Name a radioisotope that should be shielded by Plexiglass and not lead.

Phosphorous-32 is a beta-emitting radioisotope, and its primary hazard comes from the beta particles it emits. These particles can be effectively shielded by materials that are less dense than lead, such as Plexiglass, which is commonly used in laboratory settings due to its ability to attenuate beta radiation while allowing for visibility and ease of handling.

This choice is particularly appropriate because beta particles have a relatively low penetrating power and are effectively absorbed by lightweight materials. Using Plexiglass enhances safety by providing adequate shielding while also ensuring that the working environment remains visible and accessible.

In contrast, other isotopes listed as options either emit radiation that requires more robust shielding materials such as lead or may have different safety considerations. For instance, lead is primarily used for shielding against gamma radiation, which is emitted by isotopes like Iodine-125. Therefore, Plexiglass is the suitable material for shielding against beta emissions like those from Phosphorous-32.