The SHIPTRAP project: A capture and storage facility at GSI for heavy radionuclides from SHIP

SHIPTRAP is an ion trap facility which is being set up to deliver very clean and cool beams of singly-charged recoil ions produced at the SHIP velocity filter at GSI Darmstadt. SHIPTRAP consists of a gas cell for stopping and thermalizing high-energy recoil ions from SHIP, a rf ion guide for extraction of the ions from the gas cell, a linear rf trap for accumulation and bunching of the ions, and a Penning trap for isobaric purification. The physics programme of the SHIPTRAP facility comprises mass spectrometry, nuclear spectroscopy, laser spectroscopy and chemistry of transeinsteinium elements. This revised version was published online in August 2006 with corrections to the Cover Date.     

First Measurements with the Gas Cell for SHIPTRAP

SHIPTRAP is an electromagnetic transport and trapping system to provide very clean and cold beams of singly-charged recoil ions from the SHIP facility at GSI. The different components of the system are currently under development in Munich (gas cell and extraction RFQ) and GSI (Buncher RFQ and Penning traps)[1]. Design and manufacturing of the prototype buffer gas cell and the extraction RFQ based on a wide range of simulations have been completed. The results of these simulations together with the first measurements will be reported. This revised version was published online in July 2006 with corrections to the Cover Date.     

Mössbauer spectroscopy of the high-pressure phase of pure iron;ɛ-Fe

Hyperfine Interactions - First off-line tests at the ion trap facility SHIPTRAP took place. The facility is being set up to deliver very clean and cooled beams of singly-charged recoil ions (Rare...     

Laser spectroscopy of trapped 7Be and 10Be at a prototype slow RI-beam facility of RIKEN

A next-generation slow radioactive nuclear ion beam facility (SLOWRI) which provides slow, high-purity and small emittance ion beams of all elements is being build as one of the principal facilities at the RIKEN RI-beam factory (RIBF). High energy radioactive ion beams from the projectile fragment separator BigRIPS are thermalized in a large gas catcher cell. The thermalized ions in the gas cell are guided and extracted to a vacuum environment by a combination of dc electric fields and inhomogeneous rf fields (rf carpet ion guide). From there the slow ion beam is delivered via a mass separator and a switchyard to various devices: such as an ion trap, a collinear fast beam apparatus, and a multi-reflection time of flight mass spectrometer. In the R&D works at the present RIKEN facility, an overall efficiency of 5% for a 100A MeV ⁸Li ion beam from the present projectile fragment separator RIPS was achieved and the dependence of the efficiency on the ion beam intensity was investigated. Recently our first spectroscopy experiment at the prototype SLOWI was performed on Be isotopes. Energetic ions of ¹⁰Be and ⁷Be from the RIPS were trapped and laser cooled in a linear rf trap and the specific mass shifts of these isotopes were measured for the first time.     

Development of a Fourier-Transform Ion-Cyclotron-Resonance detection for short-lived radionuclides at SHIPTRAP

The Penning-trap mass spectrometer SHIPTRAP at GSI is designed to provide clean and cooled beams of singly charged radioactive ions produced in fusion-evaporation reactions and separated in-flight by the velocity filter SHIP. The scientific goals include mass spectrometry, atomic and nuclear spectroscopy, and chemistry of transuranium species which are not available at ISOL- or fragmentation facilities Penning-trap based mass measurements on radionuclides relies up to now on the destructive time-of-flight ion-cyclotron-resonance method. One of the main limitations to the experimental investigations is the low production rate of most of these exotic nuclides, for which the use of this detection scheme is not applicable. A sensitive and non-destructive method, like the narrow-band Fourier Transform ion-cyclotron-resonance technique, is ideally suited for the identification and characterization of these species. A new cryogenic trap setup for SHIPTRAP exploiting this detection technique as well as some results of first preparatory tests are presented.     

Cyclotron ion guide for energetic radioactive nuclear ions

A new ion guide, with an additional function having an infinite gas thickness by using a strong magnetic field, is proposed, which enables us to stop energetic radioactive nuclear ions in He gas. The stopped singly charged ions, guided in a dc field with a focusing force generated by an rf field in the gas, are extracted with a SPIG (sextupole rf ion guide) of a focusing device for a spectroscopic study of radioactive nuclei. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Physics Case for SHIPTRAP: Measuring the Masses of Transuranium Elements

SHIPTRAP will allow direct measurement of masses of transuranium nuclides. The method of choice is a Penning trap spectrometer coupled to the SHIP (Separator for Heavy Ion Products) facility at GSI, Darmstadt. In this paper the impact of the SHIPTRAP facility, with its capability of systematic mass measurements with high precision, is explored. Rather few masses of nuclides above uranium are presently known experimentally. In the region of nuclides above Z=100 no ground state masses were measured directly. SHIPTRAP will play an important role in systematically mapping out this area. Possible candidates for direct mass measurements, even with small or very small production cross sections, are presented. This revised version was published online in July 2006 with corrections to the Cover Date.     

An on-line ion trap for unstable nuclei at INS

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.On leave from the Institute of Atomic Energy, Academia Cinica, Shanghai, P.R. China     

Mass-selective trapping of pulsed charged particle beams

The phase-space method is used to evaluate the mass-selective ion confinement properties of the radio-frequency (rf) quadrupole ion trap with phase-synchronized switching-on of the driving rf field for pulsed ion injection from an external source. The results are of interest for on-line investigations of both short-lived isotopes and stable highly charged ions. In particular, singly charged ions with an energy of 10 eV and a mass in the neighborhood of 100 amu, injected along the gap or through an aperture on one of the electrodes, are considered. Mass-selective storage of the injected ions is possible for any trap operation point within the stability region by allowing a field-free drift distance before ion injection. It is shown that after appropriate scaling the results apply to the trapping of any pulsed beam of charged particles.     

Three-Dimensional Compensation for Minimizing Heating of the Ion in Surface-Electrode Trap

The trapped ions confined in a surface-electrode trap(SET) could be free from rf heating if they stay at the rf potential null of the potential well.We report our effort to compensate three-dimensionally for the micromotion of a single ~(40)Ca~+ ion near the rf potential null,which largely suppresses the ion's heating and thus helps to achieve the cooling of the ion down to 3.4 mK,which is very close to the Doppler limit.This is the prerequosite of the sideband cooling in our SET.     

Anharmonic oscillation of ions trapped in a rf trap with light buffer gas

The effect of a light buffer gas on the anharmonic oscillation of ions trapped in a rf trap is studied. The rf resonance absorption signals showed a change of the signal height and the hysteresis with the sweep direction of the dc voltage or the probing frequency due to the anharmonicity of the pseudopotential well of a rf trap. It was found that the signals changed drastically or even disappeared depending on the pressure of buffer gas, although almost the same number of ions were trapped. These effects indicate that the sensitivity of detection of the trapped ions can be improved by appropriately choosing the pressure of the buffer gas and the sweep direction. The trapped ions could be detected until 76 h 20 min and the storage time of 1.3×10⁵ s was determined when these parameters were optimized.     

Fluorescence Detection and Buffer Gas Cooling of Trapped Mercury Ions in Paul Trap

Pig ions are confined in a hyperboloid ion trap. With the rf discharge ^202 Hg isotope lamp, the fluorescence signal of trapped Hg ions is observed. By means of buffer gas cooling, the ionic temperature is reduced. As a result, the trapping time is increased and the signal-to-noise ratio （SNR） of the fluorescent signal is improved. The temperature of ion cloud is estimated by measuring the space charge shift.     

The energy distribution of an ion in a radiofrequency trap interacting with a nonuniform neutral buffer gas

An ion in a radiofrequency (rf) trap sympathetically cooled by a simultaneously trapped neutral buffer gas exhibits deviations from thermal statistics caused by collision-induced coupling of the rf field to the ion motion. For a uniform density distribution of the buffer gas, the energy distribution of the ion can be described by Tsallis statistics. Moreover, runaway heating of the ion occurs if the buffer gas particles are sufficiently heavy relative to the ion. In typical experiments, however, ultracold buffer gases are confined in traps resulting in localised, non-uniform density distributions. Using a superstatistical approach, we develop an analytical model for an ion interacting with a localised buffer gas. We demonstrate theoretically that limiting collisions to the centre of the ion trap enables cooling at far greater mass ratios than achievable using a uniform buffer gas, but that an upper limit to the usable mass ratio exists even in this case. Furthermore, we analytically derive the functional form of the energy distribution for an ion interacting with a buffer gas held in a harmonic potential. The analytical distribution obtained is found to be in excellent agreement with the results of numerical simulations.     

Kinetic Energy of Trapped Ions Cooled by Buffer Gas

The average kinetic energy of 40 Ca＋ ions is measured by the method of evaporating ions in an rf ion trap. The kinetic energy of the ion 40Ca＋ varies from 0.5eV to 0.2eV with changing buffer gas pressure from 10^-7 mbar to 10^-5 mbar. The Brownian motion model is also introduced to calculate the average kinetic energy of the trapped ions.     

Technical developments for an upgrade of the LEBIT Penning trap mass spectrometry facility for rare isotopes

The LEBIT (Low Energy Beam and Ion Trap) facility is the only Penning trap mass spectrometry (PTMS) facility to utilize rare isotopes produced via fast-beam fragmentation. This technique allows access to practically all elements lighter than uranium, and in particular enables the production of isotopes that are not available or that are difficult to obtain at isotope separation on-line facilities. The preparation of the high-energy rare-isotope beam produced by projectile fragmentation for low-energy PTMS experiments is achieved by gas stopping to slow down and thermalize the fast-beam ions, along with an rf quadrupole cooler and buncher and rf quadrupole ion guides to deliver the beam to the Penning trap. During its first phase of operation LEBIT has been very successful, and new developments are now underway to access rare isotopes even farther from stability, which requires dealing with extremely short lifetimes and low production rates. These developments aim at increasing delivery efficiency, minimizing delivery and measurement time, and maximizing use of available beam time. They include an upgrade to the gas-stopping station, active magnetic field monitoring and stabilization by employing a miniature Penning trap as a magnetometer, the use of stored waveform inverse Fourier transform (SWIFT) to most effectively remove unwanted ions, and charge breeding.     

Visual observation and optical cooling of electrodynamically contained ions

Resonance fluorescence from as little as 10 to 20 barium ions, spatially confined in a miniaturized rf quadrupole ion trap, has been detected visually, photographically, and photoelectrically. Gross effects of optical sideband cooling of the ions were observed.     

Trapped ion density distribution in the presence of He-buffer gas

The spatial density distribution of Ba⁺ ions, confined in a rf quadrupole trap, has been measured by laser scanning across the trap. This allows to determine the ion temperature, assuming thermal equilibrium. Under UHV conditions the average ion energy has been found to be one tenth of the trap potential well depth. Collisions with He at pressures up to 5×10⁻⁶ mbar reduce the ion temperature by a factor of 3.     

The WITCH experiment: towards weak interactions studies. Status and prospects

Primary goal of the WITCH experiment is to test the Standard Model for a possible admixture of a scalar or tensor type interaction in β-decay. This information will be inferred from the shape of the recoil energy spectrum. The experimental set-up was completed and is under intensive commissioning at ISOLDE (CERN). It combines a Penning trap to store the ions and a retardation spectrometer to probe the recoil ion energy. A brief overview of the WITCH set-up and the results of commissioning tests performed until now are presented. Finally, perspectives of the physics program are reviewed.     

Beam purification techniques for low energy rare isotope beams from a gas cell

A linear gas stopping cell has been implemented at the NSCL as part of the Low Energy Beam and Ion Trap (LEBIT) facility. The gas stopping cell is used to convert relativistic ions into low energy ions suitable for use in ion trap experiments. A common undesired property of such systems is the production of beam contaminants through charge exchange of gas impurities with the He⁺ ions produced in the stopping process. These contaminants are of particular concern for Penning trap mass spectrometry, where the simultaneous trapping of ions with different masses can cause unwanted shifts in the measured cyclotron frequency of the desired ions. In order to minimize such effects, a multi-stage beam purification system has been implemented at LEBIT.