Precise measurement of the $γ$-decay probability of the Hoyle state with a new triple coincidence-detection method

TL;DR

The paper addresses the precise measurement of the Hoyle state's γ-decay probability, a key parameter for the 3α nucleosynthesis rate in stars. It introduces a novel triple-coincidence method detecting scattered α, recoil 12C, and γ rays in inelastic α scattering on 12C, supported by PSD PID, background subtraction, and Geant4 simulations. The study finds Γ_γ/Γ = (4.00 ± 0.22 (sta) ± 0.18 (sys))×10^-4, and, using Kelley 2017's pair-decay branching, Γ_rad/Γ ≈ 4.07×10^-4, aligning with established literature values. This result reinforces the reliability of the widely used γ-decay probability in calculating the 3α reaction rate and demonstrates the effectiveness of a low-background triple-coincidence method for nuclear state decays.

Abstract

We measured the $γ$-decay probability of the Hoyle state with a new method of triple coincidence detection of a scattered $α$ particle, a recoil $\rm ^{12}C$ nucleus, and a $γ$ ray in inelastic alpha scattering on $\rm ^{12}C$. This method successfully enabled a low-background measurement and a precise determination of the $γ$-decay probability of the Hoyle state as $Γ_\mathrmγ/Γ=[4.00 \pm 0.22 \mathrm{(sta.)} \pm 0.18 \mathrm{(sys.)}]\times10^{-4}$, which is consistent with the previous literature value. Therefore, we concluded that the literature value can be reliably used in the study of nucleosynthesis in the universe.

Figures (8)

• Figure 1: Schematic view of the experimental setup.
• Figure 2: Correlation between $A_{\rm max}$ and kinetic energies of detected particles. The red and black dashed lines show the PID function to select $\alpha$-particle-like events. The gray dashed line shows the trigger threshold for the data acquisition. The red, green, and blue boxes indicate the regions where $\alpha$ particles, $\rm ^{12}C$, and $Z$ = 1 particles (p or d) are located, respectively, while the black box shows the region where PID cannot be performed. These boxes are drawn as visual guides and do not represent any event-selection conditions in the present analysis.
• Figure 3: (a) Excitation-energy spectrum of $^{12}$C and (b) expanded view around the Hoyle state. A Gaussian function to fit the Hoyle state is drawn by the thick red line, while an exponential function for high-energy states is shown by the thick black line. Contributions from the $^{13}$C and empty targets are presented by the thick and thin gray lines, respectively. The thin black line is the sum of the four components.
• Figure 4: Correlation between the excitation energy of $^{12}$C and $A^{'}_{\mathrm{max}}$.
• Figure 5: (a) Distribution of $A^{'}_{\mathrm{max}}$ for the particle-coincidence events at $E_\mathrm{r} \geq2.8$ MeV after subtracting accidental coincidence events. The thick red line shows a Gaussian function to fit the $^{12}$C events, while the thick black line presents an exponential function for the low $A^{'}_{\mathrm{max}}$ tail of the multi-$\alpha$ events. The thin black line is the sum of the two functions. (b) Distribution of $A^{'}_{\mathrm{max}}$ for the triple-coincidence events.