This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Astronomers can't see dark matter directly, but they know it's there: Its gravity shapes galaxies and the large-scale structure of the cosmos. In an effort to uncover the composition of this hidden mass, a team at Lawrence Livermore National Laboratory (LLNL) is pursuing evidence of particles that exist beyond the standard model of physics.
In a new experimental campaign called Magnetometry for Neutrino physics (Magneto-ν), scientists at LLNL are searching for the sterile neutrino using nuclear beta decays of plutonium-241. This hypothetical neutrino species is significantly heavier than ordinary neutrinos and is considered a leading candidate for warm dark matter. Initial findings of their study were recently published in the journal Physical Review C.
When a plutonium-241 atom undergoes beta decay, it transforms into an americium-241 atom, releasing an electron and an antineutrino. Different neutrino masses can be created in this process; one might have a mass in the kiloelectronvolt range—the scale relevant for warm dark matter. Detecting these neutrinos directly is extremely difficult because neutrinos rarely interact with matter.
The Magneto-ν experiment takes a different approach: Rather than trying to observe the escaping antineutrino itself, researchers precisely measure the energies of the other two decay products—the electron and the recoiling americium-241 atom. Because the total energy released in the decay is known, the energy carried away by the unseen antineutrino can be reconstructed. The technique allows the team to measure subtle signatures that could indicate the presence of sterile-neutrino dark matter.
If a sterile neutrino is emitted during beta decay, it would carry away a specific amount of energy to form its mass, leaving behind a tiny distortion in the measured beta-decay spectrum or in the reconstructed antineutrino energy spectrum. Magneto-ν uses a highly sensitive magnetic microcalorimeter to detect this kink in the beta-decay spectrum of plutonium-241. "If successful, the experiment could help us answer two of science's biggest questions: what dark matter is and how neutrinos have mass," said Geon-Bo Kim, LLNL staff physicist.
Plutonium-241 provides an ideal source because its beta-decay spectrum aligns with the expected mass range of sterile-neutrino warm dark matter, with an average emitted beta energy of about 5 keV. Magneto-ν pairs this isotope with ultra-sensitive magnetic microcalorimeters (MMCs) to capture the full decay energy, minus the antineutrinos, of plutonium-241 beta decays. "Since plutonium is a highly controlled nuclear material, few research environments can safely prepare, handle and measure it for fundamental physics," said Kim.
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