  The rare isotope 6Li is made only by cosmic rays, dominantly in alpha+alpha
fusion reactions with ISM helium. Consequently, this nuclide provides a unique
diagnostic of the history of cosmic rays in our Galaxy. The same hadronic
cosmic-ray interactions also produce high-energy gamma rays (mostly via neutral
pion production). Thus, hadronic gamma-rays and 6Li are intimately linked.
Specifically, 6Li directly encodes the local cosmic-ray fluence over cosmic
time, while extragalactic hadronic gamma rays encode an average cosmic-ray
fluence over lines of sight out to the horizon. We examine this link and show
how 6Li and gamma-rays can be used together to place important
model-independent limits on the cosmic-ray history of our Galaxy and the
universe. We first constrain gamma-ray production from ordinary Galactic cosmic
rays, using the local 6Li abundance. We find that the solar 6Li abundance
demands an accompanying extragalactic pionic gamma-ray intensity which exceeds
that of the entire observed EGRB by a factor of 2-6. Possible explanations for
this discrepancy are discussed. We then constrain Li production using recent
determinations of extragalactic gamma-ray background (EGRB). We note that
cosmic rays created during cosmic structure formation would lead to
pre-Galactic Li production, which would act as a "contaminant" to the
primordial 7Li content of metal-poor halo stars. We find the uncertainties in
the observed EGRB are so large that we cannot exclude a pre-Galactic Li which
is comparable to primordial 7Li. Our limits and their more model-dependent
extensions will improve significantly with additional observations of 6Li in
halo stars, and with improved measurements of the EGRB spectrum by GLAST.
(Abriged abstract)
