A candidate resonant tetraneutron state is found in the missing-mass spectrum obtained in the double-charge-exchange reaction ^{4}He(^{8}He,^{8}Be) at 186  MeV/u. The energy of the state is 0.83±0.65(stat)±1.25(syst)  MeV above the threshold of four-neutron decay with a significance level of 4.9σ. Utilizing the large positive Q value of the (^{8}He,^{8}Be) reaction, an almost recoilless condition of the four-neutron system was achieved so as to obtain a weakly interacting four-neutron system efficiently.

We perform the first direct mass measurements of neutron-rich calcium isotopes beyond neutron number 34 at the RIKEN Radioactive Isotope Beam Factory by using the time-of-flight magnetic-rigidity technique. The atomic mass excesses of ^{55-57}Ca are determined for the first time to be -18650(160), -13510(250), and -7370(990)  keV, respectively. We examine the emergence of neutron magicity at N=34 based on the new atomic masses. The new masses provide experimental evidence for the appearance of a sizable energy gap between the neutron 2p_{1/2} and 1f_{5/2} orbitals in ^{54}Ca, comparable to the gap between the neutron 2p_{3/2} and 2p_{1/2} orbitals in ^{52}Ca. For the ^{56}Ca nucleus, an open-shell property in neutrons is suggested.

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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-