60Fe is a radioactive isotope produced in convective shells of massive stars
by neutron capture reactions. Theoretical predictions of 60Fe yields are quite
uncertain, due to uncertainties in nuclear reaction rates and neutron sources
and stellar structure. With a 2.2 My radioactive lifetime, 60Fe accumulates
in the interstellar medium after it is ejected by the core-collapse supernova,
which ends the massive-star's life shortly after 60Fe has been synthesized.
SPI on INTEGRAL measures the two gamma-ray lines resulting from the decay
of 60Fe in interstellar space. This spectrum shows the superposition of both
lines, resulting in the most-significant detection of this isotope to date,
improving upon previous hints from RHESSI and earlier SPI measurements.
60Fe nucleosynthesis is a key diagnostic of the structure of massive stars
in their late stages. In particular, the ratio of 60Fe to 26Al gamma-rays
is a convenient relative diagnostic, if both isotopes indeed originate
predominantly from massive stars. This ratio has been predicted to lie in
the range 10-100%, and is now measured by SPI as 15%. Refinements of models
are underway, in particular re-evaluating nuclear reaction cross sections
and beta-decay lifetime.
