Kavli Affiliate: Marshall W. Bautz
| First 5 Authors: Eric D. Miller, Gregory Y. Prigozhin, Beverly J. LaMarr, Marshall W. Bautz, Richard F. Foster
| Summary:
To take advantage of high-resolution optics sensitive to a broad energy
range, future X-ray imaging instruments will require thick detectors with small
pixels. This pixel aspect ratio affects spectral response in the soft X-ray
band, vital for many science goals, as charge produced by the photon
interaction near the entrance window diffuses across multiple pixels by the
time it is collected, and is potentially lost below the imposed noise
threshold. In an effort to understand these subtle but significant effects and
inform the design and requirements of future detectors, we present simulations
of charge diffusion using a variety of detector characteristics and operational
settings, assessing spectral response at a range of X-ray energies. We validate
the simulations by comparing the performance to that of real CCD detectors
tested in the lab and deployed in space, spanning a range of thickness, pixel
size, and other characteristics. The simulations show that while larger pixels,
higher bias voltage, and optimal backside passivation improve performance,
reducing the readout noise has a dominant effect in all cases. We finally show
how high-pixel-aspect-ratio devices present challenges for measuring the
backside passivation performance due to the magnitude of other processes that
degrade spectral response, and present a method for utilizing the simulations
to qualitatively assess this performance. Since compelling science requirements
often compete technically with each other (high spatial resolution, soft X-ray
response, hard X-ray response), these results can be used to find the proper
balance for a future high-spatial-resolution X-ray instrument.
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