Decay Assisted Laser Spectroscopy of Neutron Deficient Francium

Abstract:  Collinear resonance ionization spectroscopy (CRIS) was performed on neutron-deficient francium isotopes to determine the extent that shape coexistence defines the low-energy quantum structure across the isotopic chain.[1] Recent alpha-decay measurements suggest that a proton intruder state exists in the neutron-deficient isotopes of francium, with evidence suggesting that the ground state of Fr-199 is an intruder state.[2] Intruder states are quantum states of a nucleus that would be expected to reside at high excitation energies, but due to residual interactions between valence nucleons, are stabilized and brought lower in energy. Understanding the interactions that allow this to happen is critical to developing a more predictive model of the nucleus. Hyperfine spectra were measured for francium isotopes with mass numbers 202,203,204,205 and 206. There are low-energy isomers of even-A francium, which leads to a convoluted hyperfine spectrum. In order to assign the spectrum to the correct isomer, alpha decay spectroscopy was utilized. By matching the laser frequency to a resonance peak and simultaneously recording the energy of the alpha decay, each peak in the hyperfine spectrum can be assigned to the corresponding ground or isomeric state. The nuclear structure information that is extracted from these spectra, such as the nuclear moments and charge radii, will inform the assignment of the nuclear state as having a normal or intruder configuration of nucleons. Using this method, the hyperfine spectra of francium 202-202m and 204-204m were successfully de-convoluted, and tentative assignments were given for the resonances of francium 206-206m, which have since been re-examined.[3] The isotope shift and hyperfine constants were extracted from the hyperfine spectra for ground states of the francium isotopes with mass numbers from 202-206, as well as for the low-energy isomers in the odd-A isotopes. Future studies will be directed towards francium-199, to confirm if the proton intruder state will become the ground state. References: 1. K.M. Lynch et al., Decay-Assisted Laser Spectroscopy of Neutron-Deficient Francium, Phys. Rev. X 4, 011055 (2014) 2. J. Uusitalo et al., α Decay Studies of Very Neutron-Deficient Francium and Radium Isotopes, Phys. Rev. C 71, 024306 (2005) 3. A. Voss et al., Nuclear Moments and Charge Radii of Neutron-Deficient Francium Isotopes and Isomers, Phys. Rev. C 91, 044307 (2015)