During a core-collapse supernova , absorption of \bar { \nu } _ { e } emitted from the proto-neutron star by protons in the hydrogen envelope produces neutrons and positrons .
Neutron capture on protons and positron annihilation then produce \gamma rays of 2.22 and 0.511 MeV , respectively .
We calculate the fluxes of these \gamma rays expected from a supernova with an 11 M _ { \odot } progenitor .
The flux from neutron capture on protons exponentially decays on a timescale of 564 s , which is determined by neutron decay and capture on protons and ^ { 3 } He nuclei .
The peak flux is 2.38 \times 10 ^ { -7 } { cm } ^ { -2 } { s } ^ { -1 } for a supernova at a distance of 1 kpc .
In contrast , the \gamma -ray flux from positron annihilation follows the time evolution of the \bar { \nu } _ { e } luminosity and lasts for \sim 10 s. The peak flux in this case is 6.8 \times 10 ^ { -5 } { cm } ^ { -2 } { s } ^ { -1 } for a supernova at a distance of 1 kpc .
Detection of the above \gamma -ray fluxes is beyond the capability of current instruments , and perhaps even those planned for the near future .
However , if such fluxes can be detected , they not only constitute a new kind of signals that occur during the gap of several hours between the neutrino signals and the optical display of a supernova , but may also provide a useful probe of the conditions in the surface layers of the supernova progenitor .