ReviewLaser spectroscopy for nuclear structure physics
Introduction
High-resolution laser spectroscopy has long been established as a powerful tool in the study of nuclear shape, size and multipole structure. The “Hyperfine spectroscopy of radioactive atoms” was first reviewed in 1979 by Jacquinot and Klapisch [1]. When deployed at radioactive beam facilities, where long chains of isotopes on both sides of the valley of beta stability can be produced, optical techniques still provide a unique opportunity to study changes in the structure of nuclear ground (and isomeric) states.
Otten [2] produced a comprehensive review of the field in 1989. This was updated in 1995 by Billowes and Campbell [3] and then in 2003 by Kluge and Nörtershäuser [4]. New experimental techniques, most notably radiofrequency ion beam coolers and bunchers, trapped atoms, and the greatly increased use of specialist ion sources formed the basis of a 2010 review by Cheal and Flanagan [5].
In this report we review the field with a focus on recent and projected progress with respect to measurements and facility developments, taken as that since the 2010 Cheal and Flanagan review [5]. Recent reported results, those since 2010, are highlighted in Table 1. Techniques are reviewed in Section 3 and developments at facilities are discussed in Section 4.
The reported results provide model-independent measures of the nuclear spin, multipole moments and radial extent of the charge distribution. The new results are concentrated towards the lighter and heavier mass regions of the nuclear chart. In many of the works new results for nuclear moments are reported and, where of structural pertinence, considered further here. A general review of nuclear moments is not presented. A substantial number of methods, outside of laser spectroscopy, exist for the measurements of these parameters, reviewed by Neugart and Neyens in 2006 [6]. A complete evaluation of nuclear moment results is maintained by Stone [7], Nuclear Data Section of the IAEA, Vienna. Two further nuclear parameters, the distribution of magnetism and the mass of the nucleus, produce measurable perturbations in the atomic structure. The former gives rise to the so-called “hyperfine anomaly” and is considered in Section 2.2.1. A recent compilation of the measured anomalies has been published by Persson [8]. The latter, the nuclear mass, produces the “mass shift” in atomic spectra, Section 2.3, and requires evaluation for the extraction of nuclear radial parameters. The evaluation requires knowledge of the nuclear mass. Precision measurements of the nuclear mass provide valuable structural information, akin to that provided by charge radii measurements, and are considered further.
A general review of precision atomic physics techniques applied to radioactive nuclear beams (including laser spectroscopy) has recently been made by Blaum, Dilling and Nörtershäuser [9]. Results presented in that review highlight the close structural connection between mass measurements and charge radii. Moreover the majority of the present and planned Penning-trap mass spectrometers [9] are now, or will be, sited at facilities with on-line laser spectroscopy stations with many sharing common beam preparation traps (Section 4.1). The future possible spectroscopy with such combined stations is considered in Section 8.
The current status of measurements made for contemporary nuclear physics using optical spectroscopic methods is summarised in Fig. 1. The measurements which are currently unpublished are highlighted in green. Table 1 presents an overview of optical measurements as of July 2015 (non-optical measurements are not listed but are discussed where pertinent within this report). References published since the review of Cheal and Flanagan [5] are indicated separately for clarity. All data published prior to the review of Otten [2], are referred in Table 1 to that review. For earlier data not contained in [2] the original references are provided. In the event of re-evaluation both the re-evaluation and original work are cited.
Section snippets
Nuclear perturbations of the atomic structure
In an atomic nucleus, with atomic spin , nuclear spin and total angular momentum , spectroscopic measurements of transition frequencies may be attempted between states involving electronic transitions (most typically, electric dipole) or within the same atomic state (magnetic dipole) or, in the presence of an external field, between substates, , of these levels. The following considerations are discussed in the context of the former (electronic) transitions. Spectroscopy
Excitation and detection of atomic transitions
The atomic spectroscopy reviewed in this report has been performed using laser light at frequencies spanning the infra-red to ultraviolet regions of the electromagnetic spectrum. The spectroscopy is in a number of studies combined with radio-frequency excitation (within a hyperfine multiplet) and with non-optical (de-)excitation mechanisms not involving photons (for example, collisional or thermal excitation, field ionisation or charge exchange). The sources of the laser light used are both
On-line laser spectroscopy facilities
The 1979 review of the field by Jacquinot and Klapisch [1] noted that, “a vast new field [has now] opened up” which a decade later, when reviewed by Otten [2], had matured to a large number of productive experiments, most notably at the Isolde facility, CERN, (Section 4.1.6). New facilities, upgrades to existing ones, and moves of experimental stations have been realised, planned and reviewed since [3], [4], [5]. The on-line isotope separator became the favoured, and almost exclusive, site
Light mass nuclei
Nuclei in the region that have been studied via laser spectroscopic techniques may be grouped into several distinct areas of physics. The lightest nuclei with are dominated by the one- and two-neutron halo structures. Above this, nuclei around and fall in a region of the chart known as the island of inversion where much experimental effort has been applied in order to probe the onset and evolution of shell structure of the neutron rich nuclei as well as to map the boundary
Medium mass nuclei
Medium mass isotopes are considered here to be those which have atomic numbers from to . At the low- start of the region the atomic field shifts and mass shifts are comparable, both of order a few hundred MHz [364]. By the end of the region, field shifts, of order a few GHz, are considerably larger than the mass effects and dominate the isotope shifts [364]. Experimentally, the size of the field shifts and precision required for their extraction has, at low-, required
Heavy mass nuclei
In the current review, the locality of heavy mass nuclei is defined as spanning those elements around Pb () whose neutron-deficient isotopes exhibit the phenomenon of shape coexistence, to the very few nuclei studied above U (). The experiments reported in this region of the nuclear chart may broadly be attributed to the two general techniques discussed in Sections 3.4 Collinear laser spectroscopy, 3.5 Resonance ionisation spectroscopy, namely high resolution collinear laser
Recent output
Fig. 17 shows the number of publications (contributing to the data in Table 1) per calendar year, and with these binned in 5 yearly intervals, since 1990. At face value the last five years of research in the field would appear to have been highly productive. While it is tempting to associate this with a number of experiments and techniques coming to fruition, and indeed many have in the reviewed period, a number of single experiments give rise to multiple publications (for which no correction
Conclusions
Results from optical studies of radioactive nuclei were reviewed in 1979, by Jacquinot and Klapisch [1], some 35 years ago. Since that time the field and associated nuclear structure results have been regularly or topically updated and reviewed [2], [3], [4], [5]. At present, following 40 years of development, a wealth of techniques are in regular use worldwide and all stand ready to be exploited at the next generation of facilities (notably the Fair, Frib and Spiral2 facilities are all
Acknowledgements
The authors firstly wish to thank A. Voss for her excellent support throughout the process of this review, both in terms of organising references and for providing the basis for several of the figures. Our effort was made far easier with her tireless contributions. We also wish to thank K. Marinova for providing access to all data used in her updated table of nuclear ground state charge radii, A. Barzakh for providing unpublished Bi data and for all those who kindly provided figures.
This work
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