Direct total cross section measurement of the ${}^{16}$O($\alpha ,\gamma$)${}^{20}$Ne reaction at $E_{\mathrm{c}.\mathrm{m}.}=2.26$ MeV

In stellar helium burning, ${}^{16}$O represents the endpoint of the helium-burning sequence due to the low rate of ${}^{16}$O($\alpha$,$\gamma$)${}^{20}$Ne. We present a new direct measurement of the total capture reaction rate of ${}^{16}$O($\alpha$,$\gamma$)${}^{20}$Ne at $E_{\mathrm{c}.\mathrm{m}.}=2.26\mathrm{MeV}$ employing the DRAGON recoil separator. For the first time, the total $S$ factor and its contributing direct capture transitions could be determined in one experiment.

Figure 1

Excitation levels in ${}^{20}$Ne. On the right-hand side, Gamow windows corresponding to 0.1 and 0.3 GK are indicated. The red line (second from the top) indicates the energy discussed in the present work.

Figure 2

Schematic view of the DRAGON facility.

Figure 3

Ionization chamber PID: energy deposited in anode 1 vs energy deposited in anode 0 (given in arbitrary units). The red circles are recoils detected in the ionization chamber, the blue crosses are recoils in the ionization chamber coincident with a $\gamma$ detection at the target, and the green line represents a software gate used to identify recoil events.

Figure 4

Simulation of pure $S_0$ (red solid line) and $S_2$ (blue dashed line). Shown is the maximum detected $\gamma$ energy.

Figure 5

Maximum detected $\gamma$ energy: solid line, measured; dashed line, simulation.

Figure 6

Comparison of this work to previous results: The orange dashed-dotted line denoted “C-$S$” is the total $S$ factor given in Ref. [6], and the purple dashed line marked “L-S” is the total $S$ factor from Ref. [2]. The red data points and upper limits are $S_0$ and $S$, respectively, taken from Ref. [3]. The blue data points are the results presented here.