This document describes the design, operation, testing, and calibration of the imaging proportional counters (IPCs) installed in the flight model of the Stellar X-Ray Polarimeter (SXRP). Section 2 contains a brief summary of the impact of IPC performance on the operation of the SXRP and the design goals for the IPCs. Section 3 contains a description of the IPCs and a discussion of their operation. Section 4 contains a statement of the IPC Requirements, a description of the testing done at Metorex International and Columbia University related to these requirements, and a report on the results of these tests. Section 5 contains a description of detailed IPC testing and calibration done at Columbia University.

2 IPC Design Goals

The imaging proportional counters (IPCs) are crucial elements of the SXRP. The SXRP measures x-ray polarization using a graphite crystal and a metallic lithium target as polarization analyzers. X-rays reflected or scattered from these analyzers are detected in the IPCs. There are four IPCs surrounding the polarization analyzers, as shown in Figure 2.0-1. X-rays reflected from the graphite crystal are focused onto a small region of one of the IPCs. The other three IPCs are used to detect x-rays scattered from the lithium target.

The IPCs are designed to efficiently detect x-rays from the polarization analyzers. X-rays from the graphite polarimeter are of relatively low energy (2.6 keV and 5.2 keV) and the detector efficiency is limited by the transmission of the window. A 33 mm diameter, 50 micron thick beryllium window is used for the graphite polarimeter. Such windows are installed on two IPCs for redundancy. The lithium polarimeter scatters x-rays at higher energies, 5-15 keV, and over all angles. The fill gas includes Xenon to obtain high efficiency at high energies. Large area (100 cm^2 per detector) and relatively thick (150 microns) beryllium windows are used to maximize the active area of the IPCs in order to maximize the efficiency of the lithium polarimeter.

As an x-ray's energy is not significantly changed by Thomson scattering, the accuracy with which we can measure the energy of x-rays in the lithium polarimeter is determined by the energy resolution of the IPCs. The scientific objectives of the SXRP can be met with an energy resolution near 25% at 6 keV. This energy resolution provides at least three energy bands for the lithium polarimeter and is more than adequate to separate the 2.6 keV Bragg reflection peak from the second order peak at 5.2 keV.

IPCs with 4 mm position resolution for 2.6 keV x-rays should allow us to resolve the point spread function of the SODART telescope. Position sensing of the x-rays scattered by the lithium increases the effective modulation factor because it leads to a more accurate determination of the scattering angle. Non-imaging detectors used in the same geometry would decrease the polarization sensitivity by a factor of 3. In addition, imaging allows us to continuously measure the background and its possible polarization signature.

3 IPC Description

In this section, we describe the imaging proportional counters (IPCs), including the attached electronics and the operation of the detectors and the electronics. An x-ray detector subassembly consists of an IPC, three high voltage power supplies (HVPSs), a high voltage filtering network, and front-end electronics. A photograph of the four IPCs is shown in Figure 3.0-1. The HVPSs are mounted on the back of the IPC body. The sets of HVPSs for two IPCs are visible in the photograph. The high voltage filtering network and front-end electronics are mounted in a box (the "corner E-box") which is attached to the side of the IPC. In the center of the photograph an open E-box is displayed. The high voltage network is potted to prevent electrical breakdown after attachment to the IPC body. After the potting, the E-box and IPC are treated as a mated unit, as removal of the potting compound is extremely difficult. The HVPS can be physically removed and exchanged.

There are two types of IPCs used in the SXRP. "High energy" IPCs are used only to detect x-rays scattered by the lithium target. The high energy IPCs (HEIPCs or HE1 and HE2) have a beryllium window with a uniform thickness of 150 microns. In "low energy" IPCs (LEIPCs or LE1 and LE2), a region of the beryllium window is only 50 microns thick to allow x-rays of 2.6 keV from the graphite Bragg crystal to enter the detector. The LEIPCs and the HEIPCs are identical in all aspects except the window and the window strong back.

3.1 Imaging Proportional Counters

The IPCs were fabricated at Metorex International, Oy, according to specifications provided by the IAS del CNR (Frascati, Italy) and Columbia University. The design of the IPCs is based on prototypes constructed at Columbia.

The IPCs have stainless steel bodies. Without E-boxes or HVPSs, the IPCs are 17 cm by 20 cm by 7 cm in dimension and weigh 2.7 kg. The IPCs are mounted in the SXRP using a pattern of threaded holes surrounding the beryllium window. The back sides of the IPCs have mounting studs for the HVPSs and a copper lug for electrical grounding. The electrical interface to the IPC internal components is via high voltage feedthroughs on the side of the IPC body. There are six mounting studs surrounding the feedthroughs to allow attachment of the E-box.

X-rays enter the detectors via a 111 mm by 101 mm beryllium window which is supported by a titanium strong back. The beryllium is glued to a nickel frame which is welded to the stainless steel IPC body; the thermal expansion coefficient of nickel closely matches that of beryllium. The strong back patterns and IPC windows are different for the low energy counters and the high energy counters. For the high energy IPCs, the entire window is divided by an 8 by 7 rectangular strong back grid into 11.44 mm by 11.75 mm regions where the beryllium is 150 microns thick. For the low energy IPCs, the 8 by 7 rectangular strong back grid is modified to accommodate the "low energy window". The low energy window is a circular region with 16.5 mm radius where the beryllium is 50 microns thick. The strong back pattern for the LEIPCs is shown in Figure 3.1-1.

3.1.2 Internal Components

The internal volume consists of the drift region, the multiplication region, and the anticoincidence region. A cross-section of the internal components is shown in Figure 3.1-2. Starting from the window, the components are: four field forming rings, the front cathode wire plane, the anode wire plane, the wedge and strip (W&S) plane, the guard cathode, the anticoincidence anode plane, and the anticoincidence cathode plane. The critical dimensions of the stack of internal components are given in Table 1. Note that dimensions are the relative spacing of the wire planes, i.e. from the cathode wires to the anode wires. The tolerances are plus or minus the value quoted.

Table 1: IPC Internal Component Specifications

Table 2: Legend for Figure 3.1-2

X-rays entering the windows are absorbed primarily in the drift region. A uniform electric field perpendicular to the window is maintained throughout the 2.7 cm thick drift region by surrounding the drift region with four 1 mm thick aluminum field forming rings denoted FF1 through FF4 in Figure 3.1-2. As shown in Figure 3.1-2, a 1 mm thick aluminum shield is glued to the nickel frame in order to prevent nickel fluorescence.

The front cathode wire plane separates the drift region from the multiplication region. All three cathode wire planes are identical. The planes consist of a 1.25 mm thick alumina frame with outer dimensions of 170 mm by 155 mm and a 128 mm by 118 mm inner hole. The alumina frames are coated with aluminum to maintain a uniform electric field near the inner edge of the frames. The wires are gold plated tungsten of 50 micron diameter and are welded at the two ends to stainless steel sleeves mounted on the alumina frame. The wire was obtained from Luma. The wires are placed on 0.85 mm centers with three cathode wires per anode wire. The wires run across the shorter dimension of the frame and cover the entire inner hole of the frame. All of the wires on each plane are in electrical contact.

The main anode wire plane is between the front cathode wire plane and the W&S plane. The main anode and the anticoincidence wire planes are identical. The planes consist of an alumina frame identical to the cathode wire frames. Most of the wires are gold plated tungsten of 20 micron diameter and were obtained from Luma. The two wires near each edge of the inner hole (four wires total) are 50 micron diameter. Thicker wire was used at the edges to reduce the potential for sparking. The wires are placed on 2.54 mm centers. All of the wires on each plane are in electrical contact.

The W&S cathode plane consists of a 4.3 micron thick copper plate ("1/8 oz" copper) on a 25 micron thick kapton backing glued onto a 1.25 mm thick alumina frame. The W&S pattern used is a standard wedge, strip, and zee pattern. The pattern is aligned parallel to the anode wires. The pitch of the pattern matches the anode wire spacing of 2.54 mm. The maximum wedge (strip) signal occurs at the position marked as MAX WEDGE (MAX STRIP) in Figure 3.1-1. The W&S pattern is surrounded by a rectangular copper electrode which is used for background rejection; named "side anticoincidence". The electrode is 6 mm wide. There is a 6 mm wide gap in the electrode where the W&S electrodes pass through to reach wire attachment points at the edge of the frame. The "Sideanti" gap is marked on Figure 3.1-1.

The anticoincidence region consists of an anode wire plane (henceforth the Anti wire plane) and two surrounding cathode wire planes. The plane above the Anti wire plane (immediately below the W&S plane) is the guard cathode. This plane prevents charging-up of the underside of the kapton backing of the W&S plane and maintains a uniform electric field on the anticoincidence anode. The lower cathode maintains a uniform drift field in the rear anticoincidence region between the wire planes and the back plate of the IPC.

3.1.3 Gas Fill

The gas mixture used is 50% Xenon, 40% Argon, and 10% Methane by pressure. The total internal pressure of the IPC is 1.2 atm.

3.1.4 Radioactive Sources

For each IPC, an 55Fe calibration source is mounted into the strong back. The sources were installed at Columbia. The installation dates and the measured count rates on those dates are given in Table 3.

Table 3: 55Fe Calibration Sources

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