Precision Measurement of the <sup>241</sup>Pu Beta Spectrum with Magnetic Microcalorimeters
ORAL · Invited
Abstract
In this talk, we introduce the MAGNETO-v experiment, which utilizes plutonium source to search for hypothetical heavy sterile neutrinos. An enriched 241Pu source is implanted into magnetic microcalorimeter (MMC) detectors that consist of paramagnetic sensing films and SQUID (Superconducting Quantum Interference Devices) quantum magnetometers, enabling precise measurement of the beta-decay spectrum via the Decay Energy Spectrometry (DES) technique. This method simultaneously offers both 100% detection efficiency and excellent energy resolution (E/ΔE > 1000). In the initial experimental run, we collected one billion 241Pu beta-decay events, resulting in the most precise 241Pu spectrum to date. The Q-value of the decay was determined to be 22.04(2) keV, and the shape factor for the first-forbidden non-unique beta decay was measured as C(W) = 1 − 1.919(8)W + 0.931(9)W2. In addition, we observed strong evidence for a potential low-energy excess in the decay rate. Analysis of the first 200 million events set up an upper limit of |Ue4|2 = 1.23 × 10-3 at a 95% confidence level for 10 keV sterile neutrinos. Alpha decays from other plutonium isotopes (²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu, and ²⁴²Pu) and rare ²⁴¹Pu decays (0.00237% branching) were measured with 1-2 keV FWHM (full-width-half-maximum) energy resolution, demonstrating the ability for precise plutonium isotopic analysis for nuclear safeguards and forensics applications.
*This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. This work was funded by the Laboratory Directed Research and Development program of Lawrence Livermore National Laboratory (20-LW-024 and 23-LW-043). This work was funded by the National Nuclear Security Administration of the Department of Energy, Office of Defense Nuclear Nonproliferation Research and Development (NA-22) and International Nuclear Safeguards (NA-241). This material is based upon work supported by the Consortium for Nuclear Forensics under Department of Energy, National Nuclear Security Administration award number DE-NA0004142.
–
Publication:Lee, C., et al. "Magneto-$\nu $: Heavy neutral lepton search using $^{241} $ Pu $\beta^-$ decays." arXiv preprint arXiv:2503.18350 (2025).
