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.
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...