[WilsonPatel]

How are excess neutrons formed to create isotopes? I know that one method is through the use of a cyclotron which accelerates a proton and causing a collision with a "target". This would increase the atomic number, Z and create an isotope with 1 less neutron. But how does this method produce an isotope with an excess of neutrons ? For instance Technetium-99 is formed from Mo-99, but how is Mo-99 formed?

[chenbeier]

It is formed from Niobium 99

[gippgig]

There are many ways to produce small amounts of neutrons. Large quantities can be produced in a nuclear reactor (or nuclear explosion). Fission of U or Pu produces an average of slightly over 2 neutrons. One is needed to sustain the chain reaction but the others can be used to produce isotopes on an industrial scale (altho many of the excess neutrons are absorbed by random stuff).

[Enthalpy]

Humans create big neutron amounts for isotope production essentially by uranium fission. Either in powerful reactors for electricity production, or in small reactors meant to preserve the excess neutrons, using highly enriched uranium and possibly heavy water.

⁹⁹Mo is made by neutron irradiation of ²³⁵U
https://en.wikipedia.org/wiki/Isotopes_of_molybdenum#Molybdenum-99
I imagine special targets are inserted and removed, because ⁹⁹Mo must be extracted well before the main fuel is replaced.

As compared, particle accelerators give a smaller neutron flux, but under circumstances, they might be considered for the production of medical isotopes. A highly accelerated proton smashes on some unimportant heavy nucleus like lead, breaks it somehow, and neutrons are emitted which can then be absorbed by a target material.

Historically, alpha radioactive materials were mixed with beryllium which consists of two alphas loosely hold by a very fuzzy neutron. The collision breaks the kernel and frees the neutron. The tiny flux achieves nearly nothing.

Just D-T or D-D fusion in a fusor makes neutrons. The flux is too tiny for use.

I have proposed a reduced tokamak or stellarator that burns only deuterium in a D-D reaction as a neutron source. The flux is much smaller than at a fission reactor, but caring design could let it produce enough ⁹⁹Mo and other nuclides for medical uses
https://www.chemicalforums.com/index.php?topic=92021.0
I still have to explain some day how ⁹⁸Mo shall capture the neutrons efficiently. That would be a very nice use of miniature tokamaks, a first commercial use, much easier than electricity production.