My research focuses on spectroscopy of exotic nuclei far from stability. Unstable nuclei with unusual proton-to-neutron ratios, called as “exotic nuclei”, often exhibit surprising phenomena such as nuclear halo and shape coexistence, presenting important challenges for our understanding of atomic nuclei. The goal of present-day nuclear physics is to establish the unified understanding of nuclear structure for stable and exotic nuclei. New experimental results with rare isotope beams serve as vigorous tests for modern nuclear theories, as well as providing answers to questions concerning the nature of neutron stars and the origin of the elements in the Universe. Nuclear structure information can also be used to test fundamental symmetries that describe the weak and strong forces in nature.
At NSCL, my group performs in-beam gamma and particle spectroscopy with rare isotope beams, with a current focus on lifetime measurements of nuclear excited states. Lifetimes of bound excited states are directly related to transition probabilities between the states, which provide sensitive probes for anomalies in the structure of exotic nuclei. For unbound levels, lifetimes can be associated with the energy uncertainties to be measured as the resonance widths, which play important roles in the stability of extreme quantum systems as well as in nuclear reaction rates of astrophysical interest.
Recently, a new plunger device TRIPLEX (TRIple PLunger for EXotic beams, Fig.1) dedicated for the recoil-distance Doppler-shift measurements has been developed. The device uses three thin metal foils separated by very precise distances. In this approach, Coulomb excitation or knockout reactions are used to populate excited states in exotic, projectile-like reaction residues that decay in flight after traveling a distance related to its lifetime. Two degraders are positioned downstream of the target to reduce the velocity of the ion. As a consequence, gamma-rays emitted behind each foil have different Doppler shifts. The lifetime of the state can be determined using relative gamma-ray yields measured at different foil separations. As an example, a result from recent GRETINA campaign experiment on 74Kr is shown in Fig.2. In 74Kr, a strong competition between prolate and oblate shapes has been suggested, and lifetime information obtained from the spectrum confirms this exotic nature of 74Kr.
Magnetic response of the halo nucleus 19C studied via lifetime measurement” K.Whitmore, D.Smalley, H.Iwasaki et al., Phys. Rev. C 91, 041303(R) (2015).
Evolution of collectivity in 72Kr; evidence for rapid shape transition” H.Iwasaki, A.Lemasson, et al., Phys. Rev. Lett. 112, 142502 (2014)
Enhanced Quadropole Collectivity at N=40: The Case of Neutron-Rich Fe Isotopes" W. Rother, A. Dewald, H. Iwasaki et al., Phys. Rev. Lett. 106, 022502 (2011)
Breakdown of the Z=8 Shell Closure in Unbound 12O and its Mirror Symmetry”, D.Suzuki, H.Iwasaki et al., Phys. Rev. Lett. 103, 152503 (2009)