Neutron-upscattering enhancement of the triple-alpha process

The neutron inelastic scattering of carbon-12, populating the Hoyle state, is a reaction of interest 17 for the triple-alpha process. The inverse process (neutron upscattering) can enhance the Hoyle state’s decay rate to the bound states of 12 18 C, effectively increasing the overall triple-alpha reaction rate. 19 The cross section of this reaction is impossible to measure experimentally but has been determined 20 here at astrophysically-relevant energies for the first time using detailed balance. This cross section 21 will inform astrophysical models on the importance of neutron upscattering in neutron-rich stellar 22 environments. 23 Using a highly-collimated monoenergetic beam of neutrons incident on the Texas Active Target 24 Time Projection Chamber (TexAT TPC) filled with CO2 gas, the 3α-particles (arising from the decay 25 of the Hoyle state following inelastic scattering) were measured and a cross section was extracted 26 between En = 8.15 MeV and En = 10.0 MeV. The cross section above the threshold was dominated by a few resonances in 13 27 C around Ex = 13.5 MeV. This has a significant effect on the contribution 28 of neutron upscattering in stellar environments. 29 The neutron-upscattering enhancement is observed to be much smaller than previously expected. 30 For a temperature of 1 GK, the total enhancement factor, from upscattering to the ground state and 31 first-excited state, was seen to be 5, rising to around 46 at 10 GK thereby exceeding the contribution 32 from proton enhancement. The importance of the neutron-upscattering enhancement may therefore 33 not be significant aside from in very particular astrophysical sites (e.g. neutron star mergers).