We introduce a new technique for determining x-ray fluorescence line energies and widths, and we present measurements made with this technique of 22 x-ray L lines from lanthanide-series elements. The technique uses arrays of transition-edge sensors, microcalorimeters with high energy-resolving power that simultaneously observe both calibrated x-ray standards and the x-ray emission lines under study. The uncertainty in absolute line energies is generally less than 0.4 eV in the energy range of 4.5 keV to 7.5 keV. Of the seventeen line energies of neodymium, samarium, and holmium, thirteen are found to be consistent with the available x-ray reference data measured after 1990; only two of the four lines for which reference data predate 1980, however, are consistent with our results. Five lines of terbium are measured with uncertainties that improve on those of existing data by factors of two or more. These results eliminate a significant discrepancy between measured and calculated x-ray line energies for the terbium L l line (5.551 keV). The line widths are also measured, with uncertainties of 0.6 eV or less on the full-width at half-maximum in most cases. These measurements were made with an array of approximately one hundred superconducting x-ray microcalorimeters, each sensitive to an energy band from 1 keV to 8 keV. No energy-dispersive spectrometer has previously been used for absolute-energy estimation at this level of accuracy. Future spectrometers, with superior linearity and energy resolution, will allow us to improve on these results and expand the measurements to more elements and a wider range of line energies.

Future X-ray astrophysics experiments require multiplexed readout of high fill-factor, kilo-pixel arrays of transition-edge sensors (TESs), with very high spectral resolution over a broad range of energies. In this paper we report on a prototype kilo-pixel array of Mo/Au TESs readout with 8-column by 32-row time-division multiplexing (TDM). This system is being used to demonstrate the critical detector and readout technology for ESA's Athena X-IFU, and when complete will be used in laboratory astrophysics experiments. Our array and TDM readout have demonstrated a combined full-width-at-half-maximum energy resolution, including > 200 pixels, of: 1.95 eV for Ti-Kα (4.5 keV), 1.97 eV for Mn-Kα (5.9 keV), 2.16 eV for Co-Kα (6.9 keV), 2.33 eV for Cu-Kα (8 keV), 3.26 eV for Br-Kα (11.9 keV). The 1 sigma statistical errors are ≤0.01 eV for all spectra. These results meet the broad-band resolution requirements for X-IFU with margin.

The Lynx x-ray microcalorimeter (LXM) is an imaging spectrometer proposed as part of the Lynx satellite mission concept currently under study to be a National Aeronautics and Space Administration (NASA) flagship mission.¹ Lynx will enable unique astrophysical observations into the x-ray universe due to its high angular resolution and large field of view. Because Lynx will improve sensitivity by two orders of magnitude compared to Chandra,² it will be able to penetrate into the epoch of reionization to detect x-rays from black holes lighting up the first galaxies and map the baryons in the Cosmic Web and galactic halos.³ Since the majority of the baryonic matter in the universe is visible in either x-ray emission or absorption, Lynx will be a powerful tool for understanding the origins of the cosmos.

Nondestructive analysis (NDA) based on x-ray emission is widely used, for example, in the semiconductor and concrete industries. Here, we demonstrate significant quantitative and qualitative improvements in broadband x-ray NDA by combining particle-induced emission with detection based on superconducting microcalorimeter arrays. We show that the technique offers great promise in the elemental analysis of thin-film and bulk samples, especially in the difficult cases where tens of different elements with nearly overlapping emission lines have to be identified down to trace concentrations. We demonstrate the efficiency and resolving capabilities by spectroscopy of several complex multielement samples in the energy range 1–10 keV, some of which have a trace amount of impurities not detectable with standard silicon drift detectors. The ability to distinguish the chemical environment of an element is also demonstrated by measuring the intensity differences and chemical shifts of the characteristics x-ray peaks of titanium compounds. In particular, we report measurements of the $K\alpha /K\beta$ intensity ratio of thin films of TiN and measurements of Ti $K\alpha$ satellite peak intensities in various Ti thin-film compounds. We also assess the detection limits of the technique, comment on detection limits possible in the future, and discuss possible applications.

For experiments with high arrival rates, reliable identification of nearly-coincident events can be crucial. For calorimetric measurements to directly measure the neutrino mass such as HOLMES, unidentified pulse pile-ups are expected to be a leading source of experimental error. Although Wiener filtering can be used to recognize pile-up, it suffers from errors due to pulse shape variation from detector nonlinearity, readout dependence on subsample arrival times, and stability issues from the ill-posed deconvolution problem of recovering Dirac delta-functions from smooth data. Due to these factors, we have developed a processing method that exploits singular value decomposition to (1) separate single-pulse records from piled-up records in training data and (2) construct a model of single-pulse records that accounts for varying pulse shape with amplitude, arrival time, and baseline level, suitable for detecting nearly-coincident events. We show that the resulting processing advances can reduce the required performance specifications of the detectors and readout system or, equivalently, enable larger sensor arrays and better constraints on the neutrino mass.