First investigation of excited states in the odd-proton nucleus ${}^{209}\mathrm{Fr}$

Excited states in ${}^{209}\mathrm{Fr}$ were produced following the ${}^{176}\mathrm{Yb}$(${}^{37}\mathrm{Cl}$, $4n$) reaction. An excitation function was measured with data taken at 173, 179, and 185 MeV. Recoiling fusion-evaporation products were separated using the SASSYER gas-filled spectrometer. HPGe clover detectors of the YRAST Ball array at the target position and at the focal plane were used to detect prompt and delayed gamma ray decays, providing the first spectroscopic information about prompt and delayed transitions in ${}^{209}\mathrm{Fr}$. The decay from a proposed $\pi i_{13/2}$ configuration isomer was observed in ${}^{209}\mathrm{Fr}$, and its half-life was measured to be 446(14)ns. States above the isomer were also observed.

Figure 1

The beam is incident (from the left of this figure) on a target centered at YRAST Ball. The recoils are then focused to the focal plane by setting the fields of two dipole magnets and one quadrupole magnet. Recoiling nuclei are detected in SCARY, an array of 30 solar cells. Prompt gamma rays are detected using YRAST Ball, and delayed gamma rays are detected using Ge detectors around the focal plane.

Figure 2

Excitation function measured in this work for the 3, 4, and 5 particle channels. Top panels show gamma rays detected at the focal plane. Bottom panels show gamma rays detected at the target position. The 194, 632, 568, and 563 keV transitions belong to ${}^{209}\mathrm{Fr}$. The 635, 325, 778, and 577 keV transitions belong to ${}^{208}\mathrm{Rn}$. For details, see text. (Note the suppressed zero in some of the plots.)

Figure 3

Gamma ray spectra obtained at the focal plane at 173, 179, and 185 MeV. Top: Portion of spectrum (a) of delayed gamma rays measured at the focal plane at a beam energy of 173 MeV. The spectrum is dominated by the 194 and 632 keV gamma rays belonging to ${}^{209}\mathrm{Fr}$. Bottom: Portion of the spectrum indicated by dashed box in (a) between the 194 and 632 keV gamma rays at (b) 173, (c) 179, and (d) 185 MeV beam energies. The varying strengths of the 3, 4, and 5 particle channels are evident from the relative intensities of the corresponding gamma rays.

Figure 4

Time relative to implantation for several gamma rays measured at the focal plane. The solid line shows a weighted least squares fit to decay spectrum of isomeric gamma rays: 632 keV (${}^{209}\mathrm{Fr}$), 325 keV (${}^{208}\mathrm{Rn}$), and 433 keV (${}^{210}\mathrm{Fr}$).

Figure 5

Gate on 632 keV isomeric transition (top) shows prompt gamma rays in coincidence. Gates in the recoil-gamma-gamma matrix, such as the gate on the 568 keV transition (bottom), have been used for building the level scheme.

Figure 6

Level scheme for ${}^{209}\mathrm{Fr}$ obtained in this work. Dashed lines indicate tentative assignments.

Figure 7

Calculations done using the ultimate cranker model [29] with $\hslash \omega =0.23$ MeV for positive parity proton and neutron excitations, shown with contours of 200 keV. They show two minima at low excitation energies: One very deep, slightly oblate minimum (left) and one higher-lying, strongly prolate minimum (right).

Figure 8

Levels [12, 14, 16, 18, 21] in the odd Bi, At, and Fr isotopes indicative of the relative energies of the orbitals. The ${29/2}^{+}$ and ${25/2}^{+}$ states are likely members of the $\pi i_{13/2}\otimes \pi (h_{9/2}){}^2$ configuration. The ${23/2}^{-}$ and ${21/2}^{-}$ maximally aligned states presumably have the $\pi f_{7/2}\otimes \pi (h_{9/2}){}^2$ and $\pi (h_{9/2}){}^3$ configurations, respectively.

Figure 9

Comparison of level spacings of the ($h_{9/2}){}^3$ configurations in the odd-At [14, 16, 18] and odd-Fr [21] isotopes.

Figure 10

Comparison of similar configurations in the $N=122$ Rn [15], Fr, and Ra [36] isotopes.