Radiation Seed Therapy Is a Common Method for Treating Some $\times$

Radiation seed therapy is a common method for treating some prostate cancers and brain tumors. Cesium-131 is a new isotope being proposed for this treatment. Cesium-131 decays with a half-life of 9.7 days by emitting X-rays with a photon energy of 29,000 eV. What mass of cesium-131 would have to be embedded in a 15 g tumor to produce a radiation dose of 115 Gy in 3 weeks' time (21 days) ? Note: 1 Gy = 1 J/kg, and 1 eV = 1.60 $\times$ 10^-19 J.

A) 0.37 $\mu$ g
B) 0.31 $\mu$ g
C) 0.10 $\mu$ g
D) 0.23 $\mu$ g
E) 0.16 $\mu$ g