Spectroscopy of neutron-unbound ${}^{27,28}$F

The ground state of ${}^{28}\text{F}$ has been observed as an unbound resonance $2\underline{2}0$ keV above the ground state of ${}^{27}$F. A comparison of this result with USDA/USDB shell model predictions leads to the conclusion that the ${}^{28}$F ground state is primarily dominated by $sd$-shell configurations. Here we present a detailed report on the experiment in which the ground state resonance of ${}^{28}$F was first observed. Additionally, we report the first observation of a neutron-unbound excited state in ${}^{27}$F at an excitation energy of $25\underline{0}0\left(2\underline{2}0\right)$ keV.

Figure 1

Diagram of the experimental setup.

Figure 2

Energy loss versus flight time of the secondary beam, showing its three components: ${}^{29}\text{Ne}$, ${}^{32}\text{Mg}$, and various lighter species (“wedge fragments”) produced in the A1900 wedge.

Figure 3

(a) Element separation in $\Delta E$-$E.$ (b) Element separation in $\Delta E$-$t_b$. Panels (a) and (b) are both gated on incoming ${}^{29}\text{Ne}$ and have the fluorine events circled. (c) Three-dimensional plot of $t_b$ versus dispersive position and angle after the Sweeper for fluorine fragments produced from the ${}^{32}\text{Mg}$ beam. The various bands correspond to different isotopes of fluorine. (d) Profile of the three-dimensional plot in (c), including the emittance parameter, $e(x,{\theta }_x),$ which is determined by fitting lines of constant $t_b$ with a second-order polynomial. (e) Plot of the $e(x,{\theta }_x)$ parameter determined in (c) versus $t_b$, demonstrating isotope bands in two dimensions. (f) Final corrected time of flight $\left(t_c\right)$ for fluorine elements produced from the ${}^{29}\text{Ne}$ beam, with ${}^{26}\text{F}$ and ${}^{27}\text{F}$ indicated.

Figure 4

Neutron time of flight to the front face of MoNA, for the following conditions: (a) ungated; (b) in coincidence with ${}^{26}\text{F}$; (c) in coincidence with ${}^{27}\text{F}$.

Figure 5

Measured relative energy spectrum for ${}^{26}\text{F}+n$ coincidences. The filled squares with error bars are the experimental data, the dashed red curve is the result of a 380-keV resonant simulation, the shaded grey curve is a simulation of the Maxwellian nonresonant background ($\Theta =1.48$ MeV), and the solid black curve is the sum of the resonant and nonresonant models, with a resonant/total fraction of $33\%$. The inset is a plot of the negative log-likelihood as a function of central decay energy, with each point minimized with respect to all other free parameters.

Figure 6

Comparison of ${}^{26}\text{F}+n$ simulation and data for (a) neutron time of flight, (b) charged fragment kinetic energy, (c) neutron-fragment opening angle, and (d) neutron-fragment relative velocity. The filled black squares are the experimental data and the solid black, dashed red, and shaded grey curves depict the same simulation components as in Fig. 5.

Figure 7

Summary of experimentally known levels in ${}^{26,27}\text{F}$, including the present observation of an unbound excited state in ${}^{27}\text{F}$ at 2500 keV, decaying to the ground state of ${}^{26}\text{F}$. The shaded grey boxes around the various levels indicate the total uncertainty in their placement relative to the ${}^{27}\text{F}$ ground state. The dashed lines surrounding the presently observed 2500 keV level represent the uncertainty on the decay energy only, and the total uncertainty also includes that of the ${}^{27}\text{F}$ $1n$ separation energy. All bound excited information is from [3], ground state energies are from [28], and ground state $J^{\pi }$ are from [29].

Figure 8

Measured excited levels in ${}^{27}\text{F}$ compared to predictions of the USDA and USDB shell models. The first two measured excited states are both from Ref. [3], while the state at $2.5$ MeV is from the present work.

Figure 9

Relative energy spectrum for ${}^{27}\text{F}+n$ coincidences. The filled squares with error bars are the experimental data, the dashed red curve is the the 220-keV resonance simulation, and the dotted blue curve is the 810-keV simulation. The solid black curve is the sum of the 220 and 810 keV resonances, with the relative contribution of the 220 keV resonance at $28\%.$ The inset shows the profile log-likelihood as function of the lower resonance energy.

Figure 10

Comparison of ${}^{27}\text{F}+n$ simulation and data for (a) neutron time of flight, (b) charged fragment kinetic energy, (c) neutron-fragment opening angle, and (d) neutron-fragment relative velocity. The filled black squares are the experimental data, and the solid black, dashed red, and dotted blue curves depict the same simulation components as in Fig. 9.

Figure 11

Difference between experimental and theoretical (USDA, USDB) binding energies for $N=19$ isotones, $9⩽Z⩽17.$ The error bars on the data points represent experimental errors only. The blue dotted and red dashed bands represent the respective 170 and 130 keV RMS deviations of USDA and USDB interactions. Experimental values, save for $Z=9$ which is from the present work, are taken from Ref. [28] if reported there; otherwise they are from the 2003 Atomic Mass Evaluation [31]. Figure reproduced from Ref. [9].