The two-neutron removal cross section ( ${\sigma }_{-2n}$) and the longitudinal momentum distribution of ${}^{15}\mathrm{B}$ fragments from the breakup of ${}^{17}\mathrm{B}$ on ${}^9\mathrm{Be}$ were measured at $70A\mathrm{MeV}$. The distribution in the projectile rest frame is characterized by a FWHM of $80\pm 10\mathrm{MeV}/c$ for ${}^{15}\mathrm{B}$. The ${\sigma }_{-2\mathrm{n}}$ is found to be $0.22\pm 0.05\mathrm{b}$. A Glauber–type analysis of the data provides clear evidence of a two-neutron halo structure in ${}^{17}\mathrm{B}$.

• Received 25 December 2001

DOI:https://doi.org/10.1103/PhysRevLett.89.012501