Each JEM-X detector is a microstrip gas chamber with a sensitive geometric area of 500 cm per unit. The gas inside the steel pan-shaped detector vessel is a mixture of xenon (90%) and methane (10%) at 1.5 bar pressure. The incoming photons are absorbed in the xenon gas by photo-electric absorption and the resulting ionization cloud is then amplified in an avalanche of ionizations by the strong electric field near the microstrip anodes. Significant electric charge is picked up on the strip as an electric impulse. The position of the electron avalanche in the direction perpendicular to the strip pattern is measured from the centroid of the avalanche charge. The orthogonal coordinate of an event is obtained from a set of electrodes deposited on the rear surface of the microstrip plate (MSP).
The X-ray window of the detector is composed of a thin (250 m) beryllium foil which is impermeable to the detector gas but allows a good transmission of low-energy X-rays (see dashed curve in Fig. 1). the Be window imposes an absolute lower limit of keV on the energy of X-rays coming into the detector, and hence it is meaningless to try to push the data analysis below this limit.
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A collimator structure with square-shaped cells is placed on top of the detector entrance window. It gives support to the window against the internal pressure and, at the same time, limits and defines the field of view of the detector. The collimator is important for reducing the count rate caused by the cosmic diffuse X-ray background. However, the presence of the collimator also means that sources near the edge of the field of view are attenuated with respect to on-axis sources (see Fig.3). The materials for the collimator (molybdenum, copper, aluminium) have been selected in order to minimize the detector background caused by K fluorescence. Four radioactive sources are embedded in each detector collimator in order to calibrate the energy response of the JEM-X detectors in orbit. For JEM-X1 two Fe and two Cd sources were used. For JEM-X2 all four radioactive sources are Cd. Each source illuminates a well defined spot on the microstrip plate. Cd emits 22 keV and 88 keV photons. Fe produces one unresolved doublet at 6 keV. The gain of the detector gas is monitored continuously with the help of these sources. Figure 4 shows the collimator layout and the locations of the calibration sources. There is one calibration source for each anode segment on the MSP. The 29.6 keV photons produced by Xe fluorescence can be detected all over the MSP and are used for offline monitoring of the gain correction by the software, and also to produce instrument model tables of the spatial gain (SPAG) variation across the detector plate. For the complete archive of these offline analyses see: http://outer.space.dtu.dk/users/oxborrow/sdast/GAINresults.html.
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