Development of n-type CCDs for NeXT, the next Japanese x-ray astronomical satellite mission

We have developed X-ray charge-coupled devices (CCD) for the next Japanese X-ray astronomical satellite mission, NeXT (Non-thermal energy eXploration Telescope). The hard X-ray telescope(HXT) onboard the NeXT can focus X-rays above 10 keV. Therefore, we need to develop an X-ray CCD for a focal plane detector to cover the 0.3-25 keV band in order to satisfy the capability of the telescope. We newly developed an n-type CCD fabricated on an n-type silicon wafer to expand the X-ray energy range as a focal plane detector of the HXT. It is possible to have a thick depletion layer of approx. 300μm with an n-type CCD because it is easy to obtain high resistivity with an n-type silicon wafer compared to a p-type silicon wafer. We developed prototypes of n-type CCDs and evaluated their X-ray performance, energy resolution, charge transfer inefficiency(CTI) and the thickness of the depletion layer of two devices, designated Pch15 and Pch-teg. We measured the thickness of the depletion layer of Pch15 to be 290±33μm. For Pch-teg, the energy resolution was 152±3eV full width at half maximum (FWHM) at 5.9 keV and the readout noise was 7.3 e-. The performance of the n-type CCDs was comparable to that of p-type CCDs, and their depletion layer were much thicker than those of p-type CCDs.