Conrad, J

López Noriega, M

Morsch, A

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH European Laboratory for Particle Physics     Internal Note/ ALICE reference number ALICE-INT-2006-005 version 1.0Institute reference number [-]Date of last change   16.03.2006    Simulation of the ITS 2004 test beam with FLUKA and the ROOT Geometrical Modeler TGeo    Authors:  M. López Noriega, A. Morsch, J. Conrad  Abstract:  In this note we present results of the simulation of the ITS integrated test beam experiment of November 2004. Simulations obtained with FLUKA VMC using the newly developed ROOT geometrical modeler TGeo are compared to the test beam data and to results obtained with a corresponding GEANT3 simulation using the same geometry and detector response model.          1. IntroductionFLUKA [1] plays a very important role in ALICE for all the tasks where detailed andreliable physics simulation is vital, given its thorough physics validation and its almostunique possibility to couple low-energy neutron transport with particle transport in asingle program. For this reason ALICE has implemented a FLUKA VMC implementation ofthe Virtual Monte-Carlo Interface using the ROOT geometrical modeler TGeo [2].In order to gain confidence in FLUKA VMC, we present here the results from the compar-ison of FLUKA and Geant3 [3] in the Inner Tracking System (ITS) [4]. We also comparethe results from FLUKA to some results from the Silicon Pixel Detector (SPD) of theITS test beam data of November 2004.In section 2, the geometry is presented and the class into which it was coded is described.In section 3, the results from the comparison of the two transport models are presentedand discussed. Finally, in section 4, the results from the comparison of FLUKA to dataare shown.2. Geometry2.1. Test beam geometryThe integrated ITS test beam that took place in November 2004 was the first testbeam in which layers from the three sub-detectors of the ITS: the Silicon Pixel Detector(SPD) [5], the Silicon Drift Detector (SDD) [5], and the Silicon Strip Detector (SSD) [5],were together in a test beam [6]. The following elements formed the geometry for thistest beam:• 4 SPD modules (in two parallel planes) with sensitive volume of size 70 mm × 12mm and 200 µm thickness.• 2 SDD modules (in two parallel planes) with sensitive volume of size 70.17 mm ×74.97 mm and 299.8 µm thickness.• 4 SSD modules (in two parallel planes, with a small overlap of the sensitive volumesbetween modules in the same plane) with sensitive volume of size 75 mm × 42 mmand 300 µm thickness.2.2. AliITSvITS04 geometry classThe geometry of the ITS test beam was coded into a class called AliITSvITS04, thisclass also includes the step manager. This geometry was built using the new Root Geo-metrical Modeler TGeo [2], and it has been tested in simulation and to generate clustersfrom the test beam data in the SPD. This class also includes the possibility of initializingthe geometry from an ascii file that has been previously created with the same geometryclass.Figure 1 shows four different views of the integrated ITS test beam geometry as gen-erated by the geometry class. The proton beam goes in the +Y direction. Following thedirection of the beam, the first two planes correspond to the SPD, the next two to theSDD, and the last two to the SSD. There are also two planes of scintillator (before theSPD planes and after the SSD planes) that were present for triggering purposes. In thegeometry class there is also the possibility of switching on a target in front of the firstscintillator. This target was also present in some of the runs. However, the data used tocompare with the results from FLUKA simulations in the analysis presented here is fromruns in which the target was not present, the detectors were placed directly in the beam.Therefore the target was not included in the simulations.3. Geant3 and FLUKAGEANT3 is the detector simulation Monte Carlo used extensively so far by HEP com-munity for simulation of the detector response. However, it is no longer maintainedand has several known drawbacks, both in the description of physics processes particularhadronic and of the geometry.FLUKA is the transport code used for radiation environment simulations of the AL-ICE experiment. Its Virtual Monte-Carlo implementation FLUKA VMC renders the programsuitable for full detector response simulation. It incorporates a full treatment of high en-ergy particle interactions, but special emphasis has been put on effects occurring aroundenergies of a few GeV and below: a pre-equilibrium cascade model for inelastic interac-tions below 1.3 GeV and for capture reactions at rest, nuclear evaporation and gammade-excitation after inelastic interactions, multigroup transport of neutrons below 20 MeVwith detailed kinematics for (n,p)-scattering, neutron capture reactions with explicit pho-ton emissions.For the special application of the ITS test-beam simulation hadronic interactions play aminor role. The charge deposition in the silicon layer is mainly determined by ionizationloss including explicit δ-electron production. For these processes FLUKA provides thelatest parameterizations of the density effect at high energies and shell corrections at lowenergies.3.1. FLUKA vs. Geant3 for ITS simulationIn this section we present the results from the comparison of the generated hits, wherethe hit represents a single energy deposition in the detector, and calculated digits andclusters using both Monte Carlo transport models Geant3 and FLUKA. Digits representthe digitized response of the detector and clusters are the reconstructed space points. Weuse this comparison as a technical test of the interface. Everything was done with thetools that are already in AliRoot.For both transport models, the simulation of hits was run using the AliSimulationclass. The generator used was the simple AliGenBox (a generator for particles with a flatdistribution in a preset kinematic range) and 5000 events of one proton per event withmomentum in the range between 119.5 and 120.5 GeV/c were generated (in the test beamruns, the beam was a positive beam of 120A GeV/c). The ranges in the φ and θ angleswere defined so that particles passed through, at least, one of the sub-detectors.Digits were generated directly from hits, and not from SDigits (these are digits thatcan be summed when different simulated events are superimposed), using the differentslow simulation methods for each sub-detector (coded in AliITSsimulationSPDdubna,AliITSsimulationSDD, and AliITSsimulationSSD classes) [7–10]. For the last step ofthe reconstruction process considered in this study, clusterization, the cluster finder al-gorithm for each sub-detector was called in order to group the digits into clusters (thesex (cm)-3-2-10123Y (cm)0102030405060708090Z (cm)-6-4-20246Y (cm)0 10 20 30 40 50 60 70 80 90Z (cm)0SPDSDD SSD-6-4-20246x (cm)-3 -2 -1 0 1 2 3Y (cm)0102030405060708090x (cm)-3 -2 -1 0 1 2 3Z (cm)-6-4-20246Figure 1. Four different views of the ITS elements present on the integrated ITS testbeam runs as generated with the AliITSvITS04 geometry class. The proton beam wasin the +Y direction. The planes correspond to the detectors indicated in the top rightpanel.dE/dx (GeV/cm)0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02Normalized distribution-410-310-210-110 Geant3Fluka)e(E-m10Log-5 -4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0Normalized distribution020406080100120140160180200220GeantFlukaFigure 2. Normalized distribution of the energy lost per hit (dE/dx) for hits of primaryparticles in the SPD (left figure), and normalized distribution of the kinetic energy of thesecondary particles (right figure), as simulated by Geant3 and by FLUKA.algorithms are coded in the AliITSClusterFinderSPD, AliITSClusterFinderSDD, andAliITSClusterFinderSSD classes) [7,9,10].3.2. Generated hitsFigure 2 shows the ionization energy per step normalized distribution for hits in the SPD(left), and the kinetic energy normalized distribution of the secondary particles (right) assimulated by Geant3 and by FLUKA. Both distributions are normalized to the numberof entries. A good agreement is observed between both models in those distributions.3.3. Digits and clustersAs already mentioned, digits were generated from hits using the different slow simula-tion methods for each sub-detector. In this case, the number of digits calculated from hitsfrom the two models is approximately the same. Once digits were calculated, the clusterfinder algorithm for each sub-detector was run.Figure 3 shows the normalized distributions of the size of the clusters (i.e., the numberof pixels, in a given direction, that made the cluster) in the X and Z directions in theSPD. A nice agreement between both models is seen.Figure 4 shows the normalized distribution of the deposited energy in the silicon of thedetector by each particle in each of the sides of the SSD. The expected Landau distributionis observed, as well as a nice agreement between the two transport models.4. FLUKA vs. DataIn this section we present the results from the comparison of the digits and clustersas obtained from the hits generated by FLUKA to those obtained from the test beamdata. Information from 5000 test beam events was used for this comparison. The resultspresented here are only for the SPD sub-detector. The classes that contain the algorithmsfor decoding the test beam data, and for its clusterization, are in AliRoot and are currentlyX size (# of pixels)0 2 4 6 8 10 12 14Normalized distribution-410-310-210-1101 GeantFlukaZ size (# of pixels)0 2 4 6 8 10 12 14Normalized distribution-310-210-1101 GeantFlukaFigure 3. Normalized distribution of the SPD cluster sizes in the X (column) direction,left figure; and in the Z (row) direction, right figure.SignalN0 20 40 60 80 100 120 140Normalized distribution-310-210-110GeantFlukaSignalP0 20 40 60 80 100 120 140Normalized distribution-310-210-110 GeantFlukaFigure 4. Normalized distribution of the deposited energy in the N side of the SSD, leftfigure; and in the P side of the SSD, right figure.dE/dx (GeV/cm)0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02Normalized distribution-510-410-310-2101 keV10 keV30 keVX size (# of clusters)0 2 4 6 8 10 12 14Normalized distribution-410-310-210-1101 1 keV10 keV30 keVFigure 5. Normalized distribution of the energy lost per primary particle per step (left)and of the cluster sizes in the columns of the SPD (right) as simulated by FLUKA forthree different transport threshold cuts.being tested.Before we compared with data, we studied the dependency of the final results fromFLUKA on the transport threshold for the different particles and media. Figure 5 showsthe result of this comparison for hits (left plot) and for clusters (right plot) for threedifferent threshold limits. One can see a difference in the energy lost per particle in eachstep, as expected since the energy threshold changes, but the final results are unaffectedby the change in this cut. For the comparison with data, the transport threshold cuts inFLUKA were set to 30 keV for all relevant particles and processes.In the comparison of Geant3 and FLUKA (see Figure 3), charge sharing between pixelswas treated in a rough way (the current default in AliRoot). This does not affect thecomparison between the two models as long as it is done for both models consistently.However, for the comparison with data, we decided to use the more sophisticated “Dubnamodel” [7,8] in order to get a better agreement with data, especially at small cluster size.In this model, the charge is shared between annexing pixels by drifting the electron/holecloud with a Gaussian diffusion. Since the sigma of this Gaussian is a function of the driftvelocity (which is proportional to the electric field), this diffusion strongly depends on thebias voltage parameter1 which we took to be 100 V. A tuning of this model to beam testdata (from 2002 and 2003) is currently under work [11].Figure 6 shows the normalized distributions of the cluster size in the SPD when thebias voltage is 100 V. The inclusion of diffusion does not affect the region of high clustersizes, however it makes the agreement between simulation and data better at low clustersizes.5. ConclusionThe geometry of the integrated ITS test beam based on the TGeo geometrical modelerof root has been presented. The results from a comparison of hits, digits and clusters in1for a given thickness of the sensor and a fixed temperatureX size (# of clusters)0 2 4 6 8 10 12 14Normalized distribution-410-310-210-1101DataFlukaZ size (# of clusters)0 2 4 6 8 10 12 14Normalized distribution-310-210-1101 DataFlukaFigure 6. Normalized distribution of the cluster size in the X and Z direction in the SPDmodules. In the simulation case, diffusion is allowed (see text for explanation).FLUKA and Geant3 as generated according to this geometry have been shown. Finally,results from the comparison of digits and clusters from FLUKA to those from the testbeam data in the SPD has been presented. The differences in the cluster sizes fromFLUKA simulation and test beam data become smaller when diffusion in the digitizationprocess is allowed. Therefore FLUKA can be validated in the ITS.REFERENCES1. A. Kling, F. Barao, M. Nakagawa, L. Tavora, P. Vaz, Springer-Verlag Berlin, p. 159-164 (2001). A. Kling, F. Barao, M. Nakagawa, L. Tavora, P. Vaz, Springer-VerlagBerlin, p. 955-960 (2001).2. The Geometry Package (http://root.cern.ch/root/doc/RootDoc.html).3. GEANT3 Collaboration, CERN-DD/EE/84-1, September 19874. P. Kuijer, ALICE Internal Note/2003-049 (2003).5. ITS Technical Design Report, CERN/LHCC 99-12 (1999).6. D. Nouais et al, to be submitted as an ALICE internal note.7. R. Barbera et al, ALICE-INT-2001-48.8. B. Batyunya and A. Zinchenko, ALICE-INT-94-11 (1994).9. P. Cerello et al, ALICE-INT-2001-34.10. F. Retie`re et al, ALICE-INT-1999-36.11. G. Bruno et al, to be submitted as an ALICE internal note.

Simulation of the ITS 2004 test beam with FLUKA and the ROOT Geometrical Modeler TGeo

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