Collinear Resonance Ionization Spectroscopy of Neutron-Deficient Francium Isotopes

Abstract

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...

Decay-Assisted Laser Spectroscopy of Neutron-Deficient Francium

“…This paper follows the recent publication reporting the hyperfine-structure studies of 202;203;205 Fr [39]. During the experimental campaign, the neutron-rich francium isotopes 218m;219;229;231 Fr are also studied.…”

Collinear laser spectroscopy at ISOLDE: new methods and highlights

“…While this can be partially understood in the conventional interpretation of the nuclear shell model, a full description must clearly go beyond this picture. Laser spectroscopy studies were performed on atomic Cd in the 5s5p P 5s6s S Overall, the isotopes from 100 Cd up to the shell-closure at 130 Cd were studied and nuclear magnetic moments and electric quadrupole moments were extracted from the spectra using reference dipole moments and calculated electric field gradients, respectively. The electric quadrupole moments of the 11 2 -isomers extracted from the spectra are shown in figure 6 [70].…”

Combined high-resolution laser spectroscopy and nuclear decay spectroscopy for the study of the low-lying states inFr206,At202, andLynch1, Cocolios2, Billowes3 et al. 2016Phys. Rev. CSelf Cite

“…In order to study the details of the structure of 206 Fr, we have performed an experiment with the collinear resonance ionization spectroscopy (CRIS) experiment at the CERN-ISOLDE facility [23]. The CRIS technique allows the study of the hyperfine structure of short-lived isotopes produced online, whereby using ground-state or isomer selective ionization as a purification step, decay spectroscopy can be performed in clean conditions [19].…”