${}^7\mathrm{Be}(n,p){}^7\mathrm{Li}$ Reaction and the Cosmological Lithium Problem: Measurement of the Cross Section in a Wide Energy Range at n_TOF at CERN

We report on the measurement of the ${}^7\mathrm{Be}(n,p){}^7\mathrm{Li}$ 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 ${}^7\mathrm{Be}$ 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 ${}^7\mathrm{Li}(p,n){}^7\mathrm{Be}$ 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 $p+{}^7\mathrm{Li}$ reaction is also discussed.

Figure 1

The ${}^7\mathrm{Be}(n,p){}^7\mathrm{Li}$ reduced cross section measured at n_TOF compared with the results of previous measurements [7, 8, 17, 18] and with the ENDF/B-VII.1 library [16].

Figure 2

Resonance fit of the n_TOF cross section data, complemented by data from the $(p,n)$ channel (adopted). The contribution from the first five levels (indicated in the upper-right corner at their respective excitation energy), are included in the plot (color code online), but all nine states above the neutron separation energy are included in the SLBW fit as described in the text.

Figure 3

Rates of the ${}^7\mathrm{Be}(n,p){}^7\mathrm{Li}$ reaction relative to Cyburt [22]. The present result is shown with the associated error band, and the rates of Smith et al. [23], Serpico et al. [24], and Descouvemont et al. [25] are shown for comparison. The temperature range of BBN with a larger impact on the lithium yield is indicated by the vertical band.

Figure 4

The cross section of the ${}^7\mathrm{Li}(p,n){}^7\mathrm{Be}$ (black symbols), obtained by time-reversing the n_TOF data of the ${}^7\mathrm{Be}(n,p)$ reaction. For comparison, the data from measurements of the near-threshold $(p,n)$ reaction (Refs. [19, 34]) are also shown.