In-flight calibration system for the INTEGRAL x-ray monitor JEM-X

JEM-X is the x-ray monitor serving the two gamma-ray experiments imager and spectrometer onboard the ESA's INTEGRAL satellite. Due to the intrinsic weakness of the celestial sources in the gamma energy range they will need very long integration times. During these long pointings JEM-X will be able to detect very small variations on most x-ray sources, but only if accurately calibrated. The in- flight calibration system of the JEM-X experiment is devoted to measure the response of the detection chain (detector plus electronics) in a small set of positions and energies. The data from this system, together with on ground calibrations and simulations, must provide the capability to deconvolve pulse height spectra of celestial sources. The baseline for the in-flight calibration system foresees a set of four radioactive sources, maybe by Fe 55 and Cd 109 nuclides, and a pair of Amptek Cool-X15 X-ray generators. The latter is a novel product, based on a pyroelectric crystal used to generate energetic electrons that produce fluorescence lines by hitting a metallic target. We plan to use the four low intensity radioactive sources for monitoring the four independent anode chains, and the two x- ray tubes, one with a copper and the other with a molybdenum target, shared on the two twin detectors, for a flat illumination of the whole detectors area.