The density of vacancylike defects, produced in silicon by hydrogen implantation at 15.5 keV and surviving to successive isochronal annealings, has been measured by means of a slow positron beam. The results show that the number of defects acting as positron traps is a small fraction of the Frenkel pairs produced by implantation. This number decreases, increases again, and eventually disappears after annealing at increasing temperatures. The mean depth of the positron traps in as-implanted samples is smaller than the mean depth of vacancies predicted by computer simulations, but reaches, and in some cases surpasses, this limit after annealing. A minimum in the number of the positron traps occurs around 350 °C when the number of displaced silicon atoms, produced by hydrogen agglomeration, is at maximum. Further annealing increases the number of traps, until at high temperatures, above 700 °C, all the traps disappear. This complicated behavior is interpreted as the result of several concomitant effects: the formation of vacancylike defects during implantation, their partial annealing below 350 °C, an initial passivation of the traps caused by hydrogen followed by a reactivation stage, and the formation of thermally stable hydrogen complexes.

Positron emitting radionuclides are used to label different compounds, allowing the study of the major biological systems using PET (positron emission tomography) imaging. Although there are several radionuclides suited for PET imaging, routine clinical applications are still based on a restrict group constituted by ¹⁸F, ¹¹C, and, more recently, ⁶⁸Ga. However, with the enlarged availability of low-energy cyclotrons and technical improvements in radionuclide production, the use of unconventional radionuclides is progressively more common. Several examples of unconventional radionuclides for PET imaging are being suggested, and ⁴⁵Ti could be suggested as a model, due to its interesting properties such as its abundant positron emission (85%), reduced positron energy (β⁺ endpoint energy = 1040 keV), physical half-life of 3.09 h, and interesting chemical properties. This review aims to introduce the role of cyclotrons in the production of unconventional radionuclides for PET imaging while using ⁴⁵Ti as an example to explore the potential biomedical applications of those radionuclides in PET imaging.