The best absorber to use to block gamma radiation is __________.

Lead is the best absorber to use to block gamma radiation due to its high density and atomic number. Gamma rays are highly penetrating electromagnetic radiation, and effective shielding materials must possess the ability to absorb or scatter these high-energy photons. Lead's dense structure allows it to provide substantial attenuation of gamma radiation, making it particularly useful in environments where radiation protection is critical, such as in medical radiology, nuclear power plants, and research laboratories.

While other materials like concrete, aluminum, and plastic can also provide some level of protection, they are generally less effective than lead. Concrete requires a greater thickness to achieve similar shielding effectiveness compared to lead. Aluminum is lightweight and can be beneficial for lower-energy radiation such as beta particles, but it is not dense enough to effectively attenuate gamma rays. Plastic, while useful for certain applications, typically lacks the necessary density and atomic weight to effectively block gamma radiation, resulting in less attenuation compared to lead.