developed detector to work reliably for more than three years. We investigate the CCD response to charged particles using α particles from $^{241}$Am. The CCD and the α source are put in a chamber filled with dry N$_{2}$ gas, with the distance between them fixed at 123 mm. The N$_{2}$ gas pressure is varied to change the energy of the α particles at the surface of the CCD. The energy to create an electron–hole pair in Si is 3 . 65 eV, and hence an energy deposit of 1 MeV, for instance, creates about 300,000 electron–hole pairs. Electrons are collected at the anode and stored in a charge capacitance. During storage, electrons spread to the adjacent pixels and make a cluster. Fig. 1 shows typical cluster shapes made by α particles of energy 1 MeV and 4 MeV. For α particles with energy lower than 2 MeV, the created electrons diffuse isotropically in the vertical and horizontal directions. For more en- ergetic particles, higher than 2 MeV, the cluster shapes become anisotropic. This is because so many electron–hole pairs are created that they overflow the full well capacity. As shown in Table 1, the vertical full well capacity is lower than the horizontal capacity. Overflowed electrons spread only in the vertical direction and make an anisotropic cluster. Considering the anisotropy of the full well capacity, the signal region is determined to be 7 pixels (horizontal) × 11 pixels (vertical) around the peak pixel, collecting all the created electrons. More than 99% of the diffused electrons are stored in this region. Fig. 2 shows the energy calibration curve of the total charge in the signal region to the energy of α particles. Linearity 5 3. Performance of a CCD sensor