Resonance Excitations in ${}^7\mathrm{Be}(d,p){}^8{\mathrm{Be}}^{*}$ to Address the Cosmological Lithium Problem

The anomaly in lithium abundance is a well-known unresolved problem in nuclear astrophysics. A recent revisit to the problem tried the avenue of resonance enhancement to account for the primordial ${}^7\mathrm{Li}$ abundance in standard big-bang nucleosynthesis. Prior measurements of the ${}^7\mathrm{Be}(d,p){}^8{\mathrm{Be}}^{*}$ reaction could not account for the individual contributions of the different excited states involved, particularly at higher energies close to the $Q$ value of the reaction. We carried out an experiment at HIE-ISOLDE, CERN to study this reaction at $E_{\mathrm{c}.\mathrm{m}.}=7.8\mathrm{MeV}$, populating excitations up to 22 MeV in ${}^8\mathrm{Be}$ for the first time. The angular distributions of the several excited states have been measured and the contributions of the higher excited states in the total cross section at the relevant big-bang energies were obtained by extrapolation to the Gamow window using the talys code. The results show that by including the contribution of the 16.63 MeV state, the maximum value of the total $S$ factor inside the Gamow window comes out to be 167 MeV b as compared to earlier estimate of 100 MeV b. However, this still does not account for the lithium discrepancy.

Figure 1

The detector setup for the ${}^7\mathrm{Be}+d$ experiment at $5\mathrm{MeV}/\mathrm{u}$.

Figure 2

The $\mathrm{\Delta }E\text{−}E$ spectrum of protons, deuterons, ${}^3\mathrm{He}$, and $\alpha$, detected at $\mathrm{W}1+\mathrm{MSX}25$ telescopes, from $5\mathrm{MeV}/\mathrm{u}$ ${}^7\mathrm{Be}$ on ${\mathrm{CD}}_2$ target.

Figure 3

$E$ vs $\theta$ for protons at W1 and BB7. The kinematic lines for different excited states of ${}^8\mathrm{Be}$ are shown for ${}^7\mathrm{Be}(d,p){}^8{\mathrm{Be}}^{*}$ at $5\mathrm{MeV}/\mathrm{u}$. Inset shows the excitation energy spectrum of ${}^8\mathrm{Be}$.

Figure 4

Angular distributions for ${}^7\mathrm{Be}(d,d)$ and ${}^7\mathrm{Be}(d,p)$ to 16.63 MeV (a); to 0.0, 3.03, and 11.35 MeV states (b). The corresponding fresco [25] calculations are shown by dashed and dotted lines, respectively.

Figure 5

(a) Excitation function for ${}^7\mathrm{Be}(d,p){}^8{\mathrm{Be}}^{*}$. The solid triangles, diamonds, squares, and circles correspond to total cross sections due to g.s., $\mathrm{g}.\mathrm{s}.+3.03$, $\mathrm{g}.\mathrm{s}.+3.03+11.35$, and $\mathrm{g}.\mathrm{s}.+3.03+11.35+16.63\mathrm{MeV}$ states, respectively. The data in green, brown, blue and magenta are the measurements of [7, 8, 9] and the present work. The violet (g.s.), cyan ($\mathrm{g}.\mathrm{s}.+3.03$), yellow ($\mathrm{g}.\mathrm{s}.+3.03+11.35$), and red $(\mathrm{g}.\mathrm{s}.+3.03+11.35+16.63)\mathrm{MeV}$ bands are talys calculations normalized to the present data at 7.8 MeV (green vertical line). The bands do not include systematic uncertainty due to extrapolation. (b) The $S$-factor representation of the excitation function. The red dotted line is the estimate by Parker [10].