Fluorine, the element with 9 protons, is a monoisotopic element in nature, with flourine-19 being the only stable isotope. Remove one neutron, and you have our nuclide of the day, flourine-18 (F-18), which decays with a half-life of about 2 hours.

We have already mentioned a lot of useful radioactive nuclides, but we didn’t mention yet a nuclide that decays with beta+. It is time to change this today, with F-18. So what is the beta+ decay? That is when a proton in the nucleus decays to a neutron, something that maybe could seem like an impossibility considering that the neutron is heavier than a proton… How can something lighter split apart into a fragment that is heavier than what you started with? The answer lies in the nuclear binding energy. If the nucleus has too many protons, and the binding energy increases by the decay, then this leads to the total energy of the nucleus being smaller after decay. And as we remember from Einstein, energy is equivalent to mass, so that means the decay product is still of smaller mass than the parent nucleus.

In addition to the resulting neutron, as also given away by the name of the decay, a beta+ particle is emitted in the decay. That is the positively charged antiparticle twin to the electron, also called a positron. Positrons are just like electrons in every way, but they have positive charge. It can also be mentioned that the positron and the electron have an explosive relationship. When they meet and hook up, they bind each other for a short while in an orbital motion, trapped by the electric forces of one another. This bound system is called positronium, and reminds a bit about a neutral atom. However, this state lasts not long before they annihilate. This means they disappear, just go up in smoke, and are replaced by pure energy in the form of two photons in opposite directions. The energy of the two photons is equal to the rest mass of the electrons, but just a little bit lower due to the binding energy of the positronium. About 511 kiloelectronVolt (keV) each is distributed to the photons.

Ok, some of you out there are probably going to write angry emails if we don’t say that a little neutrino is also emitted in the beta+ decay, but that is barely noticable. So let’s mention that to reduce the load on our mail servers.

The particularities of the 511 keV photons are indeed very useful. Since they split in 180 degree directions, In the event that both can be detected in some array of gamma detectors, then one can know for sure that they originated from a nearly straight path between the two detectors that were hit. This has been put to use in PET-scans (positron emission tomography) since the 50-ies.

Because of F-18 being a suitable beta+ decaying nuclide for use in PET, it is regularly produced for this purpose, using cyclotrons, which are spiral shaped particle accelerators. Protons are accelerated, and hit a target with stable oxygen-18, and fluorine-18 is formed. Since the half-life is only 2 hours, this needs to be done relatively shortly before the planned use. After extraction of the radioactive fluoride, it is used in the synthesis of a chemical substance for injection to the patient.

Often, the fluoride is attached to glucose. Glucose is fuel for the cells and as such it is consumed at a high rate by tumor cells. Thereby, the tumors start to fill themselves with the fluorine-18 tracer, and since the PET detectors can track the location of fluorine-18, the doctor may also learn about the locations of any metastases that might have spread in the patient's body. The patient may receive injections with hundreds of Mega-Bequerel of F-18, meaning that hundreds of millions nuclei decay in their body every second, and for each of the decays there is an associated probability of being registered in the detector array, helping to form the image that will guide the further treatment of the patient.