Exploring macroscopic surface structures on X-ray CCDs using silicon absorption edge quantum efficiency measurements

Abstract

We have begun an experiment to systematically record the quantum efficiency variations of a silicon CCD across the K edges of it's constituent materials using the Synchrotron Radiation Source (SRS) at the Daresbury Laboratory. The ultimate goal of the program is to provide a detailed calibration database of instrumental X-ray Absorption Fine structure (XAFS) in CCDs intended for X-ray astronomical space missions. Because X-ray CCDs are single photon counting devices, the maximum flux that can be accommodated while still retaining full spectral resolution is typically a few hundred events cm−2s−1, a requirement clearly incompatible with normal synchrotron usage. Thus, in order to carry out our experiments, the SRS had to be operated in a new low current mode. We describe how the experiment was carried out and present a preliminary analysis of data obtained on March 9, 1994. The data show considerable structure in the quantum efficiency above the Si K edge at 1839 eV and, in fact, the response of a silicon CCD in this region is dominated by the electrode and passivation structures.