New measurement of the ${}^3$He($\alpha$,$\gamma$)${}^7$Be cross section at medium energies

We report on a new cross-section measurement for the ${}^3$He($\alpha$, $\gamma$)${}^7$Be reaction at three medium energies of $E_{\mathrm{c}.\mathrm{m}.}$ between 1 and 3 MeV. The interest stems from the significant role played by the reaction in calculating an accurate solar neutrino flux and the primordial ${}^7$Li abundance. The energy dependence of the astrophysical $S_{34}$ factor observed in the present work, especially above 1 MeV, highlights the need to constrain theories in order to obtain a precise extrapolated value for $S_{34}\left(0\right)$. In this context, a comparison with the recent theoretical work in a fully microscopic fermionic molecular dynamics approach and a few other representative calculations emphasize the need for further experimental as well as theoretical work to resolve the existing conflicts.

Figure 1

Schematic diagram of the setup used in the experiment. See text for more details.

Figure 2

$\gamma$ spectra from the ambient background (a) and from the Cu catchers prepared at $E_{\mathrm{c}.\mathrm{m}.}=1.054$ (b) and 2.007 (c) MeV.

Figure 3

$S_{34}$ factor from the present work (Madrid '11) is shown along with the existing modern data. The latter are taken from Ref. [9] (Weizmann '04), [10, 11] (LUNA '06, LUNA '07), [12] (Seattle '07), and [15] (ERNA '09/'09-act). A resonance structure can be noted in the data around $E_{\mathrm{c}.\mathrm{m}.}=3$ MeV, which was first observed in Ref. [15]. This corresponds to a 7/2${}^{-}$ state residing at an excitation energy of 4570 keV in ${}^7$Be. Old measurements (Pa '63) from Parker et al. [8] at medium energies do not agree with Madrid '11 and ERNA '09/'09-act. New calculations from Ref. [17] are also shown without (Neff) and with (Neff-N) normalization together with the normalized representative calculations from Ref. [26] (Nollett-N) and [27](Kajino-N). Striking differences in the shapes of theoretical curves and discrepancies with the data can be seen.