That's the whole point. PSA should be undetectable without a prostate.
However, excess PSA is generated by Pca cells, and if these cells
"migrated" outside the prostate prior to removal, they will merrily
continue to generate PSA. A PSA reading above 0.2 is an indicator that
Pca still remains in the body. Radiation offers a second chance to zap
these cells if they still exist and are still localized.

Before surgery, you have a certain PSA concentration in your blood.
After surgery, if all went well, no significant amount of PSA is being
produced, but it does take some time for the existing PSA to decompose.
 For that reason,  doctors don't usually take PSA readings until many
weeks following surgery.  At that point it should be undetectable.
Other parts of the body produce very small amounts of PSA, but these
shouldn't show up on tests.  If it is possible to detect PSA after
surgery, it means there was some prostate tissue left behind, either
benign or cancerous.  In some prostate cancer remains in the local area,
it may be possible to zap it with radiation.  If it has already excaped
to remote sites, local radiation won't do any good, and eventually, in
almost all cases, it will grow.    Just what is considered detectable,
and when and how to treat it is not a simple matter, and you should rely
on a physician to help you make choices.   In many cases, the body's
immune system can keep even a metastatic cancer under control for
extended periods of time.  If that fails, hormone suppressing drugs can
control the cancer for some additional time.