http://research.engr.oregonstate.edu/immiscibles/sites/research.engr.oregonstate.edu.immiscibles/files/Papers/Wildenschild_Sheppard_AWR_2012.pdf

X-ray imaging and analysis techniques for quantifying pore-scale structure and processes in subsurface porous medium systems

Photon counting detectors (PCDs) with energy discrimination capabilities have been developed for medical x-ray computed tomography (CT) and x-ray (XR) imaging. Using detection mechanisms that are completely different from the current energy integrating detectors and measuring the material information of the object to be imaged, these PCDs have the potential not only to improve the current CT and XR images, such as dose reduction, but also to open revolutionary novel applications such as molecular CT and XR imaging. The performance of PCDs is not flawless, however, and it seems extremely challenging to develop PCDs with close to ideal characteristics. In this paper, the authors offer our vision for the future of PCD-CT and PCD-XR with the review of the current status and the prediction of (1) detector technologies, (2) imaging technologies, (3) system technologies, and (4) potential clinical benefits with PCDs.

/pdf/vision-20-20-single-photon-counting-x-ray-detectors-in-2ozmq4m8yc.pdf

Vision 20/20: Single photon counting x-ray detectors in medical imaging

X-ray sources are developing rapidly and their coherent output is growing correspondingly. The increased coherent flux from modern X-ray sources is being matched with an associated development in experimental methods. This article reviews the literature describing the ideas that utilize the increased brilliance from modern X-ray sources. It explores how ideas in coherent X-ray science are leading to developments in other areas, and vice versa. The article describes measurements of coherence properties and uses this discussion as a base from which to describe partially coherent diffraction and X-ray phase-contrast imaging, with applications in materials science, engineering and medicine. Coherent diffraction imaging methods are reviewed along with associated experiments in materials science. Proposals for experiments to be performed with the new X-ray free-electron lasers are briefly discussed. The literature on X-ray photon-correlation spectroscopy is described and the features it has in common with other coherent X-ray methods are identified. Many of the ideas used in the coherent X-ray literature have their origins in the optical and electron communities and these connections are explored. A review of the areas in which ideas from coherent X-ray methods are contributing to methods for the neutron, electron and optical communities is presented.

Coherent methods in the X-ray sciences

Many organisms use amorphous calcium carbonate (ACC) during crystalline calcium carbonate biomineralization, as a means to control particle shape/size and phase stability. Here, we present an in situ small- and wide-angle X-ray scattering (SAXS/WAXS) study of the mechanisms and kinetics of ACC crystallization at rapid time scales (seconds). Combined with offline solid and solution characterization, we show that ACC crystallizes to vaterite via a three-stage process. First, hydrated and disordered ACC forms, then rapidly transforms to more ordered and dehydrated ACC; in conjunction with this, vaterite forms via a spherulitic growth mechanism. Second, when the supersaturation of the solution with respect to vaterite decreases sufficiently, the mechanism changes to ACC dissolution and vaterite crystal growth. The third stage is controlled by Ostwald ripening of the vaterite particles. Combining this information with previous studies, allowed us to develop a mechanistic understanding of the abiotic crystalliz...

http://homepages.see.leeds.ac.uk/~earlgb/Publications/Bots%20et%20al%20Vaterite%20crystallization%20CG%26D%202012.pdf

Mechanistic Insights into the Crystallization of Amorphous Calcium Carbonate (ACC)

A summary is presented of the present status of several compound semiconductor radiation detectors, including detectors fabricated from GaAs, HgI2, CdTe, Cd1−xZnxTe and PbI2

Room-temperature compound semiconductor radiation detectors

Demand for time resolved experiments, of which the time information related to the detected events is an essential requirement, is rapidly increasing in synchrotron research experiments. Timepix3 is a read-out chip for pixelated detectors that places a time stamp to every detected event with a nominal resolution of 1.5625 ns and provides a measure of the energy of the same event by the Time over Threshold technique. Timepix3 can then be a promising readout chip for time resolved experiments. A prototype of a Timepix3 readout system, based on a modified version of the MERLIN readout system, has already been developed by the detector group of Diamond Light Source (DLS). This prototype was used to run a set of characterization measurements using synchrotron radiation with the Timepix3 chip. The results of the characterization measurements proposed additional modifications to optimize the Timepix3 detector system to be used for time resolved experiments. The Timepix3 detector system needs to allow longer measurable time period for the Time of Arrival since it is limited by the small amount of bits dedicated. There is a need as well for locking the clock of the readout system to the source clock in order to avoid beats due to independent frequencies and track eventual deviations from nominal clock values. Development on the readout system, mainly on the FPGA firmware, has been carried out in order to increase the number of bits dedicated to the Time of Arrival. The possibility to synchronize the detector system using external clock was added.

Timepix3 readout system for time resolved experiments at synchrotron radiation facilities

A new avalanche silicon detector concept is introduced with a low gain in the region of 5 to 10, known as a Low Gain Avalanche Detector, LGAD. The lower gain reduces noise in comparison with a standard avalanche photodiode. The LGAD can be segmented to produce hybrid pixel detectors for low energy X-ray detection. Thin LGAD′s produce the same, or larger, signals as standard PIN diode detectors produced by minimum ionising particles (mips), however the collection time is reduced. The thickness reduction factor of an LGAD sensor is equal to the gain of the LGAD. For example; a 30µm thick sensor with a gain of 10 will give the same signal from a mip as a 300µm thick PIN diode. The LGAD has a fast rise time, useful for fast silicon timing detectors with sub-ns rise times from X-ray or mip interactions. Simulation of LGADs, using Sentaurus TCAD, of a pixelated device has been performed to determine the implant structures for the required gain and high voltage characteristics and to understand the in-pixel gain uniformity for a range of pixel sizes. Devices have been fabricated at Micron Semiconductor which produce the desired gain (measured to be approximately 10) and electrical performance with a breakdown voltage > 400V. LGAD sensors compatible with the Timepix readout system with varying pixel sizes have been fabricated. X-ray measurements have been performed on 5mm pads with a minimum detetable energy of 17.5 keV. Along with the detection of low energy photons these devices have been shown to produce a very fast response. Devices with this technology will be used in both the ATLAS and CMS timing detectors using pixels of the order 1mm x 1mm. Pixels have been fabricated in 2x2, 3x3 and 5x5 arrays. The measurements of gain and gain uniformity are presented.

A novel detector for low-energy photon detection with fast response

Compact and portable X-ray imager system using Medipix3RX

A simulation toolkit developed for use at Diamond Light Source is presented, accompanied with experimental validation using a silicon pixel-array sensor coupled to a Medipix3RX chip controlled via the Merlin Readout System. The simulation makes use of Geant4, where photons are tracked in order to determine their position of interaction and energy deposition. Further to this, a Finite Element Methods package, Comsol, is used to model the Charge Induction Efficiencies of various sensors. Results are coupled to Geant4 simulations to provide an accurate method for computing the signals generated on each pixel within the sensor and then an algorithm to model the front-end electronics of the device. The validity of the simulation toolkit is tested by investigating charge-sharing effects using a Medipix3RX chip bump-bonded to a silicon pixel-array sensor. The dependence of the imaging parameters, on the energy threshold, is presented from both simulation and experiment for the Medipix3RX chip operated with and without the charge sharing compensation circuitry enabled. This simulation toolkit can be used to calculate image quality parameters for the next generation of detectors, including CdTe, as well as to improve data corrections on existing detectors on synchrotron beamlines.

Comparisons between simulation and measurements taken with the Medipix3RX detector

PERCIVAL is a detector system specifically designed for the soft X-ray regime. Although still in a development phase, it has already served its first user experiments at both a storage ring and also a free-electron laser. The device performed remarkably well in all the different techniques tested: ptychography, holography and also X-ray photon correlation spectroscopy. The results of these tests are presented.

Nicola Tartoni

Papers

Timepix3 readout system for time resolved experiments at synchrotron radiation facilities

A novel detector for low-energy photon detection with fast response

Compact and portable X-ray imager system using Medipix3RX

Comparisons between simulation and measurements taken with the Medipix3RX detector

The PERCIVAL detector: first user experiments