Is an antineutrino emitted in beta decay?
Yes, it is.
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An unstable atomic nucleus with an excess of neutrons may undergo β decay ( β- ), where a neutron is converted into a proton, an electron and an electron-type antineutrino:
n → p + e− + νe
Beta decay commonly occurs among the neutron-rich fission byproducts produced in nuclear reactors. This process is mediated by the weak interaction.
Also, there is a β+ process where a proton is converted into a neutron, a positron and an electron-type neutrino,