks. See Table 3.4 for a comparison between the number of readout frames and the number of reconstructed tracks.
The beam profile at the detector front face, as well as the lateral stopping positions of the protons of all the beam energies is shown in Fig. 3.4. Each entry do not correspond to a hit, but to the position of a reconstructed track. Some features can be seen in the figure: A lower fraction of tracks are reconstructed in the upper right quadrant, this due to a dead sensor chip in that area, and the beam has a slightly increased spread in the deeper-laying layers: this is expected from the scattering.
The spatial distribution of the beam, in terms of the standard deviation of a fitted normal distribution, is 7.5 mm at the front face and 8 mm at the stopping position. This means that the increased beam spread, calculated by taking their quadratic difference, is in the order of 3 mm. Using the integral form of the Highland formula (Eq. (1.6)), we obtain a theoretical value for the scattering angle ofθ0=126 mrad. This corresponds to a lateral deviation of 2.95 mm, which is very close to the measured value.
The test beam energies were chosen with the motivation of applying the maximum available energy, and thus measuring the corresponding maximum proton range, in the multi-layered detector. Due to the highZabsorber material, the 188 MeV proton beam is traversing through only the first 7 of the 24 layers: A beam energy of 450 MeV would
3.3 Experimental Test in a Proton Beam 57
Figure 3.5:Hitmaps from experimental data at 188 MeV. Each of the four figures repre-sents a four-chip layer: The first three layers and the layer where the proton tracks come to rest. Note that fractional areas or even whole sensor chips do not contain any data:
this is due to defect sensor chips, defect data cables or breaks in the connection between them. Note also that the size of the pixel hit clusters increase towards the Bragg peak, this can be due to the increased energy deposition.
58 3. The Digital Tracking Calorimeter Prototype
Aluminum degrader [mm] 60 45 35 27 17 8 0
Energy [MeV] 119.9 139.0 150.9 159.9 170.1 180.0 188.0 Energy spread [MeV] 1.4 1.1 1.0 0.9 0.7 0.5 ∼0
Readout frames 3719 241 819 762 4944 1334 2739
Reconstructed tracks 1576 87 408 408 3431 901 2010 Table 3.4: List of the beam energies applied at the KVI-CART beam test, the number of readout frames as well as the number of reconstructed proton tracks at each energy.
Adapted from Pettersen et al. (2017).
Figure 3.6:Left: The proton beam-energy degrader at the AGOR facility at KVI-CART.
Nine aluminum plates can be placed in the beam, controlled remotely.Right: Schematics of the beam test setup at the AGOR facility. Both figures from KVI AGORFIRM (2012).
have been needed in order for the protons to traverse the whole detector (all the 24 layers) in the longitudinal direction.
In order to deliver the different beam energies, the beams were degraded by the pres-ence by an aluminum absorber in the beam line. For details about the applied proton beams; energies, as well as the different degrader thicknesses and the number of recorded protons in each of the setups, see Table 3.4. An energy spread of up to 1.4 MeV is intro-duced by the degradation. This energy spread was calculated using GATE simulations.
The energy spread increases with the thickness of the degrader following Eq. (1.11).
The beam degrader as well a schematic drawing of the complete beam setup is shown in Fig. 3.6. More detailed beam specifications and the beam optics are described in Van der Graaf et al. (2009).