Underground experimental study finds no evidence of low-energy resonance in the ${}^6\mathrm{Li}(p,\gamma ){}^7\mathrm{Be}$ reaction

The astrophysical ${}^6\mathrm{Li}(p,\gamma ){}^7\mathrm{Be}$ reaction occurs during Big Bang nucleosynthesis and the pre-main sequence and main sequence phases of stellar evolution. The low-energy trend of its cross section remains uncertain, since different measurements have provided conflicting results. A recent experiment reported a resonancelike structure at center-of-mass energy 195 keV, associated to a positive-parity state of ${}^7\mathrm{Be}$. The existence of such resonance is still a matter of debate. We report a new measurement of the ${}^6\mathrm{Li}(p,\gamma ){}^7\mathrm{Be}$ cross section performed at the Laboratory for Underground Nuclear Astrophysics, covering the center-of-mass energy range $E=60$–350 keV. Our results rule out the existence of low-energy resonances. The astrophysical $S$-factor varies smoothly with energy, in agreement with theoretical models.

Figure 1

Sketch of the experimental setup used for the measurement of the ${}^6\mathrm{Li}(p,\gamma ){}^7\mathrm{Be}$ cross section at LUNA.

Figure 2

Ratio of the counting rates in the $\mathrm{DC}\to 0$ (${\gamma }_0$) and $\mathrm{DC}\to 429$ keV (${\gamma }_1$) for each target, corrected for detection efficiency and angular distribution. The ratio is compared to literature values [22, 23]. Shaded areas represent the uncertainties on literature values. The present error bars reflect only the statistical uncertainties.

Figure 3

Astrophysical $S$-factor for the ${}^6\mathrm{Li}(p,\gamma ){}^7\mathrm{Be}$ reaction as obtained from individual targets (left panel) and from a weighted average of the different data sets (red filled circles, right panel). Previous experimental data and theoretical evaluations are also shown for comparison in the right panel. The solid black line represents an $R$-matrix fit of the present data and data from [59].

Figure 4

Astrophysical reaction rate for the ${}^6\mathrm{Li}(p,\gamma ){}^7\mathrm{Be}$ reaction, normalized to the NACRE rate [68]. The NACRE II rate [36] is also shown for comparison. Dashed lines represent the uncertainty on the NACRE rate, while shaded areas represent the uncertainties from the present experiment (red) and from NACRE II (gray).