Detector characterization for a new $^{12}$C+$^{12}$C reaction study at LUNA

TL;DR

The paper describes detector characterization for a new study of the $^{12}$C+$^{12}$C fusion cross section at the underground Bellotti IBF, targeting $E_ extrm{cm}$ below $2\, ext{MeV}$ via gamma-ray detection. It details the GePD2 HPGe detector and a NaI(Tl) veto array, including long-term background, intrinsic contamination, and Geant4 simulations to achieve unprecedented low backgrounds. A sensitivity study shows that, with shielding and veto, the campaign can probe the $^{12}$C($^{12}$C,$p$)$^{23}$Na channel below $E_ extrm{cm}=2$ MeV and possibly down to $1.7$ MeV, while accurately accounting for intrinsic backgrounds. The results support high-precision nuclear astrophysics measurements of carbon burning relevant to stellar evolution and supernova progenitors, with simulations indicating negligible impact from internal contaminations on the GePD2 detector.

Abstract

The $^{12}$C+$^{12}$C fusion reaction plays a crucial role in stellar evolution, including the occurrence of supernova explosions, and in the synthesis of the chemical elements. However, our understanding of its cross section remains severely deficient, particularly below $E_\textrm{cm}=2.5$\,MeV, the energy range of interest for astrophysics. To address these unresolved issues, the LUNA collaboration will conduct a dedicated study of the $^{12}$C+$^{12}$C reaction at the Bellotti Ion Beam Facility (Bellotti IBF) located deep underground within the Gran Sasso National Laboratory (LNGS) in Italy. Based on the combination of passive and active shields, this campaign aims to achieve unprecedented sensitivity in measuring the cross sections of the two key reaction channels, $^{12}$C($^{12}$C,$α$)$^{20}$Ne and $^{12}$C($^{12}$C,$p$)$^{23}$Na in the low-energy regime via $γ$-ray detection. Here, we report on a sensitivity study for the upcoming campaign with a focus on the characterization of two detectors, namely a HPGe detector and a NaI(Tl) array. Furthermore, their intrinsic contamination is thoroughly investigated since this could potentially influence the overall sensitivity. Assuming typical beam intensities of the Bellotti IBF, we will be able to investigate reaction rates significantly below 100 counts per day. In case of the $^{12}$C+$^{12}$C reaction we therefore expect to acquire experimental data well below the current limit of $E_\textrm{cm}=2.1\,$MeV. The results are supported by simulations to highlight the advantageous low-background environment, essential for high-precision nuclear astrophysics studies.

Figures (18)

• Figure 1: Experimental energy calibration of GePD2 based on $^{60}$Co, $^{137}$Cs radioactive sources and $^{14}$N(p,$\gamma$)$^{15}$O reaction data. The entire data set is fitted with a second order polynomial (orange). In addition, a linear fit (blue) is used to solely represent the low-energy data and extrapolated to higher energies for comparability.
• Figure 2: Comparison of the count rates for different shieldings of GePD2, i.e. without lead shielding (black), with a lead and copper shielding (blue) and an additional nitrogen flushing (orange). As a comparison, the count rate of the unshielded GeBochum above ground is shown in grey. The most prominent peak structures are labeled and color-coded according to their origin and listed in \ref{['tab:GePD2ReamainingPeaks']}. The high energetic region from 6 MeV to 12 MeV is shown as an inset for the same spectra.
• Figure 3: Expected number of $^{12}$C($^{12}$C,$p$)$^{23}$Na reactions per day assuming 200$\,\upmu$A and three different reported extrapolations for the cross section mentioned above Tumino2018Mukhamedzhanov2019Caughlan1988. The detection sensitivity for measuring the signal at 440 keV with 50% uncertainty on a $3\sigma$ confidence interval is shown as blue and yellow horizontal lines for the present and final shielding configurations, respectively. The current lower limit for experimental investigations is indicated by a red vertical line.
• Figure 4: Time development of the count rate within an energy interval of [4800 keV, 5300 keV] for GePD2. The black dotted line represents the average of the data set and the orange dashed curve represents the expected trend for a decay which follows the half life of $^{210}$Po, as suggested by Brodzinski1987.
• Figure 5: Top: Decay scheme of $^{214}$Bi including the $\beta/\alpha$ coincidence measured within the NaI(Tl) prototype. Bottom: Decay scheme of $^{210}$Pb, which is a long-lived contaminant within GePD2 and feeds the alpha emitter $^{210}$Po. Kondev2008Basunia2014Zhu2021