Success and failure of dead-time models as applied to hybrid pixel detectors in high-flux applications

Abstract

The performance of a single-photon-counting hybrid pixel detector has been investigated at the Australian Synchrotron. Results are compared with the body of accepted analytical models previously validated with other detectors. Detector functionals are valuable for empirical calibration. It is shown that the matching of the detector dead-time with the temporal synchrotron source structure leads to substantial improvements in count rate and linearity of response. Standard implementations are linear up to $ 0.36 MHz pixel À1; the optimized linearity in this configuration has an extended range up to $ 0.71 MHz pixel À1; these are further correctable with a transfer function to $ 1.77 MHz pixel À1 . This new approach has wide application both in highaccuracy fundamental experiments and in standard crystallographic X-ray fluorescence and other X-ray measurements. The explicit use of data variance (rather than N 1/2 noise) and direct measures of goodness-of-fit ( r 2 ) are introduced, raising issues not encountered in previous literature for any detector, and suggesting that these inadequacies of models may apply to most detector types. Specifically, parametrization of models with non-physical values can lead to remarkable agreement for a range of count-rate, pulse-frequency and temporal structure. However, especially when the dead-time is near resonant with the temporal structure, limitations of these classical models become apparent. Further, a lack of agreement at extreme count rates was evident.

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The R‐factor gap in macromolecular crystallography: an untapped potential for insights on accurate structures

“…For example, correct estimation of the photoncounting error requires that the data be placed on an absolute scale because this is the only scale on which the error in the count is the square root of the intensity. For counting devices such as multi-wire or pixel array detectors, the pile-up correction and its appropriateness to the time structure of the incoming signal is also a source of error [24], but as we are only concerned here with a CCD detector, this source of error was not implemented. Sources of error such as shutter jitter, beam flicker and irregular spot shape are proportional to the signal, and therefore independent of scale, while errors such as CCD readout noise and dark current are completely independent of the intensity, and therefore must also be put on an absolute scale before they can be meaningfully combined with other errors.…”

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