${}^{19}\mathrm{Ne}$ level structure for explosive nucleosynthesis

Background: ${}^{19}\mathrm{Ne}$ is an important isotope in nuclear astrophysics due to its role in both the ${}^{18}\mathrm{F}(p,\alpha ){}^{15}\mathrm{O}$ and ${}^{15}\mathrm{O}(\alpha ,\gamma ){}^{19}\mathrm{Ne}$ reactions in novae and Type I x-ray bursts, respectively. The energy levels of ${}^{19}\mathrm{Ne}$ near the $\alpha$ and proton thresholds ($S_{\alpha }=3529$ keV, $S_p=6410$ keV) correspond to resonances in both of these reactions. Previous measurements to study the structure of ${}^{19}\mathrm{Ne}$ have focused on both regions in an effort to constrain these reaction rates.

Purpose: Discrepancies in the energies, spins, and parities for levels in ${}^{19}\mathrm{Ne}$ from previous measurements contribute to the reaction-rate uncertainties. Gamma rays from the depopulation of excited states in ${}^{19}\mathrm{Ne}$ were measured to reduce the level-energy uncertainties and inconsistencies in previous spin-parity assignments.

Methods: The ${}^{19}\mathrm{F}({}^3\mathrm{He},t){}^{19}\mathrm{Ne}$ reaction was used to elucidate the structure of ${}^{19}\mathrm{Ne}$ levels up to $E_x=6.9$ MeV. The reaction products were measured using Gammasphere ORRUBA: Dual Detectors for Experimental Structure Studies—a coupling of the Oak Ridge Rutgers University Barrel Array and Gammasphere at Argonne National Laboratory. Tritons produced in the reaction were measured in coincidence with $\gamma$ rays from the deexcitation of ${}^{19}\mathrm{Ne}$ energy levels.

Results: Previously unobserved transitions allowed for discrepancies in the resonance properties relevant to these two reactions to be resolved. In total, 41 transitions from 21 energy levels were measured in ${}^{19}\mathrm{Ne}$, with 21 of those transitions being previously unobserved. Of particular importance, transitions from two $3/2^{+}$ states with energies of 6423(3) and 6441(3) keV, crucial for accurate estimations of the ${}^{18}\mathrm{F}(p,\alpha ){}^{15}\mathrm{O}$ reaction rate, were found.

Conclusions: Energies and spin-parities of important energy levels near the proton and $\alpha$ thresholds were measured and some of the discrepancies in previous measurements were resolved. Measurement of the two near-threshold $3/2^{+}$ states reduced the calculated upper limit of the ${}^{18}\mathrm{F}(p,\alpha ){}^{15}\mathrm{O}$ reaction rate by factors of 1.5–17 in the nova temperature range.

Figure 1

Particle identification spectrum from the QQQ5 detectors, detected at ${\theta }_{\mathrm{Lab}}={20}^{\circ }$. The elastically scattered ${}^3\mathrm{He}$ beam was completely blocked by the aluminum blocker and not detected. Tritons from the reaction of interest are clearly separated from the protons and deuterons produced by the other reactions on the target. Adapted from Ref. [22].

Figure 2

${}^{19}\mathrm{Ne}$ excitation energy spectrum from one QQQ5 telescope (upper panel) produced by gating on the tritons in Fig. 1. The lower panel shows the locations of previously reported energy levels in ${}^{19}\mathrm{Ne}$, taken from Ref. [23].

Figure 3

Gamma-ray spectrum gated on tritons populating excitation energies between 4.0 and 5.0 MeV. The histogram demonstrates how the application of the Doppler correction affects the resolution of the 4140- and 4364-keV $\gamma$-ray transitions from the 4377- and 4603-keV ${}^{19}\mathrm{Ne}$ states, respectively. $\kappa$ is the fraction of ${\beta }_{\mathrm{calc}}$ used, and it is clear that short-lived states require the full correction to be applied to minimize the resolution.

Figure 4

Gamma-ray spectrum of the 2556-keV transition from the 2794-keV state (left panel) and the 1840-keV transition from the 4634-keV state (right panel). The lifetimes of these states are 100(12) fs and $>1$ ps, respectively [26]. Therefore, both transitions require a Doppler correction with $\kappa <1$ to minimize the resolution of the transitions.

Figure 5

Level structure of ${}^{19}\mathrm{Ne}$ for energy levels (keV) observed in the data, with analog states in ${}^{19}\mathrm{F}$ shown to the left with dashed lines. ${}^{19}\mathrm{F}$ levels between 5.1 and 6 MeV were omitted for clarity. Transition arrows in black were previously reported. Red arrows signify transitions newly discovered in this work. Gamma-ray energies (keV) and branching ratios (%) are listed next to each transition.

Figure 6

Random-subtracted low-energy $\gamma$-ray spectra showing all transitions observed from states with excitation energies less than 3.0 MeV. All of the spectra were Doppler corrected, except for panel (a), and gated on tritons corresponding to specific ${}^{19}\mathrm{Ne}$ excitation energy ranges. The $E_x$ ranges and other gating parameters are as follows: (a) $0\text{–}0.6$ MeV, (b) $1.3\text{–}2.1$ MeV and the 238-keV $\gamma$ ray, (c) $1.0\text{–}1.8$ MeV and the 275-keV $\gamma$ ray, (d) $1.0\text{–}1.8$ MeV and a $\gamma$-ray multiplicity of 1, (e) $2.0\text{–}2.8$ MeV and the 238-keV $\gamma$ ray.

Figure 7

Random-subtracted $\gamma$-ray spectra showing all transitions observed from states between 3.0 and 6.0 MeV. All of the spectra were Doppler corrected and gated on tritons populating specific ${}^{19}\mathrm{Ne}$ excitation energy ranges. The $E_x$ ranges and other gating parameters are as follows: (a) $3.6\text{–}4.4$ MeV and a $\gamma$-ray multiplicity of 1, (b) $3.8\text{–}4.6$ MeV and the 275-keV $\gamma$ ray, (c) $3.7\text{–}4.5$ MeV and the 238-keV $\gamma$ ray, (d) $3.7\text{–}4.5$ MeV and the 275-keV $\gamma$ ray, (e) $4.0\text{–}4.8$ MeV and the 238-keV $\gamma$ ray, (f) $3.8\text{–}4.6$ MeV and the 1297-keV $\gamma$ ray, (g) $4.2\text{–}5.0$ MeV and the 1232-keV $\gamma$ ray, (h) $4.4\text{–}4.9$ MeV and the 238-keV $\gamma$ ray, (i) $4.4\text{–}5.2$ MeV and a $\gamma$-ray multiplicity less than 6, (j) $4.3\text{–}5.1$ MeV and the 275-keV $\gamma$ ray, (k) $4.7\text{–}5.4$ MeV.

Figure 8

Random-subtracted $\gamma$-ray spectra showing all observed transitions from states above 6.0 MeV. All of the spectra were Doppler corrected and gated on tritons populating specific ${}^{19}\mathrm{Ne}$ excitation energy ranges. The $E_x$ ranges and other gating parameters are as follows: (a) $5.9\text{–}6.7$ MeV and the 238-keV $\gamma$ ray, (b) $5.7\text{–}6.5$ MeV and the 275-keV $\gamma$ ray, (c) $5.8\text{–}6.6$ MeV and the 238-keV $\gamma$ ray, (d) $6.0\text{–}6.8$ MeV and the 238-keV $\gamma$ ray, (e) $6.0\text{–}6.5$ MeV and the 275-keV $\gamma$ ray, (f) $6.0\text{–}6.5$ MeV and the 1232-keV $\gamma$ ray, (g) $6.2\text{–}7.0$ MeV and a $\gamma$-ray multiplicity of 1, (h) $5.7\text{–}6.5$ MeV and the 275-keV $\gamma$ ray, (i) $6.4\text{–}7.0$ MeV and the 275-keV $\gamma$ ray, (j) $6.5\text{–}7.0$ MeV and the 1232-keV $\gamma$ ray.

Figure 9

$S$ factors calculated for the ${}^{18}\mathrm{F}(p,\alpha ){}^{15}\mathrm{O}$ reaction assuming the combination of interference signs shown. The first (second) set of parentheses shows the interference signs for the $1/2^{+}$ ($3/2^{+}$) states. An energy resolution of 15-keV was included for comparison with experimental data from Bardayan 2001 [11], Bardayan 2002 [32], de Séréville 2009 [36], and Beer 2011 [6]. This figure was adapted from Ref. [9].

Figure 10

${}^{18}\mathrm{F}(p,\alpha ){}^{19}\mathrm{Ne}$ reaction rate calculations showing the reduction in uncertainty from constraining the energies of the $3/2^{+}$ states. The upper limit of the rate is reduced by factors of 1.5–17 in the temperature range of nova nucleosynthesis. The rate calculated by Bardayan 2015 [8] is included for comparison. This figure was adapted from Ref. [9].

Figure 11

Ratios of the ${}^{18}\mathrm{F}(p,\alpha ){}^{19}\mathrm{Ne}$ reaction rates from Fig. 10 to the upper limit from Bardayan 2015 [8].