Japan Proton Accelerator Research Complex

COMET experiment is planned to be conducted at the Hadron facility of J-PARC. This experiment searches for the neutrino-less transition of a muon to an electron in a nuclear field known as “<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>–e conversion,” which is one of the Charged Lepton Flavor Violation processes. A detector solenoid (DS), which is located at the downstream end of a muon beam line for COMET, is the world's largest cryogen-free superconducting magnet, which is directly cooled by cryocoolers. The DS was fabricated, and the commissioning tests were performed in 2024. The DS has successfully been cooled down to 4.2K and excited to the rated current without training quenches. In addition, the preliminary magnetic field measurement using a 3-axis Hall probe is carried out, and these results have proven the integrity of the magnet.

Metals/alloys with a body-centered cubic (BCC) structure usually exhibit a high yield strength but suffer from poor work-hardening capability, leading to limited uniform elongation. This limitation becomes even more pronounced at low temperatures, where a ductile-to-brittle transition is commonplace. High-entropy alloys (HEAs), benefiting from their huge compositional space, provide an opportunity to achieve a large uniform elongation in BCC alloys. However, the lack of in-situ characterization techniques, especially at low temperatures, has challenged the determination of the underlying mechanisms. Here, using in-situ neutron diffraction measurements, in conjunction with microstructure observations, we identified the deformation mechanism responsible for an exceptionally large uniform elongation at liquid nitrogen temperature in a single-phase BCC (TiZrHf) 86.4 Al 2 Nb 11.6 HEA. We found that the initial plastic deformation is driven by a BCC-to-orthorhombic (known as α″) phase transformation, while twinning of the α″ phase and deformation-induced amorphization contribute to the ductility at the later stage. The cooperation of multiple deformation modes resulting from phase transformation overcomes the undesirable work-softening caused by dislocation-mediated plasticity, enabling a large uniform elongation while maintaining a high yield strength. The mechanism revealed through neutron diffraction demonstrates a feasible strategy by engineering deformation pathways to improve the low-temperature mechanical properties, thus providing guidance for developing advanced structural materials for cryogenic applications.

This study investigates the strengthening mechanisms in duplex stainless steel (DSS), a dual-phase alloy comprising ferrite (α) and austenite (γ), produced by friction stir welding (FSW) using in situ neutron diffraction during tensile testing. Two welding conditions were applied: a lower peak temperature (FSW300) and a higher peak temperature (FSW600). Electron backscatter diffraction confirmed significant grain refinement in both phases, with γ grains reduced below 1 μm in FSW300. In situ neutron diffraction provided phase-resolved stress data during deformation, enabling direct evaluation of the load-sharing behavior of α and γ. Tensile testing revealed that both FSW conditions increased yield and tensile strength while reducing uniform and total elongation compared with the base metal (BM); however, FSW300 retained greater total elongation than FSW600, attributed to less severe local elongation loss. Neutron diffraction results revealed that γ acted as the harder phase in the BM, whereas α became the harder phase in the FSWed specimens. Phase stress analysis indicated that α is more sensitive to grain refinement strengthening than γ, shifting the dominant contribution to strength and work-hardening from γ in the BM to α in the welded specimens. Although stacking fault formation in γ was more pronounced in the ultrafine-grained microstructures, work-hardening capability of γ decreased, while α showed enhanced texture development and dislocation accumulation. These findings demonstrate that low-temperature FSW improves DSS strength primarily through α refinement, offering insights for designing stronger dual-phase alloy joints.

Abstract The potential of the intense secondary muon, neutrino, and (hypothetical) light dark matter beams at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) is explored. These are produced in the high-power dumps with high-current electron beams. Light dark matter searches with the approved Beam Dump eXperiment (BDX) are driving the realization of a new underground vault behind Hall A that could be extended to a Beamdump Facility with little additional installations. High-energy muons created via the Bethe–Heitler process uniquely do not proceed through the more common pion production and decay channels. Several possible muon physics applications are highlighted. Neutrino detector technologies and experiments suitable for a beamdump facility are outlined.

The nondestructive detection of metallic lithium (Li) within lithium-ion batteries (LIBs) is essential for both ensuring safety and assessing battery quality prior to reuse. In this study, we demonstrate that elemental analysis with muonic X-rays offers a quantitative approach for identifying metallic Li deposition on graphite anodes in a laminated LIB. The technique leverages the preferential capture of negative muons by metallic Li over Li intercalated in graphite, resulting in detection of metallic Li. By applying this method to laminated LIBs with varying amounts of metallic Li, we established a standard curve correlating the muonic X-ray signal intensity with the areal density of metallic Li. Our approach achieves a detection limit of 0.01 mg/cm2, corresponding to a few percent of the battery's theoretical capacity─well below the threshold relevant for reuse criteria. This technique provides a powerful, nondestructive diagnostic tool for monitoring and quantifying metallic Li in LIBs, offering valuable insights into degradation mechanisms and supporting the development of safer, longer-lasting battery systems.

In relativistic plasma expansion, a relative motion of plasma components causes absorption of spectral lines to be Doppler-shifted with respect to the emission line centers. The shift of spectral line can be of an order or exceeding the natural linewidth, leading to a significant reduction in self-absorption and optical thickness of the plasma. In this paper we experimentally examine the effect of femtosecond relativistic laser plasma expanding from both sides of the solid foil target. In an experiment, a hot plasma x-ray source in a 1–2 μm thin 4-chlorostyrene foil was created by high-contrast ∼ 1 × 1021 W/cm2 pulses delivered by the J-KAREN-P facility. In addition, the rear side of the target was heated by 1/100 of the main pulse power with a controlled few-nanosecond time delay. It is demonstrated that the high velocity of the downstream and upstream (from the rear side) plasma expansion essentially induces plasma transparency for K-shell emission from He-like chlorine ions dominating the ion charge distribution.