Summary Description

The High Energy and Low Energy Proportional Counter (HEPC/LEPC) detectors offer large effective area, good time resolution, and moderate spatial and spectral resolution over a broad energy range. They are well-suited for temporal studies and studies of broad-band spectra. A HEPC/LEPC detector pair will be provided for each telescope. Each detector pair will share a common gas resevoir.

The detectors are microstrip gas proportional counters (MSPC), in which one conventional wire grid plane is replaced by a grid of narrowly spaced conducting microstrips, deposited by photolithographic techniques on an insulating substrate. Pulse shape and amplitude are determined from microstrip anode signals, and positional information from the microstrip cathodes. Positional information in the orthogonal direction is determined from a conventional wire grid, suspended $\sim$1 mm above the microstrip plate. A third electrode, mounted on the rear of the plate, surrounds the useful detector area defined by the counter entrance window. It provides a veto signal for charged particles whose trajectories are nearly parallel to the microstrip plate and which thus mimic the pulse shapes of valid x-ray events. Detailed descriptions of the HEPC/LEPC detectors can be found in Budtz-Jørgensen et al. (1994).

The effective areas of the HEPC & LEPC detectors are shown in Figure 22, and detailed detector parameters are listed in Table 5. The intrinsic spatial resolution of the detectors as a function of energy is shown in Figure 23. The FWHM (or HPD for an assumed Gaussian distribution) is $\mathrel{\copy\simlessbox}$ 1 mm over most of the operating range of the detectors. In comparison, the HPD of the mirror is $\sim 3^{\prime}$ ($\sim$7 mm for an 8 m focal length telescope). The contribution of the detectors to the overall image resolution is thus negligible. The HEPC/LEPC energy resolution is $\Delta E_{FWHM} = 0.33 (E/1 {\rm keV})^{1/2}$ keV, or $\sim$13% at 6 keV. The detector time resolution depends on the selected data format (see below) and can range from 1 - 400 $\mu$sec. Absolute times will be derived post-facto, using regular spacecraft clock-UT calibrations, and are expected to be accurate to $\sim$1 msec.

Charged particle background rejection efficiency, using both pulse shape discrimination and veto information, is expected to be $\mathrel{\copy\simgreatbox}$ 99.9%. The expected particle background is $<1\times10^{-4}$ cts s^-1 cm^-2 keV^-1. X-ray-like events will also arise from Compton processes in the counter gas, produced by high energy photons transmitted through the satellite and detector structure. The combined background (excluding the diffuse X-ray background) is expected to be $\sim 5\times10^{-4}$ cts s^-1cm^-2 keV^-1 and is uncertain by a factor of $\sim$2.

 

Table 5: HEPC/LEPC Detector Parameters

	HEPC	LEPC
Field of View	60$^{\prime}$	30$^{\prime}$
Entrance Window	140 mm diameter circle	70 mm diameter circle
Window Thickness	7.5 $\mu$m polyimide, 40 nm Al	0.85 $\mu$m polyimide, 40 nm Al
Spatial Resolution	$\mathrel{\copy\simlessbox}$1 mm	$\mathrel{\copy\simlessbox}$1-3 mm
Energy Resolution	$\sim$13% @ 6 keV
Time Resolution	1 - 400 $\mu$sec, depending on data format
Energy Range	2 - 25 keV	0.2 - 8 keV, 0.5 - 16 keV
Gas Thickness	4 cm	4 cm
Gas Pressure	1 atm	0.5 atm
Gas Composition	Xe 90%, CH4 10%
Non-X-ray Bgd (2$^{\prime}$ dia. circle)	$\sim 2\times10^{-3}$ c s-1	$\sim 7\times10^{-4}$ c s-1
X-ray Bgd (2$^{\prime}$ dia. circle on-axis)	$\sim 1.8\times10^{-3}$ c s-1	$\sim 1.1\times10^{-2}$ c s-1
Storage Capacity	80 Mbytes per detector
Maximum Count Rate	$\sim$ 3000 c s-1