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Pressure Dependence of GIA Chamber Gain

William Se Hwan Kim UTA-HEP/LC-0031

High Energy Physics Group Department of Physics

The University of Texas at Arlington (Dated: June 28, 2012)

Abstract The 3 x 3 cm2 Gas Electron Multiplier (GEM) chamber, developed by the Gas Detector Development (GDD) workshop at CERN, is currently used for observing the characteristics of the GEM chambers for future use as a Digital Hadron Calorimeter [5]. For all Gas Electron Multipliers, pressure is a big foe, as it immensely influences the gain. This paper presents the general mechanism of the GEM detector, the pressure dependency of the GEM chamber, and pressure correction method for the GEM chambers.

I. INTRODUCTION

On July 4th 2012, CERN announced the results of many month worth of observation, which is a step closer to the completion of the Standard Model puzzle. As CERN announced its discovery of a new particle, possibly the Higgs Boson, the demand for the International Linear Collider’s (ILC or CLIC) construction will build up. The ILC plans to collide electrons, anti-particles, and positrons close to speed of light. Physicists believe this machine represents the next step for seeking clues to unmask the workings of the most fundamental particles.

A strong detector candidate for the upcoming ILC, the Gas Electron Multiplier (GEM), aptly serves for the high-precision measurements of jet energies [5]. A GEM detector is anticipated to be used as Digital Hadron Calorimeter’s sensitive gap detectors [5]. The GEM can also be applied as a digital imaging device: it has fast recovery and an amplifying effect [8].

The University of Texas at Arlington’s High Energy Physics group has developed prototype GEM chambers, and is currently constructing the Large Gas Electron Multiplier

(LGEM). The research team is currently conducting experiments to understand the GEM chamber’s characteristics. One of the GEM chamber’s most important characteristics is its strong pressure dependency. Specifically, because of the GEM chamber’s strong pressure dependency, this results in a direct relationship between pressure and gain value. As a result, a pressure correction method proves essential for precise data collection.

II. Experimental Setup

2.1 Gas Electron Multiplier

Gas Electron Multiplier (GEM) was first invented by Fabio Sauli at CERN in 1997 [1]. GEM was a big improvement compared to MWPC and MSGC the earlier gas detectors [5].

2.2 GEM Structure and Manufacture

GEM foil is manufactured in a clean room [1]. The GEM chamber has GEM foils (depending on the amplification), cathode and anode, (drift cathode and anode). The chamber is tightly filled with ionizable gas.

GEM foil consists of a polyimide film, layer of chrome, and layer of copper [1]. The

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chrome layer A Polyimide foil with copper metallized on both sides makes up the GEM foil:

in this case, Kapton film was used. The chrome seed coat allows the foil to be flexible, and thermal stability. The copper seed coat allows the conductivity.

Holes with a diameter of 70 µm and 140 µm pitch distance cover the GEM foil [1]. These holes are drilled through a delicate chemical etching process [1].

2.2 GEM Detection Mechanism

The GEM’s detection mechanism is very simple. The cathode and anode pad on the top serves the purpose of creating a dense electric field. High voltage, optimally 1950V, is applied to the electrodes, creating a high electric dipole field inside the hole [10][1]. When a strong electron cascade enters the chamber, the electric field accelerates the free electrons, and during the process the electrons pass through the GEM foil. In the GEM holes, these electrons collide with other electrons from an easily ionizable gas, in this case ArCO2 [8] [1]. This process amplifies the electron’s effect, causing an avalanche of electrons to reach the readout pad [9]. More amplified charge readings will require multiple layers of GEM foil [1].

2.3 GEM’s Advantage

GEM chambers possess several advantages compared to other detectors: mass production capability, quick recovery time, and efficiency in terms of the high voltage. The GEM foil can be manufacture. The quick recovery time. GEM foil is advantageous for detecting small signals. By amplifying these signals, the readout system will produce clear results.

2.4 Fe55 Source

Figure 1.

Fig 3. GEM foil magnified …

Fig  4.  Diagram    

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Fe55 radioactive source serves well for understanding the characteristics of the GEM chamber. Specifically, Fe55 is well-used for specifying the detector resolutions: X-rays are most strongly emitted at 5.895 keV. For the ArCO2 using GEM chambers, the resulting spectrum contains a 3 keV Ar-escape peak [5]. The Fe55 has strong and clear signals: Fe55 source lack continuum x-rays(bremsstrahlung) [11] . Also, the peak is very clearly observable in the charge spectrum.

2.5 GIA Chamber Structure

The GIA chamber, also known as GEM33 consists two layer of 3x3 cm2 GEM foil [5]. The GEM foils were manufactured through chemical etching technique, resulting in 55 µm diameter holes and 140 µm pitch [5]. The cathode is made from copper clad Kapton film in 25 µm thickness, and a single resistor system is used for distributing high voltage to each electrodes [5]. For anode, GIA used a 2x2 cm2 printed circuit board made of copper. The GIA chamber was supplied with ArCO2 (Ar:CO2 = 80:20) and 1950V was used [5].

2.6 GIA Chamber Electronics

The GIA chamber is readout by single channel readout electronics (SCR) [5]. GIA chamber gives out charged pulses as signals [5]. The charge sensitive preamplifier manufactured by AmpTek converts it to voltage [5]. The DAQ computer takes in the digital values that are converted from analog signals; the DAQ computer uses LabView software [5].

III. DATA SET

3.1 Pressure Influence on the Chamber

GEM uses a very easily accessible gas mixture ArCO2. After the circulating through the chamber, ArCO2 releases into open air through a bottle, in Figure 10, with constant liquid level. The buoyancy of the liquid is directly related to the atmospheric pressure. Because of this, the atmospheric pressure directly influences the output rate of the gas. Since, the GEM detector is an open gas system, the pressure dependent characteristic proves highly critical.

Figure  5.    

Figure  6.    

 

Figure  7.  3x3  GIA  Chamber.  

Figure  8.  The  GIA  Electronics  and  the  LabView  Software.  

 

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3.2 High Pressure and Low Pressure

To show the stark contrast of the charge mean value, Figures 11 plots two raw distribution data of the Fe55 data from the 3x3 cm2 GIA chamber. The green line represents the high pressure data while the blue represents the low pressure data.

Figure 12-1 shows the high pressure raw distribution data collected on July 2nd at 2:53 pm. The pressure data came from the Weather Underground website. Both sets of pressure data from Weather Underground were measured at the same corresponding time as the source run. The high pressure is anticipated to result in low charge mean as shown in Fig. 11, The low pressure data’s charge mean value comes out to 211.1 fC, whereas that of the high pressure data calculates to 190.4 fC. The stacked histogram explicitly shows definite shift in mean charge value, and it can be observed easily. The charge difference equals to 20.7 fC difference which is 10.9% increase from the high pressure data; in contrast, the pressure only deviated by 0.739%. The pressure dependence is critical.

Figure  9.  The  ArCO2 source.  The  sticker  mentions  the  ratio:  Ar:CO2  =  80:20.  

Figure 10. The ArCO2 gas is released through the bottle.

Figure 11. Stack of two Histograms: High Pressure Data and Low Pressure Data. Notice the horizontal shift.

Figure 12-1. High Pressure: 1.00251 atm

Figure 12-2. Low Pressure: 0.99516 atm.

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3.3 Pressure VS Gain (Weather Underground Data)

The pressure versus gain graph contains 64 data points that correspond from the weather underground pressure data (based on Arlington, TX). A trend can be easily seen. During the period June 7th to June 20th, 64 data points were collected, and the weather fluctuated enough for the pressure to range from 29.77 inch of hg to 30.06 inch of hg, meaning 0.99483 atm to 1.00452 atm. Table 1 contains the full data record from the GIA chamber. After June 20th, the GIA chamber had spark problems. The GEM chambers often spark. Normally, the frequency of the spark is low and it is unnoticeable on the Oscilloscope. The sparks can be observed when GEM foil’s surface creates discharge. However, at June 20th the spark’s frequency reached to a point where the gain from the source was indiscernible from the sparks. As a result, the GIA chamber’s foil had to be replaced.

During the repair period, the lab purchased a digital barometer for more accurate pressure measurement, as well as to figure out the relation between the weather underground data and the digital barometer’s indoors pressure data.

The replaced GEM foil did not have the spark problems and produced good results. However the machine had some irregularities. The new GIA chamber seemed to have unstable signal readout. The previously collected data did not match with the new data.

3.4 Weather Underground Data VS Digital Barometer

IV. ANALYSIS

4.1 Calculating Gain

Gain=𝐶ℎ𝑎𝑟𝑔𝑒  𝑀𝑒𝑎𝑛𝑒×218

4.2 Correction Method

The correction method aims to correct gain data from any pressure point to 1 atm

Figure 13. Pressure VS Gain graph.

Fig 14. Digital Barometer

Figure 15. Pressure VS Gain graph. The green line represents the 1 atm line.

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(101.325 kPa = 1 atm) gain data. The correction method uses a conversion factor that is calculated from the previously collected data. The conversion factor,   𝑐𝑓𝑝   ,derived from the ratio between each pressures’ gain value and the linearly fitted 1 atm gain value. The reference gain, 𝑔𝑟𝑒𝑓, equals the 1 atm gain value after the

linear fit. Specific pressure gain value, 𝑔𝑝  , equals

the gain value at each corresponding pressure value.

The correction factor equals to:

𝑐𝑓𝑝=  𝑔𝑝𝑔𝑟𝑒𝑓

The reference gain:

𝑔𝑟𝑒𝑓=5723.2749

The green line represents the 1 atm value. The gain value corresponding to the 1 atm pressure is the reference for correcting the data.

F𝐺𝑎𝑖𝑛=𝐺𝑎𝑖𝑛𝑐𝑓𝑝

4.3 Corrected Data

Figure 16-1 and Figure 16-2 shows the stark contrast between the uncorrected data and the corrected data. The correction method was used to the same data. The correction method looks fairly clean.

The correction data

V. CONCLUSIONs

VI. FUTURE WORK

For future work: Compare dependence with different size GEM chambers.

Figure 16-2.

Figure 16-1.

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Table 1. Data from Weather Underground Pressure Data

Date Time File name Pressure Fitted Mean Gain High Voltage Corrected Data

6/7/2012 9:51 AM 20120607-1.dat 29.97 200.4 5737.608048 1950V 5778.207114

10:28 AM 20120607-2.dat 29.98 201 5754.786515 1950V 5813.121839

11:29 AM 20120607-3.dat 29.99 201.4 5766.238826 1950V 5842.447613

12:27 AM 20120607-4.dat 29.99 200.2 5731.881892 1950V 5807.636604

1:53 AM 20120607-5.dat 29.98 198.4 5680.34649 1950V 5737.927228

2:53 AM 20120607-6.dat 29.96 198.7 5688.935724 1950V 5711.882456

3:53 PM 20120607-7.dat 29.95 199.8 5720.42958 1950V 5726.204435

6/8/2012 2:02 PM 20120608-1.dat 29.94 199.6 5714.703425 1950V 5703.294693

2:53 PM 20120608-2.dat 29.89 201.7 5774.82806 1950V 5678.047253

3:53 PM 20120608-3.dat 29.9 201.6 5771.964982 1950V 5692.071844

4:53 PM 20120608-4.dat 29.88 202.1 5786.280371 1950V 5672.525774

6/11/2012 8:53 AM 20120611-1.dat 29.83 204.5 5854.99424 1950V 5656.463871

9:53 PM 20120611-2.dat 29.84 205.4 5880.761941 1950V 5697.92083

10:53 PM 20120611-3.dat 29.84 203.6 5829.226539 1950V 5647.987736

12:53 AM 20120611-5.dat 29.84 205.9 5895.07733 1950V 5711.791134

1:53 PM 20120611-6.dat 29.84 208.6 5972.380433 1950V 5786.690775

2:53 PM 20120611-7.dat 29.83 208.2 5960.928121 1950V 5758.80576

3:53 PM 20120611-8.dat 29.83 208.1 5958.065043 1950V 5756.039763

6/12/2012 8:53 AM 20120612-1.dat 30.04 195.4 5594.454154 1950V 5756.134424

9:53 PM 20120612-2.dat 30.06 192.5 5511.424896 1950V 5706.035425

10:53 PM 20120612-3.dat 30.05 191.9 5494.246429 1950V 5670.585846

11:53 PM 20120612-4.dat 30.05 193.4 5537.192597 1950V 5714.910384

12:53 AM 20120612-5.dat 30.03 194.4 5565.823375 1950V 5709.002122

1:53 PM 20120612-6.dat 30.05 193.7 5545.78183 1950V 5723.775291

2:53 PM 20120612-7.dat 30.05 194.1 5557.234142 1950V 5735.595168

3:53 PM 20120612-8.dat 30.03 195 5583.001843 1950V 5726.622499

4:53 PM 20120612-9.dat 29.96 199 5697.524957 1950V 5720.506335

6/13/2012 8:53 AM 20120613-1.dat 30.01 197.4 5651.715711 1950V 5761.540622

9:53 AM 20120613-2.dat 29.96 199.2 5703.251113 1950V 5726.255588

10:53 AM 20120613-3.dat 29.94 200 5726.155736 1950V 5714.724141

11:53 AM 20120613-4.dat 29.92 201.7 5774.82806 1950V 5728.893112

12:53 PM 20120613-5.dat 29.92 202.1 5786.280371 1950V 5740.254328

1:53 PM 20120613-6.dat 29.9 202.9 5809.184994 1950V 5728.776672

2:53 PM 20120613-7.dat 29.87 203.6 5829.226539 1950V 5697.820677

3:53 PM 20120613-8.dat 29.87 204.3 5849.268084 1950V 5717.410434

6/14/2012 8:53 AM 20120614-1.dat 29.91 199.2 5703.251113 1950V 5641.047336

10:53 AM 20120614-3.dat 29.91 201.5 5769.101904 1950V 5706.17991

11:53 AM 20120614-4.dat 29.9 200.5 5740.471125 1950V 5661.013912

12:53 PM 20120614-5.dat 29.89 201.2 5760.51267 1950V 5663.971776

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1:53 PM 20120614-6.dat 29.87 203.6 5829.226539 1950V 5697.820677

3:53 PM 20120614-8.dat 29.83 205.4 5880.761941 1950V 5681.357844

4:53 PM 20120614-9.dat 29.82 205.9 5895.07733 1950V 5678.680771

6/15/2012 8:53 AM 20120615-1.dat 29.9 199.5 5711.840347 1950V 5632.779429

9:53 AM 20120615-2.dat 29.92 196.5 5625.948011 1950V 5581.197306

1:53 PM 20120615-3.dat 29.93 197.9 5666.031101 1950V 5637.790016

2:53 PM 20120615-4.dat 29.91 199 5697.524957 1950V 5635.383634

3:53 PM 20120615-5.dat 29.91 200 5726.155736 1950V 5663.702144

4:53 PM 20120615-6.dat 29.88 200.4 5737.608048 1950V 5624.810317

6/18/2012 9:53 AM 20120618-1.dat 29.85 205.7 5889.351175 1950V 5722.927186

1:53 PM 20120618-3.dat 29.81 209.5 5998.148134 1950V 5761.26946

2:53 PM 20120618-4.dat 29.79 211 6041.094302 1950V 5769.173254

3:53 PM 20120618-5.dat 29.78 211.1 6043.957379 1950V 5755.369768

4:53 PM 20120618-6.dat 29.77 210.8 6035.368146 1950V 5730.770828

6/19/2012 8:53 AM 20120619-1.dat 29.89 204.832 5864.499659 1950V 5766.216038

9:53 AM 20120619-2.dat 29.9 203.684 5831.631525 1950V 5750.912507

10:53 AM 20120619-3.dat 29.9 206.039 5899.057009 1950V 5817.404715

11:53 AM 20120619-4.dat 29.88 208.611 5972.695371 1950V 5855.275974

12:53 PM 20120619-5.dat 29.85 209.314 5992.822809 1950V 5823.474871

1:53 PM 20120619-6.dat 29.84 208.215 5961.357583 1950V 5776.010641

2:53 PM 20120619-7.dat 29.84 209.812 6007.080936 1950V 5820.312391

3:53 PM 20120619-8.dat 29.84 207.492 5940.65753 1950V 5755.954181

4:53 PM 20120619-9.dat 29.84 207.174 5931.552942 1950V 5747.132668

6/20/2012 8:53 AM 20120620-1.dat 29.93 202.461 5796.616082 1950V 5767.724125

12:53 PM 20120620-5.dat 29.92 200.556 5742.074449 1950V 5696.400035

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VII. BIBLIOGRAPHY

[1] Optimisation and Characterisation of Gas Electron Multiplier

[2] Application of Large Scale GEM for Digital Hadron Calorimetry.

[3] Development and applications of the gas electron multiplier.

[4] A first mass production of gas electron multipliers.

[5] Measurement of Gas Electron Multiplier (GEM) Detector Characteristics Using Cosmic Ray and Radioactive source.

[6] Fabrication of a GEM Detector and Its Signal Readout with a Homemade Readout Board.

[7] Radiation Effect on Gas Electron Multiplier Detector Performance.

[8] Applied Radiation and Isotopes

[9] Characterization of GEM Chambers Using 13bit KPiX Readout System.

[10] Analysis of T-1010 Test Beam Data from GEM Chambers with DCAL Readout System

[11] http://www.x-raymicroanalysis.com/x-ray-microanalysis-explained/pages/tutorial1/gooddetector6.htm  

[12] http://www.linearcollider.org/about/What-is-the-ILC/The-project