Study of straw tube trackers for the comet experiment

Study of Straw Tube Trackers for The COMET Experiment Master of Physics Nguyen Minh Truong Department of Physics, Graduate School of Science Osaka University, Japan July 31st 2012 Abstract A straw tube tracker is one of the two particle detector of the COMET (The COherent Muon to Electron Transition) experiment, which will be located in J-PARC to search for the muons to electrons (µ− - e− conversion) that is a lepton number violation process Since electron of µ− - e− conversion have low energy of 105 MeV, multiple scattering can dominate energy resolution of detector Therefore, it is important to reduce the tracker’s material and the tracker should operate in vacuum In order to achieve these requirements, we design the straw tube tracker which is made from Kapton films of 1m length and 5mm diameter We constructed and studied the prototype straw tube tracker to study the perform of straw tube tracker ii Acknowledgements “ First of all, I would like to express my sincere thanks to people who supported me in the development of my thesis I’m grateful for all the support and motivation of my supervisor from the Osaka University, Prof Y.Kuno He allowed my attend to his group and left me to myself with his proper advice Thank you for providing me the brain power to study in physics I also thank Prof S Mihara and Prof H Nishiguchi for their education of experimental basis for me I appreciate their interest in my work, as well as their advice and patience as this thesis evolved They was always available when I had questions and I especially would like to thank them for their clear, efficient and fast help whenever it was needed I also was supported by many friends in Kuno group I am grateful to them At last, I would like to thank my family for their unlimited support ” iii Contents Abstract ii Acknowledgements iii List of Tables vi List of Figures vii COMET EXPERIMENT AT J-PARC 1.1 Introduction 1.2 Theoretical Motivation 1.3 µ− → e− conversion 1.4 Overview of The COMET Experiment 12 1.5 Pulsed Proton Beam 14 1.6 Muon Stopping Target 15 Straw Chamber 17 2.1 Requirement for the COMET tracker 17 2.2 Gas study 20 iv 2.3 Principal operation of the straws tube tracker Study of Prototype Straw Tube Tracker 27 35 3.1 Overview of Prototype Straw Tube Tracker 35 3.2 Study the sag of the wire 40 3.3 Gas gain study 43 3.4 Efficiency of straw tube tracker 46 3.5 Study the drift velocity of straw tube tracker 52 3.5.1 r t calibration 52 3.5.2 Drift velocity of straw tube tracker 55 Position resolution 58 3.6 Conclution 60 v List of Tables 1.1 Requirement of the pulsed proton beam [2] 1.2 Lifetimes and relative strength of µ− - e− conversion process in different 14 muon stopping material [2] 16 1.3 Parameter of muon stopping target [2] 16 2.1 The drift times of various gas mixtures at a nominal operational voltage 21 vi List of Figures 1.1 History of charge LFV searches[9] 1.2 Diagram of neutrino’s mass contribute to µ− → e− transition[2] 1.3 Electron spectrum, normalized to the free-muon decay rate Γ0 The solid blue line is for carbon, the black dotted line for aluminum, the green dot-dashed line for silicon and the red dashed line for titanium [11] 1.4 End-point region of the electron spectrum for aluminum The squares correspond to the spectrum with recoil effects The triangles is the spectrum neglecting recoil The right plot is a zoom for Ee > 100 MeV, the solid (dashed) line on this plot corresponds to the Taylor expansion around the end-point with (without) recoil [10] 1.5 Cross section through SINDRUM II connected to the PMC magnet [12] 10 vii 1.6 Recent results by SINDRUM-II Momentum distributions for three different beam momenta and polarities: (i) 53 MeV/c negative, optimized for µ− stops, (ii) 63 MeV/c negative, optimized for π − stops, and (iii) 48 MeV/c positive, optimized for µ+ stops The 63 MeV/c data were scaled to the different measuring times The µ+ data were taken using a reduced spectrometer field [12] 1.7 11 The COMET experiment schematic layout of the muon beam line and the detector [2] 13 1.8 Bunched proton beam in a slow extraction mode [2] 15 2.1 Straw tube tracker perpendicular with charge particle in the COMET experiment [1] 18 2.2 Layout of straw tube layer [1] 19 2.3 Drift velocity in different Argon and Ethan mixture [19] 22 2.4 Drift velocity of Argon and Ethan was simulate by Garfield 23 2.5 Diffusion of Argon and Ethan was simulate by Garfield 25 2.6 Townsend of Argon and Ethan was simulate by Garfield 26 2.7 Two layer of Kapton over-woven straw 27 2.8 Construction of straw tube tracker [1] 27 2.9 Electric field distribution in straw [21] 28 2.10 Contours electric field in straw is simulated by Garfield 29 2.11 The drift line of electrons in the straw tube tracker 30 2.12 Cross section as the function of electron energy in Ar gas [22] 32 viii 2.13 The different drift line of electron in case no magnetic field (left) and magnetic file (right) 33 2.14 electron drift line and cluster in straw tube with magnetic field 34 3.1 The design of prototype 36 3.2 Straw tracker (left) and straws are placed in the two layer as show in the right 37 3.3 Design of the prototype end plate of the prototype 38 3.4 The gas line to flow the gas mixture to the straw chamber [23] 39 3.5 Electric read out the signal from wire anode 40 3.6 Principle of the sag of the wire 41 3.7 Garfield simulation the sag of the wire 41 3.8 Compare the sag of the wire in Garfield simulation and calculation 42 3.9 The setup experiment for the gas gain study 43 3.10 Logic trigger for gas gain study 43 3.11 QDC histogram for the gas gain study 44 3.12 Mean QDC value as a function of the high voltage of straw 44 3.13 The amplification gain of the preamplifier and the ASD buffer 45 3.14 The gas gain of straw tube tracker 45 3.15 Image of fiber [24] 46 3.16 fibers are put close together 47 3.17 Experiment setup for hit efficiency 47 3.18 Hit efficiency of straw tube tracker 48 3.19 Straw tube tracker simulation by Geant4 49 ix 3.20 β energy spectrum of Sr90 source 49 3.21 Energy deposit in straw tube tracker by simulation 50 3.22 Energy deposit spectrum on straw tube with events passed through straw tube and fiber 51 3.23 Energy deposit spectrum on straw tube with events passed through straw tube and fiber 51 3.24 Setup experiment for drift velocity study 53 3.25 Time distribution of straw tube tracker 54 3.26 r t calibration for the upper straw tube tracker 55 3.27 Setup experiment for drift velocity study 56 3.28 rt relation of straw tube trackers 57 3.29 Fit r-t relation with linear function order 6th 57 3.30 Residual distribution of straw tube tracker 59 3.31 Effect of multiple scattering on straw tube tracker 59 x Figure 3.21: Energy deposit in straw tube tracker by simulation 50 Figure 3.22: Energy deposit spectrum on straw tube with events passed through straw tube and fiber Figure 3.23: Energy deposit spectrum on straw tube with events passed through straw tube and fiber 51 pass through straw upper and fiber The right histogram is the spectrum of events which pass through straw upper, straw lower and fiber Finally, we achieve efficiency of straw tube tracker as formula 3.3 and it is same with real experiment The events as figure 3.23 will reduce the efficiency of straw tube tracker ε= 3.5 3.5.1 entryof histogramh2 94898 = = 0.936 entryof histogramh2 101218 (3.3) Study the drift velocity of straw tube tracker r t calibration An important characteristic of straw tube tracker is the drift time spectrum and it is better to take the drift time spectrum of straw tube tracker with a uniformly irradiated cell To measure the drift time spectrum with a uniformly irradiated cell, we setup experiment as figure 3.24 The shape of such a spectrum is determined by the cell geometry, gas mixture and the applied high voltage Figure 3.25 is the time spectrum for straw tube tracker in the COMET experiment in the gas mixture Ar/C2H6 : 50:50 The time spectrum of straw tube tracker shows the measurement time between a particle traversing a cell and the arrival of the first electron clusters in the avalanche region near the wire However, particle trajectories are described in space coordinates making the shortest distance at which the particle past the wire interesting Therefore, there are a relation between the measured drift time and the shortest distance This relation in general depends on many variables and parameters, 52 such as gas mixture, cell geometry and applied high voltage Figure 3.24: Setup experiment for drift velocity study To convert the TDC time (t) to isochrone radius (r), the relation is calibrated from the shape of the TDC spectrum By using the assumption of a homogeneous illumination n the formula 3.4, we have relation between time and distance of straw tube tracker as in the figure 3.26 t t1 r(t) n(t′ )dt′ = N R (3.4) In this case, t1 is the starting point of the spectrum, N is the total number entries in the spectrum, R is the straw tube radius, n(t’) is the number entries at time t’ Thus, the integrating the time spectrum from t1 to t correspond to isochrone radius 53 The time spectrum of straw tube tracker is showed in the previous figure Due to the common-start read out of TDCs, higher value is the drift time Therefore, the slow slide ≈ 60ns correspond to the drift time from the trackers close to the wall of the straw tube tracker The distribution width is 60 ns, correspond to the maximum drift time This is good agreement with the Garfiled simulation Figure 3.25: Time distribution of straw tube tracker Figure 3.26 is an example of the result for r t calibration curve of straw upper Each bin t in the time spectrum histogram is convert to the radius r by using formula 3.4 The fit function is applied with formula 3.5 Parameter from fit function in figure 3.26 are used to convert time to radius for lower straw tube r(t) = p0 + p1 ∗ t + p2 ∗ t2 + p3 ∗ t3 + p4 ∗ t4 + p5 ∗ t5 + p6 ∗ t6 + p7 ∗ t7 54 (3.5) Figure 3.26: r t calibration for the upper straw tube tracker 3.5.2 Drift velocity of straw tube tracker To study the drift velocity of straw tube tracker, we setup the experiment as figure 3.27, the trigger timing is the coincidence between the upper straw, the lower straw and fibers We will have time spectrum of two layer, the time spectrum of upper straw is used to convert to time distribution and the time spectrum of lower straw is used to convert to the distance from the wire to the wall of straw Finally, we have 2D histogram r t relation of straws tube tracker as figure 3.28 This relation is fit 55 with linear function order 6th as formula 3.6 r(t) = p0 + p1 ∗ t + p2 ∗ t2 + p3 ∗ t3 + p4 ∗ t4 + p5 ∗ t5 + p6 ∗ t6 Figure 3.27: Setup experiment for drift velocity study 56 (3.6) Figure 3.28: rt relation of straw tube trackers Figure 3.29: Fit r-t relation with linear function order 6th 57 3.6 Position resolution A track of particle is reconstructed by fitting a line through the measured hits It is requirement a prediction of precision to properly weigh the measurements of the drift time in the track reconstruction For the drift time distance relation, we can define a resolution in the time and in the distance domain The error on the measure distance and the fit function is given by the residual: a = rmeasure − rf it (3.7) Where a is the residual, rmeasure is the distance which we convert from time spectrum, rf it is the distance which we have from the fit function 3.6 Figure 3.30 shows the residual distribution of straw tube tracker The residual of straws tube tracker is fitted with Gaussian function and the position resolution is given by the square root of the variance on the residual Moreover, there are multiple scattering in straw tube tracker To study the effect of multiple scattering on straw tube tracker we use Geant4 simulation In simulation, the experiment is setup same with real experiment in figure 3.27, but the Sr90 source is design as a point source Figure 3.31 shows the effect of multiple scattering on straws tracker study by simulation Furthermore, we reconstruct the distance from the time spectrum as described in previous section, it will give an error for the measure distance Therefore, the position resolution of straw tube tracker is given by the formula 3.8 σ= 2 σresidual1 + σresidual2 − σmulSt = √ 0.332 + 0.372 − 0.342 ≈ 0.361mm 58 (3.8) Figure 3.30: Residual distribution of straw tube tracker Figure 3.31: Effect of multiple scattering on straw tube tracker 59 Chapter Conclution The performance of prototype for straw tube tracker are studied and summarized in the following: • Sag of wire: the sag of the cathode wire is simulated by GARFILED and calculate by formula Their result have good agreement and we can conclude that the sag of wire is micrometer when the tension of wire is 150g Thus, it is small enough to neglect their effect when we analysis the data • Gas gain: The gas gain of proposed gas is study by using X-ray from Fe55 source As the result, we can achieve a gas gain when we apply the high voltage of 1.75 kV • Efficiency: The efficiency of straw tube tracker as the function of high voltage for cathode wire and it has the Plateau curve shape We can achieve a efficiency 95% when high voltage of 1.75 kV - 1.8 kV is apply on cathode wire • Drift velocity and position resolution: the drift velocity is ≈ 0.04 mm/ns but 60 the position resolution is 0.36mm The reason is that multiple scattering of electron in straw tube gas reduce position resolution of straw tube tracker The efficiency of straw tube tracker prototype is close to the requirement of straw tube tracker for the COMET experiment However, the position resolution study is affected of multiple scattering Therefore, for next step, we have plane • Reduce the noise of straw tube signal • Study the best gas mixture for straw tube tracker • Study the gas leak of straw tube • Test the straw tube in vacuum and high rate environment • Test the straw tube 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COMET experiment, there are super layers of a simple straw tube tracker and each straw tube tracker consists of X, X’, Y and Y’ planes... provide X and Y information as in the figure 2.2 [1] According to the conceptual design report of the COMET experiment, there are some requirement for the straw tube tracker: • Straw tube tracker