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Cyclotrons are poorly suited to making plutonium. The neutrons per energy increases dramatically with higher energy particles; a 1 GeV beam of protons on a thick uranium target makes 60 neutrons per proton. The nuclear reaction here is called "spallation", where the protons shatter nuclei into many fragments, including free neutrons.

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"On August 20, 1942, a trace quantity of this element was isolated and measured for the first time. About 50 micrograms of plutonium-239 combined with uranium and fission products was produced and only about 1 microgram was isolated."

So small were the amounts of early produced plutonium, than new methods to study it had to be used.

https://www.scientificamerican.com/article/ultramicrochemist...

Of course we have now military plutonium production reactors.

Yes, they had to do it that way, because they had no better way at the time.

Today, if you wanted to make Pu with an accelerator you'd use energies nearly 2 orders of magnitude higher. At such high energy, the particles lose energy mostly in nuclear collisions, not to ionization.

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