Characterization of GEM Chambers Using 13bit KPiX Readout System

Transcription

1 Characterization of GEM Chambers Using bit KPiX Readout System Safat Khaled and High Energy Physics Group Physics Department, University of Texas at Arlington (Dated: February, ) The High Energy Physics Group at the University of Texas at Arlington is developing and testing prototype gas ionization detectors using Gas Electron Multiplier technology. These Digital Hadron Calorimeters implement double-layer GEM foils and are candidates to be used in experiments at future accelerators, such as the International Linear Collider. The group tested four cm x cm prototype GEM detectors in the T- experiment at Fermi National Laboratories. One of these detectors utilized a bit KPiX chip and accompanying read out system developed at the Stanford Linear Accelerator Center. This talk will present the results of the beam test data analysis to understand the characteristics and performance of the prototype detectors. More specifically, it will present the measured gain, response, and efficiency of the detectors as well as the dependence of these measurements on the ambient pressure and position at which the particle passes through the detector. I. INTRODUCTION Understanding of the universe is at an all time high. With the advancements in particle detector technology made in the past decades, exploration of the quantum realm has been made possible. The Large Hadron Collider at CERN and the Tevatron at Fermi National Accelerator Laboratories have provided extensive amounts of data to analyze and with which to understand the complexities in the universe. However, the more that is understood about the universe, the more questions that seem to arise, and to answer these questions, more sophisticated equipment sensitive to subtle signals from subatomic particles are required. The International Linear Collider is the next site for the advancement in mankind s understanding of the universe. The High Energy Physics Group at the University of Texas at Arlington is tasked to develop components for the ILC. These instruments require a very high level of precision in its detective ability so as to maximize the capability to discover intricacies not yet discovered. To make these discoveries, however, the properties of the chamber and its behavior under various natural conditions must first be understood in order to correct for any anomalies. This paper presents the results found after close examination of the chamber and its performance. II. GAS ELECTRON MULTIPLIER TECHNOLOGY In 99, Fabio Sauli created a remarkable new type of gaseous ionization detector which was dubbed as a Gas Electron Multiplier, or GEM. These GEMs are praised as a brilliant development due to the fact that they are able to efficiently accomplish their primary task of signal amplification while keeping the manufacturing costs economically viable. These GEMs can be mass produced with flexibility of shape and ease of assembly with relatively smaller risks of errors due to more automated,less labor-intensive production.[] GEMs are typically made using a thin layer of Kapton approximately µm thick which is coated with a extremely thin layer of copper approximately µm thick on each side. After these GEMs are layered with copper, they are perforated with holes approximately µm in diameter with pitch of approximately µm. Figure shows an electron microscope photo taken of a GEM foil.[] FIG. : GEM Foil The GEM foils act as amplifiers that accelerate a particle passing through the perforations. A potential difference is applied across the GEM foil which creates an extremely strong electric field which accelerates the electron cascade passing through. The accelerated particles collide with molecules of an easily ionizable gas(in this study, ArCO is used), creating an even larger electron cascade. ArCO is used due to the fact that it is easily ionizable and recombines with causing significant complications within the detector with a relatively small dead time. Then the input signal, or incoming particle passing through the detector, is proportional to the output signal, or the total number of electrons deposited onto the read out pad. The constant of proportionality is defined as the gain of the chamber.[ ]

2 III. PROTOTYPE UTA DETECTORS UTA is developing a number of particle detectors utilizing these GEM foils. There are two different prototype chambers, a cm x cm GEM chamber and a cmxcm GIA chamber, that have been developed and used in the T Experiment done at Fermi National Accelerator Laboratories, while a third is in construction, a cmxcm Large GEM chamber. The detector of interest for this paper is the cmxcm GEM chamber. This chamber implements double layer GEM foils to increase amplification of the signal and create a larger cascade for more precise read out capability and particle identification. Scintillation pads on either side of the chamber act as the triggering mechanism that convey to the chamber that a particle has passed through the entire array. The chamber stores data of an event only when both scintillation pads react to a particle in a small time integral. If the particle is detected by the scintillation pads, but no signal is read for that particle from the chamber, then the detector must be inefficient in detecting that particle. IV. KPIX READOUT SYSTEM The data acquired from the chamber is read out by two different systems; the single channel digital readout system, DCAL, and the analog read out system, KPiX. The DCAL system was collaboratively developed by Argonne National Laboratories and Fermi National Accelerator Laboratories, while the KPiX system was developed by the KPiX Group at the Stanford Linear Accelerator Center. This paper focuses on the data acquired and analyzed by the KPiX system. The data acquired and by the DCAL system can be found in the paper Analysis of T Test Beam Data from GEM Chambers with DCAL Readout System.[8] The KPiX system hinges on two key proponents, the -bit KPiX chip and the KPiX Data Acquisition System (KPiX DAQ). The readout pad sends the information of the incident particle from the detector to the KPiX chip, which converts the signal into a form that can be read by the DAQ system. The KPiX chip is a versatile and multipurpose chip capable of reading out information from channels and analyzing the information from each channel. For the preliminary testing, only channels, organized in an 8x8 grid, are used to take data from the GEM chamber using the KPiX system. The DAQ then reads, analyzes, organizes, and stores the information into a file that can be easily accessed using the analysis code. [9, ]Figure shows a basic diagram of the flow of information. The information stored in the file pertains to how much charge is deposited on each channel of the read out pad, the time the event occurred, the location at which the particle passed through the detector, and various other parameters that can be used to describe the particle or event. FIG. : KPiX System Taking Data from Incident Particle Passing Through GEM Chamber V. PRESSURE CORRECTION The gas flowing through the chamber is a 8%-% mixture of ArCO. The mechanism that regulates the flow of gas throughout the chamber arrow is dubbed the Gas Distribution system. This system utilizes a system of high capacity flow meters that allows for the control of gas flow rate, a gas path splitter that distributes the ArCO to a number of GEM chambers from a single source, and a number of bubblers that allows for visual confirmation of a constant flow rate which also allow monitoring of differences, or lack there of, flow rates between GEM chambers. The gas flows from a high volume cylinder into the gas path splitter. This splitter routes equal amounts of the gas into high capacity flow meters which can be adjusted to various flow rates. The output paths of each of these flow meters is connected to a corresponding chamber to deliver the gas to the chamber. Then the gas flows from the chamber into a bubbler which is filled with fluid. When the gas passes from the chamber to the fluid, the gas output will create bubbles within the fluid which will rise to the top and into the atmosphere. By examining the rate of bubble creation, the corresponding flow rate can be examined and observed. Figure shows the gas flow path for the Gas Distribution System. However, a problem arises in keeping normality within the experiment because the gas flow will be affected by the ambient pressure. Being open to the atmosphere creates changes in factors such as recombination rates and density of the gas within the chamber. A study was done to find the relationship between ambient pressure on the gain of the chamber. This study yields a linear relationship given by the equation. Gain = 9.9 Pressure(in kpa) () Using the gain value at various pressures, a correction coefficient can be calculated using the gain of the cham-

3 Hit Distribution Across Chamber Hit Distribution Across Chamber FIG. : T Experiment Parameters FIG. : Gas Flow Path ber at atmospheric pressure of. kpa. The coefficient is given by equation. the beam had to pass. The triggering mechanism was set up such that the detectors would recognize a particle passing through the array as a real event if and only if the particle passed through all four scintillation pads within a given time. This ensured that if the particle is detected by the triggering mechanism, but is not detected by the detector, then it is due to the inability of the chamber to recognize the particle. Figure shows the summed hit distribution map of data collected from the T experiment with the beam positioned at the center of the chamber(position, see Section XII for more information). Coefficient = Gain at Varying Pressure Gain at. kpa Dividing the charge value detected by the GEM chamber by this coefficient yields the charge value at. kpa. Then, all of the results can be normalized to atmospheric pressure. Then the results found can be examined side by side without the ambient pressure factor affecting the comparison. The plots presented in this paper are all corrected for ambient pressure and are normalized to atmospheric pressure. () Event Map Entries Mean x.9 Mean y. RMS x.8 RMS y. Event Map Entries Mean x.9 Mean y. RMS x.8 RMS y. FIG. : Hit Distribution Across Chamber using GeV Proton with Coincidence Trigger with Beam Directed Through Center of Chamber VI. T EXPERIMENTAL SET UP The T Experiment, done at FNAL during August of, exposed beams of particles to cmxcm GEM chambers and cmxcm GIA chamber to a variety of particle beams to test the characteristics of the detectors. Three of these GEM chambers were used to take data utilizing the DCAL system while was used to take data utilizing the KPiX system. The data from these tests is used to characterize the chamber behavior. For the T Experiment, the triggering mechanism utilized four different scintillation pads. Figure shows the experimental set up of the test. The detectors were placed vertically to ensure minimal interference from cosmic rays and cosmic muons with the beam test experiment. Two cmxcm scintillation pads were placed on either side of the detector array to assure full penetration of the beam through the array. Between the first cmxcm scintillation pad and the array, cmxcm scintillation pads placed perpendicular to each other were also set up. These cmxcm pads created a cmxcm cross sectional area through which As the hit map shows, most of the events are concentrated in the center, which is what is expected, but there are also hits on the outer edges of the map which can be caused by electronic noise, signifying that there was a concentration of noisy channels in that region of the chamber. In this way, analyzing the data using the KPiX readout system helps to clarify what exactly is happening within the inner workings of the chamber. For each of these response runs, the voltage across the chamber was set to 9V, which is the optimal operational voltage found through testing of the GEM chambers. VII. RESPONSE During the T experiment, the GEM chamber was exposed to three different types of particles: Muons, Protons, and Pions. The response of the chamber can be characterized by considering how the chamber reacts to the different particles passing through it. Then, based on the shape of the spectrum, the response of the chamber to various types of particles can be identified.

4 A. Muon Beam The first particle exposed to the chamber was a beam of muons. The muon beams had energies of GeV and used only the cmxcm scintillation pads as the coincidence for the triggering mechanism. The charge distribution for the summation of the muon beam data is shown in Figure. 8 Pion Entries 8 Mean. RMS..9 / Constant 8. ±.8 MPV. ±. Sigma.8 ±. Muon Entries Mean.9 RMS / Constant ±. MPV.8 ±. Sigma.9 ±. FIG. 8: Proton Charge Distribution Used to Characterize Response of GEM Chamber FIG. : Muon Charge Distribution Used to Characterize Response of GEM Chamber The charge distributions can be compared directly to see how the spectrums vary with respect to the particle type. Figure 9 shows an overlay of the distributions which have all been normalized to. Particle Charge Distributions.. Particle Entries 8 Mean Muon. RMS Pion. Proton.. B. Proton Beam The second particle exposed was the proton beams. These protons had energies of GeV and the triggering mechanism used was all four scintillation pads as the coincidence for the trigger. The summation of all the data taken using the proton beam passing through the center of the chamber is given in Figure. 8 8 Proton Entries Mean. RMS. 9. / 8 Constant ± 9. MPV. ±.8 Sigma.9 ±. FIG. : Proton Charge Distribution Used to Characterize Response of GEM Chamber.. FIG. 9: Normalized Overlay of Response Charge Distributions The charge distributions are fitted to a Landau Distribution. The gain of the chamber proportional to the χ Most Probable Value of the Landau fit while the shows how well the distribution mimics a Landau Distribution (the identical mimic occurring when χ =). Then by looking at the parameters of the fit, the response can be understood. Table I shows the parameters of the fits with respect to the particles. TABLE I: Response of Chamber to Various Particle Types χ Particle Events MPV Muon..8±. Proton..±.8 Pion 8..±. C. Pion Beam The last particle exposed to the chamber was the pion beams. The pions had energies of GeV and used all four scintillation pads as the coincidence for the triggering mechanism. The summation of the the data taken is shown in Figure 8. As the table suggest, there is a discrepancy between the protons and pions when compared to the muons. There seems to be a slight shift from the Landau distribution χ which causes the value to vary drastically and shifts the MPV down. While the proton and pion show MPVs within sigma of each other, the muon shows an MPV value over sigma from the proton MPV. This could be

5 caused by a number of things and more tests are being done to explore this anomaly. VIII. GAIN The gain is defined as the ratio of the detected signal to the incoming signal. From earlier studies, it was found that after fitting the charge distributions to a Landau Distribution, the gain can be calculated through the formula TABLE II: Response of Chamber to Various s χ Events MPV 8V pm. 8V.98.pm.99 8V.998.pm.99 9V pm. 9V.99.pm.8 9V.88.9pm.8 9V pm.98 Gain As a Function of Across GEM Gain =. MP V 9. e () Gain(Logarithmic) where e is the charge of an electron. The gain is also directly affected by the voltage across the GEM foils within the chamber, so then a study was done on how the gain varied as the voltage across the chamber was gradually increased in increments of V. Figure shows the overlay of the charge distributions at various high voltages all normalized to Voltage Across Each GEM FIG. : Variation of Gain with Respect to Charge Distributions. H.V. 8V Entries H.V. 8V 9 Mean H.V. 8V...8. RMS H.V. 9V H.V. 9V H.V. 9V H.V. 9V IX. HIT MULTIPLICITY.8... FIG. : Charge Distributions at Various s The plot shows that the charge distribution actually spreads out as the voltage across the GEM chamber increases. That means that each incident particle creates a larger cascade of electrons being deposited across the readout pad as the high voltage across the chamber increases. That is the definition of the gain. Hence the gain of the chamber increases as the voltage across the chamber increases. Fitting these distributions to a Landau Distribution similar to the method used in the response section, the gain of the chamber at various voltages can be calculated by using Equation (). Table II summarizes the information taken from the fitted distributions. Figure shows the gain of the chamber as a function of the voltage across each GEM. As the figure shows, the gain of the chamber increases exponentially as the voltage across the chamber increases. At the operational voltage of 9V, the gain of the chamber is.. Due to the relatively large size of the chamber with respect to a particle passing through it, it can not be assured that only one particle passes through the chamber at any given event. Therefore, it is necessary to take into consideration the hit multiplicity of the data when characterizing the behavior of the chamber due to its geometry. The hit multiplicity study was done using the GeV proton beam passing through the center of the chamber. Hit multiplicity distributions were made by looking at the data event by event and considering how many channels had charges deposited across it that were higher than various thresholds. Figure shows the hit multiplicity distributions for a few different high voltages at a threshold of.fc. As shown, the distribution is quite Hit Distributions High Combined Voltage at Threshold.+ of fc. fc Entries 98 H.V. 8V Mean 8. RMS H.V. 9V.88 H.V. 9V N FIG. : Hit Multiplicity Distribution at Threshold of.fc

6 spread out and as the voltage is increased, the average number of hits registered increases due to the increase in gain. Then, the average value for the distribution is quite high due to the spread of the distribution. This is caused by background noise being registered as hits. Figure shows the hit multiplicity distributions for a threshold of fc. As the threshold is increased many of Hit Distributions... High Combined Voltage at Threshold + of fc fc Entries 98 H.V. 8V Mean.8 RMS H.V. 9V. H.V. 9V Mean Mean As a Function of Threshold 9 8 H.V. 8V H.V. 9V H.V. 9V Threshold (fc) FIG. : Average Number of at Various Thresholds... N FIG. : Hit Multiplicity Distribution at Threshold of.fc the hits created due to background noise are excluded and what was taken out was due to the noise as the hits caused by noise. Figure shows the distribution as the threshold is increased further to fc. Hit Distributions High Combined Voltage at Threshold + of fc fc Entries 98 H.V. 8V Mean.8 RMS H.V. 9V.9 H.V. 9V N FIG. : Hit Multiplicity Distribution at Threshold of fc When the threshold is increased from fc to fc, there is not a drastic change. A few hits may have been excluded, but the overall distribution remains fairly even. Therefore, the vast majority of the hits shown at a threshold of fc is due to the incident beam crossing the chamber. If the threshold is set too high, then the signal from the incident particle will also start to be excluded and defeat the purpose of the detector. Therefore a balance must be found between excluding background noise and finding pure signal. Figure shows the average number of hits as a function of the threshold applied. This plot shows the average number of hits with respect to the threshold applied to the readout pad. Any events with charge deposition lower than the threshold will not be registered. At very low thresholds, the average number of hits is very large and increases asymptotically as the threshold approaches. However, as the threshold is increased, the average number of hits decreases significantly. The average number of hits at fc is at the operational high voltage. This means that at thresholds of fc or above, the chamber stays relatively quiet and reacts only when an incident particle with large energies passes through the chamber. X. EFFICIENCY The efficiency of the chamber is the ability of the chamber to detect an incident particle. If a particle is recognized by the triggering mechanism and not by the chamber, then by virtue of the triggering mechanism that assures that the particle passes through the array. Hence if the chamber is not detecting the particle then it is due to the inefficiency of the chamber. The efficiency study was done using GeV Protons passing through the center of the chamber at various high voltages. The efficiency of the chamber was calculated with respect to various charge values using two different methods of analysis. To calculate the efficiency of the chamber, the charge distributions were used. For various different charge values, the efficiency was calculated using the formula: Integral from Charge Value to Infinity Efficiency = Integral from to Infinity () Figure shows the efficiency curves as a function of the charge value for various voltages. Then the efficiency curves were calculated using the Hit Multiplicity Distributions with using the formula: Efficiency = Total Events at Total Events () Figure shows the efficiency curves as a function of the threshold applied at various voltages. At the operational voltage of 9V, both efficiency curves show consistent results. At a threshold of fc, the efficiency of the

7 Efficiency Efficiency As a Function of Charge.8 H.V. 8V H.V. 9V H.V. 9V The hit multiplicity becomes asymptotic as the efficiency approaches, however, the hit multiplicity increases gradually until efficiencies of 99%. At efficiencies of 9%, the average number of hits is.8-. This means that the chamber stays relatively quiet for efficiencies 99%... XI. CHAMBER UNIFORMITY. Charge FIG. : Efficiency Curve Calculated Using Charge Distributions Efficiency Efficiency As a Function of Threshold.8 H.V. 8V H.V. 9V H.V. 9V The uniformity study considered how the different portions of the chamber reacts to a particle passing through it. Ideally, the chambers response at any given point should be isotropic, but in reality many external factors come into play when determining the response of the chamber. Therefore the study was done to see how much each section of the chamber deviates from the others. Figure 9 shows the sections of the chamber that were tested, associating a number with each section. Figures - show the charge distributions for each position.... Threshold (fc) FIG. : Efficiency Curve Calculated Using Hit Multiplicity Distributions chamber is calculated to be 9%. This shows that the chamber has a relatively high efficiency at large thresholds at the operational voltage. Recall, at fc there was not a significant shift in the Hit Multiplicity Distribution, therefore most of the background noise was removed at that point. Because the efficiency stays very high, the chamber has a high capability of detecting particles. Figure 8 shows the hit multiplicity as a function of the efficiency. Efficiency vs Mean Mean Efficiency H.V. 8V H.V. 9V H.V. 9V FIG. 8: Variation of Hit Multiplicity with Respect to Efficiency 8 8 FIG. 9: Position Diagram Proton Entries Mean. RMS. 9. / 8 Constant ± 9. MPV. ±.8 Sigma.9 ±. FIG. : Position Table III summarizes the data collected for the chamber uniformity. There were some discrepancies between the response of the chamber from section to section. These can be caused by various factors such as ambient factors, program malfunctions, or a multitude of other possibilities. The variation can be seen in Figure, which shows the variation of the MPV as a function of the position.

8 8 Entries Mean. RMS.. / Constant. ±. MPV. ±. Sigma. ±. 8 Entries Mean.9 RMS.. / Constant 9. ±. MPV 8.8 ±. Sigma. ± FIG. : Position FIG. : Position Entries 8 Mean. RMS.. / Constant. ± 8. MPV.9 ±. Sigma. ±.8 8 Entries 8 Mean 9.9 RMS.. / Constant 99. ±.9 MPV.8 ±.9 Sigma.9 ± FIG. : Position FIG. : Position The graph was fitted with a horizontal line fit and yielded an average MPV of.±.9. XII. CONCLUSION future High Energy Physics Experiments. The high efficiencies, gains, and low noise rates along with the versatility of analyzing data that is provided by the KPiX System makes it a strong candidate for use in future particle accelerators. Numerous studies have been done to characterize different facets of the chamber. The chambers response was tested for various particles and MPVs for each particle were found. The dependence of the gain on the voltage across the chamber was found to be. at the operational voltage. The chamber efficiency was measured to be 9% at a threshold of fc. The hit multiplicity was measure to be.8- at an efficiency of 9% at operational voltage. The uniformity of the chamber had an average MPV of.±.9. This chamber is very promising in finding precise measurements and has many appealing qualities for use in 8 Entries Mean 9.8 RMS..8 / Constant. ±. MPV. ±.9 Sigma 9.8 ±. Entries 9 Mean. RMS.8 8. / Constant ±.8 MPV.9 ±. Sigma.8 ±.9 FIG. : Position FIG. : Position

9 9 TABLE III: Position Data χ Position Events MPV..±.8.8.±. 8..9±...± ± ± ±. Position vs MPV MPV χ / ndf. / p. ±.9 Position FIG. : MPV With Respect to Position [] Yu, J. Development of GEM Based Digital Hadron Calorimeter, Arlington, Tx: University of Texas at Arlington.. Print. [] Yu, J. Development of GEM Based Digital Hadron Calorimeter, arxiv.org. Cornell University Libarary.. [] White, A. Development of GEM Based Digital Hadron Calorimetry Using SLAC KPiX Chip, Kolympari, Crete, Greece: IOP Publishing,. Web. May.. ///P/pdf/8- P.pdf [] SiD Detector Concepts [] ILD Detector Concepts [] Gas Electron Multiplier, Wikipedia.. Web. May. Electron Multiplier [] Gas Electron Multipliers, A Time Projection Chamber for a Future Linear Collider.. Web. May. [8] Pray, D. Analysis of T Test Beam Data from GEM Chambers with DCAL Readout System. Arlington, Tx: University of Texas at Arlington,. Print. [9] Freytag, D. KPiX, An Array of Self Triggered Charge Sensitive Cells Generating Digital Time and Amplitude Information SLAC Publishing. 8. Web. [] Carman, J. Characterizing the Noise Performance of the KPiX ASIC Readout Chip, 8. Web. -.pdf