 Alchemists in the Middle Ages saw the creation of the Philosophus Stone, an object that was supposedly capable of converting base metals like lead into gold. We now know that the difference between lead and gold is simply the number of protons in the nucleus of each atom. Gold atoms have 79 protons in the nuclei, while lead atoms have 82. We also now know that the chemical procedures the alchemists were using back in the Middle Ages could only rearrange the electrons that form the bonds within molecules and therefore they could not have had any effect on the structure or composition of the nucleus at the center of an atom. Using the techniques of the Middle Ages, the alchemists could never have succeeded in their goal of transmutation of elements. However, we have already seen a hint of how we can transmute elements using nuclear techniques. In a nuclear alpha or beta decay, one time of a nucleus spontaneously changes into another. The question I will address in this video is whether or not we can artificially initiate nuclear transmutation and control these processes directly. As a reminder from the previous lectures, recall that nuclear decays are a type of reaction that occurs spontaneously when it is energetically feasible. That is when the masses of the products are lighter than the mass of the parent nucleus. It turns out that some spontaneous decay processes can be reversed. An example we have already looked at is the alpha decay of Polonium-212 turning into Helium-4 and lead-2O8. We can reverse this process by giving a Helium-4 nucleus some energy and directing it at a target made of lead-2O8 nuclei. When they collide, the injected energy can compensate for the mass difference and the Helium-4 and lead-2O8 can fuse together to make a new combined system. In this way we can convert a lead nucleus into a Polonium nucleus. Let us look at another example in more detail. The unstable nucleus Fluorine-18 undergoes beta-plus decay emitting positrons. These positrons can combine with electrons to produce gamma rays that can be used to image the location where the Fluorine-18 atom decayed. This is the basis of a PET scanner. PET stands for positron emission tomography and is a common medical imaging technique. However the nucleus Fluorine-18 doesn't exist in nature and has a half-life of only 109 minutes. It must be artificially created to be used in a PET scanner. The reaction that is most often used involves a proton which you will remember is just a hydrogen nucleus. This nucleus fuses with an 018 nucleus to create Fluorine-18 and a neutron. We can look up the masses of all of these species and their values are given here in atomic mass units. The masses of the proton and the oxygen-18 add up to 19.006985 mass units while the masses of the neutron and the Fluorine-18 add up to 19.0096029 mass units. Since the combined mass of the products is higher than the combined mass of the reactants this process cannot occur spontaneously and energy needs to be put into the system for this reaction to occur.