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RIKEN Accel. Próg. Rep. 24 (1990)

111-5-10. Velocity Distribution of IGISOL lon Beams

M. Koizumi, M. Azuma, T. Inamura, T. Ishizuka, H. Katsuragawa, S. Matsuki, K. Morita, T. Murayama, I. Nakamura, M. Nakaoka, K. Shimomura, I. Sugai, M. Takami, K. Yalli, and A. Yoshida

GARIS/IGISOL at RIKEN consists of an ion-guide isotope separator on-line (RIKEN IGISOL) constructed in tandem with a gas-filled recoil isotope separator (GARIS). The design of this system was described in a previous report.⁰ This system is potentially powerful in separating refractory elements as well as short-lived species because the separation time is of the order of one millisecond, and equal separation efficiency is obtained for isotopes of any Chemical elements. Therefore, we are interested in making collinear laser spectroscopy for isotopes of a wide rangę of elements using this system. It has been esti-mated from mass resolution measurements that the energy spread of IGISOL ion beams is 100 eV or more,^2,3) which corresponds to about a 1-GHz resolution at fwhm for ⁴⁰Ar¹⁺. The resolution is not sufficient enough for doing laser spectroscopy; about 500 MHz or less is desirable.

Since the energy spead has so far been esti-mated indirectly from mass resolution, it is desirable to carry out a direct measurement of the velocity distribution of IGISOL ion beams. We have measured the Doppler broadening of a reso-nance linewidth in a metastable atomie State of 4°A_r¹⁺ (611.6 nm (3d^,2G₉/2^_>4p²F_7/2) transition)⁴⁾to observe the velocity distribution of ion beams. A schematical setup is shown in Fig. 1. The energy of ions were 30 keV, and the pressure in a helium gas celi was 20 mbar. A resonance spectrum measured with 150° incidence is shown in Fig. 2. The fwhm is 0.68 GHz, which corresponds to an energy spread of 44 eV. The fwhm of laser induced fluorescence (LIF) spectra sys-tematically measured at three incident angles

IGISOt. D-magnet

Slr.s

Elecrosuiic Mm Ar.aly/er

Fig. 1. Schematic setup of an LIF experiment.

O 20 40 60 BO 100 1?0

Channtl (0.11GHx/ch.)

Fig. 2. LIF excitation spectrum of the metastable State in 30-keV Ar¹⁺ ions at 150’ to a laser beam. The resonance was observed at 16330.96 cm^-1.

(30°, 90° and 180°) are plotted as a function of the skimmer voltage in Fig. 3. The divergence of the ion beam is deduced from the measurement at 90°, and the velocity distribution from that at 180°. The fwhm measured at 30° has both contri-butions. The broadening caused by beam diver-gence inereases with inereasing skimmer poten-tial. Although the broadening caused by velocity distribution also inereases with an inerease in the skimmer potential, it is smaller than that by beam divergence. A typical value of the Doppler broadening caused by only velocity distribution (collinear (180°) at a skimmer potential of 500 V and a pressure of 20 mbar) was 1.25 GHz in fwhm, which corresponds to an energy spread of 120 eV. Since the gas pressure is too Iow to stop most of the recoil products, a series of measurements are now in progress at higher pressures.

We conclude that the linewidth in the LIF spectrum is narrower at lower skimmer potential. One way to reduce the velocity spread of ion beams in order to get a better resolution in LIF measurements is to select a part of the total velocity distribution by using slits set behind a magnetic analyzer. The spectrum in Fig. 2 was measured in this way. By this method, however, we lose a certain amount of ions for the measurements. The other possibility to reduce the veloc-