Purpose

To determine how many electron-ion pairs are produced in the IPCs for x-rays of known energy and to determine how energy resolution varies with x-ray energy.

Testing Setup and Procedure

These tests examine the response of the IPCs to x-rays of known energy. X-rays of known energy are produced in two ways: 1) Using the x-ray beam line and 2) Using sealed radioactive x-ray sources. When using the x-ray beam line, the IPC is attached to one end of a tube evacuated to a pressure below 1 mtorr, and an x-ray source sits about 3.9 meters away from the IPC at the opposite end of the tube. The x-ray source anode is an interchangable part, and anodes composed of four different materials are used in the x-ray source to produce emission excesses at the characteristic line energies of the materials. The anode materials used and their K(alpha) line energies are given below.

Further description of the x-ray beam line can be found in section 5.2.4. The sealed radioactive sources used to produce x-rays of fixed energy are also described in section 5.2.4. The radioactive isotopes used and their corresponding x-ray line energies are given below.

When using the sealed radioactive sources, the IPC is placed on a table with its window exposed, and the radioactive source in use is positioned so that the entire IPC window is illuminated. The 6.40 keV line from the 57Co source is used rather than the same line from the x-ray beam line with the Fe anode. When using either the x-ray beam line or the sealed radioactive sources, data is acquired at a rate between 150 and 300 Hz. For each energy, either 50,000 or 200,000 events are acquired for four different Gas Gains: 2500, 5000, 10,000, and 20,000. Throughout the testing, the gain of the Slow Anode shaping amplifier is set to 5.

Analysis and Results

Figures 5.4-4a to 5.4-4d display the relationship between the energies of the x-rays illuminating the IPC and the number of electron-ion pairs produced in the IPC per event for the four Gas Gains. For each point, the number of electron-ion pairs is found by binning Slow Anode channel amplitudes measured when the IPC is illuminated by the previously discussed sources. The sum of a Gaussian and a quadratic (The functional form is specified in 5.4.1.) is fit to the pulse height histogram. Figure 5.4-3 shows the resulting Slow Anode pulse height histogram and the calculated fit when HE1 is attached to the x-ray tube and illuminated by the x-ray source with the copper anode installed in the x-ray source. In addition to the copper K(alpha) line, a substantial background component from the x-ray source should be noted.

Once the mean of the Gaussian part of the function is determined from the fit, the mean is converted to a number of electron-ion pairs. 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.

Unfortunately, the Anode pulse height amplitude deviates from a linear dependence on x-ray energy, particularly at high Gas Gains. The deviation may be a space charge effect. A quadratic provides an adequate characterization of the data. A quadratic least squares fit is calculated to the data for each Gas Gain and the fits are shown in Figures 5.4-4a to 5.4-4d. The fits are of the form:

The fit parameters for the four IPCs are shown in Table 19.

Figures 5.4-5a to 5.4-5d display the relationship between the energies of the x-rays illuminating the IPC and the FWHM of the resulting Slow Anode distribution for the four gas gains. For each point, the FWHM is found by binning Slow Anode channel amplitudes measured when the IPC is illuminated by the previously discussed sources. The sum of a Gaussian and a quadratic (The functional form is specified in 5.4.1.) is fit to the pulse height histogram. Once the standard deviation of the Gaussian part of the function (the parameter a2) is determined from the fit, it is multiplied by 2.35 and converted to a number of electron-ion pairs.

A quadratic least squares fit is calculated to the data for each Gas Gain and the fits are shown in Figures 5.4-5a to 5.4-5d. The fits are of the form:

Table 19: Line Energy vs. Slow Anode Amplitude Fit Parameters

Table 20: Line Energy vs. Slow Anode FWHM Fit Parameters

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