H. Schelin

Cross section for the astrophysical 14C(n, γ) 15C reaction via the inverse reaction

The 14C(n, γ)15C reaction is important in neutron-induced CNO cycles of stellar evolution phases beyond the main sequence. The chain of reactions in primordial nucleosynthesis in the neutron-rich environment of an inhomogeneous big bang also involves the 14C(n, γ)15C reaction. We have used a beam of 15C ions at E/A = 35 MeV to measure cross sections for 15C breakup on targets of C, Al, Zn, Sn, and Pb. The Coulomb part of the breakup cross section was determined as a function of decay energy. By the principle of detailed balance, the neutron capture cross section was then determined as a function of neutron energy. This excitation function rises with energy to 7 μbarns at 300 keV and falls to 2 μbarns at 1.2 MeV. In the stellar-burning region, our cross section exceeds a previous measurement by a factor of 3. Neither the shape nor the magnitude of the excitation function agrees with those of theoretical calculations based on the direct capture model. The reaction rate is calculated reliably up to ~T9 = 5. In the temperature range of inhomogeneous big bang models, our rate exceeds the rate of the 14C(p, γ)15N reaction for T9 < 1.2.

Astrophysical reaction rate for the Li-8(n,gamma)Li-9 reaction

An attempt was made to measure the excitation function of the cross section for the Li-8(n,gamma)Li-9 reaction by performing the inverse reaction Li-9(gamma,n)Li-8, with the equivalent photons in the electric field of nuclei in a Pb target providing the gamma rays for the reaction. The energy spectrum of lithium nuclei in coincidence with neutrons had no discernible peak where any beam-velocity Li-8`s would be located. Statistically, a Gaussian-shaped Li-8 peak could have been present with 30+/-29 counts, which we interpreted as consistent with zero, with a two-standard-deviation upper limit of 87 counts. Using the fact that neutron capture on Li-8 must be dominantly s-wave capture, and applying detailed balance, we obtained, with E in eV, sigma(n,gamma)>930E(-1/2) mu b. The corresponding limit on the astrophysical reaction rate is >790 cm(3) mol(-1) s(-1). Theoretical predictions of the reaction rate have exceeded our upper limit by factors of 3-50.

Measurements of astrophysical neutron capture cross sections via the inverse reaction

Abstract Neutron capture of short-lived isotopes can be important within several astrophysical environments. The laboratory investigation of these reactions can be difficult since these isotopes cannot form a target. We used radioactive beams and measured the inverse reaction instead. Particularly we used 15 C and 9 Li beams to measure the 14 C(n , γ) 15 C , and 8 Li ( n , γ) 9 Li reactions. We determined the reaction rates within the astrophysically interesting temperature range. Our data for the 14 C capture rises quickly up to 8 μb and then it decreases by a factor of two at 1 MeV. We investigated neutron capture by 8 Li which is one possible leak of the bridging reaction 8 Li (α, n ) 11 B of the A=8 gap. We found that this leak is lower than the theoretical expectations.

Production rates of excited fragment isotopes from the36Ar+Ag reaction at 35 MeV/nucleon

Previous studies showed that the binding energy plays a systematic and important role in the production of ground-state fragments in intermediate energy, heavy ion reactions. The production rates were measured as a function of fragment kinetic energy at angles of 15°, 30°, 45° and 60° for excited fragments of7Li,8Li,11Be and12B. Using a thermal model the total production of neutron unbound excited states was determined, and it was found that their production rates correspond to the previous systematic behaviour using the binding energies corrected by the excitation energies.