岡田 邦宏

Although the projectile fragment separator (RIPS) at the RIKEN accelerator facility (RARF) provides wide variety of energetic radioactive beams, it is not adequate for low energy beam experiments such as precision spectroscopy of trapped ions, Aiming for an efficient deceleration and cooling of energetic radioactive ions, the development of an rf ion guide system which comprises a large He gas cell and an rf funnel structure in the cell is under progress. A proof of the principle (pop) machine has been successfully tested on-line for collection of 70-MeV/u Li-8 ions. A compact gas cell of 70 cm in length and 10 cm in diameter with a He gas pressure of 30 Torr was placed after a variable wedge shaped energy degrader. The ions were stopped in the gas and were transported a superposition of dc and rf electric field toward a small exit hole, The overall efficiency was 10(-4) which can be separated into a gas stopping efficiency of 0.43 % and an ion-guide efficiency of 2.4 %. The former is limited by the volume of the cell and the pressure of the He gas. The latter is due to the present rf voltage limitation of 40 V. Both efficiency could be increased up to 10 % range by larger cell, higher pressure. and higher rf voltage, which would yield an expected overall efficiency of 1 %.

A cryogenic linear ion trap was developed for the minimization of the loss of trapped Be+ ions. Buffer gas cooling as well as laser cooling for light element ions was tested in the cryogenic trap. The lifetime for the He-gas-cooled Be+ ions was longer than 15 min with a liquid-He-cooled trap, although they are known to be difficult to cool with a buffer gas in a conventional trap. A laser-cooled ion crystal was maintained for approximately 30 min without any ion loss.

Ion traps have been pivotal in opening new frontiers for the precision spectroscopy of stable ions. We report on the demonstration of an additional ion trap: the linear combined trap. This device is particularly well suited for trapping ions with unstable nuclei, due to its large range of stability parameters that facilitates external injection online to an accelerator. The motion of an ion in such a linear combined trap is investigated theoretically and experimentally. In the trap ions oscillate harmonically in the radial direction and move nearly harmonically between fixed boundaries along the longitudinal axis. The presence of a homogeneous magnetic field and the applied dc and rf electric fields, lead to a set of coupled Mathieu equations. Their approximate solutions exhibit motional frequencies, which are combinations of shifted macromotion frequencies and the cyclotron frequency. The dependence of these motional frequencies on the applied trapping fields was studied in detail. For the measurements we used small clouds of laser cooled Be-9(+) ions and crystallized Be-9(+) ions. The observed oscillation frequencies are compared to both the results of zeroth order analytical solutions and to numerical simulations in which the frequency spectrum was obtained from the ion orbits by using the fast fourier transform formalism. The various motional resonances were experimentally recorded by applying a weak dipole excitation field to one of the trap electrodes, and by simultaneously observing at resonance the changes in the fluorescence intensity. Depending on the detuning of the cooling laser the ions gain energy during the motional excitation process in such a way that they are shifted further in or out of optical resonance by the Doppler effect. This leads to either positive or negative ion motion signals at the various ion oscillation frequencies. (C) 2001 American Institute of Physics.

A long series of simulations of hairpin-filament guns as used in electron spectrometers was performed for filament-anode distances from 2 to 10 mm and acceleration voltages from 10 to 100 V using Child's law approximation, space-charge effects and rotational symmetry. The role of the Wehnelt electrode is clarified. Two distinct operation modes, the laminar (with positive Wehnelt voltages) and scrambled (with negative Wehnelt voltages) modes, are described and, for each, typical equations for the dependence of the current on the filament-anode distance and anode voltages are given.

Laser-microwave double-resonance spectroscopy of laser-cooled Be-9(+) ions in a linear Paul trap has been performed aiming at the development of both highly sensitive and precise methods to be applied to the unstable isotopes. By the alternative irradiation of laser and microwave radiation (pulsed-wave method), high accuracy of 4 x 10(-7) was obtained in the measurement of the ground-state magnetic-dipole hyperfine constant (A) of Be-9(+) ion in a weak magnetic field. By the continuous-wave method, high sensitive detection of the microwave resonance was demonstrated. These methods can be complementarily applied to the measurements of unknown hyperfine constants of the unstable isotopes for studying the hyperfine anomaly.

In order to investigate the nuclear magnetic moments and the magnetization distributions in the unstable nuclei of Be-7,Be-11, laser-microwave spectroscopy experiments using an ion-trap are under progress at IPNS, KEK-Tanashi. Status of the experiment for the first unstable isotope Be-7 is reported.

For an investigation concerning the nuclear magnetic moments and their distributions in the nuclei of Be-7,Be-11, an experimental project using laser-microwave double resonance spectroscopy for trapped ions is under progress at INS. Laser cooled ion crystals consisting of a few Be-9(+) ions were observed and the ground state hyperfine splitting of Be-9(+) was measured with a precision of 10(-5). Experiments on the unstable isotope Be-7 have been started.

A new type of linear ion trap system in combination with a sextupole ion beam guide (SPIG) has been developed for the accumulation of a continuous ion beam. An axial trapping potential is generated by three cylindrical electrodes mounted outside, but concentric to, the SPIG. The ions in a continuous beam are trapped in the potential well via collisions with He gas. This linear ion trap is called the ''SPIG-trap''. The properties of the SPIG-trap were tested off-fine with a discharge ion source in a gas cell. Up to 10(6) ions were trapped and extracted as a bunch with an efficiency of 10% for 10 ms of accumulation. The SPIG-trap system was then connected to a gas filled recoil mass separator (GARIS) and tested with an energetic primary beam from a cyclotron (O-16, 107 MeV). The overall efficiency of the SPIG-trap including ion losses in an energy degrader, which was placed in front of the He cell, was measured to be of the order of 10(-5) for 10 ms accumulation.

We report the results of laser-microwave double resonance spectroscopy of laser cooled Be-9(+) ions trapped in a linear rf trap. The microwave transition frequency between the sublevels of the Be-9(+) ground state hyperfine structure in a weak magnetic field was measured with a precision of 5 x 10(-6). This experiment has been carried out as a first step of our project to study the hyperfine anomaly of unstable nuclear ions at INS.

The hyperfine structure of atoms informs us various static characteristics of nuclei, particularly for electro-magnetic moments and their distributions. We have been developing an experimental method to perform laser-microwave double-resonance spectroscopy for the hyperfine structure of Be and Ca isotopes, including unstable nuclei. The purpose and the status of the experiments are described.

Multiply charged ions of Ta were produced by using laser ablation,and were stored in a radio-frequency ion trap. The charge states of the ions stored range from q = +1 to +6 for tantalum. The experimental apparatus for trapping and probing multiply charged ions is described, and results for the measurements on the charged-state spectra of trapped ions and storage time are presented.

An on-line ion trap system is under construction at INS in order to study properties of the trapped unstable nuclei produced by a cyclotron beam. Among several subjects on the system, development of a direct trapping method for unstable nuclear ions from a recoil separator is taken to be of first priority. We have successfully developed a new ion cooling device for this purpose using an ion-guide followed by an RF multipole ion beam guide.