NavList:

Subjectively, ANY ionizing radiation can be considered harmful.  To what degree is the question.  Its the dose of the radiation that matters.  The sextants will be releasing detectable radioactivity, a very small but continuous dose, so I would say relatively safe.  You aren't going to get radiation sickness, but you are increasing your background radiation level.  If it was non-ionizing radiation, I would have zero concern, but that simply isn't the case here.  You most definitely are increasing your background level and thus increasing your risk.

I would strongly consider storing the sextants in a metal case.  Perhaps a gun safe.  That will decrease your exposure.  

There are medical professionals on-board.  Perhaps they can shed some light.

Brad