Kei Minamisono

One of my current research interests is to determine fundamental nuclear properties, the magnetism and shape, of radioactive isotopes away from the beta stability line and towards the nucleon drip lines in the nuclear chart. The nuclear magnetic-dipole moment arises from the orbital angular momentum and intrinsic spin, and sensitive to the valence-nucleon configuration in a nucleus. The nuclear-quadrupole moment and charge radius represent the charge distribution inside the nucleus and sensitive to the shape and onset of deformation. They are essential to gain critical insights into the driving force of the structural change in rare isotopes.

Our immediate interests are twofold: determination of unknown charge radii of radioactive Fe isotopes around neutron magic number N = 28 and towards N = 42; investigation into the abnormal behavior of variation in chains of charge radii at N = 20 around Ca region. The latter is a continuing effort in our group and we recently confirmed a monotonic variation of the charge radii of K (one proton less from Ca) isotopic chain at N = 20, whereas characteristic discontinuities in chains of charge radii are commonly seen at other nucleon magic numbers.  

Hyperfine interaction is an interplay between nuclear and atomic spins/angular momenta, and results in a hyperfine structure (HFS), which contains information about nuclear moments and charge radii. Laser assisted techniques are used to measure the HFS; the collinear laser spectroscopy (CLS) and beta-ray detecting Nuclear Magnetic Resonance (bNMR). Such experimental techniques are available at the BEam COoling and LAser spectroscopy (BECOLA) facility at NSCL, where the atomic/nuclear spin manipulation for production of nuclear polarization as well as CLS with cooled/bunched beams are performed. High precision/resolution laser systems and a detection system with great sensitivity are required to resolve HFS of rare isotopes at low-production rates.

Technical development is another essential aspect of our group to enhance overall sensitivity, namely extending the reach of radioactive isotopes. Some of ongoing efforts include manipulation of atomic population in an RF ion trap, resonant laser ionization spectroscopy and offline production of stable-isotope beams for studies of laser excitation schemes.

Hyperfine structures of ³⁶K and ³⁷K are shown for the D1 transition, measured by optical pumping to produce nuclear polarization and subsequent beta-decay asymmetry detection. The pattern of spectum is the HFS and contains information about nuclear moments, and the relative shift of centroids of HFS between ³⁶K and ³⁷K is the isotope shift, from which charge radii can be extracted.