Research for Radiation Damage of CCD Vertex Detector

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Two subjects were studied for an application of CCD's as vertex detector at nearby room temperature in future e^+e^- linear collider JLC.1) Noise hits should be suppressed to less than 10^<-5> for total number of pixels more than 10^8 for the vertex detector. As dark currents strongly depend on an operating temperature, can we obtain enough S/N ratio at the room temperature?A dependence of the dark current on accumulation time and S/N using ^<55>Fe X-ray (5.89 keV) source were measured in order to verify the applicability. Four CCS's with pixel size of 12×12, 24×24, and 48×48 μm^2, which are commercially available, were used in the study. A mode of Multi-Pinned-Phase (MPP) were adapted to reduce the dark current. We concluded that the enough S/N could be obtained with the CCD of pixel size smaller than 24×24 μm^2 and a faster driver circuit of CCDs with charge transfer speed more than 5 MHz.2) Radiation of 150 krad is expected for 10 year operation of JLC.Can CCD has sufficient radiation hardness in the environment near collision point of JLC?Negative driving voltage V^<MPP>_<cc> at which CCD is converted to MPP mode and Charge Transfer Inefficiency (CTI) were measured before and after irradiation of ^<60>Co γ-ray of about 200 Ci. After irradiation of 27 krad V^<MPP>_<cc> moved from -6.5 V to -9.8 V and CTI increased from 1.0×10^<-6> to 3.5×10^<-6>. After an additional exposure of 18 krad the operation of the CCD became difficult. This means that this type of CCDs tested now can not be used to operation of JLC for more than two years.We conclude that enough S/N can be obtained for the operation of CCDs at the room temperature. However we need more extensive R＆D to get adequate radiation hardness for the operation of JLC over ten years.