X-ray CCDs are widely used as the focal plane detectors of the X-ray telescopes. Among them, backside illuminated CCDs with a deep depletion layer are preferred because of their high quantum efficiency in both soft and hard X-ray bands. However, they tend to have poorer energy resolution and higher background due to the relatively large charge diffusion. We carried out simple experiments to apply a magnetic field of 0.25 T or 0.4 T to the CCD, which is expected to suppress the charge diffusion very slightly and to bring subtle improvement in the performance of the CCD. We found unexpectedly that grade branching ratios of Grade 3 and Grade 4, both are horizontal split events, symmetrically changed depending on the direction of the applied magnetic field. Although the cause of the change is not understand yet, it clearly demonstrate that the charge cloud in the CCD is affected by the externally applied magnetic field. We also found a decrease of Grade 7 only in the experiment 2. We consider this may be caused by the supress of the charge diffusion by the magnetic field, although other possibilities can not be excluded. No significant improvement was detected in the energy resolution. We could show with these experiments that the charge cloud in the CCD may be controlled by the externally applied magnetic field. Magnetic field may become useful tool in future to improve the performance of CCDs.

Date Published: 24 July 2014
PDF: 9 pages
Proc. SPIE 9144, Space Telescopes and Instrumentation 2014: Ultraviolet to Gamma Ray, 91443W (24 July 2014); doi: 10.1117/12.2055705

Published in SPIE Proceedings Vol. 9144:
Space Telescopes and Instrumentation 2014: Ultraviolet to Gamma Ray
Tadayuki Takahashi; Jan-Willem A. den Herder; Mark Bautz, Editor(s)