Cosmological constraints on the radiative decay of heavy neutrinos in the fourth generation

Cosmological constraints are investigated for neutrinos in the fourth generation. From the requirement that the emitted photon flux should not exceed the observed γ-ray background and the photofission of light elements (4H, 3H, 2H) should not overproduce 3H and 2H or should not destroy 2H, it is concluded that unstable heavy neutrinos in the fourth generation must have a lifetime shorter than about 10/sup 12/ sec for a mass greater than about 300 GeV or shorter than ∼104 (m/sub ν//GeV)/sup 1.3/ sec in the mass range between 1 and 300 GeV. If lepton-number asymmetry exists the above constraints become much more stringent