STAR Inner TPC MWPC Upgrade And Studies On Ion-back-flow Of TPC Based On GEM And Anode Wire Grid

In order to study the phase transition process of QCD at a deeper level and search for the possible critical point,RHIC STAR plans to upgrade the detector after the Beam Energy Scan-I and the upgrade of the inner TPC(iTPC)becomes the core part of this project.The iTPC upgrade program plans to increase the pad coverage from 20%to 100%and upgrade the corresponding readout electronics.After the upgrade,the detector is expected to achieve better momentum resolu-tion,better dE/dx resolution,and the reconstruction efficiency of the particles in the high pseudo rapidity range(1<|?|<1.5)will be greatly enhanced.Before the iTPC mass production,considering the stringent requirement of upgrade index on three-layer wires of MWPC,We built and automated the wire winding system and wire tension test system respectively.After accuracy and stability tests,we found that the systems can meet the higher physical accura-cy requirements.Aiming at the non-uniformity of wire tension in the winding process,the author developed a real-time tension compensation module based on LabVIEW after using ANSYS to analyze the force and deformation of the frame.Then after entering the formal stage of mass production,every step of production has been strictly controlled.In the performance test,we built a set of hardware equipment which is is based on 55Fe radiation source and high frequency X-ray tube,supplemented by MCA,oscilloscope,STAR electronics and other advanced data acquisition instruments.The iTPC performance tests include wire readout test and pad readout test.In the wire readout test,the gain could be stabilized at about 2500(anode wire working voltage is 1120V)and the detector has good proportional near the gain of about 2500.The gain uniformity is hotter than 2%,and the onergy resolution could roach about 10%.Under high-internsity X-ray irradiation(X-ray frequency 2.3×105HZ,ray tube voltage 25kV.ray tube current 500?A),the detector could work stably and the leakage current is maintained at around 500nA.In the test of pad readout using STAR electronics system,it was found that the background of each pad of iTPC fluctuated within 35-60ADC,the ratio of total induced signal charge on all pad to that on anode wire is 1:1.98,the peak value of PRF response function(the ratio of induced signal charge on one pad to that on anode wire)is about,30%,and the position resolution could reach about 2.5mm.Then one iTPC sector was shipped to BNL Laboratory for Run-18 test and the reconstruction of 200 GeV Isobar and 27 GeV Au+Au collisions show that,compared with the old iTPC,the upgraded iTPC's hit points increase from 45 to 72.The measurement threshold of transverse momentum PT is reduced to 60 MeV/c,the pseudo-rapidity acceptance interval ? extends forward by 0.4 units,and the resolution of dE/dx has also been significantly improved.By February 2019,STAR has completed the installation and commissioning of all iTPC detec-tors and launched the Beam Energy Scan-II officially.The STAR iTPC upgrade project was completed successfully.Future colliders(such as EIC)will have several orders of magnitude higher luminosity and collision rate than the colliders currently in operation in the world.The traditional MWPC has periodic delay due to its gating grid mechanism,which makes the detector unreadable.Therefore,the development of a continuous readable TPC has become a task.Based on the STAR iTPC upgrade project,we proposes a new time projection chamber structure:the anode wire is kept,and the cathode wire and gated wire are replaced by GEM,then the traditional door opening and closing mechanism is abandoned.In this way,the anode wire still provides the main signal amplification function.One or more layers of GEM provide auxiliary amplification under low working voltage which are mainly used to restrain the ions flowing back.Then the continuous readout of signal could be realized to adapt to high luminosity environment.The TPC prototype was built,and the test system including MCA test sys-tem,gas mixing system,temperature and pressure monitoring system were also established.According to the regular hexagonal arrangement rules of holes on GEM.the author tried two overlapping schemes.The first two-layer GEM trans-lation overlapping scheme can reach the 100%overlapping in simulation,but it is limited by assembly accuracy in experiment and is not suitable for practical application.It serves as a reference for simulation theory.The second two-layer GEM rotation overlapping scheme is less affected assembly accuracy which was applied in pcrformancc testing.With the help of APDL Garfield++simulation,we firstly studied single-layer GEM and double-layer GEM.Impor-tant properties such as electron and ion transport,electron collection efficiency,electron extraction efficdency,gas gain has been quantified.In the final simulation of double-layer GEM plus anode wire grid,it is found that the configuration is stable and the IBF ratio could be suppressed to a low level by optimizing the electric field.The parameters similar to the STAR TPC environment variables(gas composition,temperature,pressure)have also been proved to be possible for providing a stable working environment for the detector.A comprehensive performance tests show that the effective gain of the de-tector increases exponentially with the increase of the anode wire voltage(fix GEM working voltage).During the process of the working voltage increasing from 940V to 1120V,the effective gain of the detector increases from about 2500 to about 44000.And low offective gain is more suitable for practical applica-tion.When the effective gain is about 2500,the energy resolution of the de-tector could reach about 10%.The accuracy of IBF ratio measurement could reach about 5%.When Edrift=0.1kV/cm,Etransfer=4.0kV/cm,Vanode wires=940V,?VGEMupper=?VGEMlower=255V,the IBF ratio could be suppressed to the best of 0.58%,and the parameter ?(Geff*IBF)could reach about 14.After evaluation of the ion density distribution resulted from the new TPC in the working environ-ment,we found that the design has the greatest potential for practical application in e+p collisions on EIC in the future.Although the collision rate of EIC is high enough,the multiplicity of e+p collisions is much less than that of p+p collisions.and is much lower than that of Au+Au collisions.And the space charge density will be similar to that of Au+Au collisions in RHIC-STAR.The level of space charge density is in an accep table range,so the design can be used as one of the schemes for EIC in the future.