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

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(-1); the optimized linearity in this configuration has an extended range up to ∼0.71 MHz pixel(-1); these are further correctable with a transfer function to ∼1.77 MHz pixel(-1). This new approach has wide application both in high-accuracy 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 (χ(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.

Novel energy resolving x-ray pinhole camera on Alcator C-Mod.

A new energy resolving x-ray pinhole camera has been recently installed on Alcator C-Mod. This diagnostic is capable of 1D or 2D imaging with a spatial resolution of ≈1 cm, an energy resolution of ≈1 keV in the range of 3.5-15 keV and a maximum time resolution of 5 ms. A novel use of a Pilatus 2 hybrid-pixel x-ray detector [P. Kraft et al., J. Synchrotron Rad. 16, 368 (2009)] is employed in which the lower energy threshold of individual pixels is adjusted, allowing regions of a single detector to be sensitive to different x-ray energy ranges. Development of this new detector calibration technique was done as a collaboration between PPPL and Dectris Ltd. The calibration procedure is described, and the energy resolution of the detector is characterized. Initial data from this installation on Alcator C-Mod is presented. This diagnostic provides line-integrated measurements of impurity emission which can be used to determine impurity concentrations as well as the electron energy distribution.

Improved count rate corrections for highest data quality with PILATUS detectors.

The PILATUS detector system is widely used for X-ray experiments at third-generation synchrotrons. It is based on a hybrid technology combining a pixelated silicon sensor with a CMOS readout chip. Its single-photon-counting capability ensures precise and noise-free measurements. The counting mechanism introduces a short dead-time after each hit, which becomes significant for rates above 10(6) photons s(-1) pixel(-1). The resulting loss in the number of counted photons is corrected for by applying corresponding rate correction factors. This article presents the results of a Monte Carlo simulation which computes the correction factors taking into account the detector settings as well as the time structure of the X-ray beam at the synchrotron. The results of the simulation show good agreement with experimentally determined correction factors for various detector settings at different synchrotrons. The application of accurate rate correction factors improves the X-ray data quality acquired at high photon fluxes. Furthermore, it is shown that the use of fast detector settings in combination with an optimized time structure of the X-ray beam allows for measurements up to rates of 10(7) photons s(-1) pixel(-1).

Hydranencephaly in newborn calves in Zambia.

Hydranencephaly without arthrogryposis was observed in three newborn Holstein-Friesian calves, born during one week in a 400-cow dairy herd in Zambia. The affected calves were blind and exhibited nystagmus and depressed behavior. Akabane virus infection was suspected based on the pathological findings and etiological data, however, we could not determine the exact causative agent.

Outbreaks of Salmonella Dublin infection among calves on a dairy farm applying Salmonella bacterins in Zambia.

In Zambia, a dairy farm keeping about 600 cows and self-contained calves had applied S. Dublin and S. Typhimurium bacterins to pregnant cows and calves in combination with all-in all-out pen system for rearing calves. Only relatively small scale outbreaks of S. Dublin infection occurred repeatedly in these years from 1989 to 1991 among fattening calves on the farm. The results obtained from the epizootiological study suggest that the preventive measures including the vaccination with Salmonella bacterins gave insufficient protection against S. Dublin infection to the calves, but they might have prevented large scale outbreak of the disease. This is the first report of the epizootiological study on outbreak of bovine S. Dublin infection on farm in Zambia.

Preliminary observations on ovine paratuberculosis (Johne's disease) in Zambia.

In an imported flock of sheep from South Africa, an ewe became partially anorexic and gradually losing weight and conditions. The paratuberculosis (Johne's disease) was confirmed on clinical, faecal and histopathological examination. Cultural examination remained doubtful. Serological investigation of other sheep in the flock and at other farm reacted to antibodies of Mycobacterium paratuberculosis on complement fixation test. The study suggests that the disease is actively spreading. This is the first report of ovine paratuberculosis in the Republic of Zambia.