2.4. Lithium-7 - The Lithium Valley
The trend of the BBN-predicted primordial abundance of lithium
(almost entirely 7Li) with
is more `interesting' than
that of the other light nuclides (see
Figs. 1 & 4).
The ``lithium valley'', centered near
10
2 - 3, is the result of
the competition between production and destruction in the two paths
to mass-7 synthesis. At relatively low baryon abundance
(10
2) mass-7 is mainly
synthesized as 7Li via 3H + 4He ->
7Li + 
.
As the baryon abundance increases at
low , 7Li
is destroyed rapidly by (p,
)2
reactions.
Hence the decrease in (Li/H)P
y7P with increasing
seen (at low
) in
Figs. 1 &
4. Were this the only route
to primordial synthesis of mass-7, this monotonic trend would continue,
similar to those for D and 3He. However, mass-7 may also be
synthesized via 3He + 4He -> 7Be +
The
7Be will later
capture an electron to become 7Li. This channel is very important
because it is much easier to destroy 7Li than
7Be. As a result, for relatively high baryon abundance
(10
3) this latter
channel dominates mass-7 production and y7P increases with
increasing .
|
Figure 4. The predicted 7Li
abundance (solid curve) and the
2 |
As may been seen in Figure 4, the BBN-predicted
uncertainties for
y7P are much larger than those for D, 3He,
or 4He. In the interval 1
10
10 the
1
uncertainties
are typically ~ 20%, although in a narrow range of
near
the bottom of the `valley' they are somewhat smaller (~ 12 - 15%).