Research for Radiation Damage of CCD Vertex Detector

研究了CCD在近室温条件下作为顶点检测器在未来的e^+e^-线性对撞机jlc中的应用。1)顶点检测器的总像素数大于10^8时，噪声命中应抑制到小于10^<-5>。由于暗电流对工作温度有很大的依赖性，我们能否在室温下获得足够的信噪比？利用^<55>Fe x射线（5.89 keV）源测量了暗电流与积累时间和信噪比的关系，以验证该方法的适用性。研究中使用了4种商品化的CCS，其像素尺寸分别为12×12、24×24和48×48 μm^2。采用多管相（MPP）模式来减小暗电流。研究结果表明，采用像素尺寸小于24×24 μm^2的CCD和电荷传输速度大于5 MHz的CCD驱动电路，可以获得足够的S/N。2) JLC运行10年预计辐射量为150克拉。CCD在JLC碰撞点附近的环境中是否有足够的辐射硬度？测量了200 Ci Co γ射线^<60>辐照前后CCD转换为MPP模式的负驱动电压V^<MPP>_<cc>和电荷转移效率（CTI）。27 krad辐照后，V^<MPP>_<cc>从-6.5 V增加到-9.8 V， CTI从1.0×10^<-6>增加到3.5×10^<-6>。在额外曝光18克拉后，CCD的操作变得困难。这意味着现在测试的这种类型的ccd不能用于JLC的操作超过两年。我们得出结论，在室温下可以获得足够的信噪比。然而，为了使JLC在十多年的运行中获得足够的辐射硬度，还需要更广泛的研发。

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.