A typical tumor treated with radiotherapy may receive 400 Gy per 250g.

How many disintegrations of caesium-137 atoms are required to provide that absorbed dose?

Help would be appreciated!

cesium-137: half-life 30.2 years, 0.3 MeV beta and 0.66 MeV gamma-ray emitter

1 Gray (Gy) = 1 joule of ionizing energy per kilogram

Energy = 400 Gy•0.25 kg= 100 J

100J/0.3•10⁶•10⁻¹⁹=3.33•10¹⁵ disintegrations