Figure 4.

Schematic illustration of possible interactions of emitted $e^{+}$ with radiolabeled SPIONs in water. (a) Different routes of positron annihilation: instantaneous annihilation with an electron in the surrounding water, formation of positronium (Ps) in the open volume defects within the SPION core Fe₃O₄ crystal structure or in surrounding water with two possible quantum states: the singlet state called para-Ps (p-Ps) decaying into two gammas and the triplet state called ortho-Ps (o-Ps) decaying in three gammas. (b,c) Corresponding schematic photon spectra: two back-to-back photons with equal energy of 511 keV for p-Ps decay and three annihilation photons with energies between 0 and 511 keV summing to 1022 keV for o-Ps respectively. Feynman diagrams for both p-Ps and o-Ps decay are illustrated beneath each spectrum.