Production of low energy spin-polarized radioactive nuclear beam using superfluid helium

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This study is aiming at developing a superfluid helium target for radioactive nuclear beam and meson beam. We have realized that approximately 30% of implanted particles remain as an ion state in the helium. We have planed to extract these ions from the helium target and generate low-energy polarized nuclear beam by introducing a spin polarizer with a polarized electron transfer reaction. In order to transport unstable nuclei into the polarizer with high efficiency it is necessary to monitor their positions. We have measured the decay position by reconstructing the beta-ray trajectory. Moreover, we installed a range-stack type detector with 6 plastic layers to measure end-point energy of beta decays. Scintillation photons produced in each layer were transferred through optical fiber embedded in the counter, which enable us not to interfere with other detector elements. By making coincidence this end-point detector and gamma detector, it can be extracted much more detailed information for the nuclear spectroscopy. Also, we are planing to use superfluid helium as a stopping target for K beam. This is why energy loss of secondary particles by the target material is drastically reduced comparing with standard solid target. The feasibility study was performed by tuning of the K beam line and by measuring momentum spectra of the secondary particles. As a consequence, superfluid helium is applicable fox the stopping target at J-PARC under the condition of low momentum dispersion of K beam at the target position.