Barium sulfate (BaSO₄) is a good shield against gamma rays primarily due to its high atomic number and high density. Here's why:

* High Atomic Number: Barium has a high atomic number (56), meaning its atoms have many electrons. Gamma rays interact with matter primarily through the photoelectric effect, where a gamma ray interacts with an electron, transferring its energy to the electron and being absorbed. The probability of the photoelectric effect is proportional to the atomic number raised to the fourth power (Z⁴). This means that materials with high atomic numbers are much more likely to absorb gamma rays.

* High Density: The density of barium sulfate is around 4.5 g/cm³, which is relatively high. This means there are many barium atoms packed into a small volume, increasing the probability of gamma ray interaction.

In summary: Barium sulfate's high atomic number and density make it effective at absorbing gamma rays through the photoelectric effect. This makes it a suitable material for shielding against this type of radiation.

Note: While barium sulfate is effective at shielding against gamma rays, it's not a perfect solution. The effectiveness of shielding also depends on the energy of the gamma rays, the thickness of the shielding material, and other factors.