We, the QUARTET Collaboration, propose an experiment to measure the nuclear charge radii of light elements with up to 20 times higher accuracy. These are essential both for understanding nuclear physics at low energies, and for experimental and theoretical applications in simple atomic systems. Such comparisons advance the understanding of bound-state quantum electrodynamics and are useful for searching for new physics beyond the Standard Model. The energy levels of muonic atoms are highly susceptible to nuclear structure, especially to the mean square charge radius. The radii of the lightest nuclei (with the atomic number, $Z=1,2$) have been determined with high accuracy using laser spectroscopy in muonic atoms, while those of medium mass and above were determined using X-ray spectroscopy with semiconductor detectors. In this communication, we present a new experiment, aiming to obtain precision measurements of the radii of light nuclei $3\le Z\le 10$ using single-photon energy measurements with cryogenic microcalorimeters; a quantum-sensing technology capable of high efficiency with outstanding resolution for low-energy X-rays.

Battery research often encounters the challenge of determining chemical information, such as composition and elemental oxidation state, of a layer buried within a cell stack in a non-destructive manner. Spectroscopic techniques based on X-ray emission or absorption are well-suited and commonly employed to reveal this information. However, the attenuation of X-rays as they travel through matter creates a challenge when trying to analyze layers buried at depths exceeding hundred micrometers from the sample's surface. In the context of battery research, the limited escape depth of X-rays often imposes to design experiment-specific cells with thinner inner layers, despite the risk that these tailored cells may not exactly replicate the cycling behavior of larger commercial cells. Muon-induced X-ray emission (MIXE) is a non-destructive spectroscopic technique that involves implanting negative muons into a sample and detecting the highly energetic muonic X-rays generated when these muons are captured by the sample's atoms. By virtue of the high energy of muonic X-rays, the depth of analysis of MIXE greatly exceeds other X-ray based techniques. In this article, we introduce the technique and lay the groundwork for employing MIXE in future in-situ/operando analyses of batteries. We demonstrate that MIXE can detect nearly all elements, including low atomic number ones such as Li. Additionally, we establish the quantitative nature of MIXE through the precise determination of LiNi_xMn_yCo_1-x-yO₂ (NMC) electrode stoichiometries. Finally, we demonstrate that MIXE enables the acquisition of depth-resolved chemical information from 100-1,000 μm thick cells, in good agreement with simulation results.

We present a novel method for accurately measuring the absolute electric quadrupole moments of light transition elements (23≤Z≤30). Our approach is based on performing precision muonic x-ray spectroscopy of the 2s-2p manifold, which is also referred to as the Lamb shift. These transitions are too weak to be detected with dispersive methods and too overlapping to be resolved by solid-state detectors. Here, we propose the use of cryogenic microcalorimeters, which possess high efficiency and excellent energy resolution in the relevant energy regime, coupled with state-of-the-art theoretical calculations. We demonstrate the feasibility of this approach by performing extensive calculations and realistic simulations. In this way, we establish that the uncertainty in the absolute moment, which is transferred to the quadrupole moments of all isotopes in the chain, could be reduced by up to an order of magnitude within a day of measurement. These precise reference quadrupole moments serve as valuable inputs for nuclear structure studies and for benchmarking state-of-the-art quantum chemistry calculations in open-shell elements.

Background: ²²⁵ Ac is one of the most promising short-lived radionuclides for application in targeted alpha therapy (TαT). However, quantitative detection of ²²⁵ Ac and its daughter radionuclides by γ-spectroscopy has so far been unattainable. Achieving this would enable precise organ dosimetry and better targeting of the therapeutic potential of the ²²⁵ Ac decay series. Methods: In this study, an X-ray and γ-spectrum of an ²²⁵ Ac sample (89 kBq) was recorded with a Metallic Magnetic Calorimeter (MMC) detector over a wide energy range of 5 to 125 keV. Results: Here, we show the feasibility of detecting γ-lines of ²²⁵ Ac, ²²¹ Fr, ²¹³ Bi, and ²⁰⁹ Tl with ultra-high-resolution spectroscopy (FWHM of 23 eV @ 5.9 keV). By analyzing characteristic X-ray fluorescence lines, it is also possible to distinguish ²²¹ Fr, ²¹⁷ At, ²¹³ Bi, ²¹³ Po, and ²⁰⁹ Pb in this sample. Additionally, particle induced X-ray emission from ²²⁵ Ac is found. Conclusions: To the best of our knowledge, this first MMC based X-ray and γ-spectrum of ²²⁵ Ac demonstrates the detector’s potential, as it enables the separate detection of most nuclides in the ²²⁵ Ac decay chain (all except ²²¹ Ra and ²¹⁷ Rn) with high sensitivity, excellent energy resolution, and precise energy calibration, paving the way for key technological advancements in research and clinical applications in nuclear medicine.

We propose an experiment to measure the nuclear charge radii of light elements with up to 20~times higher accuracy. These are essential both for understanding nuclear physics at low energies, and for experimental and theoretical applications in simple atomic systems. Such comparisons advance the understanding of bound-state quantum electrodynamics and are useful for searching for new physics beyond the Standard Model. The energy levels of muonic atoms are highly susceptible to nuclear structure, especially to the mean square charge radius. The radii of the lightest nuclei (with the atomic number, $Z=1,2$) have been determined with high accuracy using laser spectroscopy in muonic atoms, while those of medium mass and above were determined using X-ray spectroscopy with semiconductor detectors. In this communication, we present a new experiment, aiming to obtain precision measurements of the radii of light nuclei $3 \leq Z \leq 10$ using single-photon energy measurements with cryogenic microcalorimeters; a quantum-sensing technology capable of high efficiency with outstanding resolution for low-energy X-rays.