[1009.3932] Time of primordial Be−7 conversion into Li−7, energy release and doublet of narrow cosmological neutrino lines

Authors:  Rishi Khatri, Rashid A. Sunyaev
Abstract:  One of the important light elements created during the big bang nucleosynthesis is Be-7 which then decays to Li-7 by electron capture when recombination becomes effective but well before the Saha equilibrium recombination is reached. This means that Be-7 should wait until its recombination epoch even though the half-life of the hydrogenic beryllium atom is only 106.4 days. We calculate when the conversion from primordial Be-7 to Li-7 occurs taking into account the population of the hyperfine structure sublevels and solving the kinetic equations for recombination, photoionization and conversion rate. We also calculate the energies and the spectrum of narrow neutrino doublet lines resulting from Be-7 decay.
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Neal Dalal
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Joined: September 25 2004
Affiliation: CITA

[1009.3932] Time of primordial Be-7 conversion into Li-7, e

Post by Neal Dalal » September 23 2010

This is a neat result! This paper is about the final stage of Big Bang Nucleosynthesis (BBN). We normally think of BBN as occurring at extremely high redshift, e.g. [tex]z\approx 10^8[/tex]. Here they discuss a BBN process that occurs much later, at redshifts closer to [tex]10^4[/tex] or [tex]10^5[/tex].

Most of the [tex]^7[/tex]Li produced during BBN arises from the decay of [tex]^7[/tex]Be, through electron capture. The authors point out that this decay of [tex]^7[/tex]Be occurs much later than naively expected, because the electron capture rate is too low when Be atoms are fully ionized. Only at 100 eV temperatures are there sufficient numbers of bound electrons that [tex]^7[/tex]Be can decay to [tex]^7[/tex]Li.

It's unclear if this effect has any observable consequences, just because the number of [tex]^7[/tex]Be nuclei is so low. Still, at least for non-experts like me, it's surprising that nucleosynthetic processes can operate at such low temperatures.

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