<>By Taif Mohamed and Shaikha Al-Qahtani

FIGURE 1. (a) Electron multiplication in GEM hole (b) Triple GEM detector (c) Asymmetrical GEM holes due to single mask

Gas Electron Multiplier

The Gas Electron Multiplier (GEM) is a gas-based detector that identifies external charged particles when they enter the detector and collide with gas particles, effectively ionizing them. The ionization of gas particles in the detector leads to the release of electrons, which can be detected as an electric signal. Additionally, layers of GEM foil, consisting of a copper/Kapton/copper setup and a high density of holes, amplify the electrons’ signal before reading it at the printed circuit board (PCB).2 Figure 1A illustrates the amplification of the signal in the holes due to the high electric field existing there. Because of this functionality, the triple GEM detector, made up of three GEM layers as shown in Figure 1B, is a strong candidate to be installed in the Compact Muon Solenoid (CMS) experiment in the next upgrade.

​ The goal of using GEM detectors is to create a signal strong enough to be differentiated from background noise. The performance of the detector, in that sense, can be affected by several factors, including hole size and shape, the electric field existing in gas gaps, and the gas mixture, among others. Our study focused on the effect of different manufacturing processes on hole size and shape.

Foreword

The work performed by this team contributes to the studies by the European Council for Nuclear Research (CERN) to better understand the fundamental particles that make up our universe. The team in Qatar is part of the Compact Muon Solenoid team, an international scientific collaboration bringing more than 2,000 scientists to work together.​

Introduction

CERN is a French abbreviation, which stands for The European Council for Nuclear Research. It is the largest center for scientific research in the world. The purpose of the experiments in CERN is to answer questions about the origins of the universe and matter. The experiments are done with particle accelerators and detectors. CERN is known for containing the largest particle accelerator in the world, the Large Hadron Collider (LHC). In 2012, after three years of operation, the LHC made history in the physics world; a new particle was observed in the accelerator that resembled the Higgs boson, which had been predicted by the standard model theory.1 This achievement opened the door for more discoveries and a gateway to new physics. In order to continue the discovery of new physics, scientists must increase the energy and luminosity of the LHC. However, current technologies might not be able to cope with the changes, and thus, an upgrade is needed. One of the experiments on the LHC that will be upgraded is the Compact Muon Solenoid (CMS) experiment. The CMS’s detector technology was designed to accurately detect muon particles. Muons are charged particles that resemble electrons but have a much greater mass; they are present in the most important physical processes. A new detector known as the Gas Electron Multiplier (GEM) would be added onto the muon chambers. This research aims to optimize GEM operation and efficiency by comparing the effect of two different etching techniques on the detector gain.

THE PURPOSE OF THE EXPERIMENTS... IS TO ANSWER QUESTIONS ABOUT THE ORIGINS OF THE UNIVERSE AND MATTER...

GEM Manufacturing Process

Holes are made in the GEM layers using chemical etching, which presents the difficulty of getting precise hole size and shape. The standard manufacturing process for the GEM has been the double mask technique, which results in a symmetric double-conical shape of the holes. For the required size of GEM layers (1 m x 0.5 m), however, this process is too complicated and expensive. Alternatively, we propose the single mask technique to produce these GEM layers. The effect of using the single mask is different hole diameters on the upper and lower sides of the GEM.3 Two orientations of the GEM were identified, where orientation A exhibited a larger upper hole diameter than B, as shown in Figure 1C. Changing the orientation of the GEM layer can cause a significant difference in gain, which is defined as the number of electrons produced through primary and secondary ionizations due to the detection of a single muon divided by the number of primary ionizations. The ultimate goal of this study is to identify the most optimal configuration of triple GEM that maximizes the gain.

Methods

We used three main simulation software to complete our studies—HEED, ANSYS, and Garfield++—and we made use of a supercomputing facility due to the need of high computing power. Figure 2 shows the logical flow of the simulation. In order to compare the different manufacturing processes of the GEM, three main configurations were defined for every simulation: double mask, single mask orientation A with the wider hole on the top, and single mask orientation B with the wider hole on the bottom.

Figure 2. Logical flow of the simulation.

ANSYS

ANSYS is a computational fluid dynamics package based on the Finite Element Method.4 We used ANSYS to define the detector geometry and to compute the electromagnetic field map throughout the detector. To obtain an electromagnetic field map, ANSYS divides the entire detector into a mesh of nodes and computes the electromagnetic field values at these nodes. Then, ANSYS displays the solution in the form of four list files, which are essential to the Garfield++ simulation.

Garfield++

The list files, obtained from ANSYS, are then used in Garfield++, which employs Monte-Carlo simulation methods to simulate the interactions of the electrons as they are produced and flow through the detector, ionizing more gas particles on the way.5 Garfield++ uses Magboltz to calculate the electron properties as they interact with the gas in the detector.6 It can then conclude how many secondary ionizations may result from each primary ionization and compute the gain.

HEED

HEED is a software tool that simulates charged particles passing through a gas and computes the energy loss by taking into account the collisions between the atoms.7 This program shows the number of primary electrons produced and the number of clusters per cm in the GEM. The geometry of the GEM from ANSYS is the input to HEED. Then, the output of HEED, the number and position of primary electrons, is the input of Garfield++. Thus, instead of using the random function to generate random electrons, HEED simulates the actual primary electrons, which makes the simulations more realistic.

FIGURE 3. Results of drift field study (top) and high voltage study (bottom).

Results

For a triple GEM detector, we conducted simulations that studied the gain at different values of the electric field in the drift gas gap and for different values of voltage at the drift cathode, called the high voltage. The gain of each simulation was recorded, and the graphs displayed in Figure 3 were produced to compare the effect of each configuration on the performance of the GEM detector. Results show that with the current voltage divider, installing the GEM in orientation A would yield the highest gain, and consequently, the highest chances of accurate detection.

THE RESULTS OF OUR SIMULATIONS AND ANALYSIS WILL BE CRUCIAL DURING THE ASSEMBLY PROCESS...

Conclusion

Our work is still ongoing as we are enhancing the simulations for a more thorough study of these two manufacturing processes for the triple GEM detector. The results clearly showed that the GEM layer orientation has a significant effect on the performance of the detector. Therefore, the results of our simulations and analysis will be crucial during the assembly process, and we aim to share our findings in order to facilitate a successful upgrade for the CMS muon detector.

Acknowledgements

We would like to thank our research group, the High Energy Physics research team at Texas A&M University at Qatar. The team consists of researchers and undergraduate students. The head of the team is the Director of Research Computing and Research Professor at TAMUQ Dr. Othmane Bouhali, with the assistance of the Physics Lab Coordinator Maya Abi Akl. The researcher working with us is Munizza Sayed Afaque. Finally, the undergraduate students in the team are Taif Mohamed, Shaihka Al-Qahtani, Abdulaziz Al-Qahtani, Meera Al-Sulaiti, Maryam Al-Buainain, and Alaa Abdulla.

​WE AIM TO SHARE OUR FINDINGS IN ORDER TO FACILITATE A SUCCESSFUL UPGRADE FOR THE CMS MUON DETECTOR.  

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Taif Mohamed  '18

Taif Mohamed is a graduating senior electrical engineering major with minors in physics and math at the Texas A&M Qatar campus. Taif’s hometown is Ismailia, Egypt. After graduation, Taif plans to attend graduate school for power engineering.

Shaikha Al-Quatani '18

Shaikha Al-Qahtani is a graduating senior electrical and computer engineering major with minors in physics and math at the Texas A&M Qatar campus. Shaika is originally from Alkhor, Qatar. Shaika has spent two summers working at CERN, which has developed her interest in electrical engineering. Shaika plans to pursue doctoral studies after graduation.

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References
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References
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6. Garfield++ simulation of tracking detectors, available on (http://garfieldpp.web.cern.ch/garfieldpp/).
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