I don't have the ref. chart in front of me. Aren't gamma rays about 10^15 Hertz?
X-Rays are around 10^15 Hz. Gammas are more like 10^20 Hz. They are penetrating suckers. I just know that we had a 1/4-inch tantalum shield ready to fly on New Horizons, but ditched it because it only dropped the gamma flux by 1/3 to 1/2. It wasn't worth the mass hit to fly it...