The radioactive element astatine exists only in trace amounts in nature. Its properties can therefore only be explored by study of the minute quantities of artificially produced isotopes or by performing theoretical calculations. One of the most important properties influencing the chemical behaviour is the energy required to remove one electron from the valence shell, referred to as the ionization potential. Here we use laser spectroscopy to probe the optical spectrum of astatine near the ionization threshold. The observed series of Rydberg states enabled the first determination of the ionization potential of the astatine atom, 9.31751(8) eV. New ab initio calculations are performed to support the experimental result. The measured value serves as a benchmark for quantum chemistry calculations of the properties of astatine as well as for the theoretical prediction of the ionization potential of superheavy element 117, the heaviest homologue of astatine.

The magnetic moments and isotope shifts of the neutron-deficient francium isotopes [202][203][204][205] Fr were measured at ISOLDE-CERN with use of collinear resonance ionization spectroscopy. A production-todetection efficiency of 1% was measured for 202 Fr. The background from nonresonant and collisional ionization was maintained below one ion in 10 5 beam particles. There are surprisingly few nuclear observables with which theorists can elucidate the nuclear force and interacting many-fermion problem. The laser spectroscopy technique reported here has measured two of these (the magnetic moment and mean-square charge radius) via the hyperfine interaction and isotope shift. This is achieved without introducing assumptions associated with any particular approach, making these measurements suitable for testing modern nuclear models. A variety of laser spectroscopy techniques now exists for studying shortlived radioactive isotopes, which broadly focus on either high resolution (< 100 MHz linewidth) or high sensitivity (< 1 atom=s) [1,2].We report here the first measurements of 202;203;205 Fr, reaching 11 neutrons from the N ¼ 126 shell closure. This has been made possible by a new highly sensitive, highresolution technique of bunched collinear-beam resonance ionization spectroscopy (CRIS). The CRIS technique combines for the first time velocity bunching (provided by the collinear geometry [3,4]) and time bunching (to eliminate the duty loss of required pulsed laser systems). The high sensitivity is reached through a combination of the excellent overlap of laser and beam, and the high quantum efficiency of ion detectors. This new technique may be applied generally to all nuclides, but it is at the limits of nuclear stability that it will manifest its particular advantages.The CRIS method was first proposed more than 30 years ago [5], and its sub-Doppler resolution was demonstrated by Schultz et al., who measured the radioactive isotopes of ytterbium [6]. Since these isotopes were produced as a continuous beam, the duty cycle losses associated with pulsed lasers introduced a loss in efficiency by a factor of 30, which contributed to a low total experimental efficiency of 0.001%. With the installation of a gas-filled radio frequency quadrupole ion trap (ISCOOL) at ISOLDE, bunched ion beams that match the duty cycle of the pulsed lasers are now available [7]. This motivated the development of a dedicated experiment to exploit the CRIS technique applied to time-bunched beams. In the present case it allowed for the first time measurements of the neutrondeficient francium isotopes with half-lives as short as 300 ms and production rates below 100 atoms=s.In our work the francium isotopes were produced through spallation reactions induced by 1.4 GeV protons, incident on a high temperature uranium carbide target (2000 C) and surface ionized (ionization potential 4.07 eV [8]) with use of a tantalum ionizer tube. A schematic of the experiment is presented in Fig. 1. The beam was accelerated to 50 keV, mass separated, and sub...

This paper reports on the hyperfine-structure and radioactive-decay studies of the neutron-deficient francium isotopes [202][203][204][205][206] Fr performed with the Collinear Resonance Ionization Spectroscopy (CRIS) experiment at the ISOLDE facility, CERN. The high resolution innate to collinear laser spectroscopy is combined with the high efficiency of ion detection to provide a highly sensitive technique to probe the hyperfine structure of exotic isotopes. The technique of decay-assisted laser spectroscopy is presented, whereby the isomeric ion beam is deflected to a decay-spectroscopy station for alpha-decay tagging of the hyperfine components. Here, we present the first hyperfine-structure measurements of the neutron-deficient francium isotopes [202][203][204][205][206] Fr, in addition to the identification of the low-lying states of 202;204 Fr performed at the CRIS experiment.

Hyperfine structure and isotope shifts have been measured for the ground and isomeric states in the neutrondeficient isotopes [179][180][181][182][183][184] Tl using the 276.9-nm transition. The experiment has been performed at the CERNIsotope Separator On-Line facility using the in-source resonance-ionization laser spectroscopy technique. Spins for the ground states in 179,181,183 Tl have been determined as I = 1/2. Magnetic moments and changes in the nuclear mean-square charge radii have been deduced. By applying the additivity relation for magnetic moments of the odd-odd Tl nuclei the leading configuration assignments were confirmed. A deviation of magnetic moments for isomeric states in 182,184 Tl from the trend of the heavier Tl nuclei is observed. The charge radii of the ground states of the isotopes [179][180][181][182][183][184] Tl follow the trend for isotonic (spherical) lead nuclei. The noticeable difference in charge radii for ground and isomeric states of 183,184 Tl has been observed, suggesting a larger deformation for the intruder-based 9/2 − and 10 − states compared to the ground states. An unexpected growth of the isomer shift for 183 Tl has been found.

α-decay spectroscopy of 182,184 Tl has been performed at the CERN isotope separator on-line (ISOLDE) facility. New fine-structure α decays have been observed for both isotopes. α-decay branching ratios of 0.089(19)%, 0.047 (6)% and 1.22(30)% have been deduced for the (10 − ), (7 + ) and (2 − ) states respectively in 184 Tl and a lower limit of 0.49% for the α-decay branching ratio of 182 Tl. A new half-life of 9.5(2) s for the (2 − ) state in 184 Tl and 1.9(1) s for the low-spin state in 182 Tl has been deduced. Using α-γ coincidence analysis, multiple γ rays were observed de-exciting levels in 178,180 Au fed by 182,184 Tl α decays. The γ transitions connecting these low-lying states in 178,180 Au are essential to sort the data and possibly identify bands from inbeam studies in these isotopes. Owing to the complex fine-structure α decays and limited knowledge about the structure of the daughter nuclei, only partial