Y. Abe

Nuclear structure of 30 S and its implications for nucleosynthesis in classical novae

The uncertainty in the 29P(p,gamma)30S reaction rate over the temperature range of 0.1 - 1.3 GK was previously determined to span ~4 orders of magnitude due to the uncertain location of two previously unobserved 3+ and 2+ resonances in the 4.7 - 4.8 MeV excitation region in 30S. Therefore, the abundances of silicon isotopes synthesized in novae, which are relevant for the identification of presolar grains of putative nova origin, were uncertain by a factor of 3. To investigate the level structure of 30S above the proton threshold (4394.9(7) keV), a charged-particle spectroscopy and an in-beam gamma-ray spectroscopy experiments were performed. Differential cross sections of the 32S(p,t)30S reaction were measured at 34.5 MeV. Distorted wave Born approximation calculations were performed to constrain the spin-parity assignments of the observed levels. An energy level scheme was deduced from gamma-gamma coincidence measurements using the 28Si(3He,n-gamma)30S reaction. Spin-parity assignments based on measurements of gamma-ray angular distributions and gamma-gamma directional correlation from oriented nuclei were made for most of the observed levels of 30S. As a result, the resonance energies corresponding to the excited states in 4.5 MeV - 6 MeV region, including the two astrophysically important states predicted previously, are measured with significantly better precision than before. The uncertainty in the rate of the 29P(p,gamma)30S reaction is substantially reduced over the temperature range of interest. Finally, the influence of this rate on the abundance ratios of silicon isotopes synthesized in novae are obtained via 1D hydrodynamic nova simulations.

Fast-kicker system for rare-RI ring

We are developing a new fast-kicker system for rare-RI ring at RIKEN RI beam factory. New fast-kicker system enables us to inject a rare particle into the ring individually, and also to extract the rare particle from the ring quickly. It consists of a kicker power supply, which has a fast-response mechanism and a hybrid charging system, and a large acceptance kicker magnet. Propagation time from a trigger signal input to the power supply until the flat-top center of the kicker magnetic field is approximately 465 ns, the result is sufficient to achieve an individual injection with an energy of 200 MeV/nucleon. Reliable operation of the hybrid charging system makes it possible to extract a particle from the ring in 700 μs by using the same kicker magnet. A waveform of the magnetic field is under investigation by using a prototype kicker magnet.

Performance of a Resonant Schottky Pick-Up for the Rare-RI Ring Project

Construction of a new storage ring called “Rare-RI Ring” was started in 2012 at RIBF. This project aims at precise isochronous mass measurements for extremely neutron-rich exotic nuclei in the r-process nucleosynthesis. To precisely tune the ion-optical condition to be isochronous, the resonant Schottky noise pick-up technique will be employed. We performed an off-line test of the resonant Schottky pick-up. Figure 1 shows the resonant Schottky pick-up that will be installed in the Rare-RI Ring. It consists of a pillbox-type resonant cavity electrically isolated from the beam pipe by a ceramic tube. A schematic view of the pick-up is shown in Fig. 2(a): a chamber shown in blue is the beam pipe and the shaded cylinder surrounding the beam pipe is the cavity equipped with two ports (yellow). The ports are movable plunger pistons that can adjust the resonance frequency (fres) of the eigenmode. Fig. 2(b) shows the cross-sectional view of the cavity, and the detailed structure of the gap can be seen at the center. The cavity itself is filled with air and has the shape of a pillbox with an outer diameter of 750 mm and length of 200 mm. The inner diameter is 320 mm. The lower flanges ( see Fig. 1 ) are prepared for feedthroughs to take out signals from a loop coil that magnetically couples to the cavity field induced by the beam. Using a network analyzer, we measured the basic quantities characterizing the resonant cavity: the resonance frequency, the shunt impedance Rsh, and the unloaded Q factor Q0. To measure Rsh, the perturbation method was adopted. From the measurements, fres = 171.54(±0.44) MHz, Rsh = 169 kΩ, and Q0 = 1884 were obtained. For tuning the isochronous field settings, the proposed pick-up is required to have an excellent singleion sensitivity. By using the results of the off-line test, the output signal power corresponding to a single ion with charge q at resonance is estimated to be P = q × 2.8 × 10−21 W , and the power of thermal noise Pnoise is 7.1 × 10−19 W. For q ≥ 16, the signal power exceeds the noise floor, and the signal from the beam can be detected by the present Schottky pickup. Therefore, the performance is sufficient for precise tuning of isochronus field settings of the Rare-RI Ring. The resonant Schottky pick-up will be soon installed into the Rare-RI Ring. Detailed results of the off-line test and online beam performance test will be reported

A resonant Schottky pick-up for Rare-RI Ring at RIKEN

The Rare-RI Ring project has been launched at RIKEN. The Rare-RI Ring is a storage ring specially designed for the isochronous mass spectrometry of unstable nuclei. Precise mass measurements () are necessary to reveal the r-process path and, therefore, the ion-optical conditions must be tuned to yield isochronicity of order 10−6. For this purpose, we employ a highly sensitive resonant Schottky cavity as a probe for single-ion detection. Here, we first explain this technique theoretically and derive the necessary equations. Then, based on the results of off-line tests, we determine the sensitivity of the Schottky pick-up and estimate the intensities of the signals induced inside the cavity.

Isochronous field study of the Rare-RI Ring

Construction of the Rare-RI Ring to measure masses of short-lived rare-RI with a relative precision of 10−6 is in progress at RIKEN. The Rare-RI Ring consists of six sectors where each sector consists of four dipole magnets. Since the mass measurement is done by the isochronous mass spectrometry in the Rare-RI Ring, creating isochronous magnetic field is one of the important issues in mass measurements with the Rare-RI Ring. In order to make an isochronous field, we installed ten trim coils in the two outer dipoles among the four dipoles in each sector magnet. The isochronism of the magnetic field have been confirmed by measuring time-of-flight (TOF) of alpha particles from an alpha-source (241Am). We measured TOF of alpha particles while changing the radial gradient of the magnetic field by trim coils and evaluated the isochronism from standard deviation of the TOF spectrum. The TOF width is minimum for a radial gradient of magnetic field (/)/B0 = 0.205 m−1, which is in good agreement with the simulated value.