Upper limit of the lifetime of 16B.

The exact location of the drip lines is one of the most stringent tests of nuclear structure models. The predictions for stability of nuclei along the proton drip line can be tested up to very heavy nuclei. Beyond the proton drip line, the large Coulomb barrier and the angular momentum barrier can lead to very long lifetimes for proton-unbound nuclei ~proton radioactivity! @1#. Several of these ground state proton emitters have been observed and they serve as important probes of the drip line since the lifetimes are sensitive to the nuclear potential @2,3#. On the neutron-rich side of stability the search for neutron radioactivity, which arises solely due to the angular momentum barrier, is extremely difficult because of the shorter lifetimes expected and the current inaccessibility of the neutron drip line beyond Z;8 @4,5#. One interesting candidate for neutron radioactivity is B. This nucleus was first reported to be neutron-unstable by Bowman et al. who observed the isotopes B but not B @6# from the spallation of uranium by 4.8 GeV protons. This result was later confirmed by Langevin et al. who studied the fragmentation of 44 MeV/nucleon Ar on a tantalum target @7#. More recently, Bohlen et al. studied B produced via the multiparticle transfer reaction C~C,N!B @8#. They reported that the ground state of B is neutron unbound by only 40 6 60 keV and thus is nearly particle stable. Furthermore, the simple shell model picture for the structure of B suggests a d5/2 orbital for the last neutron. The low neutron binding energy and the l 5 2 angular momentum barrier may then yield a quasi-stationary ground state for B with a relatively long lifetime. By reexamining the previous experimental studies of B we can estimate limits on its lifetime. In the work of Bowman et al. @6# the B fragments were emitted with ; 2 MeV/nucleon energy over a flight path of ; 40 cm. From a combination of the B flight time with an estimate of the number of B that should have been observed relative to the B in this experiment, we deduce the B lifetime to be shorter than ; 9 ns. In the work of Langevin et al., fragmentation products from reactions of the 44 MeV/nucleon Ar beam were separated from the incident beam using a fragment separator and were detected approximately 18 m down-