First investigation of excited states in the odd-proton nucleusFr209

Abstract

Excited states in 209 Fr were produced following the 176 Yb( 37 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 Fr. The decay from a proposed πi 13/2 configuration isomer was observed in 209 Fr, and its half-life was measured to be 446(14)ns. States above the isomer were also observed.

Spectroscopy and high-spin structure ofFr209

“…The 203, 257, 520, 701, 721, and 817 keV lines were also observed in the γ -γ -coincidence studies of our earlier work with 12 Although the present γ -γ -time coincidence results confirm the arrangement of transitions and levels claimed in Ref. [4] with an isomeric state decaying through a 194 and 632 keV cascade, from our excitation function results and cross bombardments, we assign it to 208 Fr, rather than to 209 Fr. (A report on 208 Fr has been published [5], and 210 Fr will be the subject of a later study.)…”

“…For still lighter isotopes, the valence neutron holes are expected to play a more important role. In 210 Fr(N = 123), a number of excited states have been assigned from the α decay of 214 Ac [3], but these are necessarily of relatively low spin, while a recent high-spin study by the Yale group reported a set of transitions for 210 Fr and a partial level scheme for 209 Fr [4]. However, our study does not agree with the latter assignments.…”

“…2, Ref. [4]) but as noted earlier, fission competition which becomes more prevalent at higher energies has the effect of favoring channels that involve proton emission. The competition can result in a difference in energy dependence (and therefore peak energies) for 5n evaporation compared to 1p4n evaporation, for example, and compromise interpretations based on the shape of excitation functions.…”

“…The measurements and analyses were carried out in conjunction with a study of the spectroscopy of 212 Rn as reported recently [7,8]. To address the discrepancy that has arisen between the present work and the earlier assignments [4] of transitions to 209 Fr and 210 Fr, our study also included measurements with 17 O and 18 O beams, the latter leading to the well-established case of 211 Fr, to provide an indirect calibration of the 16 O-induced reaction, as will be outlined below.…”

“…The upper state is suggested to be the expected πh 4 9/2 i 13/2 multiplet member (consistent with the preferential decays to this state from the higher-lying states in the main cascade), but there are few other possibilities for obtaining states of this spin and parity at such a low energy, and no equivalent is observed in 211 Fr [1]. This leads us to the suggestion of an excitation involving the i −1 13/2 neutron hole, which is less significant in the heavier isotopes.…”