Precision Measurement of the ²⁴¹Pu Beta Spectrum with Magnetic Microcalorimeters

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 |U_e4|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.