Abstract
The defects are observed in the optical spectrum of irradiated diamond. TR12 has many very sharp phonon replicas in absorption, but fewer in photoluminescence. During electron irradiation at room temperature the defect is produced initially as the cube of the dose. It can be induced only in type II diamonds, and is probably an interstitial complex. The photoluminescence spectrum of 3H has been recorded for the first time. The defect is quite distinct from the H3 and S1 centres, in spite of the similarity in energy of their zero phonon lines.