The apparatus would need to immediately separate the two to prevent an extremely large explosion.
1kg of mass to energy (assuming every single particle met a corresponding antiparticle) would release a little over half the energy of the Krakatoa volcano eruption, similar to a 21 megaton bomb, so around the upper end of fusion bombs. Helium is about 99.5% the mass of the hydrogen that goes into it, so I guess those big fusion bombs must produce about 200kg of helium?
either way super cool
(megaton, to be clear, is not a measure of mass when talking about bombs, it's a measure of energy released - a 1 megaton bomb is equivalent to the energy released by a million tons of TNT, if such a volume could be detonated)