Spin-polarization of radioactive 123Ing.s. by the tilted-foil method

Abstract

Abstract. A spin-polarized radioactive123 In (I π = 9/2 + , g = 1.220(2), T 1/2 = 5.97(5) s) beam has been successfully generated by the tilted-foil method. This nuclide is the heaviest ever polarized in its ground state by this method. Using the ISOL-based re-acceleration-type facility TRIAC, an 123 Ing.s. beam of 305 keV/nucleon went through a stack of 15 carbon foils with a tilt angle of 70• , and an asymmetry of 0.76 ± 0.25% of β-decays was observed by the β-NMR technique. The asymmetry shows that the tilted-foil method combined with a re-acceleration facility is useful for producing spin-polarized beams for applications such as nuclear physics and materials science.Spin-polarized radioactive isotope beams (RIBs) provide an opportunity to explore nuclear structure through β-decay spectroscopy as well as probing the electromagnetic properties of materials using the β-NMR technique. The tilted-foil (TF) method [1] is one of the most useful techniques for producing spin-polarized RIBs, because the method can be applied in principle to any element of short-lived RIB generated at isotope separation on-line (ISOL)-based re-acceleration-type facilities, except for the nuclei with I = 0 and/or a lifetime shorter than a few nanoseconds.The process of nuclear polarization by the TF method is considered as follows: First, atomic spin-polarization is introduced through anisotropic atomic collisions of incident ions with electrons at the exit surface of the foil. Then, the atomic spin-polarization is transferred to nuclear spin-polarization via the hyperfine interaction during flight in vacuum. From the successful polarization of 147 Gd nuclei in isomeric states with a half-lifetime of 26.8 ns [2], the time scale for the production of nuclear polarization by the TF method can be inferred to be fast (the order of nanoseconds). RIBs of a few hundred keV/nucleon are considered to be most suitable for the effective capture of electrons, which is essential for the primary atomic polarization [2,3]. a e-mail: yoshikazu.hirayama@kek.jp The induced nuclear polarization by using multi-tilted foils increases with increase of the number of foils and saturates according to a classical model equation [4] as follows:Here N is the number of foils, I and J are nuclear and averaged atomic spins, respectively. The atomic and nuclear polarizations are conveniently expressed by P S = P S (S + 1)/S (S = I, J) using the usual polarization P S in these equations. Nuclear polarization could be estimated by using the eq. (1) with a reasonable averaged atomic spin J and the atomic polarization P J . The degree of polarization also increases with increasing the tilt angle φ between the beam axis ( − → v ) and the axis normal ( − → n ) to the foil surface [5]. The direction of polarization, which is well defined by − → n × − → v , is easily reversed by reversing the normal axis of the foil surface [5]. Thus, the nuclear polarization produced by the TF method is controllable and the