5.4.1 Gas Gain vs. High Voltage

Purpose

To determine the relationship between Gas Gain and the High Voltage settings ("gain curves") and to see how this relationship depends on temperature. The data is analyzed to find the dependence of Gas Gain on the difference between the Anode or Anti and Cathode voltages and also to look for any second order dependence on the Cathode voltage. Note that the Gas Gain is defined as the number of electron-ion pairs produced near the Anode or Anti wire planes for each electron produced in the drift region.

Testing Setup and Procedure

For this test, the entire IPC face is illuminated by an 55Fe source during data acquisition. In the cases where data is taken at elevated temperature, the IPC sits on a hot plate as described in section 5.3.3. While it is illuminated, the Cathode voltage is fixed while the Anode or Anti voltages are varied. For fixed Cathode voltages of 200 V and 400 V and a fixed Anti voltage of 500 V, the Anode voltage is varied to produce Gas Gains ranging from about 400 to 20000. In this case, the Anode wire plane is the "multiplication wire plane". Similarly, for fixed Cathode voltages of 200 V and 400 V and a fixed Anode voltage of 500 V, the Anti voltage is varied to produce Gas Gains ranging from about 400 to 20000. In this case, the Anti wire plane is the multiplication wire plane. In all cases, between 5000 and 50000 events are collected at each of five, six, or seven voltage settings. When possible, the position of the source is adjusted so that data is acquired at 100 to 200 Hz. It should be noted that a much stronger 55Fe source must be used to get count rates near 100 Hz while acquiring data for the Anti gain curves than is used to produce the Anode gain curves.

For LE1 and LE2, data is taken to produce Anode and Anti gain curves with the Cathode voltage at 400 V for three different temperatures (60°C, 45°C, and 25°C). For HE1 and HE2, data is taken at the three temperatures only with the Anode wire plane as the multiplication wire plane. The data taken with the Cathode voltage at 200 V is acquired while the IPC is at room temperature. When acquiring data for the Anti gain curves, the output from the Backanti preamplifier is connected to the Slow Anode shaping amplifier so that data acquisition is still triggered. Throughout the testing, the gain of the Slow Anode shaping amplifier is set to 5.

Table 15: High Voltage Fit Parameters: Input and Feedthrough Voltage vs. High Voltage Setting

Table 16: High Voltage Fit Parameters: HVPS Monitor Reading vs. Input Voltage

Table 17: High Voltage Fit Parameters: E-Box Monitor Reading vs. Feedthrough Voltage

Analysis and Results

Figures 5.4-2a to 5.4-2d display gain curves in four plots for each IPC. The top left graph contains the Anode gain curves for a Cathode voltage of 400 V for the three temperatures, while the top right graph contains the same for the Anti. The bottom left graph contains the Anode gain curve for a Cathode voltage of 200 V, while the bottom right graph contains the same for the Anti. For each point, the Gas Gain is found by binning Slow Anode or Backanti channel amplitudes measured when the IPC is illuminated by 5.90 keV x-rays produced by an 55Fe source. A function of the form:

(the sum of a Gaussian and a quadratic) is fit to the pulse height histogram. Figure 5.4-1 shows the resulting Slow Anode pulse height histogram and the calculated fit for illumination of HE1 with the 55Fe source. Once the mean of the Gaussian part of the function (the parameter a1 ) is determined from the fit, the mean is converted to a Gas Gain. The conversion is calculated as follows: The Slow Anode offset, found from the calibration described in section 5.3.2, is subtracted from the mean of the Slow Anode channel amplitudes after the mean is converted to a voltage. The result is divided by the Slow Anode channel gain. To find the number of electron-ion pairs, the voltage is multiplied by 1 pF (since this is the capacitance of the preamplifier feedback capacitor) and divided by the constant factor 1.6×10^-19 C/electron. The Gas Gain is found by dividing the number of electron-ion pairs by the number of electrons expected to be produced in the drift region of the IPC. On average, a 5.90 keV x-ray should produce 244 electrons in the drift region.

A linear least squares fit is calculated to the base 10 logarithm of the Gas Gain vs. the Anode or Anti to Cathode feedthrough voltage difference. Defining m and b to be the slope and offset parameters respectively and V to be the Anode or Anti to Cathode voltage difference (in volts), the equation used to calculate the fit is: Gas Gain = 10^mV+b . The fits for the data taken at 25°C are shown in the plots. The slopes and offsets for these fits are shown in the figure. For LE1 and LE2, no significant variation in the fit parameters is observed with changes in temperature or in Cathode voltage. For HE1 and HE2, the Anode fit parameters show no significant variation with temperature or Cathode voltage, while the Anti fit parameters show a marginally significant variation with Cathode voltage and no significant temperature variation. The fit parameters for the 25°C Anode and Anti gain curves with the Cathode voltage at 400 V for all four IPCs are given in Table 18.

Table 18: Gain Curve Fit Parameters at a Temperature of 25°C
and a Cathode Voltage of 400 V

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