Application of transition-edge sensors for micro-X-ray fluorescence measurements and micro-X-ray absorption near edge structure spectroscopy: a case study of uranium speciation in biotite obtained from a uranium mine.

In this study, we successfully applied a transition-edge sensor (TES) spectrometer as a detector for microbeam X-ray measurements from a synchrotron X-ray light source in the hard X-ray region to determine uranium (U) distribution at the micro-scale and its chemical species in biotite obtained from a U mine. It is difficult to separate the fluorescent X-ray of the U Lα1 line at 13.615 keV from that of the Rb Kα line at 13.395 keV in the X-ray fluorescence spectrum with an energy resolution of approximately 220 eV using a conventional silicon drift detector (SDD). Meanwhile, the fluorescent X-rays of U Lα1 and Rb Kα were fully separated by a TES with 50 eV energy resolution at an energy of around 13 keV. The successful peak separation by the TES led to an accurate mapping analysis of trace U in micro-X-ray fluorescence measurements and a decrease in the signal-to-background ratio in micro-X-ray absorption near edge structure spectroscopy. Thus, it could be a powerful tool for studying the U distribution and speciation in various environmental samples.

Understanding the Phase of Responsivity and Noise Sources in Frequency-Domain Multiplexed Readout of Transition Edge Sensor Bolometers.

Cosmic microwave background (CMB) experiments have deployed focal planes with O ( 10 4 ) transition edge sensor (TES) bolometers cooled to sub-Kelvin temperatures by multiplexing the readout of many TES channels onto a single pair of wires. Digital Frequency-domain Multiplexing (DfMux) is a multiplexing technique used in many CMB polarization experiments, such as the Simons Array, SPT-3 G, and EBEX. The DfMux system studied here uses LC filters with resonant frequencies ranging from 1.5 to 4.5 MHz connected to an array of TESs. Each detector has an amplitude-modulated carrier tone at the resonant frequency of its accompanying LC resonator. The signal is recovered via quadrature demodulation where the in-phase (I) component of the demodulated current is in phase with the complex admittance of the circuit and the quadrature (Q) component is orthogonal to I. Observed excess current noise in the Q component is consistent with fluctuations in the resonant frequency. This noise has been shown to be non-orthogonal to the phase of the detector's responsivity. We present a detailed analysis of the phase of responsivity of the TES and noise sources in our DfMux readout system. Further, we investigate how modifications to the TES operating resistance and bias frequency can affect the phase of noise relative to the phase of the detector responsivity, using data from Simons Array to evaluate our predictions. We find that both the phase of responsivity and phase of noise are functions of the two tuning parameters, which can be purposefully selected to maximize signal-to-noise (SNR) ratio.

Controlling 229Th isomeric state population in a VUV transparent crystal.

The radioisotope thorium-229 (229Th) is renowned for its extraordinarily low-energy, long-lived nuclear first-excited state. This isomeric state can be excited by vacuum ultraviolet (VUV) lasers and 229Th has been proposed as a reference transition for ultra-precise nuclear clocks. To assess the feasibility and performance of the nuclear clock concept, time-controlled excitation and depopulation of the 229Th isomer are imperative. Here we report the population of the 229Th isomeric state through resonant X-ray pumping and detection of the radiative decay in a VUV transparent 229Th-doped CaF2 crystal. The decay half-life is measured to 447(25) s, with a transition wavelength of 148.18(42) nm and a radiative decay fraction consistent with unity. Furthermore, we report a new "X-ray quenching" effect which allows to de-populate the isomer on demand and effectively reduce the half-life. Such controlled quenching can be used to significantly speed up the interrogation cycle in future nuclear clock schemes.

High-sensitive XANES analysis at Ce L2-edge for Ce in bauxites using transition-edge sensors: Implications for Ti-rich geological samples.

Accurate determination of cerium (Ce) valence state is important for interpreting the Ce anomaly in geological archives for (paleo)redox reconstruction. However, the routine application of Ce L3-edge X-ray absorption near-edge structure (XANES) spectroscopy for detecting trace Ce in geological samples can often be restricted by coexisting titanium (Ti) due to the proximity of their fluorescence emission lines. Therefore, the signal-to-noise ratio of Ce L3-edge XANES spectra may not be sufficiently high for high-quality spectroscopic analysis. This study introduces a semi-quantitative approach appropriate for Ti-rich, Ce-dilute geological materials by synchrotron-based X-ray measurement at the Ce L2-edge. First, the results confirm that Ce L2-edge XANES spectra are able to avoid overlapping Ti Kß emissions and provide more reliable information on the Ce valence state in Ti-rich materials relative to L3-edge XANES. Moreover, the application of transition-edge sensor (TES) could reach the higher sensitivity with better energy resolution than conventional silicon drift detector (SDD) to detect fluorescence X-ray (Ce Lß1). The investigation on bauxites developed from the Columbia River Basalts shows that combining Ce L2-edge XANES and TES allows for resolving weak Ce fluorescence lines at the L2-edge from Ti-rich, Ce-dilute samples (Ti/Ce mass ratio up to ∼6000, tens of ppm Ce). The outcome emphasizes the practical possibility of investigating Ce redox state in Ti-rich geological samples.