Speaker
Description
The triple-alpha reaction is important not only for determining the abundance of $^{12}$C, but also for understanding the overall nucleosynthesis in the universe. Right after the Big Bang, elements heavier than helium were scarcely synthesized due to the absence of stable nuclei with mass numbers 5 and 8. The triple-alpha reaction overcomes this bottleneck and is considered one of the starting points for the synthesis of heavier elements. To determine the triple-alpha reaction rate, it is necessary to determine the probability of deexcitation of $^{12}$C from the $3\alpha$ resonance states to the ground state.
So far, only electromagnetic decay has been considered for deexcitation from the $3\alpha$ resonance states of $^{12}$C. However, in high-density environments, such as supernova explosions, the contribution of inelastic scattering with neutrons, which are not affected by the Coulomb barrier, becomes significant[1]. Therefore, we aim to determine the rate enhancement of the triple-alpha reaction in dense conditions.
Since the lifetime of the $3\alpha$ resonance state is extremely short, a direct measurement of the deexcitation process induced by neutron inelastic scattering is impossible. Instead, we measure the time-reversal reaction, $^{12}$C$(n,n')$, where the ground state of $^{12}$C is excited to the $3\alpha$ resonance states by neutron inelastic scattering, and then derive the deexcitation cross section using the principle of detailed balance.
We performed a cross-section measurement of the $^{12}$C$(n,n')$ reaction at the cyclotron facility of the Research Center for Accelerator and Radioisotope Science, Tohoku University. Quasi-monoenergetic neutron beams with $E_n = 9.4$, 10.9, 11.7, and 12.5 MeV were produced via the $^1$H$(^{13}$C,$n)$ reaction and injected into the MAIKo+ active-target TPC[2]. The MAIKo+ TPC was operated with $i$-$\mathrm{C}4\mathrm{H}{10}$(10%) + $\mathrm{H}_2$(90%) at 0.1 atm, enabling the detection of low-energy $\alpha$ particles emitted from the excited $^{12}$C $3\alpha$ resonance states.
In addition, two liquid scintillation counters were installed downstream of the MAIKo+ TPC at 170 mm and 4505 mm. Using these detectors, we measured the neutron beam intensity and evaluated the beam energy.
In this presentation, we will show TPC data of $^{12}$C neutron inelastic scattering events recorded in MAIKo+, present the analysis results of neutron counting and energy determination using the liquid scintillation counters, and report the measured cross section of the $^{12}$C$(n,n')$ reaction.
References
[1] Beard et al., Phys. Rev. Lett. 119, 112701 (2017).
[2] T.Furuno et al., Nucl. Instrum. Methods Phys. Res. A 908, 215 (2018).