It is shown that the continuum missing-mass spectra for the (${\mathrm{\pi }}^{\mathrm{-}}$,${\mathrm{\pi }}^{+}$) and (${\mathrm{\pi }}^{\mathrm{-}}$,p) reactions leading to extremely neutron-rich exotic nuclei can be explained in terms of phase-space distributions by invoking the presence of dineutrons as one of the products of the breakup. It is suggested that this indicates the presence of the dineutron as a cluster in these neutron-rich systems during their breakup. It is noted that these observations in weakly unbound systems may be analogs of the dineutron halos for which evidence has been found in weakly bound nuclei near the neutron drip line.

• Received 18 June 1990

DOI:https://doi.org/10.1103/PhysRevLett.66.2448