Lifetimes of states in ${}^{19}\mathrm{Ne}$ above the ${}^{15}\mathrm{O}$$+\alpha$ breakup threshold

The ${}^{15}\mathrm{O}$($\alpha ,\gamma$)${}^{19}\mathrm{Ne}$ reaction plays a role in the ignition of type I x-ray bursts on accreting neutron stars. The lifetimes of states in ${}^{19}\mathrm{Ne}$ above the ${}^{15}\mathrm{O}$$+\alpha$ threshold of 3.53 MeV are important inputs to calculations of the astrophysical reaction rate. These levels in ${}^{19}\mathrm{Ne}$ were populated in the ${}^3\mathrm{He}$(${}^{20}\mathrm{Ne}$,$\alpha$)${}^{19}\mathrm{Ne}$ reaction at a ${}^{20}\mathrm{Ne}$ beam energy of 34 MeV. The lifetimes of six states above the threshold were measured with the Doppler-shift attenuation method. The present measurements agree with previous determinations of the lifetimes of these states and in some cases are considerably more precise.

Figure 1

Experimental setup (not to scale). A layer of ${}^3\mathrm{He}$ was implanted in a Au foil. α particles from the ${}^3\mathrm{He}$(${}^{20}\mathrm{Ne}$,α)${}^{19}\mathrm{Ne}$ reaction were detected in 25 μm $\Delta E$ and $500\mu$m $E$ detectors. The Ge detector aligned to the beam axis was used to detect ${}^{19}\mathrm{Ne}$ γ rays. The second Ge detector viewed a weak ${}^{88}\mathrm{Y}$ source shielded from the target by a Pb absorber.

Figure 2

Particle identification spectrum from the Si $\Delta E$ and $E$ detectors. The dominant particle groups are (a) α particles and (b) elastically scattered ${}^3\mathrm{He}$.

Figure 3

Kinetic energy spectrum of detected α particles. The three peaks correspond to the population of ${}^{19}\mathrm{Ne}$ states (a) above 4 MeV, (b) around 1.5 MeV, and (c) below 0.3 MeV.

Figure 4

γ rays detected in coincidence with α particles having kinetic energies between 11 and 14.5 MeV. Transitions between states in ${}^{19}\mathrm{Ne}$ are labeled by the excitation energies of the states in MeV.

Figure 5

Doppler-shifted transition of the 1536 keV level to the 238 keV level in ${}^{19}\mathrm{Ne}$. The lifetime of this state was determined to be $19.1_{-0.6}^{+0.7}\pm 1.1$ fs.

Figure 6

Doppler-shifted line shapes due to two transitions of the 4035 keV level. The experimental data are shown along with the calculated line shape and background that best fit them. (a) Decay to the ground state with a lifetime of $7.1_{-1.9}^{+1.9}\pm 0.6$ fs. (b) Decay to the 1536 keV level with a lifetime of $6.6_{-2.1}^{+2.4}\pm 0.7$ fs.

Figure 7

γ rays detected in coincidence with α particles having kinetic energies between 11 and 14.5 MeV, showing the Doppler-shifted γ ray due to the transition of the 4035 keV state to the 1536 keV state and that due to the the deexcitation of the 4144 keV level to the 1508 keV state.

Figure 8

Joint likelihood for the lifetime of the 4035 keV state in ${}^{19}\mathrm{Ne}$, taking into account the two transitions of the state observed in this experiment. The combined measurements yield $\tau =6.9_{-1.5}^{+1.5}\pm 0.7$ fs.

Figure 9

γ-ray transition due to the decay of the state at 4144 keV to the 1508 keV state along with the best fitting line shape calculation corresponding to a lifetime of 14.0 fs.

Figure 10

γ-ray transition due to the decay of the state at 4378 keV to the 238 keV state along with the best fitting line shape calculation corresponding to a lifetime of 2.9 fs.

Figure 11

Doppler-shifted line shapes due to two transitions of the 4548 keV level. The experimental data are shown along with the calculated line shape and background that best fit them. (a) Decay to the 275 keV state with a lifetime of $16.6_{-3.6}^{+4.4}\pm 1.6$ fs. (b) Decay to the ground state with a lifetime of $19.9_{-3.6}^{+3.0}\pm 2.3$ fs.

Figure 12

γ-ray transition due to the decay of the state at 4602 keV to the 238 keV state along with the best fitting line shape calculation corresponding to a lifetime of 7.6 fs.

Figure 13

Transitions of states in ${}^{19}\mathrm{Ne}$ studied in this experiment. The energy levels are labeled by their spins, parities, and excitation energies in keV.