The problem you will have is the low dose rate of the beam. Not exactly sure how much dose is required (??5kGy??) and getting that from a therapeutic treatment unit may be challenging when it may only deliver 2Gy/min at isocentre. If your sample was located as close to the source as possibly I don't thing even by an inverse square you will get a high enough dose rate to deliver the dose required in an acceptable time.

Dear Zsolt The minimum dose seems to be at least ten times higher than what you wrote. "Generally, 25 kGy can achieve sterility with a sterility assurance level (SAL) of 10–6. Even with elevated bioburden levels, bioburden reduction can be achieved with lower probabilities of sterility (e.g., SAL of 10–5 or 10–4)."

Cobalt treatment machines have low dose rates as compared to irradiators used for sterlization as the activity of these sterilzers is about ten times or more than regular treatment units.