During the process of designing the MVD and its software, the layout properties have been studied [1, 6] using the SDS package in PandaRoot. These encompass a coverage map of the MVD data rates as well as a detailed material budget study.
3.3.1 Detector coverage
Covering the most of the angular region with the MVD is necessary to achieve the design goal of at least four hits recorded per track. Meeting that condition enables the standalone tracking by the MVD without waiting for slower components. This can be exploited for the necessary data reduction by applying filters based on fast detector analyses.
In figure 3.2 the number of hits created per track (1 GeV/c pions) is plotted in a detector map, showing the dependence on the polar and azimutal angles with the interaction point as point of view. Most notable are the two gaps in the barrel part where the target pipe (around θ = 90◦ and φ = ±90◦) fits through. Because the first
pixel layer is very close to the interaction point this acceptance hole is for that layer relatively big reducing the number of hits per track in the MVD in that region. In the forward region, where the highest particle flux is expected (cf. figure 1.5), several barrel and disk layers will produce at least four and up to seven hits per track.
For vertex reconstruction it is preferred to have a precisely measured hit as close as possible to the decay point. Deviations due to the track direction or curvature measurement will be less important with the precise anchor point. As most particles decay very close to their creation point the distance of the first hit to the interaction zone governs the overall vertex resolution. Figure 3.3 shows the distance from the interaction point to the first hit in the MVD for slow pions. The first layer of pixel sensors is at 2 to 3 cm distance, featuring the prominent holes for the target pipe as well as gaps in the transition region between the barrel part and the disks which are covered by the second layer of pixel sensors (about 6 cm distance). In the very forward part one can see how the acceptance of the disks ends until only the strip disks can contribute with a precise first hit. This will make vertexing with shallow angled tracks going to the forward spectrometer of PANDA more challenging, especially the reconstruction of the z coordinate.
3.3.2 Material Budget
Each piece of material in a particle flight path introduces deviations in form of scat- tering and absorption. Usually a material’s effect is quantized by the radiation length X0, defined as the distance where an electron would loose a fraction of (1 − 1/e) of its
energy by bremsstrahlung. For elements usually a density-independent form ˜
X0= ρ · X0 (3.1)
is used. Composite materials are described by the mass fraction wi of each element:
1 X0 =
X wi X0,i
. (3.2)
Table 3.3 summarizes selected radiation length values for the MVD . To compare different subsequent detector components a common practice is to give the fraction of traversed radiation length X/X0. This number should be high for detectors measuring
the total particle energy (the EMC) and it should be as low as possible on other systems (basically all inner components).
Figure 3.2: Detector coverage: Number of hits per track in different angular regions (pions with 1 GeV/c from the interaction point) [1].
Figure 3.3: First hit distance to the interaction point. (pions with 100 MeV/c from the interaction point) [1].
˜ X0 ρ X0 Material [ g/cm2] [ g/cm3] [ cm] C 42.7 2.21 19.3 Al 24.0 2.70 8.89 Si 21.8 2.33 9.36 Cu 12.9 8.96 1.44 In 8.85 7.31 1.21 Water 83.3 1.00 83.3 C (foam filling) 42.7 0.227 188 C (fibre structures) 42.7 2.27 18.8 PVC 25.5 1.30 19.6
Table 3.3: Radiation length values for the most common materials in the sensitive region of the MVD.
There is the demand of as few material inside the measuring region as possible, however, front-end chips, support structure and cooling add their material to the sensors themselves. To gain insight which components introduce most influence on particle tracks a study was performed with PandaRoot and the detailed geometry description [6]. Virtual particles (“geantinos”1) are ejected randomly from the in-
teraction point with a flat angular distribution. Each trajectory piece in a volume contributes with its radiation length to the accumulated value X. The angular ra- diation length distribution viewed from the interaction point, shown in the left-hand part of figure 3.4, is not flat but varies strongly with the polar angle region as well as with the azimutal angle.
Most of the barrel and disk part are covered with a relatively small amount of material with two dense regions in the transition between the two parts. Data cables and cooling pipes are routed there (25◦ < Θ < 45◦) to the inner disks. A striped
structure is visible in the barrel part. Data cables with metallic signal lines, cooling pipes and the support frames of all components are routed along that way. In the backward part at Θ > 145◦ very dense hot-spots are present in form of the converging
cable and pipe routing as well as connectors for the MVD services. This unavoidable load of material, together with the services of the STT, will be the biggest challenge for the backward calorimeter cap.
Classifying the radiation length values by the corresponding component in the MVD produces an overview of which parts are benefitting most from optimization (figure 3.4, right-hand side). The largest contribution to the sensitive region is actually given by the cables and the support structures. As a consequence the development of low-X0 (i.e. thin) cables or local data concentration chips (which reduce the required
signal lines) will have a large beneficial impact, whereas thinning the sensors will not contribute much to reduce the material budget.
1Internal particle definition in GEANT which is not processed with the transport model’s physics processes, thus speeding up the simulations significantly.
Figure 3.4: Material budget distributions in units of the radiation length X0. Angular view
from the interaction point (left) and contributions of different components along the polar angle. [1]
3.3.3 Rate Studies
Data rates are one of the fundamental challenges in the design process of the PANDA apparatus and the MVD is no exception. As the sensitive element closest to the inter- action point the particle flux is highest. Data reduction, compression and transport have to be optimized according to the expected data rates in the different regions and building blocks of the MVD. Such rate studies have accompanied the MVD design process and software development ([60, 61]), the latest results being reported in the TDR [1].
The maximum rates are found at the long strips of the strip modules and at the large-area pixels on the pixel sensor rims, their data rates resulting from their large solid angle covered per channel and the position in the setup (cf. angular distribution of particles figure 1.5). In figure 3.5 data rate distributions on the sensors with the highest load are shown, obtained from antiproton-proton collisions simulated by the DPM event generator. The average rate per channel peaks in the order of 8 kHz for the strip sensors and at 2 kHz for the pixels.