Which material is least effective for shielding against gamma radiation?

In the context of shielding against gamma radiation, wood is least effective because it has a low atomic number and density compared to the other materials listed. Gamma radiation is highly penetrating due to its electromagnetic nature and requires denser materials with higher atomic numbers for effective attenuation.

Lead is commonly used for shielding because it is dense and has a high atomic number, which makes it very effective at absorbing gamma photons. Concrete, while also effective to a degree, is used primarily because of its availability and cost, but it requires a substantial thickness to achieve adequate shielding due to its lower density compared to lead. Iron, although it provides some shielding, is less effective than lead and entails greater thickness to achieve a similar attenuation of gamma rays.

In contrast, wood lacks the necessary density and atomic characteristics to significantly reduce gamma radiation exposure, making it the least effective choice among common shielding materials.