The nuclear physics input from the ${}^3\mathrm{He}(\alpha ,\gamma ){}^7\mathrm{Be}$ cross section is a major uncertainty in the fluxes of ${}^7\mathrm{Be}$ and ${}^8\mathrm{B}$ neutrinos from the Sun predicted by solar models and in the ${}^7\mathrm{Li}$ abundance obtained in big-bang nucleosynthesis calculations. The present work reports on a new precision experiment using the activation technique at energies directly relevant to big-bang nucleosynthesis. Previously such low energies had been reached experimentally only by the prompt-$\gamma$ technique and with inferior precision. Using a windowless gas target, high beam intensity, and low background $\gamma$-counting facilities, the ${}^3\mathrm{He}(\alpha ,\gamma ){}^7\mathrm{Be}$ cross section has been determined at 127, 148, and 169 keV center-of-mass energy with a total uncertainty of 4%. The sources of systematic uncertainty are discussed in detail. The present data can be used in big-bang nucleosynthesis calculations and to constrain the extrapolation of the ${}^3\mathrm{He}(\alpha ,\gamma ){}^7\mathrm{Be}$ astrophysical $S$ factor to solar energies.

• Received 5 July 2006

DOI:https://doi.org/10.1103/PhysRevLett.97.122502