N. Imai

Measurement of the first ionization potential of astatine by laser ionization spectroscopy

The radioactive element astatine exists only in trace amounts in nature. Its properties can therefore only be explored by study of the minute quantities of artificially produced isotopes or by performing theoretical calculations. One of the most important properties influencing the chemical behaviour is the energy required to remove one electron from the valence shell, referred to as the ionization potential. Here we use laser spectroscopy to probe the optical spectrum of astatine near the ionization threshold. The observed series of Rydberg states enabled the first determination of the ionization potential of the astatine atom, 9.31751(8) eV. New ab initio calculations are performed to support the experimental result. The measured value serves as a benchmark for quantum chemistry calculations of the properties of astatine as well as for the theoretical prediction of the ionization potential of superheavy element 117, the heaviest homologue of astatine.

Pathway for the Production of Neutron-Rich Isotopes around the N=126 Shell Closure

Absolute cross sections for isotopically identified products formed in multinucleon transfer in the (136)Xe+(198)Pt system at ∼8u2009u2009MeV/nucleon are reported. The isotopic distributions obtained using a large acceptance spectrometer demonstrated the production of the hard-to-reach neutron-rich isotopes for Z<78 around the N=126 shell closure far from stability. The main contribution to the formation of these exotic nuclei is shown to arise in collisions with a small kinetic energy dissipation. The present experimental finding corroborates for the first time recent predictions that multinucleon transfer reactions would be the optimum method to populate and characterize neutron-rich isotopes around N=126 which are crucial for understanding both astrophysically relevant processes and the evolution of magic numbers far from stability.

KEKCB electron cyclotron resonance charge breeder at TRIACa)

The KEKCB is an electron cyclotron resonance (ECR) ion source for converting singly charged ions to multicharged ones at Tokai Radioactive Ion Accelerator Complex. By using the KEKCB, singly charged gaseous and nongaseous ions were converted to multicharged ones of A/q approximately 7 with efficiencies of 7% and 2%, respectively. The conversion efficiency was found to be independent of the lifetime of the radioactive nuclei having lifetimes of the order of one second. Three collimators located at the entrance and the exit of the KEKCB defined the beam axis and facilitated beam injection. Grinding and washing the surfaces of aluminum electrode and plasma chamber dramatically reduced impurities originating from the ECR plasma of the KEKCB.

Ionization cross section measurements for autoionizing states of iridium and rhenium

New autoionizing states for neutral iridium and rhenium atoms were observed in the continuum structure near the first ionization limit by using a two-colour two-step laser resonance ionization technique. The saturation curves for the autoionizing states were measured, and the ionization cross sections were experimentally determined by solving the rate equations for the ground, intermediate, and autoionizing state populations. Efficient ionization schemes are expected to be useful for applications such as laser ion sources of unstable nuclei.

Toward Online Nanoscale Diffusion Measurements Using Radioactive 8Li Tracer

We have examined the feasibility of a new online nanoscale diffusion measurement method using a radioactive 8Li tracer by computer simulations. We have found that the detection limit of the lithium diffusion coefficient can be improved to a low value of 1×10-12 cm2/s by detecting α particles emitted at a small angle relative to a sample surface that is irradiated with a low-energy 8Li beam of about 8 keV.

GEM-MSTPC: An active-target type detector in low-pressure He/CO2 mixed gas

We developed an active-target type gas counter operating with low pressure He/CO2(10%) detector gas for application in studying low-energy nuclear reactions using radioactive nuclear beams (RNBs). A 400-?m-thick gas electron multiplier (THGEM) was used as the proportional counter for high injection rate capability. We examined the gas gain stability and the influence of ion feedback on particle tracks at high beam injection rates of up to 105 particles per second (pps) using a low-energy 12C beam. By means of modifications to the THGEM, the shift in the relative pulse height of the output signal normalized to that measured at the 12C injection rate of several hundred pps improved to within 2% at the rate of 105 pps. The position distortion induced by ion feedback was suppressed to within 1.5 mm because of the relatively high drift field value of 1 kV/cm?atm and the use of the double GEM configuration. The THGEM was found to be applicable for our active target at high injection rates of up to 105 pps.

Nanoscale diffusion tracing by radioactive 8Li tracer

We have developed a nanoscale diffusion measurement method using an α-emitting radioactive 8Li tracer. In this method, while implanting a pulsed 8 keV 8Li beam, the α particles emitted at a small angle (10°) relative to the sample surface were detected as a function of time. The method has been successfully applied to measuring lithium diffusion coefficients for an amorphous Li4SiO4–Li3VO4 (LVSO) thin film with a thickness of several hundred nanometers, demonstrating that the present method is sensitive to diffusion coefficients down on the order of 10−12 cm2/s, which is more sensitive by about two orders of magnitude than that previously achieved.

Development of the GEM‐MSTPC for studies of astrophysical nuclear reaction rates

We have developed an active‐target type gas‐detector, a Gas Electron Multiplier Multiple‐Sampling and Tracking Proportional Chamber (GEM‐MSTPC) operating with low‐pressure He‐base mixed gas, where He is used as a target for studies of astrophysical nuclear reaction rates. Different kinds of 400 μm thick GEMs were examined. The gain stability was examined with a configuration of GEMs of 400 μm in thickness fabricated in different ways, against the injection rate of low‐energy heavy ions of 105 particles per second. The gain of GEM with Cu electrodes coated by Au was observed to be stable up to the injection rate of 105 particles per second.We have developed an active‐target type gas‐detector, a Gas Electron Multiplier Multiple‐Sampling and Tracking Proportional Chamber (GEM‐MSTPC) operating with low‐pressure He‐base mixed gas, where He is used as a target for studies of astrophysical nuclear reaction rates. Different kinds of 400 μm thick GEMs were examined. The gain stability was examined with a configuration of GEMs of 400 μm in thickness fabricated in different ways, against the injection rate of low‐energy heavy ions of 105 particles per second. The gain of GEM with Cu electrodes coated by Au was observed to be stable up to the injection rate of 105 particles per second.

Diffusion Experiment in Lithium Ionic Conductors with the Radiotracer of 8Li:from Micro‐ to Nano‐diffusion

We have developed a radiotracer method for diffusion studies in lithium ionic conductors, by using, as the tracer, the short‐lived α‐emitting radioisotope of 8Li from TRIAC (Tokai Radioactive Ion Accelerator Complex). In the method, we measured α‐particles coming out of the sample of interest and have found that the time‐dependent yields of α‐particle from the diffusing 8Li primarily implanted is a good measure of the Li diffusion in the sample. The method has been successfully applied to measure the lithium diffusion coefficients in a typical defect‐mediated lithium ionic conductor of LiGa, well demonstrating that the method is very efficient to measure the diffusion in the micro‐meter regime per second. Further development, as an extension of the present method, was proposed to measure the diffusion on the nanoscale in lithium ionic conductors.

A Systematic Study of Astrophysical Reaction Rates through 8Li

Excitation functions of 8Li(α,n), (d, t) and 12B(α,n) reactions were directly measured in the energy region of astrophysical interest using low‐energy radioactive nuclear beams of 8Li and 12B. Each measured excitation function is strongly affected by one or more resonances through a compound nucleus. The measured excitation functions are presented. Dominant r‐process paths through 8Li at various temperatures are discussed and our future experimental plan is also presented.