Sun-Chan Jeong

Simulation Study on the Measurements of Diffusion Coefficients in Solid Materials by Short-lived Radiotracer Beams

We have examined, by a computer simulation, an on-line measurement of diffusion coefficients by using a short-lived alpha particle emitter, 8Li (half life of 0.84 s), as a radiotracer. The energy spectra of alpha particles emitted from diffusing 8Li primarily implanted in the sample of LiAl are simulated as a measure of the diffusion of 8Li in the sample. As a possible time sequence for the measurement, a time cycle of 6 s, i.e. the implantation of 8Li for 1.5 s and subsequent diffusion for 4.5 s, is supposed. The sample is primarily set on a given temperature for the measurement. The time-dependent yields of alpha particles during the time cycle reveal the possibility to measure the diffusion coefficient with an accuracy of 10% if larger than 1×10-9 cm2/s, by the comparison with the experimental spectra measured at the temperature, i.e. at a certain diffusion coefficient.

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.

Advanced focal plane detector with improved particle identification

Abstract A focal plane detector for a magnetic spectrograph was developed, which consists of two position counters, two energy-loss counters, and large-area silicon detectors. The energy, energy-loss and time-of-flight signals from this detector system depend on the particle incident angle into the counter and the position on the focal plane. Corrections to the angle and position dependence have improved the resolutions of these signals significantly and given unambiguous particle identification for studying reactions which have very small yields under intense backgrounds. The incident angle into the focal plane detector is found to be very useful to determine the scattering angle of reaction products.

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.

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.

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.

On-Line Diffusion Tracing in Li Ionic Conductors by the Short-Lived Radioactive Beam of 8Li

A non-destructive and on-line diffusion tracing in Li ionic conductors has been demonstrated. As the tracer, the pulsed beam of the short-lived α-emitting radioisotope of 8Li was implanted into a typical Li ionic conductor LiGa. By analyzing the time-dependent yields of the α-particles measured in coincident with the repetition cycle of the beam, the tracer diffusion coefficients were extracted with a good accuracy. The ordering of the Li vacancies in the Li-deficient β phase of LiGa was observed for the first time in terms of the Li diffusion by the present method.

A New Measurement of the 8Li(α,n)11B Reaction for Astrophysical Interest

The 8Li(α,n)11B reaction has been measured directly and exclusively in the energy region of Ecm=0.45–1.75 MeV by using highly efficient detector system covering Ecm= 0.56 MeV, which corresponds to the Gamow window at T9=1. This experiment has been performed in the condition of inverse kinematics by using low‐energy radioactive 8Li beam at the Tandem accelerator facility of Japan Atomic Energy Research Institute. The reaction cross section obtained in the present measurement is consistent with that of the previous exclusive measurements within the errors in an overlapping energy region, but is less than half of that of the inclusive measurements, in particular for lower energy region.

Study of astrophysical (α, n) reactions using light‐neutron rich radioactive nuclear beams

A systematic study of astrophysical reaction rates of (α, n) reactions on light neutron‐rich nuclei using low‐energy radioactive nuclear beams is in progress at the tandem facility of Japan Atomic Energy Agency. Exclusive measurements of 8Li(α, n)11B and 12B(α, n)15N reaction cross sections have been performed successfully. Their excitation functions together with the experimental method are presented.