Development of the X-ray CCD in Japan

We review the progress of X-ray charge-coupled device (CCD) development and related issues in Japan. They are summarized into the following topics: (1) We demonstrated, for the first time, that a CCD could detect X-ray polarization, by studying the event profiles generated from individual X-ray photons. This is based on the fact that a photoelectron is preferentially injected in the direction of X-ray polarization. (2) Before developing a CCD with a thick depletion layer, we modified a CCD for optical use by removing the absorption layer. This made it possible to detect emission lines from Ge-L (53 eV) in detail. (3) A mesh experiment was introduced to measure the quantum efficiency (QE) in detail within a pixel. We could directly measure the thickness of the gates and their overlaps with each other. This experimentally revealed how the X-ray event grades were produced within a pixel. (4) As an application of the mesh experiment, we could also measure the shape of the charge cloud generated by an X-ray photon. Once we know the charge-cloud shape, we can precisely determine the point of interaction of the X-ray photon within a pixel. This makes the position resolution of the CCD much better than the pixel size. (5) So far, we have developed X-ray astronomy satellites, ASCA, ASTRO-E2 and MAXI, equipped with CCD cameras. CCD cameras on ASCA and ASTRO-E2 were developed through collaborative work with MIT. A future Japanese satellite, NeXT, will also be equipped with a CCD camera. We will design the CCD such that it can function simultaneously with a high-energy detector in order to cover a wide energy range.