Measurement of theLi8(α,n)11

Abstract

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-

8 Li + α decay of 12 B and its possible astrophysical implications

“…In all of the cases of direct measurements 11 B ions were detected and only in one measurement [7] neutrons were also detected. As can be seen, the quality of the data, as well as the agreement between the inclusive [19,20] and exclusive measurements [7] are not good. Also, only three data points fall into the upper part of the most interesting energy region (10-11 MeV) for big bang nucleosynthesis analyses.…”

“…[7,19,20], shown in Fig. 3 (bottom), represent the sums of transitions involving 8 Li ground state and different 11 B states.…”

Study of the nucleon radiative captures 8Li(n,γ)9Li, 9Be(p,γ)10B, 10Be(n,γ)11Be, 10B(p,γ)11C, and 16O(p,γ)17F at thermal and astrophysical energies

“…As in our previous works 25,47,80 we use the Gaussian potential (2). The GS of 9 Li in the n 8 Li channel is the P 3/2 level and this potential should describe the AС for this channel correctly.…”

“…1 In the process, primordial nucleosynthesis can occur after the nuclear formation with А = 7 the further way of formation of the elements with А = 11 and higher, for example, with help of the 7 Li(n,γ) 8 Li(α,n) 11 B reaction or due to less likely branch 7 Li(α,γ) 11 B. 2,3 However, the neutron capture on 8 Li, i.e., the n+ 8 Li 9 Li+ reaction can reduce the amount of these nuclei in the given chain of fusion; 4 this reduction can reach the value of 50%. In other words, there are at least two variants of the primordial synthesis of elements with mass of A = 11-12 and greater.…”

“…As usual, 37,38,42,[46][47][48][49][80][81][82] we use the potential of the Gaussian form with the point-like Coulomb term (2) in the capacity of the n 10 Be interaction in each partial wave with a given orbital angular moment L.…”