The capture cross section of the reaction ${}^{16}\mathrm{O}(p,\gamma {)}^{17}\mathrm{F}$ was measured in the energy range from $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=200\mathrm{}–3750\mathrm{keV}$ using the windowless gas target facility Rhinoceros. The low-energy $S$ factor that is dominated by the transition to the $1/2^{+}$ first excited state in ${}^{17}\mathrm{F}$ increases strikingly with decreasing energy. This behavior is explained by the halo properties of this $1/2^{+}$ state within the framework of the direct capture model.

• Received 3 July 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.3837