From Commissioning to Precision Data-Taking: Resolving Operational Challenges in the Nab Detector Systems

TL;DR

The Nab paper tackles the CKM unitarity tension by pursuing a precision neutron-based determination of $V_{ud}$ through neutron lifetime measurements and the electron-neutrino correlation parameter via $\lambda = g_A/g_V$. It details the Nab approach, using an asymmetric magnetic spectrometer to reconstruct the full momentum phase space of neutron beta decay and extract the electron-neutrino correlation from beta decay kinematics. The commissioning phase identified three operational challenges—proton energy miscalibration, electronics instability, and segment loss—and documents targeted upgrades that mitigated these issues. With the 2024–2025 improvements, Nab has achieved robust detector performance, enabling high-precision inputs to the $V_{ud}$ determination and contributing to resolving neutron-decay datasets tensions and CKM unitarity tests.

Abstract

Our understanding of the weak mixing of quarks, described by the Cabibbo Kobayashi Maskawa (CKM) matrix, currently presents an anomaly. Thanks to major strides in both theory and experiment, improved precision in determinations of the first row of matrix elements has revealed disagreement with the expectation of unitarity. The Nab experiment at the Spallation Neutron Source is designed to precisely extract the first matrix element $V_{ud}$ and shed light on experimental tensions within the neutron beta decay dataset. Nab's asymmetric spectrometer allows coincident reconstruction of the decay proton and electron energies, which will be used to determine the electron-neutrino correlation coefficient, and thus (with the neutron lifetime) determine $V_{ud}$. This unique approach has provided a more comprehensive view of neutron beta decay, including a first observation of the full momentum phase space of the decay above detector thresholds and limits on exotic neutron states. Recent upgrades to the Nab detector system have improved the robustness and stability of the detector performance in terms of proton detection efficiency, noise performance, and detector segment availability, setting the stage for high precision physics data-taking.

Figures (4)

• Figure 1: Detected events as a function of proton Time Of Flight (TOF) and proton energy during commissioning in 2023 (left) and after improvements to detector qualification procedures in 2024 (right), assuming a rough energy calibration, and mean proton energy as observed in 2023 (left line) and the range of means expected based on 50--100 nm deadlayer (right hatch).
• Figure 2: (Left) Mapping of preamplifier circuit board to detector segment during the 2023 commissioning dataset and (center) the redesigned mapping used in 2024 and later. Colors separate different boards. Hexagonal segments are enumerated by board identifier letter plus channel number. (Right) Updated layout of the 6.54 in diameter adapter board which maps the 6 channel circuit boards and thermometers (23 + 1 outer red connectors) to a vacuum feedthrough (12 inner blue connectors). Different colored traces reside on different board layers. (color online)
• Figure 3: The detected total trigger rate (color scale) for detector segments for typical runs during the 2023 commissioning data-collection (left) and during the 2025 data-collection (right). Darker colors indicate segments which are non-reporting. Noise and threshold conditions varied among pixels and datasets, in particular in 2023.
• Figure 4: Updated electronics package including 8 channel circuit boards with Field Effect Transistors (FET), 6 channel boards with amplification stages, adapter boards, and cabling.