F. Suzaki

High-resolution measurement of the time-modulated orbital electron capture and of the β+ decay of hydrogen-like 142Pm60+ ions

Abstract The periodic time modulations, found recently in the two-body orbital electron capture (EC) decay of both, hydrogen-like 140Pr58+ and 142Pm60+ ions, with periods near to 7 s and amplitudes of about 20%, were re-investigated for the case of 142Pm60+ by using a 245 MHz resonator cavity with a much improved sensitivity and time resolution. We observed that the exponential EC decay is modulated with a period T = 7.11 ( 11 ) s , in accordance with a modulation period T = 7.12 ( 11 ) s as obtained from simultaneous observations with a capacitive pick-up, employed also in the previous experiments. The modulation amplitudes amount to a R = 0.107 ( 24 ) and a P = 0.134 ( 27 ) for the 245 MHz resonator and the capacitive pick-up, respectively. These new results corroborate for both detectors exactly our previous findings of modulation periods near to 7 s , though with distinctly smaller amplitudes. Also the three-body β + decays have been analyzed. For a supposed modulation period near to 7 s we found an amplitude a = 0.027 ( 27 ) , compatible with a = 0 and in agreement with the preliminary result a = 0.030 ( 30 ) of our previous experiment. These observations could point at weak interaction as origin of the observed 7 s -modulation of the EC decay. Furthermore, the data suggest that interference terms occur in the two-body EC decay, although the neutrinos are not directly observed.

Between atomic and nuclear physics: radioactive decays of highly-charged ions

Highly charged radioactive ions can be stored for extended periods of time in storage rings which allows for precision measurements of their decay modes. The straightforward motivation for performing such studies is that fully ionised nuclei or few-electron ions can be viewed as clean quantum-mechanical systems, in which the interactions of the many electrons can be either excluded or treated precisely. Thus, the influence of the electron shell on the decay probability can be investigated. Another important motivation is stellar nucleosynthesis, which proceeds at high temperatures and the involved atoms are therefore highly ionised. Presented here is a compact review of the relevant experiments conducted at heavy-ion storage rings. Furthermore, we outline the perspectives for future experiments at new-generation storage-ring facilities.

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.

Commissioning of the Rare-RI Ring at RIKEN RI Beam Factory

The Rare-RI Ring (R3) is an isochronous storage ring to measure masses of short-lived rare nuclei by using a TOF method*. The expected precision of the measured mass will be of the order of ppm. A commissioning run using a 78Kr beam was performed in June 2015 and basic performances of R3 were verified. We succeeded in injecting a particle, which was randomly produced from a DC beam from cyclotrons, into the R3 individually** with a fast kicker system***, and in extracting the particle from the R3 1 ms after the injection. We measured TOF of the 78Kr particles between the entrance and the exit of the R3 to check the isochronism. Through the first-order adjustment with trim-coils imbedded on the dipole magnets of the R3, the isochronism on the 10-ppm order was achieved for the momentum spread of ±0.2 %. Higher-order adjustment employed in future will lead us to the isochronism on the order of ppm. In addition, we confirmed that a resonance-type Schottky pick-up successfully acquired the revolution frequency information of one particle in a storage mode. In this conference, the technical aspects of the R3 and prospects from the results of the beam commissioning will be discussed.

Performance of a Resonant Schottky Pick-up in the Commissioning of Rare-RI Ring

Rare-RI Ring was constructed at RIKEN RIBF for precise isochronous mass spectrometry of unstable nuclei. In June 2015, we performed the first commissioning of the ring using 78Kr beam with the energy of 168 MeV/nucleon. We successfully carried out the individual injection which is one of the characteristics of the ring, and also we succeeded in the storage of 78Kr ions for a few seconds. We evaluated the performance of the resonant Schottky pick-up which was installed in the Rare-RI Ring. The purpose of the resonant Schottky pick-up is a monitor for tuning of the isochronous field in the ring. The resonant Schottky pick-up detected single 78Kr ions, where the frequency resolution was 1.29×10−6 (FWHM). The resolution is in the same order of the required isochronicity. The sensitivity and resolution of the resonant Schottky pick-up are sufficient for the tuning of isochronous optics.

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.