Wrapping up
7.1 Materials and Methods .1 The Micromegas detectors
7.1.3 Analysis methods: Stand-alone identification of RPs and beam-spot shape of proton and carbon-ion beamsof proton and carbon-ion beams
Figure 7.4 schematically shows the reference planes used to reconstruct the scanning pattern of both proton and carbon-ion beams and discern the RPs.
In the case of proton beams, from left to right in Figure 7.3, the first Micromegas doublet (1) provides the combined spot information for the x-reference plane before the phantom.
The second Micromegas doublet (2) gives the y-reference before the phantom and the third Micromegas doublet (3) determines only the y-reference after the phantom, since the x-stripes did not yield the sufficient statistics to afford the2Dretrieving of all the beam-hits (cf. Figure 7.4, upper panel). Either due to an unexpected disconnection or mistake, no drift HV was applied to the Micromegas during these measurements, which made it impossible to track the protons individually. Nonetheless, only simultaneous hits on two detectors are required to reconstruct a beam-spot before and after the phantom, which considerably increases the number of particles registered for each RP. The combined hits constitute a tracklet of two detectors, using an extrapolated hit in the middle of both detectors. For carbon-ions tracking, only two reference-planes were necessary to obtain the 2D information before and after the phantom (cf. Figure 7.4, lower panel), owed to their larger stopping power.
7.1. Materials and Methods
Phantom space
Phantom space
RRD
RRD x-reference y-reference y-reference
x-y-reference x-y-reference
doublet 1 doublet 2 doublet 3
proton beam
carbon-ion beam
Figure 7.4: Sketch of the virtual-planes defined to analyze automatically the data extracted from combined Mi-cromegas-doublets according to Figure7.3. The reference-panels used for the proton acquisitions are shown in the upper panel, whereas the lower panel outlines the arrangement of the virtual-planes used for the carbon-ion data analysis.
The next challenging task in the analysis procedure, before the identification of single-particles or beam-spots, is to determine the scanning pattern followed by the ion-beam and extract the information corresponding to eachRP. For this purpose, an automatized ROOTA script was developedB. The methodology consists of recognizing high-quality tracks with cus-tomized thresholds and differentiating theRP sequence followed by the ion-beam. Figure 7.5 displays exemplaryMicromegas data of the reconstructed x- and y-position of a proton-beam before crossing the phantom. The ion-beam moves originally in a horizontal zig-zagged pat-tern to cover theFOV. Thus, this trajectory is translated in short beam-steps in y (cf. Figure 7.5b) with the beam staying for a longer time in one x-position (cf. Figure 7.5a). A so-called sliding-window technique automatically splits this sequence in the scannedRPs. The underly-ing principle of the average runnunderly-ing-window consists of calculatunderly-ing and comparunderly-ing the mean position in two different windows, before and after an optimized number of recognized events in the windows, and look for the maximum difference (within a given threshold), which indicates that a transition to a newRPhas occurred.
Once theRPshave been identified, the hits belonging to each event fill single-RPhistograms, that are fit either to two1D-Gaussian, one in each direction (protons) (cf. Figure7.6) or to a 2D-Gaussian (carbon-ions) (cf. Figure 7.7). In the following, the x- and y- mean-position and width (𝜎) of the beam-spot are then individually extracted from the Gaussian parametrization.
The Micromegas detector single particle tracking capability is determined by three main factorsC:
1. Readout electronics: The APV25-based electronics used for data-acquisition in these measurements has a maximum readout rate of about 1 kHz. Therefore, although the Micromegasdetectors have a higher detection rate, the readout system is bounded to the
Ahttps://root.cern.ch/
BThanks to F. Klitzner (LMU) and J. Bortfeld (CERN) for developing the analysis strategy to be applied on theMicromegasexperimental data.
CPrivate communication with J. Bortfeld, December, 2016
Figure 7.5: Reconstructed two-dimensional beam-position (scanning pattern) of a proton beam in theMicromegas active area before the image target. Data provided by F. Klitzner.
beam scanning pattern x sliding mean x
(a)Reconstructed beam hit position in the Micromegasx-direction.
0 1 2 3 4 5 6 7 8
Trigger number #105
0 10 20 30 40 50 60 70 80
MMs hit position [mm]
combined hit y sliding mean y
(b)Reconstructed beam hit position in the Micromegasy-direction.
Figure 7.6: Exemplary x- and y-beam-spot distributions of a 157.43 MeV/u proton beam fitted to a Gaussian curve.
The beam has a 10.7 mmFWHMat isocenter, which corresponds to a𝜎 ∼4.54 mm. ThisRPcharacteristics were extracted from a 5×5 FOV scanned in 2 mm steps detected with theMicromegas-doublet placed in front of the phantom. Since the detection point is located before the isocenter, a smaller beam-size than the one reported in the HIT’sLIBCis expected. Figures courtesy of F. Klitzner.
Entries 662
Mean 40.55
RMS 4.008
/[ndf
χ2 26.09[/[26
Mean 40.8±0.1 Sigma 2.666±0.094
hit[position[x[[mm]
20
− 0 20 40 60 80
Entries
0 10 20 30 40 50 60
(a)Exemplary spot-distribution in x-direction of a pro-ton beam corresponding to theRPnumber 5 and cor-responding Gaussian fit parameters.
hit9position9y9[mm]
20
− 0 20 40 60 80
Entries
0 5 10 15 20
25 Entries 170
Mean 40.22
RMS 4.251
/9ndf
χ2 7.6829/99
± Mean
Sigma 3.101±0.331 χ
0.31 40.08
(b)Exemplary spot-distribution in y-direction of a pro-ton beam corresponding to theRPnumber 5 and cor-responding Gaussian fit parameters.
electronics limit. In order to increase this rate to around 1 MHz a different electronics, based on the so-called VMM3 chips (cf. Section8.2.3), must be considered.
2. Detector rate capability: The maximum particle flux, that any strip detector can handle (in the sense that coincident particle hits are reconstructed as individual hits), is deter-mined by the strip pitch and the signal duration. These floating-strip Micromegas are able to separate all particle hits up to particle rates of 10 MHz. This corresponds to a particle flux of about 7 MHz×𝑐𝑚−2 [Bortfeldt et al. 2015]. If the rate becomes larger, individual particle hits are merged into clusters. Nevertheless, up to a rate of 80 MHz individual particles can still be registered, although with reduced efficiency.
3. Drift field: The drift field produced by the HV considerably improves the collection of charge in the the detector and thus greatly enhances the signal behavior. Due to the high ionization of carbon ions, the detectors were working well with the carbon-ion beams and
7.1. Materials and Methods
Figure 7.7: Detected carbon-ion beam spots using the sliding-window technique. Two RPsextracted from a 5×5 cm2FOVactively scanned with a 299.94 MeV/u carbon-ion beam are shown. Figures courtesy of J. Bortfeld.
reconstructed y-position [mm]
8 9 10 11 12 13 14 15 16
reconstructed x-position [mm]
47 48 49 50 51 52 53 54
55 Entries 1823
/ ndf
χ2 157.3 / 82
p0 61.21 ± 2.852 p1 12.14 ± 0.03015 p2 0.9849 ± 0.0269 p3 49.78 ± 0.02613 p4 0.8807 ± 0.02508
0 10 20 30 40 50 60
Entries 1823 70
/ ndf
χ2 157.3 / 82
p0 61.21 ± 2.852 p1 12.14 ± 0.03015 p2 0.9849 ± 0.0269 p3 49.78 ± 0.02613 p4 0.8807 ± 0.02508
hx hits, rasterpoint 11
(a)RPnumber 11
reconstructed y-position [mm]
8 9 10 11 12 13 14 15 16
reconstructed x-position [mm]
42 43 44 45 46 47 48 49
50 Entries 2378
/ ndf
χ2 172.1 / 95
p0 69.74 ± 2.662 p1 12.06 ± 0.0271 p2 1.046 ± 0.02802 p3 45.34 ± 0.02532 p4 0.9896 ± 0.01792
0 10 20 30 40 50 60 70
Entries 2378
/ ndf
χ2 172.1 / 95
p0 69.74 ± 2.662 p1 12.06 ± 0.0271 p2 1.046 ± 0.02802 p3 45.34 ± 0.02532 p4 0.9896 ± 0.01792
hx hits, rasterpoint 12
(b)RPnumber 12
were able to track all single ions. For the protons on the other hand, the drift field would have been necessary.
Proton beam irradiation
In order to avoid the overflow of the trigger and readout systems due to high particle-rate, all the acquisitions were taken in the synchrotronresearch mode, that allows to irradiate low-fluence beamsnecessary for lower-dose scenarios(cf. Section3.3) than the levels typically used in the clinical practice. The beam-intensity has been reduced to 1% of the lowest nominal intensity (8×107pps) by decreasing the current threshold allowed by the firstICof theBAMS (cf. Section 2.1). The irradiated plans covered a FOV of 5×5 cm2 in 2 mm scanning-steps.
Three different irradiations were conducted in this study:
1. Air-projection (no phantom) with a 157.43 MeV proton-beam of the smallest focus (10.7 mm FWHM at isocenter, corresponding to a ∼ 4.5 mm 𝜎), allocating 8×105 primaries perRP (cf. Figure7.8a and7.8b).
2. Slab-phantom projection with a proton beam of the smallest focus at 157.43 MeV and 8×105 primaries per RP(cf. Figure7.8).
3. PMMA stepped-wedge-phantomprojection with a proton beam of the smallest focus (8.1 mmFWHMat isocenter∼3.4 mm𝜎) at the highest beam-energy 221.06 MeV and 8×105 primaries perRP (cf. Figure7.10).
Carbon-ion beam irradiation
Carbon-ion beams at 299.94 MeV/u with the smallest focus (3.9 mm FWHM at isocenter, corresponding to a∼1.66 mm) and the lowest beam-intensity available (2×106pps) were used to scan a 5×5 cm2 extended-field in 5 mm RP-steps to image the following configurations:
1. Air-projection (no phantom between the tracking system).
2. Slab-phantomprojection (cf. Figure 7.11).
3. PMMA stepped-wedge-phantom projection (cf. Figure7.13).