Measuring the Cosmic X-ray Background accurately

Nuclei population, their absorption distribution and their average spectra. However, hard X-ray instruments suffer from time-dependent backgrounds and cross-calibration issues. The uncertainty of the CXB normalization remain of the order of 20%. To obtain a more accurate measurement, the Monitor Vsego Neba (MVN) instrument was built in Russia but not yet launched to the ISS. We follow the same ideas to develop a CXB detector made of four collimated spectrometers with a rotating obturator on top. The collimators block off-axis photons below 100 keV and the obturator modulates on-axis photons allowing to separate the CXB from the instrumental background. Our spectrometers are made of 20 mm thick CeBr3 crystals on top of a SiPM array. One tube features a similar to 20 cm(2) effective area and more energy coverage than MVN, leading to a CXB count rate improved by a factor of similar to 10 and a statistical uncertainty similar to 0.5% on the CXB flux. A prototype is being built and we are seeking for a launch opportunity.