An alpha particle. The tritium.
Work Step by Step
When Li-6 absorbs a neutron, it gets more massive but the atomic number stays the same. This is discussed on page 643. It becomes Li-7, made of 3 protons and 4 neutrons. It then splits into two parts, one of which is a tritium nucleus (one proton and two neutrons). By subtraction, the other product of the nuclear reaction must contain two protons and two neutrons: it's an alpha particle, the nucleus of ordinary helium. The tritium fuels the explosive reaction.