Speaker
Description
Low energy neutron (thermal neutron) sources are widely used in various fields such as neutron radiography, neutron diffraction, and Boron Neutron Capture Therapy (BNCT). Spin-polarized neutrons are also considered as one of the next generation quantum sources, enabling analysis of magnetic structures of materials. Laser-driven neutron sources are considered promising due to their point source and short pulse. The generation of thermal neutrons by lasers has been studied by using specially designed moderators [1-3]. In this study, we have proposed a novel method based on the Stern–Gerlach principle, where neutrons are spatially separated into fully spin-polarized states by a magnetic field gradient. By combining a laser-driven neutron source with a laser-driven magnetic field, this method enables generation of spin-polarized neutrons having both short pulse duration and point-source characteristics.
The experiment is conducted at GEKKO XII–LFEX laser facility at Institute of Laser Engineering, The University of Osaka. Photonuclear reactions induced by deuterons were utilized to produce thermal neutrons without any moderators. The LFEX laser generates x-rays, which then induce neutron production at a deuterated plastic target. The resulting neutron spectrum spans a broad energy range from meV to MeV. A magnetic field is subsequently generated using GEKKO XII laser triggered after the LFEX pulse. A delay time is applied to ensure the arrival of the thermal neutrons at the magnetic field region, and then short pulse, monoenergetic, spin polarized neutrons are extracted.
The detector package is also developed for the experiment. The thermal neutron beam pattern is measured using a stack of radiochromic film (RCF), CR-39, and a ⁶LiF sheet, placed 15 mm from the neutron source target. The CR-39/⁶LiF combination provides high selectivity and sensitivity for thermal neutrons. Preliminary experimental data confirm that thermal neutrons are successfully detected by the detector package, although spin-polarized neutrons have not yet been identified due to an insufficient signal-to-background ratio. We also discuss further experiments to improve this ratio and enable the clear detection of spin-polarized neutrons.
References
[1] S.R. Mirfayzi, et, al, Sci. Rep, 10, 20157 (2020). [2] S. R. Mirfayzi et al., Appl. Phys. Lett. 116, 174102 (2020). [3] A. Yogo et al., Appl. Phys. Express 14 106001 (2021).
