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Register a new userSpectroscopy of $^{26}$F
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M. Stanoiu
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The structure of the weakly-bound $^{26}_{;;9}$F$_{17}$ odd-odd nucleus, produced from $^{27,28}$Na nuclei, has been investigated at GANIL by means of the in-beam $gamma$-ray spectroscopy technique. A single $gamma$-line is observed at 657(7) keV in $^{26}_{9}$F which has been ascribed to the decay of the excited J=$2^+$ state to the J=1$^+$ ground state. The possible presence of intruder negative parity states in $^{26}$F is also discussed.
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A long-lived $J^{pi}=4_1^+$ isomer, $T_{1/2}=2.2(1)$ms, has been discovered at 643.4(1) keV in the weakly-bound $^{26}_{9}$F nucleus. It was populated at GANIL in the fragmentation of a $^{36}$S beam. It decays by an internal transition to the $J^{pi}=1_1^+$ ground state (82(14)%), by $beta$-decay to $^{26}$Ne, or beta-delayed neutron emission to $^{25}$Ne. From the beta-decay studies of the $J^{pi}=1_1^+$ and $J^{pi}=4_1^+$ states, new excited states have been discovered in $^{25,26}$Ne. Gathering the measured binding energies of the $J^{pi}=1_1^+-4_1^+$ multiplet in $^{26}_{9}$F, we find that the proton-neutron $pi 0d_{5/2} u 0d_{3/2}$ effective force used in shell-model calculations should be reduced to properly account for the weak binding of $^{26}_{9}$F. Microscopic coupled cluster theory calculations using interactions derived from chiral effective field theory are in very good agreement with the energy of the low-lying $1_1^+,2_1^+,4_1^+$ states in $^{26}$F. Including three-body forces and coupling to the continuum effects improve the agreement between experiment and theory as compared to the use of two-body forces only.
The ground state of $^{28}$F has been observed as an unbound resonance $2underline{2}0$ keV above the ground state of $^{27}$F. 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 $25underline{0}0 (2underline{2}0)$ keV.
$beta$ decay of $^{26}$P was used to populate the astrophysically important $E_x=$5929.4(8) keV $J^{pi}=3{^+}$ state of $^{26}$Si. Both $beta$-delayed proton at 418(8) keV and gamma ray at 1742(2) keV emitted from this state were measured simultaneously for the first time with corresponding absolute intensities of 11.1(12)% and 0.59(44)%, respectively. Besides, shell model calculations with weakly bound effects were performed to investigate the decay properties of other resonant states and a spin-parity of $4^+$ rather than $0^+$ was favored for the $E_x=$5945.9(40) keV state. Combining the experimental results and theoretical calculations, $^{25}$Al($p,gamma$)$^{26}$Si reaction rate in explosive hydrogen burning environments was calculated and compared with previous studies.
The $beta$-decay properties of the neutron-deficient nuclei $^{25}$Si and $^{26}$P have been investigated at the GANIL/LISE3 facility by means of charged-particle and $gamma$-ray spectroscopy. The decay schemes obtained and the Gamow-Teller strength distributions are compared to shell-model calculations based on the USD interaction. B(GT) values derived from the absolute measurement of the $beta$-decay branching ratios give rise to a quenching factor of the Gamow-Teller strength of 0.6. A precise half-life of 43.7 (6) ms was determined for $^{26}$P, the $beta$- (2)p decay mode of which is described.
Classical novae are expected to contribute to the 1809-keV Galactic $gamma$-ray emission by producing its precursor $^{26}$Al, but the yield depends on the thermonuclear rate of the unmeasured $^{25}$Al($p,gamma$)$^{26}$Si reaction. Using the $beta$ decay of $^{26}$P to populate the key $J^{pi}=3^+$ resonance in this reaction, we report the first evidence for the observation of its exit channel via a $1741.6 pm 0.6 (textrm{stat}) pm 0.3 (textrm{syst})$ keV primary $gamma$ ray, where the uncertainties are statistical and systematic, respectively. By combining the measured $gamma$-ray energy and intensity with other experimental data on $^{26}$Si, we find the center-of-mass energy and strength of the resonance to be $E_r = 414.9 pm 0.6(textrm{stat}) pm 0.3 (textrm{syst}) pm 0.6(textrm{lit.})$ keV and $omegagamma = 23 pm 6 (textrm{stat})^{+11}_{-10}(textrm{lit.})$ meV, respectively, where the last uncertainties are from adopted literature data. We use hydrodynamic nova simulations to model $^{26}$Al production showing that these measurements effectively eliminate the dominant experimental nuclear-physics uncertainty and we estimate that novae may contribute up to 30% of the Galactic $^{26}$Al.
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