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The primordial nucleosynthesis right after the big bang (BB) is one of the key elements that support the BB model. BBN is well known that it produced primarily light elements, and explains reasonably well the element abundance. However, there remains one serious problem, call the Li problem in BBN. The BBN simulation together with recent detailed micro-wave background measurements BBN explain most light elements including D, ⁴He, etc, but the ⁷Li abundance is over predicted roughly by a factor of three.

The primordial ⁷Li was considered to be produced mostly by the electron capture of ⁷Be in the late stage of BBN. Thus, the question for nuclear physics side is whether ⁷Be is overproduced or less destructed during the BBN stage. Since the production of ⁷Be is by the ³He(α,γ) which is quite well known, the problem may be the destruction rate of ⁷Be. The ⁷Be(n,p) reaction is considered to be the main destruction reaction, but it is well investigated and thus it seems hard to change the rate by a factor of three. The next possible channel was ⁷Be(n,α) reaction, and almost no experimental works reported before. Recently, several experiments were performed or being performed by the direct and indirect methods. I will discuss these reaction studies of ⁷Be(n,α), including our activities and the result. The conclusion is that the reaction rate proposed by Wagoner et al. and currently being used is roughly one order of magnitude too large. Thus, the Li problem has not been solved, giving a more difficulty.

Coffee and tea will be served 20 minutes prior to the seminar.