It depends on where the spacecraft is going as radiation environments differ. I've taken parts to be exposed by a proton line at a particle accelerator. For some environments they just use Cobalt-60 as a radiation source.

This reminds me of a relevant anecdote: back in the naughts, I was doing research in cosmic rays at a large nuclear research facility. I did simulation and data analysis - office/computer work mostly. One day, a person with a clipboard comes into my office and asks about the whereabouts of some radiation source. I look at them confused - I had not touched sources since teaching nuclear physics labs. They show me their clipboard and lo and behold, it has my name next to a really high intensity source and they're looking to locate it.

After a few minutes of awkward shock and denying all involvement, we realized it was a colleague at the same research institute (and of the same name). He was, one building down, doing his PhD research on radiation hardened detectors for the CMS experiment at CERN(1). He was using the source for the testing. But I had a minute of real stress before that came together in my mind...

(1) I think this was his work: https://onlinelibrary.wiley.com/doi/abs/10.1002/pssa.2007763...

Pretty much!