Velocity distribution of ion beams from the RIKEN ion-guide isotope separator on-line 理研イオンガイド型オンライン同位体分離器からのイオンビームの速度分布

博士論文

The nuclei close to the N = 104 show an interesting feature. Extremely large shape staggering in the isotope and isomer shifts was observed for Hg and Tl isotopes, and shape coexistence at low excitation energy was observed for Hg, Au, and Pt isotopes. It should also be noted that many high-spin isomers exist in the nuclei of this region, such as Hf, W and Os. For many elements in this region, however, laser spectroscopy has not been reported, because these elements are refractory ones. At RIKEN, we have constructed an ISOL system GARIS/IGISOL which is powerful to extract radioactive isotopes of refractory elements. Taking full advantages of this feature, we aim at performing collinear laser spectroscopy to study exotic nuclei of these elements. To make on-line collinear laser spectroscopy, the radioactive beams from the GARIS/IGISOL is required to have adequate quality (velocity spread) and quantity. Therefore, direct measurement of the velocity spread of Ar1+ ion beams from the IGISOL was carried out with a technique of laser spectroscopy. The skimmer-potential and the gas-cell pressure dependences of the velocity spread were systematically investigated. A new focusing device SQUEEZER was introduced to reduce the velocity spread, and the effect of this was experimentally examined. From these measurements, it was pointed. out that the ion beam from the GARIS/IGISOL has adequate quality for making collinear laser spectroscopy. An estimation of the yield of radioactive isotopes from the GARIS/IGISOL was also made for a fusion reaction 9Be(166Er, xn)175-xHf . The estimated yield was expected to be more than 104 particles/s. The yield of the radioactive isotope is adequate to make collinear laser spectroscopy with a coincidence method. We have for the first time in the world extracted the radioactive isotope of refractory element 169Hf with the GARIS/IGISOL. We conclude, thus, that the collinear laser spectroscopy of refractory elements with the GARIS/IGISOL is promising.

Contents / p1 1 Introduction / p1 2 Principle of measurement / p5 　2.1 Doppler shift and Doppler broadening / p5 　2.2 Mass resolving power (MRP) / p8 3 Experimental / p11 　3.1 Location of IGISOL / p11 　3.2 SQUEEZER / p11 　3.3 Discharge ion source / p14 　3.4 Laser system / p18 　3.5 Experimental setup for velocity measurement / p18 　3.6 Production of radioactive isotope of a refractory element: ¹⁶⁹Hf / p22 4 Results and discussion / p24 　4.1 LIF spectrum of Ar¹⁺ / p24 　4.2 Velocity spread and beam divergence / p24 　4.3 Skimmer-voltage dependence of the velocity spread / p27 　4.4 The effect of SQUEEZER / p28 　4.5 He-gas pressure dependence of the velocity spread / p29 　4.6 Comparison with the mass resolving power / p30 　4.7 Velocity distribution of a collimated beam / p35 　4.8 Mass dependence of the MRP / p35 　4.9 Yield of radioactive isotopes from the GARIS/IGISOL / p37 　4.10 Feasibility of on-line collinear laser spectroscopy / p38 5 Conclusion / p41 Appendix / p43 　A Collinear laser spectroscopy setup for on-line experiment / p43 　B High-resolution off-line laser spectroscopy setup for refractory elements / p48 Acknowledgment / p52 References / p53