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An attempt was made to find the very neutron-rich isotope 21 B among the fragmentation of a 94A MeV 40 Ar beam. Evidence for the particle instability of 21 B was obtained. As part of the search, the production cross sections of 13 light neutron-rich nuclei with Be and Ta targets are presented and compared to previous data. A large enhancement of the production cross sections for a Ta target is shown and a clear energy dependence of the production cross sections is shown for very neutron-rich nuclei.

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The cross section for the *L'\(a,n) n B reaction, which is crucial to predictions of primordial nucleosynthesis in inhomogeneous models, has been measured using the radioactive-beam facility of the Institute for Physical and Chemical Research (RIKEN). The reaction cross section to all allowed n B states was found to be larger than that to just the M B ground state by about a factor of 5. (7) PACS numbers: 25.55.Hp, 95.30.Cq, 98.80.Ft The standard (homogeneous) model (SM) [1] of primordial nucleosynthesis has been known for some time to give reasonable predictions of the abundances of the nuclides up to 4 He (although recent work [2] on 4 He has raised questions about the agreement between theory and observation), and arguably valid predictions [3] for 7 Li as well. However, consideration of density inhomogeneities, possibly resulting from the quark-hadron phase transition thought to have occurred 10 ~5 s after the big bang, has led to a set of alternate models, the inhomogeneous models (IMs) [4,5]. In the IMs, the abundances predicted for light nuclides are similar to those predicted by the SM, but those for 7 Li, 9 Be, M B, and heavier nuclides are considerably higher for most of the IM parameter space. Unfortunately, the abundance of 7 Li, which is fairly easily measured by astronomers, is difficult to interpret [6-9]. Recent studies [10,11] of 9 Be, however, have pushed its abundance in metal poor stars to potentially interesting levels [12]. But M B and heavier nuclides may ultimately provide important tests of primordial nucleosynthesis; indeed the relative insensitivity [13] of the predicted M B abundance to the IM parameters may make it an ideal test of those models. Furthermore, a recent observation [14] has shown n B can be detected in metal poor stars, using the Hubble Space Telescope, at levels relevant to predictions of primordial nucleosynthesis [5]. A critical reaction in predicting abundances of n B and heavier nuclides in the IMs is 8 Li(a,Az) n B, as n B is the nuclide through which most heavier nuclides must pass, and that reaction apparently regulates the dominant pathway by which M B is made [5]. Observation of this reaction, however, is complicated by the 840.3-ms halflife [15] of 8 Li. A recent measurement [16] of the inverse reaction n B(«,a) 8 Li gives the ground-stateground-state cross section for 8 Li(a,Ai) M B. However, several M B excited states can be populated in 8 Li(a,A*) n B, so inference of the cross section of interest from measurement of the inverse reaction may underesti-