S. C. Jeong

Development of a resonant laser ionization gas cell for high-energy, short-lived nuclei

Abstract A new laser ion source configuration based on resonant photoionization in a gas cell has been developed at RIBF RIKEN. This system is intended for the future PArasitic RI-beam production by Laser Ion-Source (PALIS) project which will be installed at RIKEN’s fragment separator, BigRIPS. A novel implementation of differential pumping, in combination with a sextupole ion beam guide (SPIG), has been developed. A few small scroll pumps create a pressure difference from 1000xa0hPa–10−3xa0Pa within a geometry drastically miniaturized compared to conventional systems. This system can utilize a large exit hole for fast evacuation times, minimizing the decay loss for short-lived nuclei during extraction from a buffer gas cell, while sufficient gas cell pressure is maintained for stopping high energy RI-beams. In spite of the motion in a dense pressure gradient, the photo-ionized ions inside the gas cell are ejected with an assisting force gas jet and successfully transported to a high-vacuum region via SPIG followed by a quadrupole mass separator. Observed behaviors agree with the results of gas flow and Monte Carlo simulations.

Ion source development for the JAERI on-line isotope separator

Abstract In accordance with the JAERI-KEK joint radioactive nuclear beam (RNB) project, we have installed a forced electron beam induced arc discharge (FEBIAD) type integrated-target–ion source in the JAERI-ISOL to produce heavy neutron-rich RNBs with particle-induced fission of 238 U. In the present ion source, a 2.6-g 238 UC 2 target is directly attached to a plasma chamber. RNBs of 8 Li, 18 F and 20 F are also planned to be used for the study of thermal diffusion in materials in the joint project. Release profiles of 7 Li have been studied using a cavity type thermal ion source. For mass-separation of F atoms, the molecular ion formations have been examined using the FEBIAD ion source.

Test results of 18 GHz ECR charge breeder for KEK–JAERI RNB facility

Some of the test results of KEK ECR charge breeder made for KEK–JAERI RNB facility are presented and discussed. The charge breeding efficiencies for singly charged ions of noble gases have been measured; typically 13.5% of Ar9+, 10.4% of Kr12+, and 6.8% of Xe20+ with a global efficiency of about 50%. By injecting pulsed Xe1+ ion beam into the charge breeder, the time necessary for charge breeding has been also measured and found to be about 60 ms for Xe21+ ions. As an effort to reduce the residual gas ions incidentally existing in the plasma chamber of the charge breeder a nonevaporable getter has been tested, and the preliminary results are discussed.

KEKCB electron cyclotron resonance charge breeder at TRIACa)

The KEKCB is an electron cyclotron resonance (ECR) ion source for converting singly charged ions to multicharged ones at Tokai Radioactive Ion Accelerator Complex. By using the KEKCB, singly charged gaseous and nongaseous ions were converted to multicharged ones of A/q approximately 7 with efficiencies of 7% and 2%, respectively. The conversion efficiency was found to be independent of the lifetime of the radioactive nuclei having lifetimes of the order of one second. Three collimators located at the entrance and the exit of the KEKCB defined the beam axis and facilitated beam injection. Grinding and washing the surfaces of aluminum electrode and plasma chamber dramatically reduced impurities originating from the ECR plasma of the KEKCB.

The KEK–JAERI joint RNB project

A new ISOL-based radioactive nuclear beam (RNB) facility has been constructed as a joint project of the High Energy Accelerator Research Organization (KEK) and the Japan Atomic Energy Research Institute (JAERI) at the Tandem accelerator of the Tokai site of JAERI. It uses heavy-ion linacs and related equipment of the presently closed KEK-RNB facility. Fission fragments with energy from 0.1 to 8 MeV/u will be available at this facility.

The development of an ECR charge breeder for KEK-JAERI joint RNB project

For the construction of an electron cyclotron resonance (ECR) charge breeder (CB) at KEK-JAERI radioactive nuclear beam facility, we have made a pilot charge breeding system consisting of two compact-sized ECR ion sources. In order to find the necessary conditions for an ECRCB with high efficiency, the charge breeding efficiency for Ar ions in nitrogen plasma has been measured and compared with the results from the simulation of the trajectories of the ions in the plasma. By explicitly taking into account the cumulative small angle scattering of charged ions as well as the confinement magnetic field of the pilot CB, the simulation predicts the stopping efficiency. This is defined as a probability that the injected ions are still within the plasma volume when their initial directional motions have become randomized. Incorporating the ionization efficiency for the ions, the overall breeding efficiency has been estimated. Finally, the characteristics of the ECRCB recently installed for tests at KEK is described.

Beam bunching of the radioactive nuclear beam in a 6.4 GHz electron cyclotron resonance ion source

A single stage 6.4 GHz ECR ion source for an isotope separator on-line based radioactive nuclear beam facility at KEK-Tanashi, is under an on-line test for the effective production of 19Ne2+ ions by using alpha beams from a cyclotron. Synchronized with the pulsed operation of heavy ion linacs, the millisecond beam bunching is now being performed to reduce the loss of radioactive ions in acceleration by using a pulsed-gating potential method. The highest beam intensity at 100 Hz and 20% duty factor reaches 60% of that obtained in a continuous-wave mode.

GEM-MSTPC: An active-target type detector in low-pressure He/CO2 mixed gas

We developed an active-target type gas counter operating with low pressure He/CO2(10%) detector gas for application in studying low-energy nuclear reactions using radioactive nuclear beams (RNBs). A 400-?m-thick gas electron multiplier (THGEM) was used as the proportional counter for high injection rate capability. We examined the gas gain stability and the influence of ion feedback on particle tracks at high beam injection rates of up to 105 particles per second (pps) using a low-energy 12C beam. By means of modifications to the THGEM, the shift in the relative pulse height of the output signal normalized to that measured at the 12C injection rate of several hundred pps improved to within 2% at the rate of 105 pps. The position distortion induced by ion feedback was suppressed to within 1.5 mm because of the relatively high drift field value of 1 kV/cm?atm and the use of the double GEM configuration. The THGEM was found to be applicable for our active target at high injection rates of up to 105 pps.

Direct measurements of (α, n) and (p, n) reaction cross sections using light neutron-rich radioactive nuclear beams

Abstract A project to acquire directly nuclear cross sections of (α, n) and (p, n) reactions using low energy light neutron-rich radioactive nuclear beams (RNB) is in progress at JAERI-Tandem facility. The method of the production of neutron-rich RNB, an improvement of a gas counter (MSTPC) in order to work effectively under the high RNB-injection rate, and the feasibility of our experiments are discussed.

Stopping efficiency of an ECR charge breeder for axially injected ions

We have simulated the trajectories of ions injected into an ECR plasma, by taking into account explicitly the cumulative small angle scattering of charged ions as well as the confinement magnetic field of the ECR source. Assuming homogeneous and uniform plasma in a given volume, the stopping efficiency, defined as a probability that the injected ions are alive in the plasma volume until their initial directional motions become at random, has been extracted and discussed.