Kunihiro Okada

Slow RI-beams from projectile fragment separators

Abstract The projectile fragment separator provides a wide variety of short-lived RI-ions with less restrictions on their chemical property or lifetime limit. The beam energy and quality is, however, not adequate for low-energy beam experiments, in particular for trapping experiments. Recently, one has proposed to obtain a low-energy beam from an energetic RI-beam leaving a projectile fragment separator by using a large gas-catcher and an rf ion-guide system. In off-line and in on-line test experiments, the principle of the rf ion-guide was proven. An overall efficiency of 0.2% for 70 MeV/u 8 Li from the RIKEN projectile fragment separator (RIPS) was obtained so far.

Space-charge effects in the catcher gas cell of a rf ion guide

Slow radioactive ion beams have been produced with an overall efficiency of 4% by thermalizing energetic ions produced by a projectile fragment separator in a He-gas cell and guiding them to a vacuum vessel by dc and rf fields. Space charge was observed to have a limiting effect. Since the ionization of He atoms by energetic ions creates a region of high space charge, many thermalized ions of interest are pushed toward the walls of the gas cell. Such losses have been investigated for different incoming ion intensities.

Accumulation of ions from a recoil mass separator in a new type of linear ion trap

Abstract 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-line with a discharge ion source in a gas cell. Up to 106 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 (16O, 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.

Precision spectroscopy of the Zeeman splittings of the 9Be+22S1/2 hyperfine structure for nuclear structure studies

Abstract A precision measurement for the hyperfine Zeeman splittings of 9 Be + ions, trapped and laser cooled in a linear combined ion trap was performed by using laser–rf double and triple resonance methods. The accuracies of 10−9 for the nuclear spin flip transitions and of 10−8 for the electron spin flip transitions were achieved at an arbitrary value of strong magnetic field of ∼0.47 T. This enables us to determine the hyperfine constant A and the ratio of nuclear g-factor to electronic g-factor with accuracies of 10−9 and 10−7, respectively. To show the wider applicability of our method, the particular strength of the magnetic field where the nuclear spin flip transition is independent of the magnetic field strength was intensionally not used. The confining fields of the linear combined trap have the advantage of simple and efficient concentration of the ions into the potential minimum at the center of the trap, when laser cooling of the stored ions is performed. The feasibility of single ion spectroscopy was also demonstrated and, when fully implemented will facilitate the on-line study of short lived isotopes, which can only be produced in minute numbers.

Restoration of RI‐beams from a projectile fragment separator by Laser Ionization gas Catcher‐PALIS‐

A fragment separator at heavy ion accelerator facilities is a versatile instrument to provide wide variety of radioactive isotope (RI) beams. However, more than 99.99% of precious RI‐ions are simply dumped in the slits or elsewhere in the fragment separator. A novel concept to restore such RI‐ions for parasitic slow RI‐beams is proposed. Installation of a laser ionization gas catcher in the vicinity of the first or second focal point of the fragment separator enables to collect dead isotopes in the slits. The design concept and expected performance are discussed.

Cryogenic Ion Trap for Minimization of Trapped Ion Loss.

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 dynamics and oscillation frequencies in a linear combined trap

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 w...

Resolution improvements for hemispherical energy analyzers

An electron tracing code in spherical coordinates was used to treat irregular, nonspherical boundaries in the “Herzog regions” at the exit and entrance planes of hemispherical energy analyzers. Some of the classical designs, i.e., the sudden cutoff “ideal field” approximation, real apertures in the 180° plane, the Herzog correction, the Jost correction, and shortened spheres were compared and checked for improvements. Ratios of gap to average radius from 0.2 to 0.55 are covered. Focusing, maximum trace width, base resolution, and the banana-shaped asymmetries from the circular reference path are investigated. Nearly ideal focusing and beam width can be restored in all devices by simply rotating the entrance beams through a small negative angle. Design equations are provided.

First Direct Mass Measurements of Nuclides around Z=100 with a Multireflection Time-of-Flight Mass Spectrograph

The masses of ^{246}Es, ^{251}Fm, and the transfermium nuclei ^{249-252}Md and ^{254}No, produced by hot- and cold-fusion reactions, in the vicinity of the deformed N=152 neutron shell closure, have been directly measured using a multireflection time-of-flight mass spectrograph. The masses of ^{246}Es and ^{249,250,252}Md were measured for the first time. Using the masses of ^{249,250}Md as anchor points for α decay chains, the masses of heavier nuclei, up to ^{261}Bh and ^{266}Mt, were determined. These new masses were compared with theoretical global mass models and demonstrated to be in good agreement with macroscopic-microscopic models in this region. The empirical shell gap parameter δ_{2n} derived from three isotopic masses was updated with the new masses and corroborates the existence of the deformed N=152 neutron shell closure for Md and Lr.

Acceleration of the chemical reaction of trapped Ca+ ions with H2O molecules by laser excitation

The reactivity of laser-excited Ca+* ions in 3 2DJ (J = 3/2, 5/2) states with neutral H2O molecules was experimentally investigated. The reaction-product ions were sympathetically cooled by laser-cooled 40Ca+ ions and detected by laser-induced fluorescence mass spectroscopy. The reaction rate coefficient of Ca+*(DJ) + H2O → products was found to be 1.8(1) × 103 s−1 Pa−1 at an ion temperature of T = 3.1(1) × 103 K. The ion temperature dependence of the reaction rate coefficient was also measured for buffer-gas-cooled Ca+ ions.