A GEM based TPC for beam monitoring
PPrepared for submission to JINST
International Conference on Instrumentation for Colliding BeamPhysics24-28 February 2020Novosibirsk, Russia
A GEM based TPC for beam monitoring
G. Galgóczi, a,b, G. Hamar, a P. Pázmándi, a and D. Varga a a Wigner Research Centre for Physics,1121 Budapest, Konkoly-Thege Miklósút 29-33., Budapest, Hungary b Eotvos Loránd University, Faculty of Sciences, Department of Physics,Egyetem tér 1-3, 1053, Budapest, Hungary
E-mail: [email protected]
Abstract : In recent years Gas Electron Multipliers [1] have proven to be reliable am-plification stages at high beam rates, and can be used also in Time Projection Chambers[2]. Our group developed a 1 dm active volume double-GEM TPC, with spatial resolutionof 50 µm and 280 µ m. Custom designed FPGA data acquisition system enables rate ca-pability for about 100 kHz · mm − , providing excellent track-by-track position and angularinformation, better than 0.1 mm and 1 mrad respectively. The wide dynamic range of thesystem enables identification from He up to Kr using ionization measurement. Two ofthese TPCs are planned to operate in tandem mode [3, 4] to filter off-time particles and toachieve a superior angular resolution.
Keywords: beam-line instrumentation, micropattern gaseous detectors, Time ProjectionChambers Corresponding author. a r X i v : . [ phy s i c s . i n s - d e t ] S e p ontents We present the test results of a newly developed Time Projection Chamber (TPC) whichhas a gas amplification stage of a double Gas Electron Multipliers (GEMs) [1]. The ideais to exploit the advantages of GEMs for gaining high granularity with an outstandingion feedback suppression. Several such detector systems are currently being developedand tested for several applications including beam monitoring [5, 6] hadron therapy [7] andparticle identification. In our case a set of commercially available FPGAs (Zynq series) witha custom designed front end electronics are capable of reading out the electronic signal with5 ns time resolution. The results of the first test measurements conducted at RIBLL (theradioactive ion beam line at the Institute of Modern Physics, in Lanzhou, China) [8] arepresented and a spatial resolution of 50 µ m along the pads and 280 µ m (for a drift velocityof 1.3 cm/ µ s) in the time "direction" is derived. The active volume of the TPC is 10x10x10 cm . Charge amplification is done by the twostandard 10x10 cm GEMs. Each GEM has a hole diameter of 70 µ m, thickness of 50 µ mand a pitch of 140 µ m. Five segmented pad rows with a spacing of 20 mm collect the chargeafter the initial amplification. Each pad row consists of 64 pads. The 32 pads in the middleof the row have a size of 1.2x20 mm and the outer 32 have a size of 2x20 mm . Finerresolution is needed in the inner pads as the beam crosses the detector there. In the outerareas we expect much lower particle multiplicity as only the scattered particles cross there.The beam enters the TPC through a 15 µ m mylar window and the field cage twice (50 µ mkapton and 10 µ m copper). The beam-detector interactions are minimized with such a thinwindow and field cage. To ensure the high purity of the gas inside the active volume theTPC has a double wall system: field cage and gas cage. The high purity gas enters into the– 1 –ctive volume from the cathode, flows towards the GEMs, closed sideways via the gas-tightfield cage; then enters in between the two cage walls before the outlet. The drift field ofthe TPC and the amplification gain of the GEMs are set through a resistor chain. Figure 1 . (Left) The PadPlane of the TPC, containing the 5 padrows, the HV connections pointsof the GEMs lead to the place of the resistor chain. (Right) The kapton field cage of the activevolume, and 2mm away the support walls of the thin outer gas cage.
The signal from each pad is amplified by a preamplifier on the FEE card and thendigitized with a 12 bit ADC at a 2.5 MHz sampling rate. The ADCs are read out withfive Zynq-7020 FPGAs with a time resolution of 5 ns in order to achieve a good spatialresolution perpendicular to the pad plane.
Figure 2 . The TPC during lab testing with a β source (left). GEM-TPC test measurement in theRIBLL beam in Lanzhou (right). The FEE electronics with one preamplifier per channel connectedto the FPGA board tower are visible. The performance of the detector and the read-out was tested at the RIBLL [8] duringmeasurements in 2017 and 2019. An Ar/CO mixture (85% / 15% respectively) was usedin the TPC. The beam consisted of Kr ions, varying between 10,000 and 100,000 ineach spill. Since these ions create a considerably large primary signal low GEM amplification– 2 –as required. The beamspot was 3x3 mm on the detector. The drift velocity was variedin the range of 0.2-1.5 cm/ µ s. Figure 3 . (Left) The distribution of the center of clusters parallel to the pads, for each pad rowindependently. The detector was rotated by 100 mrad thus the shift of the clusters in consequentlayers. (Right) Figure distribution of the center of clusters in the vertical (time) direction, for eachpad row independently. Some off-time ions are observable.
In order to process the raw data, a custom C++ analysis code was developed. Since eachpad row is read out by its dedicated FPGA board which utilizes its internal clock basedon a quartz oscillator, the time stamp of each FPGA is shifted compared to the others.Therefore analysis code matches up events in each file. Afterwards peaks in the signals foreach board with a significance of at least 3 σ are identified. Flood fill method is used tobuild clusters around the peaks. Linear regression is applied to all permutations of thesecluster in each event. The permutations with a χ per degree of freedom less than oneare treated as tracks. In figure 3. a set of 10000 clusters belonging to tracks are shownprojected parellel to the pads (left) and in the vertical (time) direction (right). In figure 4.the difference of the observed cluster positions and the predicted values are plotted. Fromthis, one can derive a spatial resolution of 50 µ m parallel to the pad rows and 280 µ m in thevertical direction. In this case the drift velocity was set to 1.35 cm/ µ s corresponding to atime resolution of 20 ns. The drift velocity can be set to any practical value in the detector,by adjusting the cathode voltage. The choice of the current drift velocity considers thefollowing aspects: slower drift improves position resolution (as relative time measurementimproves due to longer drift time), faster drift reduces off-time background and improvesrate tolerance (drift electrons clear faster), whereas practical reduction of cathode voltagesimplifies the high voltage system design, preferring slower drift. In our case, the off-timebackground was already reduced by the short drift length, so a relatively slow drift, suchas 1.35 cm/ µ s provided sufficiently good performance.– 3 – fi t Sigma: 0.048 mm Position: 0.003 mm fi t Sigma: 0.281 mm Position: -0.013 mm Figure 4 . Residuals of 2000 tracks for all pad rows in the direction of the pad rows (left). Residualof the same tracks in the vertical (time) direction (right). A drift velocity of 1.35 cm/ µ s was set inthis case. The first results of a double-GEM based TPC - designed for ion beam monitoring - arepresented. Performance was tested in the ion beam of RIBLL at the Institute of ModernPhysics, in Lanzhou, China. The spatial resolution of the TPC was deretmined to be50 µ m parallel to the pad rows and 280 µ m in the vertical direction. A second TPC iscurrently under construction, to be operated together in tandem configuration in order toto discriminate off-time tracks and to increase angular resolution. This work was funded by the bilateral science and technology project between Hungaryand the People’s Republic of China, TÉT 16 CN-1-2016-0008. The authors acknowledgeand thankful for the experimental team at IMP, Lanzhou, namely dr. Zheng Yong, dr.Yapeng Zhang, dr. Pengming Zhang and Fengyi Zhao during the beam setup and themeasurements.
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