Sr-90 shield and bremsstrahlung radiation question?

A Sr-90 eye applicator used to treat ptergia (eye condition) is placed in an aluminum shield (low Z). What fraction of it's energy is converted into bremsstrahlung radiation? Show work......This is for my radiation protection class and this is all the information provided. No info on shield thickness or strength of source. Probably and easy answer but I'm stuck on this one. Any help would be greatly appreciated!