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: http://xmm.vilspa.esa.es/user/socpub/ccd_review/node10.html
Дата изменения: Fri Aug 30 15:54:43 1996 Дата индексирования: Tue Oct 2 12:13:31 2012 Кодировка: |
The CCD acts as a storage medium, and with frame times of the order seconds, there is no obvious method of employing prompt co-incidence rejection of charged particle events. Instead, recognition of a peculiar event signature within the CCD itself must be relied upon for rejecting unwanted data.
In silicon, minimally ionizing particles deposit approximately 80 electrons per
micron along their track. In conventional CCDs, a 
m track length
results in an equivalent energy deposition of 5 - 6 keV, and significant
confusion with X-ray events arises. The enhancement of depletion depths
described above has the added benefit of increasing track lengths sufficiently
to place the mean energy deposition significantly outside the energy band of
the X-ray optics.
In addition to an energy veto mechanism, enhanced rejection can be obtained by recognizing that charged particle events tend to be split between pixels, particularly if the ratio of pixel dimension:detection depth is small. Implicit in this approach is the fact that CCDs which are efficient at rejecting particle events split between pixels, will unfortunately have a high proportion of split X-ray events. A trade-off between rejection efficiency and spectroscopic efficiency is required.
Experiments with high-resistivity MIT Lincoln Laboratory CCDs give an indication of the potential efficiency of background rejection algorithms based on the distribution of charge produced by a minimally ionizing particle. In these experiments the CCD was exposed to 1.2 and 1.3 MeV gamma rays from a Co-60 source. Energetic electrons excited in Compton scattering events produced ionization tracks in the CCD. The X-ray event selection criteria described in section 3.1 also reject the gamma-induced events with an efficiency of better than 99%. The precise rejection efficiency depends on the details of the processing algorithm. The adjacent pixels provide some measure of anti-coincidence rejection, but the most efficienct rejection is provided by the signature of charge blooming from the undepleted regions beneath the pixels. An incompletely depleted CCD offers 5-sided anti-coincidence.
This feature is eventually limited if the frame integration time and particle event size combine to spread low levels of charge into so many pixels that the recognition of valid X-ray events is hampered by the overlap of events.