Experimental Setup of Gain Measurements

While the TPC prototype was originally constructed at the Forschungszentrum Karlsruhe, it was modified later on at the University of Aachen and Bonn [Lot01]. The anode consists of a simple copper plate and the cathode of an aluminum coated Mylar foil. On top of the cathode, a Mylar foil is built-in the chamber to protect the amplification system and close the gas volume. There are three standard CERN GEMs (cf. section 4.1) mounted in the chamber. The whole setup is illustrated in figure 36.

Figure 37: Sketch of distances and current denotations of the GEM stack in the prototype [Zen14].

The current denotations and distances between the GEMs, anode and cathode are sketched in Figure 37. The GEMs have to be framed [Hal07] for precise measurements.

The framing ensures more homogeneous electric fields, by providing flat GEM foils with a well known distance between each other. This is needed in order to provide the fore-seen fields (transfer and induction) via the voltage settings. All potentials can be set individually.

The infrastructure needed for effective gain measurements is explained in the following.

Gas System:

The choice of the appropriate gas mixture in TPCs is important, since the gas has to fulfill the requirements of the experiment. It affects the signal production and the transportation and as well the gas gain in the amplification structure. The used gas is called Tesla TDR gas (TDR) [BBHS01]. It consists of 93% argon, 5% methane and 2% carbon dioxide.

Argon based gases are often used in TPCs due to the required relative low ionization

for carbon connections between the both sides of a GEM is promoted by the carbon in the compound. The development of so-called Diehard GEMs with polytetrafluoroethyelene (PTFE) as insulator does not show the formation of carbon wires during discharges even in gas mixtures containing 30 % CO₂ [Wak14]. In addition, carbon connections also form in pure N₂ atmospheres. Here, the connection is formed by the carbon in the Kapton insulator. Therefore, although it has not been studied in detail, the carbon contained in gas compounds seems to have little or no effect on the formation of carbon wires in the GEM holes.

The gas bottle is connected with the chamber via a gas rack, which is part of the slow control system and measures the gas pressure and flow. After flushing the cham-ber, the gas again passes the gas rack and the water vapor content is measured. In this way, it is easy to monitor the quality of the gas in the chamber and no leaks are occurring.

Power Supply:

All GEMs as well as cathode and anode are connected to individual channels of a CAEN SY2527 power supply. Between the power supply and the GEM electrodes, 10 MΩ protection resistors are installed to limit the current during a discharge (cf.

section 4.4). The voltage monitor versus output voltage accuracy amounts to 0.3%± 1 V and the voltage set versus voltage monitor accuracy is 0.3% ±0.5 V [CAE14].

Current Measurements:

All used channels of the power supply are connected to the electrodes via a current measurement device. The Current Monitor (CUMO) is constructed out of NIM modules and was designed at the University of Aachen. The current is defined as follows: positive current appears when electrons are collected. If the measured particles are ions, the CUMOs show a negative current. There are five different range modes to measure the current: 20 nA, 200 nA, 20 µA, 200 µA and an auto-range mode. The auto-range mode is used in all measurements to result in the best resolution. The CUMOs provide information about the measured current and the chosen range [Zen14]. The system-atic uncertainty depends on the range used by the CUMOs and provides a resolution of 0.1 digit of the current range meaning an error of 0.01 nA for the smallest range [Bei99].

Software:

The measurements are steered with a C++ based program called XTC. The XTC soft-ware was designed in Aachen and has a graphical user interface. All voltages are adjusted via the software and the resulting voltages and currents are displayed and recorded.

Via the XTC interface, measurements can be started, stopped and monitored. On top, the CUMOs are calibrated regarding their ranges with XTC. The data - meaning all parameter settings, measured currents, the calibration information and the chosen range - are stored in ASCII files [Zen14].

Signal Creation:

A ⁵⁵Fe source is mounted above the protection Mylar foil in the test chamber. The iron source produces radiation as described by the following decay equation:

55F e+e⁻→⁵⁵ M n^∗ →⁵⁵M n+γ (22)

Iron-55 decays via electron capture to manganese-55. Manganese-55 is not stable and mostly sends out photons with a characteristic radiation ofE_γ= 5.9 keV. This is sufficient energy to ionize the gas. Between the source and the protection Mylar foil, a shutter is built-in. This shutter is also controlled with the XTC software. If the shutter is open, the photons reach the sensitive volume of the TPC. If the shutter is closed, a aluminum plate slides between the source and the Mylar foil, so that the radiation is blocked and does not reach inside the TPC [Hal10, Zen14].

Gain Calculations:

The amplification coefficients were introduced in section 3.3. In this setup, a GEM stack with three GEMs is used. In order to compare the amplification of two different GEMs, the calculation of the gain of GEM III is introduced here. Measuring the effective gain of GEM III and not of GEM I or GEM II has two reasons. On one side, there is no GEM beneath GEM III, so no produced ions distort the current between GEM III and anode and with it the measurement. On the other side, the signal is already amplified by GEM I and GEM II. Due to the higher electron statistics, the variations of the measured current of GEM III are smaller. The voltage settings of GEM I and GEM II stay the same during

Drift Field 200 V/cm

Table 9: Voltage settings for the gain measurements.

constant for the whole measurement, so that the incoming signal on GEM III stays the same. GEM III is ramped from 0 V up to a certain voltage, that depends on the used gas and distances between the GEMs. In this setup, it is ramped up to 360 V. The current of the anode side of GEM III IGEM III,anode and the current of the anodeI_anode are measured.

The effective gain is calculated by the measured currents as follows:

G_GEMIII(U_{GEM III}) = I_anode+IGEM III,anode

IGEM III, cathode⁰

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