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Why do Scientists Bombard Specific Elements to Create New Ones?

For Oganesson, it was Calcium and Californium: 20 + 98 = 118. But why not just Praseodymium and Praseodymium? If you bombard element 59 with element 59, shouldn't it also give you element 118?
gpt
There's a lot of things to consider when designing these kinds of experiments. They're fixed-target experiments, so you have a beam and a target, and you have to choose materials for both that are convenient/possible to use.

Also, you have to consider what nuclides are likely to be produced by the reactions. The typical way that these experiments are done involves heavy ion fusion-evaporation reactions. That means that the beam energy is in the range such that the two heavy ions can fuse together and create an excited compound nucleus. The compound nucleus then "evaporates" particles (primarily neutrons and gamma rays, but also protons, alpha particles, etc.). You want to maximize the probability that the final product is the nuclide of interest.

There's some relatively narrow range of energies that will work, because if the energy is too low, the Coulomb barrier will make the fusion-evaporation cross section too low to finish the experiment in a reasonable amount of time, and if it's too high, the compound nucleus will be more likely to just fission rather than ideally losing a few neutrons and remaining a superheavy element.

Since the Coulomb barrier energy is proportional to Z*_b_*Z*_t_*, where those are the atomic numbers of the beam and target, it's worse when the two have roughly equal Z. You get the benefit of a lower Coulomb barrier, so more manageable cross section, if there's a big asymmetry in the beam and target Z.

So with all of that in mind it turned out that for this particular case with the accelerator available, it was determined that a calcium-48 beam (a doubly-magic, effectively stable nucleus with a bunch of extra neutrons) with a californium-249 target (a heavy nucleus with a lot of protons to bring the charge of the compound nucleus up to 118) was the optimal choice.