We studied how the configuration parameters of a CCD (pixel size and depletion layer thickness) affect the instrumental background of an X-ray CCD camera in the space environment through the Monte-Carlo simulation. X-ray detectors are in general sensitive not only to X-rays but also to charged particles. The latter produce pseudo-signal indistinguishable from that of X-rays, which is called instrumental background. It is essential to reduce the instrumental background for the observations of dim and diffuse X-ray sources, but the low background was not considered as a design goal of an X-ray CCD camera so far. We utilized the Monte-Carlo simulator, which could successfully reproduce the Suzaku XIS background, for the current analysis. We found that thicker depletion layer tends to increase the background except for the >5 keV band of the backside-illuminated CCD. On the other hand, pixel-size dependence was different between the frontside and backside illuminated CCDs. These results are interpreted in terms of the interaction of cosmic/X-rays with the CCD.
We report on the origin of the instrumental background of the X-ray CCD camera in space obtained from the Monte Carlo simulation with GEANT4. In the space environment, CCD detects many non-X-ray events, which are produced by the interactions of high-energy particles with the materials surrounding CCD. Most of these events are rejected through the analysis of the charge split pattern, but some are remained to be background. Such instrumental background need to be reduced to achieve higher sensitivity especially above several keV. We simulated the interactions of the cosmic-rays with the CCD housing, and extracted the background events which escaped from the screening process by the charge split pattern. We could reproduce the observed spectral shape of the instrumental background of Suzaku XIS on orbit with the Monte Carlo simulation. This means
that the simulation succeeded to duplicate the background production process in space. From the simulation, we found that the major components of the background in the front-side illuminated CCD are the recoil electrons produced by the Compton-scattering of the hard X-ray photons in the CCD. On the other hand, for the backside illuminated CCD, contribution from the low energy electrons becomes dominant, which are produced by the interactions of cosmic-ray protons or hard X-rays with the housing. These results may be important to design the X-ray CCD camera for the future missions, such as NeXT.
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