Position Readout Methods

Several methods can be employed to measure the position of X-ray events in multiwire proportional counters. Only readout methods used in satellite borne imaging X-ray detectors are addressed here. In general, the signals of the two cathode grids provide the event position as shown in Figs. 4.2 and 4.3. The wire grids of the two cathodes are oriented orthogonal to each other to get both coordinates. One method is to use the wires of the continuous Z-wound cathode grid as distributed R-C delay line as shown in Fig. 4.2. Measuring the rise time of the signals arriving at both ends of each cathode permits the determination of both coordinates of the event [2]. This method was used in the IPC (imaging proportional counter) detector aboard the Einstein satellite. The IPC had a position resolution of about 1 mm FWHM [9].

Another method is to measure the charge signals induced on the individual stripes of the segmented cathodes as shown in Fig. 4.3. The charge distribution on the cathode has a (FWHM) width of about twice the distance between anode and cathode grid. Cathode strips with a width equal to the grid distance of anode to cathode reproduce this charge distribution by their signal magnitudes on several (2-5 depending on the event energy) strips. By a centroid computation of the signals,

Fig. 4.2 Distributed delay line readout (Einstein IPC)
Fig. 4.3 Center of gravity readout via a segmented cathode (ROSAT PSPC)

the event position can be determined with a resolution small compared with the strip width [4,10]. This was the readout method of the ROSAT PSPC (position sensitive proportional counter) with a spatial resolution of 250 |im (FWHM) at 0.93 keV [18].

The PSD of EXOSAT used a parallel plate proportional counter. In this detector type, the charge amplification takes place in a homogeneous high electrical field between a wire grid and a resistive plate acting as anode. By signal rise time measurements on four pick-up electrodes of the resistive anode, the event position is derived [20]. This detector, although very attractive due to its compact set up, suffered shortly after launch radiation damage by heavy ionizing particles [13].

A new detector concept was used in the JEM-X instrument aboard INTEGRAL [11]. The position sensitive proportional counter in the coded aperture mask telescope is a microstrip gas chamber [14]. This detector type uses, instead of wire grids, conductive micro structures on a partially isolating glass substrate. The micro structures can be manufactured by photolithographic processes with high accuracy resulting in high gain uniformity and fast response time. Alternating thin anode and broader cathode structures as shown in Fig. 4.4 form the intense electrical field around the anode, where gas amplification takes place. A second set of cathode stripes is placed on the rear side of the glass substrate orthogonal to the cathodes on the front side. The event position is derived from both sets of cathode stripes by capacitive charge division. In space environment this detector suffered by heavy ionizing cosmic rays. During the first week of operation about one anode per day was lost because of discharges triggered by heavy ionizing events. After lowering the gain by a factor of three, the radiation damage by heavy ionizing events was

4 Imaging Proportional Counters window rt c O

glass substrate

Be front side anodes (a)

front side cathodes (e)

Fig. 4.4 Electrode structure of a microstrip gas chamber

Fig. 4.4 Electrode structure of a microstrip gas chamber

reduced to a level of one anode loss in 2 months assuring a life time of more than 5 years [11]. Characteristics of these four IPC are summarized in Table 4.1.

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