Aiko Takamine

We demonstrated the production of high-intensity and low-energy neutral francium (Fr221) by developing an atomic source based on actinium (Ac225) covered with an yttrium (Y) layer. Low-energy Fr+221 ions and neutral Fr221 atoms were produced and measured. We investigated the flux and number of Fr221 atoms from the experimental results under different experimental conditions. The proposed atomic source is promising for Fr trapping experiments and Fr electric dipole moment measurements.

We developed a 400-m polarization-maintaining optical fiber delivery system for magneto-optical trapping of radioactive Fr atoms at RIKEN in Japan. The optical system is useful for the precise studies of the electron’s electric dipole moment and the parity nonconservation effect in atomic systems. The power saturation effect due to the stimulated Brillouin scattering in the fiber was investigated. Polarization fluctuation was optimized to be below 5 × 10−3. The absolute frequency of the stabilized laser was measured with an absolute accuracy of 1.5 MHz using a high-performance commercial wavelength meter. In addition, the absolute frequencies of hyperfine transitions of iodine molecules (127I2) near 718 nm were determined. We demonstrated laser cooling of Rb with 440-m optical fiber delivery.

A PLASEN (Precision LAser Spectroscopy for Exotic Nuclei) system, consisting of a compact radio-frequency quadrupole cooler buncher (RFQ-cb) and a collinear resonance ionization spectroscopy setup, has now been fully commissioned with radioactive ion beams at the Beijing Radioactive Ion-beam Facility (BRIF). Using both stable and radioactive Rb ion beams from BRIF, we demonstrated that the large beam energy spread observed at BRIF has been successfully handled by employing the RFQ-cb, enabling the delivery of high-quality bunched radioactive ion beams for collinear resonance ionization spectroscopy experiments. Under these conditions, we performed laser spectroscopy of exotic nuclei, achieving high resolution ( ∼ 100 MHz spectral linewidth) and high sensitivity ( ∼ 1 : 200 efficiency). This fully operational PLASEN system will serve as a state-of-the-art experimental platform at BRIF for research in multiple fields such as nuclear, atomic, and molecular physics.

The half-lives of 24 isotopes ranging from sulfur to potassium were measured using the ZeroDegree Advanced Decay Station at the Radioactive Isotope Beam Factory, including six of the most neutron-rich-^{47}S, ^{48,49}Cl, ^{51,52}Ar, ^{55}K-for the first time, while the precision for ^{48}Ar and ^{53,54}K was significantly improved. Two prominent features are found: First, a half-life drop at ^{54}K_{35} relative to ^{52,53}K, supporting the persistence of the N=34 subshell closure in potassium with Z=19. Second, a high β-decay rate for ^{48}Cl_{31} compared to ^{47}Cl and ^{49}Cl, which can be explained by the critical role of the ν1p_{1/2} orbital in shell-model calculations. The present calculations indicate that the N=32 subshell gap in chlorine with Z=17 is slightly smaller than that in calcium (Z=20), and the significant excitation across the N=32 subshell in ^{48}Cl_{31} is mainly due to strong configuration mixing.

The rapid proton-capture process is one of the primary, explosive thermonuclear burning processes that drive type I x-ray bursts. A possible termination of the rapid proton-capture process at around ^{84}Mo was previously suggested by the formation of a ZrNb cycle. We report here precision mass measurements at around ^{84}Mo that conclude the issue regarding the possibility of the cycle. The experiment was conducted using the multireflection time-of-flight spectrograph at RIKEN Radioactive Isotope Beam Factory, and the masses of ^{79}Y, ^{83}Nb, ^{84}Mo, ^{88}Ru, and an isomer in ^{78}Y were measured. For ^{84}Mo, ^{88}Ru, and the isomeric state of ^{78}Y, their masses were experimentally determined for the first time with uncertainties of δm≈20 keV/c^{2}. The mass precisions of ^{79}Y and ^{83}Nb were improved to 13 and 9.6 keV/c^{2}, respectively. The new α-separation energy of ^{84}Mo, 1.434(83) MeV, unambiguously rules out the possibility of forming the ZrNb cycle. The x-ray burst simulation with the new masses shows that our measurements effectively remove the large final abundance uncertainties in the A=80-90 mass region. The new mass values improve the prediction power for the composition of the nuclear ashes in x-ray bursts.