Primordial Heavy Element Production

A number of possible mechanisms have been suggested to generate density inhomogeneities in the early Universe which could survive until the onset of primordial nucleosynthesis 1]. We studied the eeect of such inho-mogeneities on primordial nucleosynthesis. One of the proposed signatures of inhomogeneity, the synthesis of very heavy elements by neutron capture, was analyzed for varying baryon to photon ratios and length scales L. A detailed discussion is published in 2]. After weak decoupling the vastly diierent mean free paths of protons and neutrons create a very proton rich environment in the initially high density regions, whereas the low density regions are almost entirely lled with diiused neutrons. Since the aim of the present investigation was to explore the production of heavy elements we considered only the neutron rich low density zones. However, we included the eeects of the diiusion of neutrons into the proton rich zones, using an analytical treatment comparable in accuracy to numerical methods using high resolution grids. The only open parameter in the neutron loss due to diiusion is the comoving length scale L of the inhomogeneities. Small separation lengths between high density zones make the neutron leakage out of the small low density zones most eeective. Our reaction network consists of two parts, one part for light and intermediate nuclei (Z 36), being a general nuclear network of 655 nuclei. The second part is an r-process code (including ssion) extending up to Z = 114 and containing all (6033) nuclei from the line of stability to the neutron-drip line. These two networks were coupled together such that they both ran simultaneously at each time step, and the number of neutrons produced and captured was transmitted back and forth between them. Four sets of calculations have been performed, employ-we considered four diierent cases of L: (0) 1 (negligible neutron back diiusion), (1) 10 7:5 cm MeV, (2) 10 6:5 cm MeV, and (3) 10 5:5 cm MeV. (This corresponds to distances between nucleation sites of 1, 2700, 270, and 27 m, respectively, at the time of the quark-hadron phase transition). An exponential increase in r-process abundances with increasing was found. This is due to \\ssion cycling", whereby each of the ssion fragments can form a s-sionable nucleus again by neutron captures. This is of particular importance in environments with a long duration of high neutron densities. In an r-process with ssion cycling the production of heavy …