Direct and valence neutron capture byLi7

Abstract

We have measured the cross section for neutron radiative capture by Li at thermal neutron energy as 45.4+3.0 mb. We have compared this value, and the available fast-neutron-capture cross-section data on Li, with directand valence-capture theory. The mechanism of direct capture involving simple neutron-orbital transitions within a potential well can adequately account for the magnitude of the thermal-neutron-capture cross sections and the shape of the fast-neutron-capture cross sections. We recommend that the capture cross sections from the theoretical calculation, instead of those from a recent measurement, be used for nuclear astrophysics calculations. If the fast-neutron-capture data are normalized at 25 keV to the theoretical value, the magnetic-dipole (M 1) radiation width of the 255-keV p-wave resonance can be deduced. It is in agreement with the value calculated from a valence theory for M1 capture, thus lending support for this neutron-capture mechanism as an important one, at least for light nuclei. We also find some evidence from our analysis of the total cross-section data for a possible energy dependence of the potential required to describe the almost pure single-particle s-wave resonances underlying the Li cross section.

Leading E1 and M1 contributions to radiative neutron capture on lithium-7

“…We include explicitly the E1 capture from s-waves to the excited state in 8 Li that contributes about 10% to the cross-section at very low energy. The new EFT coupling constants associated to this process are completely determined by the binding energy of the excited state and the E1 thermal capture rate to the excited state [28]. For energies below 100 keV, our results show the expected 1/v behavior also seen in potential models, however, differing by a sizable overall normalization, directly related to the effective range in the ground-state channel r…”

“…The leading uncertainty in potential model results seems to be associated with the poorly known effective range r (2 + ) 1 that we determine from the thermal capture rate. We also notice that the data set that we call ImhofB is more consistent with the 1/v behavior suggested by the Blackmon [41] and Lynn [28] data than ImhofA.…”

“…The leading contribution to the capture reaction, which comes from the initial 3 S 1 and 5 S 2 states to the 2 + ground state, proceeds through E1 transition due to electromagnetic selection rules. There is a small contribution from E1 capture to the excited 1 + state with a branching ratio of 0.106 [28]. Finally, around 200 keV there is the M1 contribution from the 3 + resonance to the 2 + ground state.…”

The 7Li (n,γ)8Li radiative capture at astrophysical energies

“…The calculation results were compared with the experimental measurements of the total cross sections of the radiative capture reaction in the energy range from 5 meV to 1.0 MeV, laid down in works [5,[42][43][44][45][46][47].…”

Determining the7Li(n,γ) cross section via Coulomb dissociation of8Li

“…The solid black line in Figure 10 shows the best fit to the available non-resonant data in the range from 10 -1000 keV and where s 0 was fixed at 6.7 µb(MeV) 1/2 to fit the measured cross section at thermal neutron energies [49]. The extracted values for s 1 = −0.53(44) MeV , respectively.…”