Cooling of short-lived, radioactive, highly charged ions with the TITAN cooler Penning trap

TITAN is an on-line facility dedicated to precision experiments with short-lived radioactive isotopes, in particular mass measurements. The achievable resolution on mass measurement, which depends on the excitation time, is limited by the half life of the radioactive ion. One way to bypass this is by increasing the charge state of the ion of interest. TITAN has the unique capability of charge-breeding radioactive ions using an electron-beam ion trap (EBIT) in combination with Penning trap mass spectrometry. However, the breeding process leads to an increase in energy spread, ??E, which in turn negatively influences the mass uncertainty. We report on the development of a cooler Penning trap which aims at reducing the energy spread of the highly charged ions prior to injection into the precision mass measurement trap. Electron and proton cooling will be tested as possible routes. Mass selective cooling techniques are also envisioned.     

The TITAN mass measurement facility at TRIUMF-ISAC

The TITAN facility at TRIUMF-ISAC will use four ion traps with the primary goal of determining nuclear masses with high precision, particularly for short lived isotopes with lifetimes down to approximately 10 ms. The design value for the accuracy of the mass measurement is 1 ×10^???8. The four main components in the facility are an RF cooler/buncher (RFCT) receiving the incoming ion beam, an electron beam ion trap (EBIT) to breed the ions to higher charge states, a cooler Penning trap (CPET) to cool the highly charged ions, and finally the measurement Penning trap (MPET) for the precision mass determination. Additional goals for this system are laser spectroscopy on ions extracted from the RFCT and beta spectroscopy in the EBIT (in Penning trap mode) on ions that are purified using selective buffer gas cooling in the CPET. The physics motivation for the mass measurements are manifold, from unitarity tests of the CKM matrix to nuclear structure very far from the valley of stability, nuclear astrophysics and the study of halo-nuclei. As a first measurement the mass of ¹¹Li will be determined. With a lifetime of 8.7 ms and a demonstrated production rate of 4×10⁴ ions/sec at ISAC the goal for this measurement at TITAN is a relative uncertainty of 5×10^???8. This would check previous conflicting measurements and provide information for nuclear theory and models.     

First use of high charge states for mass measurements of short-lived nuclides in a Penning trap

Penning trap mass measurements of short-lived nuclides have been performed for the first time with highly charged ions, using the TITAN facility at TRIUMF. Compared to singly charged ions, this provides an improvement in experimental precision that scales with the charge state q. Neutron-deficient Rb isotopes have been charge bred in an electron beam ion trap to q=8-12+ prior to injection into the Penning trap. In combination with the Ramsey excitation scheme, this unique setup creating low energy, highly charged ions at a radioactive beam facility opens the door to unrivaled precision with gains of 1-2 orders of magnitude. The method is particularly suited for short-lived nuclides such as the superallowed β emitter 74Rb (T(1/2)=65 ms). The determination of its atomic mass and an improved Q(EC) value are presented.     

An EBIS/T with integrated Penning trap

A special problem in atomic physics research with highly charged ions is to prepare ions with a unique charge state inside of EBIS or EBIT devices. On the other hand, there are great losses resulting from the transport of the ions from the source to an external trap. Therefore we are setting up an EBIS/T with internal Penning trap. This new set-up will be able to study electron–ion interaction with well-defined initial and final charge states, distinguishing between single step successive ionisation and multiple step ionisation of charge states similar to the crossed beams method but for much higher charge states. Another feature of this system is to determine with high precision the ion charge state distribution in the EBIS/T by application of Fourier Transform Ion Cyclotron Resonance (FT-ICR). This method allows the on-line monitoring of the ion distribution and the evolution of the charge state population together with its dependence on the degree of space charge compensation of the electron beam in the EBIS/T. It will be possible to study ion dynamics in compensated space charge potentials. In case of high homogeneity of the magnetic field in the trap region, experiments may be considered to measure directly binding energies of highly-charged ions and other topics of high resolution mass spectroscopy. This revised version was published online in July 2006 with corrections to the Cover Date.     

A cooler ion trap for the TITAN on-line trapping facility at TRIUMF

We present simulations of electron and proton cooling of highly charged ions in a Penning trap, including the potentially detrimental effects of radiative, dielectronic, and three-body recombination in electron cooling. We show a preliminary design for a cooler trap accommodating both electron and proton cooling, which will be a component of the TITAN ion-trap facility under construction at TRIUMF for precision mass measurements of short-lived radioactive nuclei.      

The REX-ISOLDE project

The Radioactive Beam Experiment REX-ISOLDE [1–3] is a pilot experiment at ISOLDE (CERN) testing the new concept of post acceleration of radioactive ion beams by using charge breeding of the ions in a high charge state ion source and the efficient acceleration of the highly charged ions in a short LINAC using modern ion accelerator structures. In order to prepare the ions for the experiments singly charged radioactive ions from the on-line mass separator ISOLDE will be cooled and bunched in a Penning trap, charge bred in an electron beam ion source (EBIS) and finally accelerated in the LINAC. The LINAC consists of a radiofrequency quadrupole (RFQ) accelerator, which accelerates the ions up to 0.3 MeV/u, an interdigital H-type (IH) structure with a final energy between 1.1 and 1.2 MeV/u and three seven gap resonators, which allow the variation of the final energy. With an energy of the radioactive beams between 0.8 MeV/u and 2.2 MeV/u a wide range of experiments in the field of nuclear spectroscopy, astrophysics and solid state physics will be addressed by REX-ISOLDE. This revised version was published online in July 2006 with corrections to the Cover Date.     

The SMILETRAP (Stockholm-Mainz-Ion-LEvitation-TRAP) facility

Described in this paper is an experimental facility which measures atomic masses by using multiply charged ions from an electron beam ion source. The ions are injected into a Penning trap and the cyclotron frequencies measured. A precision of 2×10^–9 has been reached using highly charged carbon, nitrogen, oxygen and neon.     

电子束离子阱对中性气体电荷交换的光谱诊断

在实验室天体物理研究中,电子束离子阱(EBIT)是极端紫外(EUV)和X射线波段能谱分析的重要实验平台,其中EBIT中心残余的中性气体对离子产生存在显著影响。研究了阱区中心残余中性气体对电荷态分布的影响,发现阱区中心残余中性气体和高电荷态离子之间的电荷/能量交换过程不仅影响离子的电荷分布, 而且对激发函数(离子分布比例随电子能量关系曲线)有着极大的影响。利用电离平衡分析方法成功诊断出阱区中心区域残留的中性气体分子数密度,以及内腔室的真空度。     

聚焦慢速高荷态重离子束微束斑X射线源

利用电子束离子源(EBIS)或者电子束离子陷阱(EBIT)产生的慢速高电荷态重离子束轰击金属靶面,离子束与靶面作用并复合辐射特征X射线;并将高荷态离子束采用离子光学系统会聚为微细束后再与靶面作用,能够辐射出微米甚至亚微米级、纳米级的微束斑X射线.本文介绍这一新型微束斑X射线源的结构、机理及其特性等.     

The new ClusterTrap setup

ClusterTrap has been designed to investigate properties of atomic clusters in the gas phase with particular emphasis on the dependence on the cluster size and charge state. The combination of cluster source, Penning trap and time-of-flight mass spectrometry allows a variety of experimental schemes including collision-induced dissociation, photo-dissociation, further ionization by electron impact, and electron attachment. Due to the storage capability of the trap extended-delay reaction experiments can be performed. Several recent modifications have resulted in an improved setup. In particular, an electrostatic quadrupole deflector allows the coupling of several sources or detectors to the Penning trap. Furthermore, a linear radio-frequency quadrupole trap has been added for accumulation and ion bunching and by switching the potential of a drift tube the kinetic energy of the cluster ions can be adjusted on their way towards or from the Penning trap. Recently, experiments on multiply negatively charged clusters have been resumed.     

New experimental initiatives using very highly charged ions from an “electron beam ion trap”

A short review of the experimental program in highly-charged heavy ion physics conducted at the Lawrence Livermore National Laboratory Electron Beam Ion Trap (EBIT) facility is presented. The heavy-ion research, involving ions up to fully stripped U⁹²⁺, includes precision x-ray spectroscopy and lifetime studies, electron impact ionization and excitation cross section measurements. The investigations of ion-surface interactions following the impact of high-Z highly charged ions on surfaces are aimed to study the neutralization dynamics effecting the ion and the response of the surface as well. The combination of an EBIT% with a cryogenic Penning Trap (“RETRSP”) allows to conduct experiments with ultra cold (e.g. 10 K) very highly charged ions. These studies of charge exchange processes, Coulomb crystals and measurements of hyperfine transitions using laser spectroscopy are in progress.     

Testing atomic structure theories with high accuracy mass measurements on highly charged ions

The mass of a highly charged ion is the sum of the mass of the nucleus, the mass of the electrons and the electronic binding energies. High accuracy mass measurements on highly charged ions in a sequence of different charge states yield informations on atomic binding energies, i.e., the ionisation potentials. In our contribution we discuss the possibility of determining atomic binding energies of highly charged ions to better than 20 eV via cyclotron frequency measurements in a Penning trap. At this level of accuracy different contributions to the binding energies, like relativistic corrections, Breit corrections and QED corrections, can be measured. This revised version was published online in August 2006 with corrections to the Cover Date.     

EBIT as a versatile experimental facility

The Electron Beam Ion Trap (EBIT) produces ions, confined within the electron beam, with charges ranging up to U⁹²⁺ at near rest energies. This allows to study the interaction of a monoenergetic electron beam with any trapped ion to a high degree of precision via X-ray spectroscopy. The development of the EBIT into an ion (trap) source enables the possibility to perform for the first time studies of the interaction dynamics in strong fields of ions with matter where the ions carry hundreds of keV potential energy at very low kinetic energies (eV).     

TITAN: An ion trap facility for on-line mass measurement experiments

Precision determinations of ground state or even isomeric state masses reveal fingerprints of nuclear structure. In particular, at the limits of existence for very neutron-rich or -deficient isotopes, one can extract detailed information about nuclear structure from separation energies or binding energies. Mass measurements are important to uncover new phenomena, to test new theoretical predictions, or to refine model approaches. For example, the N?=?28 shell has proven more stable than previously expected; however, the predicted new “magic” number at N?=?34 in the K and Ca isotopes has yet to be confirmed experimentally. For these neutron-rich nuclei, the inclusion of three-body forces leads to significantly better predictions of the ground-state mass. Similarly, halo nuclei present an excellent application for ab-initio theory, where ground state properties, like masses and radii, test our understanding of nuclear structure. Precision mass determinations at TRIUMF are carried out with the TITAN (TRIUMF’s Ion Traps for Atomic and Nuclear science) facility. It is an ion-trap setup coupled to the on-line facility ISAC. TITAN has measured masses of isotopes as short-lived as 9 ms (almost an order of magnitude shorter-lived than any other Penning trap system), and it is the only one with charge breeding capabilities, which allow us to boost the precision by almost 2 orders of magnitude. We recently made use of this feature by measuring short-lived, proton-rich Rb-isotopes, up to ⁷⁴Rb while reaching the 12?+ charge state, which together with other improvements led to an increase in precision by a factor 36.     

X-ray, visible and electron spectroscopy with the NIST EBIT

An overview is given of recent activities at the NIST electron beam ion trap (EBIT) facility. The machine has been operational for almost three years. Important characteristics and demonstrated capabilities of our EBIT are presented. Selected results include experiments with trapped highly charged ions (X-ray and visible spectroscopy), and with extracted ions (ion-surface collision studies). This revised version was published online in August 2006 with corrections to the Cover Date.     

KLn Dielectronic Recombination Process of B- Through He-like Cu Ions

The KLn dielectronic recombination processes of trapped highly charged B-like through He-like Cu ions are studied theoretically, and the theoretical results are used to analyse our previous experimental data at Heidelberg electron beam ion trap （EBIT）. The theoretical resonant positions agree with the experimental resonant positions to a precision of 0.4%, in comparison with the resonant positions of those highest peaks between theory and experiment. The experimental spectra are then fitted using a formula with the theoretical resonant energies and strengths, the result shows good overall agreement between theory and experiment over a wide electron energy range. The distribution of highly charged states is obtained from the fitting parameters.     

Cold highly charged ions in a cryogenic Paul trap

Narrow optical transitions in highly charged ions (HCIs) are of particular interest for metrology and fundamental physics, exploiting the high sensitivity of HCIs to new physics. The highest sensitivity for a changing fine structure constant ever predicted for a stable atomic system is found in Ir^17?+?. However, laser spectroscopy of HCIs is hindered by the large (～ 10⁶ K) temperatures at which they are produced and trapped. An unprecedented improvement in such laser spectroscopy can be obtained when HCIs are cooled down to the mK range in a linear Paul trap. We have developed a cryogenic linear Paul trap in which HCIs will be sympathetically cooled by ⁹Be^?+? ions. Optimized optical access for laser light is provided while maintaining excellent UHV conditions. The Paul trap will be connected to an electron beam ion trap (EBIT) which is able to produce a wide range of HCIs. This EBIT will also provide the first experimental input needed for the determination of the transition energies in Ir^17?+?, enabling further laser-spectroscopic investigations of this promising HCI.     

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.     

电子束离子阱及高电荷态离子相关物理

文章简要介绍了电子束离子阱（EBIT）的发展背景及其在国际上的状况，较详细地解释了它的结构和工作原理，介绍了它在分解研究等离子体方面的特别优势以及在EBIT上能够实现的高电荷态离子相关的前沿物理学问题研究。