Jon Headspith

Energy dispersive XAFS: characterization of electronically excited states of copper(I) complexes.

Energy dispersive X-ray absorption spectroscopy (ED-XAS), in which the whole XAS spectrum is acquired simultaneously, has been applied to reduce the real-time for acquisition of spectra of photoinduced excited states by using a germanium microstrip detector gated around one X-ray bunch of the ESRF (100 ps). Cu K-edge XAS was used to investigate the MLCT states of [Cu(dmp)2]+ (dmp =2,9-dimethyl-1,10-phenanthroline) and [Cu(dbtmp)2]+ (dbtmp =2,9-di-n-butyl-3,4,7,8-tetramethyl-1,10-phenanthroline) with the excited states created by excitation at 450 nm (10 Hz). The decay of the longer lived complex with bulky ligands, was monitored for up to 100 ns. DFT calculations of the longer lived MLCT excited state of [Cu(dbp)2]+ (dbp =2,9-di-n-butyl-1,10-phenanthroline) with the bulkier diimine ligands, indicated that the excited state behaves as a Jahn–Teller distorted Cu(II) site, with the interligand dihedral angle changing from 83 to 60° as the tetrahedral coordination geometry flattens and a reduction in the Cu–N distance of 0.03 Å.

XSPRESS — X-ray signal processing electronics for solid state detectors

Abstract With recent improvements in synchrotron sources and X-ray optics great pressures have been placed on detector systems to produce higher count rates and better resolutions. Present high performance 13 element germanium detector systems can give reasonable count rates with good resolution (∼ 10 4 –10 5 Hz per channel and ∼ 250 eV FWHM @ 55 Fe with 0.5 μs shaping time). However, these systems are restricted by limitations in both the detector and in the analogue pulse processing after the detector. With respect to the detector, increasing the number of channels without degrading the energy resolution is a great challenge due to increased crosstalk and capacitance. The analogue pulse processing electronics are significantly limited by the dead time introduced by the shaping amplifier. This dead time causes pulse pile-up at higher rates which leads to non-linearity and poor resolution. This paper describes the XSPRESS system which has been developed at Daresbury Laboratory for the new Wiggler II beamline 16. This system overcomes previous limits in both signal processing and detector fabrication to give great improvements in system performance. The signal processing electronics departs from standard analogue processing techniques and employs sophisticated adaptive digital signal processing hardware to reduce the dead time associated with each event to a minimum. This VME based technology allows us to vastly increase the count rate for each channel yet still retain the ability to gain very good resolution. The detector has been developed through a collaborative agreement with EG & G Ortec and packs an unprecedented 30 germanium crystals into an extremely small area whilst still retaining the energy resolution of smaller arrays. This system has increased throughput rate by an order of magnitude per channel and when all channels are implemented, an increase of at least two orders of magnitude for the whole array should be seen. Data has been taken using this system on the SRS at Daresbury Laboratory and these results will be given along with a detailed explanation of the operation of this system.

XSTRIP: a silicon microstrip-based X-ray detector for ultra-fast X-ray spectroscopy studies

For a number of years, an exciting and important area of synchrotron radiation science has been X-ray absorption spectroscopy fine structure studies of dynamically changing samples on the sub-second time-scales. By utilizing this technique, precise measurement of detailed structural changes can be investigated during a chemical or phase change reaction without the need for repeated experiments or expensive stopped flow techniques. Until recently, instrumentation to facilitate these studies has been based on commercially available detectors developed predominantly for other applications. Whilst these systems have yielded quality science, they have been subject to a number of fundamental limitations, particularly their speed, linearity and dynamic range. We have developed a new detector, XSTRIP, to overcome some of these. This new instrument marries dedicated silicon microstrip technology with specialist low noise, custom developed, fast readout integrated circuits, to yield an instrument that will unlock whole new areas of science to researchers. This paper will discuss some of the drawbacks of historical systems, give details of the XSTRIP system and also present the operating parameters of the system. In addition, some of the initial scientific experimental results will also be presented.

Probing local and electronic structure in Warm Dense Matter: single pulse synchrotron x-ray absorption spectroscopy on shocked Fe

Understanding Warm Dense Matter (WDM), the state of planetary interiors, is a new frontier in scientific research. There exists very little experimental data probing WDM states at the atomic level to test current models and those performed up to now are limited in quality. Here, we report a proof-of-principle experiment that makes microscopic investigations of materials under dynamic compression easily accessible to users and with data quality close to that achievable at ambient. Using a single 100 ps synchrotron x-ray pulse, we have measured, by K-edge absorption spectroscopy, ns-lived equilibrium states of WDM Fe. Structural and electronic changes in Fe are clearly observed for the first time at such extreme conditions. The amplitude of the EXAFS oscillations persists up to 500 GPa and 17000 K, suggesting an enduring local order. Moreover, a discrepancy exists with respect to theoretical calculations in the value of the energy shift of the absorption onset and so this comparison should help to refine the approximations used in models.

Microsecond time-resolved energy-dispersive EXAFS measurement and its application to film the thermolysis of (NH4)2[PtCl6]

Microsecond (μs) time-resolved extended X-ray absorption fine structure spectroscopy (EXAFS) has been developed using an energy-dispersive EXAFS (EDE) setup equipped with a silicon Quantum Detector ULTRA. The feasibility was investigated with a prototypical thermally driven redox reaction, the thermal decomposition of (NH4)2[PtCl6]. EXAFS data were collected with snapshots every 60 μs during the course of the thermolysis reaction, then averaged for 100 times along the reaction to get better signal to noise ratio which reduces the time resolution to 6 millisecond (ms). Our results provide direct structural evidence of cis-PtCl2(NH3)2 as the intermediate, together with continuous electronic and geometric structure dynamics of the reactant, intermediate and final product during the course of the thermolysis of (NH4)2[PtCl6]. The thermal effect on EXAFS signals at high temperatures is considered in the data analysis, which is essential to follow the reaction process correctly. This method could also be applied to other reaction dynamics.

Initial data from the 30-element ORTEC HPGe detector array and the XSPRESS pulse-processing electronics at the SRS, Daresbury Laboratory

Following the completion of the collaborative project between CLRC Daresbury Laboratory and EG&G ORTEC to develop the worlds first 30-element HPGe detector for fluorescence XAFS, it has now been tested and commissioned at the SRS. The system was commissioned with the XSPRESS digital pulse-processing electronics and this has demonstrated processed count rates in excess of 10 MHz. Initial data have been recorded and are presented.

Time-resolved studies of diffusion via energy dispersive X-ray absorption spectroscopy

Abstract The challenges facing the application of X-ray absorption spectroscopy to the study of electrochemically initiated processes in solution are discussed. The results from a feasibility study of the diffusion of Cu 2+ from a planar electrode are described. These show that millisecond time resolution can be achieved at a 3rd generation source, but delivery of the full potential of the experiment rests upon the availability of suitable detectors.

Development of the ProSPECTus semiconductor Compton camera for medical imaging

The ProSPECTus project is the development of a prototype semiconductor Compton camera for use in nuclear medical imaging applications. The proposed system has the potential to improve the sensitivity of conventional mechanically col-limated Single Photon Emission Computed Tomography (SPECT) systems through the use of electronic collimation techniques. In addition, the use of compatible semiconductor technology within a Magnetic Resonance Imaging (MRI) system could potentially lead to simultanous SPECT/MRI data acquisition. This paper outlines the consideration of key design features for the new system. Such design factors include the geometrical setup, suitable energy and position resolution values for the detectors and the ability of the system to function in a magnetic field. The ProSPECTus protoype imaging system will now be built according to optimised specifications.

Semiconductor detectors for Compton imaging in nuclear medicine

An investigation is underway at the University of Liverpool to assess the suitability of two position sensitive semiconductor detectors as components of a Compton camera for nuclear medical imaging. The ProSPECTus project aims to improve image quality, provide shorter data acquisition times and lower patient doses by replacing conventional Single Photon Emission Computed Tomography (SPECT) systems. These mechanically collimated systems are employed to locate a radioactive tracer that has been administered to a patient to study specifically targeted physiological processes. The ProSPECTus system will be composed of a Si(Li) detector and a High Purity Germanium (HPGe) detector, a configuration deemed optimum using a validated Geant4 simulation package. Characterising the response of the detectors to gamma irradiation is essential in maximising the sensitivity and image resolution of the system. To this end, the performance of the HPGe ProSPECTus detector and a suitable Si(Li) detector has been assessed at the University of Liverpool. The energy resolution of the detectors has been measured and a surface scan of the Si(Li) detector has been performed using a finely collimated 241Am gamma ray source. Results from the investigation will be presented.

High pressure dynamic XAS studies using an energy-dispersive spectrometer

ABSTRACT We present in this paper recent advances in the high pressure domain provided by the introduction of time-resolved energy-dispersive XAS (EDXAS) techniques at synchrotrons. We highlight technical aspects and describe two modes of acquisition: the ‘movie’ mode, where the time resolution is given by the detector acquisition speed and the ‘pump-and-probe’ mode, where the time resolution is given by the delay between the pump and the probe. These two modes define a frontier in the time resolution, respectively above and below the ∼10 μs regime. In the former, examples of applications are chemical stability and reactions at high pressure and high temperature or probing the warm dense matter regime using rapid current ramps. In the latter, an example is given on studies of dynamically compressed matter, by coupling single-bunch EDXAS at high-brilliance synchrotron to a nanosecond high-power laser.