Elsevier

Physics Reports

Volume 425, Issue 1, March 2006, Pages 1-78
Physics Reports

High-accuracy mass spectrometry with stored ions

https://doi.org/10.1016/j.physrep.2005.10.011Get rights and content

Abstract

Like few other parameters, the mass of an atom, and its inherent connection with the atomic and nuclear binding energy is a fundamental property, a unique fingerprint of the atomic nucleus. Each nuclide comes with its own mass value different from all others. For short-lived exotic atomic nuclei the importance of its mass ranges from the verification of nuclear models to a test of the Standard Model, in particular with regard to the weak interaction and the unitarity of the Cabibbo–Kobayashi–Maskawa quark mixing matrix. In addition, accurate mass values are important for a variety of applications that extend beyond nuclear physics. Mass measurements on stable atoms now reach a relative uncertainty of about 10-11. This extreme accuracy contributes, among other things, to metrology, for example the determination of fundamental constants and a new definition of the kilogram, and to tests of quantum electrodynamics and fundamental charge, parity, and time reversal symmetry. The introduction of Penning traps and storage rings into the field of mass spectrometry has made this method a prime choice for high-accuracy measurements on short-lived and stable nuclides. This is reflected in the large number of traps in operation, under construction, or planned world-wide. With the development and application of proper cooling and detection methods the trapping technique has the potential to provide the highest sensitivity and accuracy, even for very short-lived nuclides far from stability. This review describes the basics and recent progress made in ion trapping, cooling, and detection for high-accuracy mass measurements with emphasis on Penning traps. Special attention is devoted to the applications of accurate mass values in different fields of physics.

Introduction

High-accuracy mass values for atoms give us access to atomic and nuclear binding energies that represent the sum of all the atomic and nucleonic interactions [1]. Thus, accurate mass values, together with other information, provide stringent tests of the weak interaction, quantum-electrodynamics, and the Standard Model (see Table 1). Accuracy required for the mass depends on the physics being investigated and ranges from δm/m=10-5 to better than 10-8 for radionuclides which often have half-lives considerably less than a second [2], [3], [4], and even down to δm/m=10-11 for stable nuclides [5], [6], [7].

Presently the masses of about 3200 nuclides are known or estimated, as listed in the recent Atomic-Mass Evaluation AME2003 [8], [9] and summarized in Fig. 1. They are obtained by different methods in mass spectrometry. The highest mass accuracy on stable and radioactive atomic ions to date is achieved with ion traps. Not only in mass spectrometry but also in many other areas of science ion traps play an increasingly important role. They are employed for example for studies in atomic, nuclear, and particle physics such as g-factor measurements [11], [12], [13], quantum optics and computing [14], [15], realization of frequency standards [14], [16], laser spectroscopy [14], [16], [17], as well as for tailoring and improving the properties of radioactive ion beams [18], [19].

The reasons for the high accuracy obtained and this broad usage of trapping devices are the manifold advantages of a three-dimensional ion confinement in well controlled fields [18]: First, an extended time of observation is available, in principle for radionuclides only limited by the half-life. Second, single-ion sensitivity can be reached and third, ions can be stored in an ideal controlled environment. Apart from their use for direct mass determination, ion traps serve as tools to improve the performance of other methods and techniques: By accumulation and cooling of ions from sources outside the trap, ion beams of high brilliance can be obtained, which in particular enables effective use of rare species. Charge breeding inside traps allows the investigation of highly charged ions, and polarized ion bunches can be effectively produced [19].

As will be outlined in detail below, aside from storage rings two types of traps are generally used for mass determination: dynamical Paul (radiofrequency) traps and static Penning traps. The first type is widely used in analytical chemistry. A number of textbooks and reviews are available which deal with the techniques and results obtained with these devices [20], [21]. Penning traps find applications where extremely high mass resolution and accuracy are required. Some recent articles [1], [2], [18], [19], [22] give overviews and details of its application. A general summary of the use of ion traps especially for mass spectrometry can be obtained by the proceedings of the following conferences: “Trapped Charged Particles and Fundamental Physics”, Asilomar, 1998 [23], “Atomic Physics at Accelerators: Mass Spectrometry (APAC 2000)”, Cargèse, 2000 [24], “14th International Conference on Electromagnetic Isotope Separation and Techniques Related to their Application (EMIS-14)”, Victoria, 2002 [25], “Trapped Charged Particles and Fundamental Physics”, Wildbad Kreuth, 2002 [26], and “The Fourth International Conference on Exotic Nuclei and Atomic Masses (ENAM2004)”, Pine Mountain/Georgia, 2004 [27].

In this review particular emphasis is given to the application of Penning traps and storage rings for high-accuracy mass determination of short-lived and stable nuclides. This is justified by the strong efforts in recent years to construct new storage rings and Penning trap mass spectrometers at accelerators. These devices will add new results to the exciting ones that have been obtained at existing facilities. In Fig. 2 these world-wide facilities are displayed (more details are available from Table 2). All existing and planned trap facilities will be briefly described in Chapter 8. Other high-accuracy mass spectrometry devices for short-lived radionuclides like radiofrequency spectrometers, as for example the Smith-type rf-spectrometer MISTRAL [28], [29] installed at ISOLDE/CERN, or time-of-flight mass spectrometers like the magnetic-rigidity spectrometer SPEG at GANIL [30], [31] and the coupled-cyclotron complex at GANIL [32], [33], are described in [1], [34] and will not be discussed in the following.

In the first part of this review a brief history of mass spectrometry will be given, starting with the determination of the charge-to-mass ratio of an electron and ending with high-accuracy Penning trap mass spectrometry.

The second part deals with the basics and challenges of ion trapping, cooling, and detection. Three major types of storage devices and their underlying principles will be discussed: radiofrequency quadrupole (Paul) traps [35] where a time-varying quadrupole electric field is applied to the electrodes for confinement, Penning traps [36] which use a combination of a homogeneous magnetic field with a static electric quadrupole field, and storage rings [37] consisting of dipole magnets and magnetic lenses. For comparison, Fig. 3 shows a sketch of both kind of traps: a “little” Penning trap with about 3 cm in diameter and a “large” storage ring with a diameter of 20 m. Since ion cooling and efficient ion detection is essential for many applications of these devices the most important cooling and detection techniques will be presented. Meanwhile many experiments take advantage of the use of highly charged ions, or deal with radionuclides. Thus, the major production mechanisms of highly charged ions and radionuclides will be addressed as well.

In the third part fundamental applications and major achievements of high-accuracy mass measurements on short-lived and stable nuclides in different fields of physics (see also Table 1) are summarized. With the accuracy that is reached, a number of new domains could be accessed beside nuclear physics, in which the determination of atomic or nuclear masses is of importance. These are particle physics, nuclear astrophysics, stellar nucleosynthesis, neutrino physics, and metrology. One example is the study of super-allowed β-decays which enables a determination of the weak vector coupling constant and in this way to test the unitarity of the Cabibbo–Kobayashi–Maskawa (CKM) quark mixing matrix [38], [39]. Beside accurate branching ratios and half-lives, very accurate Q-values, i.e. mass differences of mother and daughter nuclei, are required. In metrology, mass measurements on stable nuclides contribute to a new definition of the kilogram, the last SI unit that is still based on a prototype, and to accurate determinations of fundamental constants such as the fine-structure constant α. Since some of the applications require mass accuracies for specific nuclides or high production yields that are not yet available but will become available in the near future at new radioactive ion beam facilities such as GSI-FAIR (Germany) [40], RIA (USA) [41], RIKEN (Japan) [42], and SPIRAL2 (France) [43], a number of Penning trap and storage ring experiments are under construction or planned (see Table 2). Therefore an overview on new trap experiments and technical challenges associated with them will be given as well.

Section snippets

History of atomic and nuclear mass measurements

The mass spectrometry branch of science goes back to the year 1897, when Joseph John Thomson (1856–1940) demonstrated the existence of the electron as an electrically charged particle, and measured the ratio of its mass to its charge [44], [45]. This is exactly the kind of measurement used in modern mass determinations, where what is actually measured is the charge-to-mass ratio q/m of individual atomic or molecular ions. For his studies, Thomson was awarded the Nobel Prize for physics in 1906.

Principles of storage devices for charged particles

To obtain full spatial confinement requires a potential minimum in all three dimensions. Moreover, the most desirable confining force is one that causes simple harmonic motion of the confined particle, i.e., one that is proportional to the distance of the particle from the center of confinement. Since no simultaneous trapping in three dimensions is possible by purely electrostatic potentials, three-dimensional confinement is achieved in a Penning trap by the superposition of a homogeneous

Ion manipulation and mass measurement techniques

Accumulation, storing, and cooling techniques play an increasingly important role in many areas of science. This is reflected by the awards of Nobel prizes within the last two decades to more than ten pioneers of particle, ion, or atom cooling. In this chapter the main ion manipulation, cooling, and frequency determination techniques in ion traps for mass measurements will be discussed.

Production and separation techniques for exotic nuclides

Mass spectrometry on radioactive nuclides requires their production and separation, for which several complementary techniques exist. Now, more than 3000 nuclides can be produced at the different radioactive ion beam facilities worldwide, with energies ranging from a few 10 keV to the relativistic regime. For the production of heavy, super-heavy, and neutron-deficient medium-mass nuclides, heavy-ion collisions, near the energy of the Coulomb barrier, are the mechanisms of choice. This technique

Production and storage of highly charged ions

As described in Section 4.5 the resolving power and accuracy in mass spectrometry with ion traps depend on the charge state of the stored ion. To this end, the use of highly charged ions is advantageous, although charge exchange losses make the vacuum requirements for the storage device much higher than for the storage of singly charged ions. There are presently three different means to produce highly charged ions:

  • 1.

    In an accelerator facility a high-energy ion beam is send through a stripper-foil

Challenges in high-accuracy mass spectrometry with ion traps

Experiments on high-accuracy mass measurements with ion traps deal with problems common to all high-accuracy experiments. These range from mundane stability problems regarding temperature, pressure, high voltage, magnetic field, beam intensity or electronic thresholds, to the effective suppression of vibrations, electric and magnetic field inhomogeneities, and electronic noise. Some of these challenges in ion trap mass spectrometry are addressed in more detail:

  • 1.

    Excellent vacuum conditions, in

Ion trap mass spectrometers worldwide

Presently about 20 ion traps for high-accuracy mass measurements are in operation, under construction, or planned at different places worldwide (see Fig. 2 and Table 2). A brief summary of the experimental setups shall be given here, but the list might not be complete.

Penning traps can be applied to short-lived radionuclides delivered by on-line isotope separators as demonstrated first by ISOLTRAP [53]. The success of this experiment has triggered many more ion trap projects to be installed at

Mass spectrometry on radioactive ions

The development of direct mass measurement techniques such as Penning trap and storage ring mass spectrometry has provided sensitive and high-precision tools for a detailed study of nuclear binding far from the valley of stability [1], [48], [226], [269], [270]. Mass measurements in long isotopic and isotonic chains has allowed the study of the fine structure of the mass surface and clarified discontinuities in order to extract nuclear structure information such as shell and subshell closures

Mass spectrometry on stable ions

High-accuracy mass measurements and mass comparisons of stable or very long-lived nuclides on the level of 10-9 and better have a wide variety of applications in physics and metrology, including new determinations of the fine structure constant [192], [380], [381], [382] and a new definition of the kilogram [64], [383], the provision of input data for the determination of the electron neutrino rest mass [384], [385] and the search for neutrinoless double beta-decays [386], [387]. Furthermore

Future projects

High-accuracy mass spectrometry especially of radionuclides appears to have a very bright future. There are at least eight new proposed mass measurement projects, six based on Penning traps (HITRAP, MAFF-TRAP, MATS, RIKEN-TRAP, SPIRAL2-TRAP, TITAN), two on storage rings (ILIMA, Lanzhou-SR), some of which are scheduled for data taking within the next few years. They are listed in Table 2 and briefly described in Chapter 8. Common to all projects are the aims to get access to even shorter-lived

Concluding remarks

This review has summarized the state-of-the art in high-accuracy ion trap mass spectrometry, a field that started about 20 years ago and that has now a large variety of exciting applications in different fields of science. The main focus was put on Penning trap mass spectrometry since it provides the highest mass accuracy, but the importance of storage ring mass spectrometry was pointed out as well. The basics of ion trapping, cooling, and detection techniques and their applications are

Acknowledgements

I have been grateful to many colleagues over the years for their input into high-accuracy mass spectrometry with ion traps. I am particularly indebted to H.-J. Kluge (GSI Darmstadt, Germany) for his permanent support and advice during all the years we worked together. The author thank many people from different facilities for providing me with material to be included into this review and especially G. Werth (University of Mainz, Germany) and R.B. Moore (McGill University, Canada) for their many

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