Feasibility study of TPC detector at high luminosity Z pole on the circular collider
Zhiyang Yuan, Huirong Qi, Yue Chang, Ye Wu, Hongyu Zhang, Jian Zhang, Yuanbo Chen, Yiming Cai, Yulan Li, Zhi Deng, Hui Gong
FFebruary 22, 2021 1:32 WSPC/INSTRUCTION FILETPC˙CEPC˙2020˙Huirong
International Journal of Modern Physics A © World Scientific Publishing Company
Feasibility study of TPC detector at high luminosity Z pole on thecircular collider Zhiyang Yuan, Huirong Qi ∗ , Yue Chang, Ye Wu, Hongyu Zhang, Jian Zhang, Yuanbo Chen State Key Laboratory of Particle Detection and Electronics,Institute of High Energy Physics, Chinese Academy of Sciences,19B Yuquan Road, Beijing 100049, ChinaUniversity of Chinese Academy of Sciences,19A Yuquan Road, Beijing 100049, China
Yiming Cai, Yulan Li, Zhi Deng, Hui Gong
Department of Engineering Physics, Tsinghua University,Hai Dian District, Beijing 100084, China
Received Day Month YearRevised Day Month YearWith the development of the circular collider, it is necessary to make accurate physicsexperimental measurements of particle properties at higher luminosity Z pole. Micro-pattern gaseous detectors (MPGDs), which contain Gaseous Electron Multiplier (GEM)and Micro-mesh gaseous structures (Micromegas), have excellent potential for develop-ment as the readout devices of the time projection chamber (TPC) tracker detector. Tomeet the updated physics requirements of the high luminosity Z from the preliminaryconcept design report (preCDR) to concept design report (CDR) at the circular electronpositron collider (CEPC), In this paper, the space charge distortion of the TPC detectoris simulated with the CEPC beam structure. Using the multi-physics simulation softwarepackage, the distribution of ion estimated by Geant4 is used as the input for the differen-tial equation, and the relationship between the ion density distribution and electric fieldin the detector chamber is simulated. These simulation results show that the maximumdeviation for Higgs O (25 µ m) meets the performance requirements in CEPC TPC de-tector at the high luminosity Z pole, while it is still a considerable challenge for Z pole,with the maximum deviation O ( > µ m). According to the previous developments,the cascaded structure of GEM and Micromegas detector has been measured. The newconsiderations of the detector’s requirements were given, the gain needs to be reachedto about 2000 with IBF × Gain under 0.1, and IBF means the ions back flow ratio of thedetector. The pixel TPC is a potential option to replace the traditional MPGDs withthe low gain, low occupancy, and outstanding pattern recognition. Finally, some updateparameters and experiments results were compared.
Keywords : Micro-pattern gaseous detector; Ions back flow; Future circular collider.PACS numbers: 29.40.Cs ∗ Huirong Qi, [email protected] 1 a r X i v : . [ phy s i c s . i n s - d e t ] F e b ebruary 22, 2021 1:32 WSPC/INSTRUCTION FILETPC˙CEPC˙2020˙Huirong Zhiyang Yuan, Huirong Qi
1. Introduction
As one option of the CEPC track detector concept, TPC can meet the require-ments of three-dimensional track reconstruction and charged particle identification,and has the advantages of the low material budget, outstanding pattern recognitioncapability and high position resolution. As the popularity and complexity of TPCapplications in particle physics increases, its calibration and monitoring become im-portant too. For the field distortion, although most sources of distortion are static,space charge distortion is a dynamic process due to the accumulation of positiveions in the TPC chamber. Most of the positive ions in the chamber are derivedfrom the avalanche multiplication of electron clusters in the TPC readout struc-ture. Under the action of an electric field, positive ions move slowly due to the driftvelocity ( O (5 m/s)) compared to electrons ( O (10 m/ms)). The change in the typeand quantity of charged particles entering the TPC chamber affects the distributionof positive ions in the chamber, and the electric field distortion generated by thisdistribution is eliminated until the cathode absorbs them. Continuous positive ionaccumulation can indirectly cause changes in the transport properties of subsequentcharged particles in the chamber. The CEPC baseline design uses a similar inter-national linear collider detector (ILD) design concept. However, its detectors needto operate in continuous mode. For the TPC detector to meet the requirements ofhigh position resolution and three-dimensional track reconstruction under continu-ous high count rate operation, correspondingly, the low IBF and sufficient coolingcapacity are required. TPC readouts use cascaded MPGDs, especially GEM andMicromegas structures, which achieve very low IBF compared to original MWPCsreadout structures. Fig. 1. The beam structure of CEPC with the specific bunch spacing for the Higgs, W and Zbosons (Left) and the structure of ion disk with equal spacing and thickness (Right).
The CEPC is a circular electron-positron collider with a 100 km circumferenceand two interaction points (IP). In the CEPC CDR, the bunch spacing as Fig. 1(Left) of the Higgs, W and Z is approximately 760 ns, 200 ns and 25 ns , respec-tively. On the one hand, the CEPC beam structure will cause the subsequent ionsdisk as Fig. 1 (Right). The longitudinal width of the ions disk is mainly affectedebruary 22, 2021 1:32 WSPC/INSTRUCTION FILETPC˙CEPC˙2020˙Huirong Feasibility study of TPC detector at high luminosity Z pole on the circular collider by the bunch spacing. On the other hand, the luminosity for the Higgs, W andZ boson will strongly affect the ions density. To eliminate space charge effects inthe CEPC TPC, we adopt a hybrid structure cascaded GEM with a Micromegasdetector (GEM-MM) to suppress the back flow ions from the amplification pro-cess passively. Similarly, a structure cascaded double micro-mesh with Micromegasdetector (DMM) has the same suppress function on IBF. Comparing to the tra-ditional MPGDs, the pixel TPC combining a pixel ASIC with a MPGD is alsoan ideal selection. To evaluate the inhibitory effect, we need to simulate the spacecharge effects of primary and backflow ions. Conversely, the ILD TPC adopts agate structure nearby the amplification to stop the ion disk entering the drift re-gion actively. In the CEPC CDR, the luminosity for Higgs, W and Z bosons are3 × cm − s − , 10 × cm − s − and 32 × cm − s − respectively. Com-pared with the luminosity reported in the CDR for the CEPC, those of the ILC and FCC are 1 . × cm − s − and 100 × cm − s − .In the following section, the space charge distortion of CEPC TPC is simulatedand explained. In section 2, the theoretical model for space charge distortion isdefined, the simulation results of φ deviation are presented, and some explanationsare given. In section 3, the conclusion is finally reached.
2. Feasibility Study Of The Tacker Module2.1.
Motivation and physics requirements
The CEPC CDR and updated parameters are listed in the Table 1. The luminos-ity increase factor for Higgs and Z are 1 . . Simulation principles
To study the interaction between the electric field E and the ion density n e,i + inthe detector, the multi-physics software COSMOL was used to simulate the TPCgeometry.
10, 11
It is a finite element analysis software, which can study transient andanalyse the complex interaction process. In the detector geometry, the software canbe used to calculate the effect of the ion density n i + on the electric field. Throughoutthe physical process of the detector, COMSOL gives a numerical approximation ofdifferential equations as the following:ebruary 22, 2021 1:32 WSPC/INSTRUCTION FILETPC˙CEPC˙2020˙Huirong Zhiyang Yuan, Huirong Qi
Table 1. Updated parameters of collider ring since CDR.Parameter Higgs ZCDR Updated CDR UpdatedBeam energy (
GeV ) 120 - 45.5 -Synchrotron radiation loss/turn (
GeV ) 1.73 1.68 0.036 -Piwinski angle 2.58 3.78 23.8 33Number of particles/bunch N e (10 ) 15.0 17 8.0 15Bunch number (bunch spacing) 242 (0.68 µ s) 218 (0.68 µ s) 12000 15000Beam current ( mA ) 17.4 17.8 461.0 1081.4Synchrotron radiation power /beam ( MW ) 30 - 16.5 38.6Cell number/cavity 2 - 2 1 β function at IP β ∗ x /β ∗ y ( m ) 0.36/0.0015 0.33/0.001 0.2/0.001 -Emittance ε x /ε y ( nm ) 1.21/0.0031 0.89/0.0018 0.18/0.0016 -Beam size at IP σ x /σ y ( µ m) 20.9/0.068 17.1/0.042 6.0/0.04 -Bunch length σ z (mm) 3.26 3.93 8.5 11.8Lifetime ( hour ) 0.67 0.22 2.1 1.8Luminosity/IP L (10 cm − s − ) 2.93 5.2 32.1 101.6 (cid:42) ∇ · (cid:42) E = ρε , (1) (cid:42) n · ( (cid:42) D − (cid:42) D ) = σ, (2) ρ = (1 + k ) LV ion / ( ms − ) ( 2 . × − r/mm − . − . × − )[ f C/mm ] , (3)where (cid:42) E is the electric field, ε is the dielectric constant, (cid:42) D is the electric displace-ment vector, k is the ion backflow ratio factor (IBF × Gain), L is the luminositynormalized to 1 × cm − s − , V ion is the velocity of ions and r is the radius.E q . (1) is the Poisson equation which describes the potential field due to a givencharge distribution. The presence of a large number of ions in the TPC causes theredistribution of electric charge in the outer shell, known as electrostatic induction.The charge on the surface of the outer shell is called induced charge, and its surfacecharge density σ is described in E q . (2). The change in the normal component of theelectric displacement vector is equal to the surface charge density. In this paper, thespace charge density ρ as E q . (3). Nine thousands Z → q ¯ q events are simulatedwith Mokka, the Geant4 simulation package. The distribution of the ion density isshown in Fig. 2 (Left) in CEPC TPC under the luminosity for Higgs. Consideringthe CEPC TPC is a columnar structure with an inner and outer radius of 0.3 m,1.8 m and a full length of 4.7 m, the Poisson equation can be specifically written inthe form of components with boundary conditions,the 3D geometry is approximatedby 2D axial symmetry, and the axis of symmetry is the Z-axis. The electric field (cid:42) E and magnetic field (cid:42) B are parallel to the Z-axis. In the CEPC TPC, the driftebruary 22, 2021 1:32 WSPC/INSTRUCTION FILETPC˙CEPC˙2020˙Huirong Feasibility study of TPC detector at high luminosity Z pole on the circular collider chamber is filled with T2K gas (Ar/CF /iC H =95/3/2). To reach a suitable driftvelocity in the drift region, TPC adopts that the central cathode plane is at a highpotential of 50 kV, and the two anodes at the two end-plates with the readouts areat ground potential. Electric field distortion and φ deviation z [mm] r [ mm ] ] - i on d e n s it y [ mm z [mm] r [ mm ] - - - - - - - · E r / E z [ no . un it ] Fig. 2. The distribution of the ion density (Left) and the value of E r /E z (Right) in CEPC TPCunder the luminosity for Higgs. The motion of charged particles in an electromagnetic field calculated usingMaxwell’s equations. Considering that the electric field and the magnetic field areparallel, the relationship can be easily achieved between the moving distance in thez and φ direction. The small change of φ deviation ∆ D φ is considered to be due tothe small moving distance ∆ Z in the z-direction and calculated by the E q . (4).∆ D φ = − ωτ ωτ ) × E r E z × ∆ Z, (4)where ω ≡ eB/m , τ is the mean free time of electrons, and E are are the r and z components of the electric field. The value of ωτ equals 10 using T2K gas and thevalue of E r /E z is shown in Fig. 2 (Right).The deviation in the φ direction is shown in Fig. 3. The simulation resultsconsider the variation of radius and different luminosity for Higgs (Left) and Z(Right). Considering that the induced charge of the TPC cylinder will produce E (cid:48) r ,contrary to the direction of E r (offset effect), the opposite positional deviation willoccur in the positive half axis. The E r includes the electric field caused by positiveion E i + and the induced charge E (cid:48) r . Related experimental research
IBF research on MPGD has been tested and optimized at the Institute of HighEnergy Physics (IHEP) and the University of Science and Technology (USTC) ofebruary 22, 2021 1:32 WSPC/INSTRUCTION FILETPC˙CEPC˙2020˙Huirong Zhiyang Yuan, Huirong Qi
Fig. 3. The deviation in φ direction as a function of drift length with different luminosity forHiggs (Left) and Z (Right). China. Experimental results of IHEP are carried out with mixture gases of T2Kand Ar/iC H =95/5 separately and show that the IBF rate of GEM-MM can bereduced to about 0.1% at the gain of about 5000. Similarly, results of USTC arecarried out with different tilt angles between the double meshes and show that theIBF rate of DMM can be as low as about 0.03% at a gain of about 20000. Fig. 4. The IBF × Gain as a function of V GEM for GEM-MM (Left) and the IBF ratio as afunction of gain with different tilt angles between the double meshes of a DMM (Right).
3. Conclusion
In this paper, we have simulated the D φ of CEPC TPC with different luminosity forHiggs and Z. The maximum φ deviation for Higgs and Z is under 25 µ m and 400 µ mrespectively. For Higgs luminosity, TPC only needs to ensure that the IBF × Gainis less than 2 to achieve a less deviation of 10 µ m. The deviation will be negligibleenough to meet the performance requirements of CEPC TPC under O (100 µ m).However, it is a considerable challenge for Z with the similar physics requirements.The gain needs to be small enough ( < × Gain under 0.1.Nevertheless, IBF × Gain has the limitation ratio from the detector R&D at highgain. The new idea for pixel TPC is being considered as the option to take theebruary 22, 2021 1:32 WSPC/INSTRUCTION FILETPC˙CEPC˙2020˙Huirong
Feasibility study of TPC detector at high luminosity Z pole on the circular collider place of the traditional MPGDs. Its gain is less than 2000, and there is almost noIBF × Gain. It can handle the massive data rates during CEPC Z running. The pixeloccupancies are low, and the pattern recognition will have no problem separatingevents and finding tracks. If CEPC produces close to one trillion or not one millionZ bosons, the technology of TPC needs to be adopted. Moreover, the pixel TPCneeds to be considered under a higher luminosity.
Acknowledgments
The author thanks for Prof. Yuanning Gao, Prof. Yulan Li and Dr. Yiming Cai forsome discussions of details. This study was supported by the National Natural Sci-ence Foundation of China (Grant NO.: 11975256), the National Key Programme for S & T Research and Development (Grant NO.: 2016YFA0400400) and the NationalNatural Science Foundation of China (Grant NO.: 11775242)
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