Galen C. O'Neil

High-resolution X-ray emission spectroscopy with transition-edge sensors: present performance and future potential

X-ray emission spectroscopy (XES) is a powerful element-selective tool to analyze the oxidation states of atoms in complex compounds, determine their electronic configuration, and identify unknown compounds in challenging environments. Until now the low efficiency of wavelength-dispersive X-ray spectrometer technology has limited the use of XES, especially in combination with weaker laboratory X-ray sources. More efficient energy-dispersive detectors have either insufficient energy resolution because of the statistical limits described by Fano or too low counting rates to be of practical use. This paper updates an approach to high-resolution X-ray emission spectroscopy that uses a microcalorimeter detector array of superconducting transition-edge sensors (TESs). TES arrays are discussed and compared with conventional methods, and shown under which circumstances they are superior. It is also shown that a TES array can be integrated into a table-top time-resolved X-ray source and a soft X-ray synchrotron beamline to perform emission spectroscopy with good chemical sensitivity over a very wide range of energies.

Laser plasma x-ray source for ultrafast time-resolved x-ray absorption spectroscopy.

We describe a laser-driven x-ray plasma source designed for ultrafast x-ray absorption spectroscopy. The source is comprised of a 1 kHz, 20 W, femtosecond pulsed infrared laser and a water target. We present the x-ray spectra as a function of laser energy and pulse duration. Additionally, we investigate the plasma temperature and photon flux as we vary the laser energy. We obtain a 75 μm FWHM x-ray spot size, containing ∼106 photons/s, by focusing the produced x-rays with a polycapillary optic. Since the acquisition of x-ray absorption spectra requires the averaging of measurements from >107 laser pulses, we also present data on the source stability, including single pulse measurements of the x-ray yield and the x-ray spectral shape. In single pulse measurements, the x-ray flux has a measured standard deviation of 8%, where the laser pointing is the main cause of variability. Further, we show that the variability in x-ray spectral shape from single pulses is low, thus justifying the combining of x-rays obtained from different laser pulses into a single spectrum. Finally, we show a static x-ray absorption spectrum of a ferrioxalate solution as detected by a microcalorimeter array. Altogether, our results demonstrate that this water-jet based plasma source is a suitable candidate for laboratory-based time-resolved x-ray absorption spectroscopy experiments.

Code-division-multiplexed readout of large arrays of TES microcalorimeters

Code-division multiplexing (CDM) offers a path to reading out large arrays of transition edge sensor (TES) X-ray microcalorimeters with excellent energy and timing resolution. We demonstrate the readout of X-ray TESs with a 32-channel flux-summed code-division multiplexing circuit based on superconducting quantum interference device (SQUID) amplifiers. The best detector has energy resolution of 2.28 ± 0.12 eV FWHM at 5.9 keV and the array has mean energy resolution of 2.77 ± 0.02 eV over 30 working sensors. The readout channels are sampled sequentially at 160 ns/row, for an effective sampling rate of 5.12 μs/channel. The SQUID amplifiers have a measured flux noise of 0.17 μΦ0/√Hz (non-multiplexed, referred to the first stage SQUID). The multiplexed noise level and signal slew rate are sufficient to allow readout of more than 40 pixels per column, making CDM compatible with requirements outlined for future space missions. Additionally, because the modulated data from the 32 SQUID readout channels provide i...

Insensitivity of sub-Kelvin electron-phonon coupling to substrate properties.

We have examined the role of the substrate on electron-phonon coupling in normal-metal films of Mn-doped Al at temperatures below 1 K. Normal metal-insulator-superconductor junctions were used to measure the electron temperature in the films as a function of Joule heating power and phonon temperature. Theory suggests that the distribution of phonons available for interaction with electrons in metal films may depend on the acoustic properties of the substrate, namely, that the electron-phonon coupling constant Σ would be larger on the substrate with smaller sound speed. In contrast, our results indicate that within experimental error (typically ±10%), Σ is unchanged among the two acoustically distinct substrates used in our investigation.

Beamline Test of a Transition-Edge-Sensor Spectrometer in Preparation for Kaonic-Atom Measurements

We are developing a new technique to apply transition-edge sensors (TESs) to X-ray spectroscopy of exotic atoms, especially of kaonic atoms. To demonstrate the feasibility of this pioneering project, performance of a TES-based X-ray detector was evaluated in pion- and kaon-beam environments at particle accelerators. We successfully observed X-rays from pionic-carbon atoms with a resolution as good as 7 eV FWHM at 6 keV. Also at a kaon beamline, we confirmed that the TES spectrometer will be able to achieve our resolution goal, 6 eV, in our first scientific campaign to measure X-rays from kaonic-helium atoms.

Beating Darwin-Bragg losses in lab-based ultrafast x-ray experiments

The use of low temperature thermal detectors for avoiding Darwin-Bragg losses in lab-based ultrafast experiments has begun. An outline of the background of this new development is offered, showing the relevant history and initiative taken by this work.

Table-top ultrafast x-ray spectroscopy using a laser plasma source and superconducting microcalorimeters

We present x-ray absorption and emission measurements of Fe-based compounds using ionizing radiation generated by a femtosecond pulsed laser source in combination with superconducting microcalorimeters.

Ultrafast X-ray Absorption Spectroscopy using Superconducting Microcalorimeter Sensors

We are developing a tabletop apparatus for time-resolved X-ray absorption spectroscopy. Short X-ray pulses are generated by focusing a pulsed 800 nm laser onto a water jet target. X-rays that pass through a sample of interest are detected using energy-resolving superconducting microcalorimeter detectors. We have successfully observed EXAFS features from iron tris(bipyridine) in a water solution. We discuss details of the microcalorimeter spectra and describe the steps that remain before the intrinsic sub-picosecond sensitivity of the apparatus can be realized.