  Core collapse of massive stars and binary neutron stars or black hole-neutron
star binary mergers are likely progenitors of long and short duration gamma-ray
bursts respectively. Neutronized material in the former and neutron star
material in the latter are ejected by the central engine implying a
neutron-rich jet outflow. A free neutron, however, beta decays to a proton, an
electron (beta) and an anti-neutrino in about fifteen minutes in its rest
frame. Sudden creation of a relativistic electron is accompanied by radiation
with unique temporal and spectral signature. We calculate here this radiation
signature collectively emitted by all beta decay electrons from neutron-rich
outflow. Detection of this signature may thus provide strong evidence for not
only neutron but also for proton content in the relativistic gamma-ray burst
jets.
