Simulation of a Compton-based detector for low-dose high-resolution time-of-flight positron emission tomography.

Two major challenges in time-of-flight positron emission tomography (TOF-PET) are low spatial resolution and high radioactive dose to the patient, both of which result from limitations in detection technology rather than fundamental physics. A new type of TOF-PET detector employing low-atomic number (low-Z) scintillation media and large-area, high-resolution photodetectors to record Compton scattering locations in the detector has been proposed as a promising alternative, but the minimum technical requirements for such a system have not yet been established. Here we present a simulation study evaluating the potential of a proposed low-Z detection medium, linear alkylbenzene (LAB) doped with a switchable molecular recorder, for next-generation TOF-PET detection. We developed a custom Monte Carlo simulation of full-body TOF-PET using the TOPAS Geant4 software package. By quantifying contributions and tradeoffs for energy, spatial, and timing resolution of the detector, we show that at reasonable combination of specifications, our likelihood-based identification of pairs of first interaction locations in the simulated detector identifies 87.1% of pairs with zero or negligible error, and correctly rejects 90% of all in-patient scatters. The same specifications give TOF-PET sensitivity of ~66.7% and PSF width 4.6 mm with clear contrast. A detector with these specifications provides a clear image of a brain phantom simulated at less than 1% of a standard radiotracer dose.

Ultrafast X-ray absorption study of longitudinal-transverse phonon coupling in electrolyte aqueous solution.

Ultrafast X-ray absorption spectroscopy is applied to study the conversion of longitudinal to transverse phonons in aqueous solution. Permanganate solutes serve as X-ray probe molecules that permit the measurement of the conversion of 13.5 GHz, longitudinal phonons to 27 GHz, transverse phonons that propagate with high-frequency sound speed. The experimental results, combined with QM/MM MD simulations, show that the hydrogen bond network around the charged solutes has a glass-like stiffness that persists for at least tens of picoseconds.

Towards a microchannel-based X-ray detector with two-dimensional spatial and time resolution and high dynamic range.

X-ray detectors that combine two-dimensional spatial resolution with a high time resolution are needed in numerous applications of synchrotron radiation. Most detectors with this combination of capabilities are based on semiconductor technology and are therefore limited in size. Furthermore, the time resolution is often realised through rapid time-gating of the acquisition, followed by a slower readout. Here, a detector technology is realised based on relatively inexpensive microchannel plates that uses GHz waveform sampling for a millimeter-scale spatial resolution and better than 100 ps time resolution. The technology is capable of continuous streaming of time- and location-tagged events at rates greater than 10(7) events per cm(2). Time-gating can be used for improved dynamic range.

Picosecond-resolved X-ray absorption spectroscopy at low signal contrast using a hard X-ray streak camera.

A picosecond-resolving hard-X-ray streak camera has been in operation for several years at Sector 7 of the Advanced Photon Source (APS). Several upgrades have been implemented over the past few years to optimize integration into the beamline, reduce the timing jitter, and improve the signal-to-noise ratio. These include the development of X-ray optics for focusing the X-rays into the sample and the entrance slit of the streak camera, and measures to minimize the amount of laser light needed to generate the deflection-voltage ramp. For the latter, the photoconductive switch generating the deflection ramp was replaced with microwave power electronics. With these, the streak camera operates routinely at 88 MHz repetition rate, thus making it compatible with all of the APS fill patterns including use of all the X-rays in the 324-bunch mode. Sample data are shown to demonstrate the performance.

X-ray focusing scheme with continuously variable lens.

A novel hybrid X-ray focusing scheme was developed for operation of the X-ray streak camera at the Advanced Photon Source: an X-ray lens focuses vertically from a long distance of 16 m and produces an extended focus that has a small footprint on an inexpensive sagittal mirror. A patented method is used to continuously adjust the focal length of the lens and compensate for chromatic dispersion in energy scans.

Field-dependent domain distortion and interlayer polarization distribution in PbTiO3/SrTiO3 superlattices.

The remnant polarization of weakly coupled ferroelectric-dielectric superlattices is distributed unequally between the component layers, and as a result the components respond differently to applied electric fields. The difference is apparent in both the nanometer-scale structure of striped polarization domains and in the development of piezoelectric strain and field-induced polarization. Both effects are probed with in situ time-resolved synchrotron x-ray diffraction in a PbTiO(3)/SrTiO(3) superlattice in fields up to 2.38 MV/cm. Domains are initially distorted to increase the polarization in the SrTiO(3) layer while retaining the striped motif. The subsequent transformation to a uniform polarization state at a later time leads to piezoelectric expansion dominated by the field-induced polarization of the SrTiO(3) layers. The results are consistent with theoretical predictions of the field dependence of the domain structure and electrical polarization.

An active-optic x-ray fluorescence analyzer with high energy resolution, large solid angle coverage, and a large tuning range.

A crystal-optic x-ray fluorescence energy analyzer has been designed and tested, which combines the features of electron-volt energy resolution, large solid angle coverage, and tunability over several kilo-electron-volts. The design is based upon the principle of active optics, with ten actuators available to optimally adjust the shape of a silicon crystal used in the Bragg geometry. In most applications the shape is that of a logarithmic spiral for high energy resolution with a spatially nonresolving detector, but a wide range of other shapes is also possible for applications such as imaging or single-shot spectroscopy in a spectral range of the operator's choosing.

Time-dependent Takagi-Taupin eikonal theory of X-ray diffraction in rapidly changing crystal structures.

The Takagi-Taupin theory is extended by synthesizing it with the eikonal theory in a unified space-time approach based upon microscopic electromagnetism. The principal goal is the description of X-ray diffraction in a crystal undergoing subpicosecond and few-femtosecond changes.