  A photon-counting silicon strip detector with two energy thresholds was
investigated for spectral X-ray imaging in a mammography system. Preliminary
studies already indicate clinical benefit of the detector, and the purpose of
the present study is optimization with respect to energy resolution. Factors
relevant for the energy response were measured, simulated, or gathered from
previous studies, and used as input parameters to a cascaded detector model.
Threshold scans over several X-ray spectra were used to calibrate threshold
levels to energy, and to validate the model. The energy resolution of the
detector assembly was assessed to range over DeltaE/E = 0.12-0.26 in the
mammography region. Electronic noise dominated the peak broadening, followed by
charge sharing between adjacent detector strips, and a channel-to-channel
threshold spread. The energy resolution may be improved substantially if these
effects are reduced to a minimum. Anti-coincidence logic mitigated double
counting from charge sharing, but erased the energy resolution of all detected
events, and optimization of the logic is desirable. Pile-up was found to be of
minor importance at typical mammography rates.
