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2 The Medium Energy detector array


2.1 The detectors

  The performance of the ME is summarized in Table 1. The ME comprised 8 individual detectors, each of which consisted of an Ar/CO2 and a Xe/CO2 gas filled multi-wire proportional counter separated by a 1.5 mm thick Be intermediate window (Fig. 1). An X-ray collimator, made from lead-glass microchannel plates was mounted in front of each detector. Anticoincidence and pulse rise time techniques were used to reduce the particle background. The ME operated well throughout the mission with one of the detectors failing on 1985 August 20. Problems with detector breakdown at the start of the mission were solved by operating the Ar counters at a lower overall gain setting. This resulted in pulse-height analyzer (PHA) channel 128 corresponding to an energy of $\sim$50 keV.


\epsfig {file=ds1551f1.eps,width=8.8cm}
\vspace*{-2mm}\end{figure} Figure 1: Cross-section of an ME detector

2.2 Background

  The ME background was usually stable with time and dominated by particle-induced events from the solar wind and events from the radioactive lines of residual Plutonium in the Be windows and detector bodies. The contribution of the extragalactic X-ray background was <1% of the total Ar background counting rate. After anticoincidence rejection of particle-induced events the typical 1-8 keV Ar background count rates was 3.8 s-1 detector-1. Occasional background flares occurred simultaneously in some or all of the detectors and were caused by enhancements in the solar wind. For normal observations longer than $\sim$5000 s, the ME was limited by systematic effects in the background subtraction to detections of $\mathrel{\hbox{\rlap{\lower.55ex \hbox {$\sim$}}
\kern-.3em \raise.4ex \hbox{$\gt$}}}$0.5 10-11 erg cm-2 s-1.

2.3 Field of view

  The ME field of view (FOV) was defined by collimators which had a rectangular aperture with an average full-width half-maximum (FWHM) of 45' and a flat top of $\sim$7' (Fig. 2). Details are to be found in Gottwald (1984) and Kuulkers (1995). Figure 2 shows a labeled schematic model of the collimator response. The 8 detectors were grouped into four pairs or "quadrants'', each of which could be offset by up to 120' from the aligned position. For most targets, two quadrants (one half of the ME) would be aligned and pointed at the target, while the remaining quadrants would be offset and pointed at two adjacent regions of sky in order to monitor the background counting rate. For bright sources, where background subtraction is not so critical, the ME could be operated with all four quadrants observing the source. The orientations of the quadrants are stored in the spacecraft pointing files, which were updated every 60 s.

2.4 Additional slew observations

  As well as the slew manoevers required to move between scheduled pointings, EXOSAT performed a series of scans along parts of the galactic plane as part of the scientific program. The results are presented in Warwick et al. (1985, 1988) and are not included in the EXMS.


\epsfig {file=ds1551f2.eps,width=8.8cm}\end{figure} Figure 2: A schematic model of an idealized ME collimator response (Kuulkers 1995). Slew directions were always parallel to the Y or Z axes. The flat top ($Y_{\rm F}T$, $Z_{\rm F}T$) to the collimator response had approximately equal sides of length 7 arcmin. The numbers indicate the relative transmission

Table 1: Properties of the EXOSAT ME

Geometric Area (8 detectors) & 1600~cm$^{2}$\\ Field...
 ...ackground & 21 counts~s$^{-1}$\space \\  
(8 detectors) & \\ \hline\end{tabular}

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