Summer Student Project Report

Transcription

1 European Organization for Nuclear Research Summer Student Project Report Vlasios Vasileiou CERN EP/NOE Aristotle University of Thessaloniki August 2002 Supervisors: Igor Garcia Irastorza Luigi Di Lella Work can be found at /afs/cern/ch/users/v/vlasisva/public/summer_project

2 CERN Summer Student Report Vlasios Vasileiou 2 Introduction During my stay 1 as a Summer Student at CERN I worked mainly on the Time Projection Chamber (TPC) of the CAST [1] experiment. Among others, my work included Simulation of the chamber and refining of the offline analysis cuts. 1. Simulation A simulation of the CAST Time Projection Chamber was carried out by means of Geant4 [2] simulation toolkit. The main purpose of the simulation was the estimation of the chamber s efficiency. 1.1 Simulated Geometry The model of the TPC used for the simulations is shown in fig.1. It consisted of a Plexiglas case the drift gas (95% Argon 5% Methane, 15x30x10 cm 3, Standard conditions) two Mylar foils (thickness 5 micron) supported by two stainless steel windows (diameter 6cm) Figure 1. Front and side views of the TPC model used for the simulation Figure 2. Mylar supporting mesh 1 2 nd July to 6 th September 2002

3 CERN Summer Student Report Vlasios Vasileiou X-rays source The x-rays point-like source was fixed above one of the TPC windows. All the primaries were shot on the same trajectory; a straight line perpendicular to the window. 1.3 Physics Processes For improved validity of the simulation results, the low energy extension [3] of Geant4 s electromagnetic processes was used. This extension is valid for energies from 250eV up to 100GeV. The activated physical processes for x-rays included photoelectric effect, Compton and Rayleigh scattering and for electrons multiple scattering and ionization. Furthermore, auger electrons emission and fluorescence were activated for the de-excitation of atoms. 1.4 Results - efficiency For each energy, some photons were shot into the chamber. After each photon was shot, the program recorded it s energy deposition in the gas. Finally, two plots (fig. 3) were produced: a. the ratio of the energy deposited in the gas over the total energy injected into the detector (=number of photons(20000) * photon energy), for each energy. b. the ratio of photons that fully deposited their energy in the gas over the number of photons injected (=20000), for each energy. These two plots were created for two kinds of runs; with and without the window. When the window was activated, the plots included the absorption of the mylar foil and the screening effect of the supporting mesh 1 (ª8.3%). For the TPC model without window (fig 4.) the plots show only the x-ray absorption efficiency of the gas mixture and not the efficiency of the whole detector (ª combined effect of mylar foil + mesh + gas). Figure 3. Efficiency of the TPC 1 This 8.3% was subtracted after the simulation, during the offline analysis of the results

4 CERN Summer Student Report Vlasios Vasileiou 4 Figure 4. Absorption efficiency of the TPC gas The TPC chamber was tested experimentally in Max Plank Institute/Panter facility in Munich, where the efficiency of the chamber and the transmitivity of the window were measured. Fig. 5 shows a comparison of the experimental efficiency results with the efficiency derived from simulation. The blue line of fig. 5 represents the theoretical estimation of the TPC window transmitivity. This is equal to the theoretical transmitivity of mylar reduced by 8.3% because of the screening effect of the supporting mesh. The gas absorption efficiency reduced by the window transmitivity (=mylar+mesh) is equal to the expected TPC efficiency (red line). As seen in the plot, the efficiency from simulation (green dots) nearly matches the expected efficiency, verifying the validity of the simulation results. The show the experimental results for the TPC efficiency. As it is shown, there is large divergence between simulation and experiment for most of the energy range. The possible effect causing that, is that the simulated efficiency is rather the combined absorption probability of the TPC s window and gas than the real efficiency of the TPC detector. The latter includes charge detection efficiency, multi cluster events due to fluorescence or Compton scattering etc; effects that if calculated, would narrow the gap between simulation and real experiment. Efficiency Efficiency (theory) Gas Absorbtivity (theory) Window transmitivity (theory) Efficiency (simulation) Efficiency (experiment) Window transmitivity (experiment) Energy (kev) Figure 5. Simulation versus experiment

5 CERN Summer Student Report Vlasios Vasileiou Results X-rays absorption per depth distribution It is interesting to know the depth that the x-rays interact (mainly via photoelectric effect) with the chamber s gas. Figure 6 shows the percentage of x-rays that interact with the gas per mm of depth (after any absorptions on mylar). Zero depth corresponds to the plane of the entrance of the x- rays. In fig. 7 some photon interaction cross sections [4] for Argon are shown. Figure 6. Percentage of x-rays that interact with the gas per mm of depth Figure 7. Photon interaction cross sections for Argon

6 CERN Summer Student Report Vlasios Vasileiou Results Spread of ionization area Suppose that a photon interacts with the gas, ionizing it. The electrons produced can produce further ionization. If we project the volume of the gas where ionizations have occurred on a plane parallel to the entrance plane of the detector, then the shape of this projection could be a circle. This circle would be a rough approximation of the mean cluster on the real detector, supposing a minimal diffusion while the electrons drift down to the wires. If the diameter of the circle is bigger than about 1.5 mean cluster size, then this event could be a multi cluster event. We conclude that the diameter of the circle compared to the size of a usual cluster (about three wire distances = 3*3 mm), gives an estimation of whether an ionization would produce single cluster or multi cluster events. The number of the to-be multi cluster events is very interesting because these events are cut in the offline analysis, resulting in a reduction of the efficiency. Figure 8. Percentage of ionizations with diameter over R Figure 9. Mean Ionization area diameter

7 CERN Summer Student Report Vlasios Vasileiou 7 2. Analysis My contribution on TPC offline analysis was the refining of the cuts. The available data (table 1) were from tests in Max Planck Institute/Munich. The program used for my work was ROOT [5]. Run Energy (kev) Bins Background -- Table 1. Runs used Cuts used: Number of wires per anode cluster (min, max) Number of wires per cathode cluster (min, max) Anode/cathode charge ratio (min, max) Number of anode clusters per event Number of cathode clusters per event Table 2. Cuts used For every signal run only a specific part of it s data were used; the region of interest (ROI) was the source s peak. After that, the background data were divided in eight parts, each corresponding to a ROI taken from the signal runs. Every cut was applied on every signal run and a plot with the remaining signal was created for every cut value. This number should be at least over 90% for an accepted cut. The same procedure was carried out for all of the background run fragments. Then a plot showing the ratio of the remaining signal over the remaining background for the same ROI was created for every run and every cut. By definition this ratio is unity for a background run. If this number is, for a specific cut and energy combination, under unity then this means that this cut rejects more signal than background, something unacceptable. On the contrary, a ratio over unity shows a good working cut. By checking these two numbers, an optimum cut value can be selected for every energy and for every cut. Then with these values, energy depended cuts can be created, maximizing the signal/noise ratio of the data.

8 CERN Summer Student Report Vlasios Vasileiou Number of cathode wires per cluster Figure 10. Number of cathode wires per cluster Figure 11. Scanning for the optimum minimum value

9 CERN Summer Student Report Vlasios Vasileiou 9 Figure 12. Scanning for the optimum maximum value Energy (kev) Minimum Maximum Table 3. Proposed cut values 1 Minimum cathode multiplicity If energy Œ(2, 2.5) then minimum=2 If energy > 2.5 then minimum=3 Maximum cathode multiplicity If energy Œ(0.7, 1.7) then maximum=5 Table 4. Proposed cuts conditions 1 If no number is shown then no effective cut value can be selected for this energy-cut combination

10 CERN Summer Student Report Vlasios Vasileiou Number of anode wires per cluster Figure 13. Number of wires per anode cluster Figure 14. Scanning for the optimum minimum value

11 CERN Summer Student Report Vlasios Vasileiou 11 Figure 15. Scanning for the optimum maximum value Energy (kev) Minimum Maximum Table 5. Proposed cut values Minimum anode multiplicity Maximum anode multiplicity If energy < 0.7 then minimum=1 No effective cut Table 6. Proposed cuts conditions

12 CERN Summer Student Report Vlasios Vasileiou Number of anode and cathode wires per cluster Figure 16. Number of anode and cathode wires per cluster Figure 17. Scanning for the optimum minimum value

13 CERN Summer Student Report Vlasios Vasileiou 13 Figure 18. Scanning for the optimum maximum value Energy (kev) Minimum Maximum Table 7. Proposed cut values Minimum anode & cathode multiplicity If energy < 1.2 then minimum=3 If energy Œ(1.2, 2) then minimum=4 If energy Œ(2, 4.7) then minimum=5 Maximum anode & cathode multiplicity No effective cut Table 8. Proposed cuts conditions

14 CERN Summer Student Report Vlasios Vasileiou Anode/cathode charge ratio 1 Figure 19. Anode & cathode charge ratios Figure 20. Scanning for the optimum minimum value 1 Data used for this cut were already filtered by the default set of cuts

15 CERN Summer Student Report Vlasios Vasileiou 15 Figure 21. Scanning for the optimum maximum value Energy (kev) Minimum Maximum 0.3 No data available Table 9. Proposed cut values Minimum charge ratio No cut Maximum charge ratio Maximum=2.9731*energy Table 10. Proposed cuts conditions

16 CERN Summer Student Report Vlasios Vasileiou 16 Figure 22. Anode/cathode charge ratio gaussian fits Figure 21. Anode/cathode gaussian fit parameters

17 CERN Summer Student Report Vlasios Vasileiou Number of zero anode and cathode cluster events These plots are given as a reference. If an event has no anode or cathode clusters then it is rejected. So these plots show the percentage of events to be rejected by this condition. Figure 23. Percentage of zero anode cluster events Figure 24. Percentage of zero cathode cluster events

18 CERN Summer Student Report Vlasios Vasileiou 18 References: [1] CERN Axion Solar Telescope ( [2] GEANT4 Simulation Toolkit ( [3] GEANT4 Low Energy Electromagnetic Physics ( [4] XCOM: Photon Cross Sections Database ( [5] ROOT : An Object-Oriented Data Analysis Framework (