Parallel Momentum Distributions as a Probe of Halo Wave Functions

Distributions of the parallel momentum of ${}_{}{}^{10}{}_{}{}^{}\mathrm{Be}$ fragments from the breakup of $63A$ MeV ${}_{}{}^{11}{}_{}{}^{}\mathrm{Be}$ have been measured for ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$, ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$, ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$, and ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ targets. The distributions have similar narrow widths with a mean value of 43.6 ± 1.1 MeV/c FWHM, and agree with a theoretical momentum distribution for the valence neutron in ${}_{}{}^{11}{}_{}{}^{}\mathrm{Be}$. The breakup mechanisms do not appear to distort the parallel momentum distribution of the ${}_{}{}^{10}{}_{}{}^{}\mathrm{Be}$ core. These findings support the existence of a one-neutron halo in ${}_{}{}^{11}{}_{}{}^{}\mathrm{Be}$ and the use of the parallel momenta of the heavy breakup products as a reliable probe of halo neutron momenta.