Next Generation Ta-STJ Sensor Arrays for BSM Physics Searches

TL;DR

The paper addresses calibration artifacts in BeEST Be-7 recoil spectroscopy using superconducting tunnel junctions. It identifies resistive crosstalk from shared ground wiring and substrate-absorption of calibration photons as the main sources of systematic errors and demonstrates design and operation changes to mitigate them. By implementing per-pixel ground wires and employing a more stable, collimated UV laser, the BeEST collaboration achieved robust energy resolution in the $1$–$2$ eV range below $100$ eV and suppressed the phase-III calibration artifacts, enabling more reliable searches for BSM neutrinos in Phase-IV and related experiments like SALER at FRIB. These improvements have clear practical impact for precision recoil spectroscopy and rare-process studies using STJ sensors, with potential for further gains through improved laser collimation.

Abstract

The Beryllium Electron capture in Superconducting Tunnel junctions (BeEST) experiment uses superconducting tunnel junction (STJ) sensors to search for physics beyond the standard model (BSM) with recoil spectroscopy of the $\mathbf{^7}$Be EC decay into $\mathbf{^7}$Li. A pulsed UV laser is used to calibrate the STJs throughout the experiment with $\sim$20 meV precision. Phase-III of the BeEST experiment revealed a systematic calibration discrepancy between STJs. We found these artifacts to be caused by resistive crosstalk and by intensity variations of the calibration laser. For phase-IV of the BeEST experiment, we have removed the crosstalk by designing the STJ array so that each pixel has its own ground wire. We now also use a more stable UV laser for calibration. The new STJ arrays were fabricated at STAR Cryoelectronics and tested at LLNL and FRIB. They have the same high energy resolution of $\sim$1\textendash2~eV in the energy range of interest below 100~eV as before, and they no longer exhibit the earlier calibration artifacts. We discuss the design changes and the STJ array performance for the next phase of the BeEST experiment.

Figures (8)

• Figure 1: Schematic band diagram and charge flow in an STJ: The EC decay of $^7$Be generates excess quasiparticles in Ta in proportion to the deposited energy (1), which diffuse into the Al trap and are confined by inelastic scattering (2). As they tunnel through the barrier (3), they generate a measurable signal current until they eventually recombine into Cooper pairs (4). Here, the dashed arrow indicates the direction of negative current flow.
• Figure 2: Phase-III $^{\mathrm{7}}$Be decay spectrum from a single STJ pixel (black) and associated laser calibration spectrum (red) Inwook. The four peaks correspond to K-electron capture to the $^{\mathrm{7}}$Li ground state (K-GS), K-electron capture to the $^{\mathrm{7}}$Li excited state (K-ES), and the two corresponding L-capture peaks (L-GS) and (L-ES).
• Figure 3: Photopeak calibration plot demonstrating two unexpected artifacts: The photopeaks here are slanted, indicating some degree of inter-pixel correlation, and higher-order photopeaks are offset to a higher average laser intensity Inwook. Here, $\mathrm{E_{\gamma}=3.49865(15)}$ eV ponce-thesis and $\mathrm{FWHM\approx}$ 1.5– 2.7 eV Inwook.
• Figure 4: Schematic of the readout electronics for the calibration of two STJs with a common ground wire. Here, STJ$_{\mathrm{other}}$ contributes to the current in the resistive ground plane for simultaneous laser events as a result of $\sum \mathrm{ I_{other}}$ in Equation 1. However, this is not the case for randomized $^{\mathrm{7}}$Be decay events.
• Figure 5: Cross section and film thicknesses for a Ta-Al-AlOx-Al-Ta STJ from STAR Cryoelectronics. The 4.5 nm Nb layer nucleates the base Ta film in the desired bcc $\mathrm{\alpha}$-Ta phase.