Be7(n,p)Li7

Abstract

We report on the measurement of the ^{7}Be(n,p)^{7}Li cross section from thermal to approximately 325 keV neutron energy, performed in the high-flux experimental area (EAR2) of the n_TOF facility at CERN. This reaction plays a key role in the lithium yield of the big bang nucleosynthesis (BBN) for standard cosmology. The only two previous time-of-flight measurements performed on this reaction did not cover the energy window of interest for BBN, and they showed a large discrepancy between each other. The measurement was performed with a Si telescope and a high-purity sample produced by implantation of a ^{7}Be ion beam at the ISOLDE facility at CERN. While a significantly higher cross section is found at low energy, relative to current evaluations, in the region of BBN interest, the present results are consistent with the values inferred from the time-reversal ^{7}Li(p,n)^{7}Be reaction, thus yielding only a relatively minor improvement on the so-called cosmological lithium problem. The relevance of these results on the near-threshold neutron production in the p+^{7}Li reaction is also discussed.

Big-Bang Nucleosynthesis after Planck

“…Figure 1 shows the data for 7 Be(n, p) 7 Li. We see that the new n TOF [40] points are higher than those of Sekharan et al [42] in and around the BBN energy range; this difference reaches substantially beyond the quoted errors. The former are direct measurements of the forward reaction, while the latter are from the reverse reaction, but we are not aware of a reason to exclude either.…”

“…In brief: we use NACRE-II for the rates listed in Table I. We use [38] for p(n, γ)d. And below we will discuss the remaining major rate, 7 Be(n, p) 7 Li, as well as the neutron lifetime, and other new rates for subdominant reactions. The 7 Be(n, p) 7 Li reaction was measured at the n TOF experiment at CERN [40]. The experiment used a neutron beam and a 7 Be target, both of which are radioactive.…”

“…Both the direct and inverse method has been pursued over the years. Most recently, the n TOF experiment at CERN has added new data on the forward reaction [40]. We combined this forward-kinematics determination of 7 Be(n, p) 7 Li with existing and largely inverse kinematics data, appropriately weighting for statistical and systematic uncertainties, following the procedure in [29].…”

“…Our rate is the thick black line, with the 1σ error range shown in dotted lines. For comparison we also show in red the results for Cyburt [29], and in cyan the n TOF recommended rate [40].…”

“…19 also shows the rates for Cyburt [29], which lacked the n TOF data. We also show the n TOF recommended rate tabulated in [40], which uses their own data along with that of Sekharan [42] but not the other data in Fig. 1.…”