Fig. 2.

Scheme of the time sequence in the processes used for positronium imaging. A ⁴⁴Sc nucleus undergoes β⁺ decay. Next, on the average after about 3 ps, excited ⁴⁴Ca ^∗ emits prompt gamma with energy of 1160 keV (dotted blue arrow). Parallelly, positron travels through matter, thermalizes, and forms an ortho-posironium bound state. Interaction with surrounding molecules or conversion process leads to emission of two photons. The mean ortho-positronium lifetime is in the order of nanoseconds, in contrast to the duration of thermalization [42] and deexcitation [43] processes which are in the order of 10 ps