Which of the following materials is recommended for shielding syringes containing positron-emitting radionuclides?

Tungsten is the recommended material for shielding syringes containing positron-emitting radionuclides. This is primarily due to its high density and effectiveness in attenuating the gamma radiation produced during the decay of positron emitters. Positron emitters release positrons, which are a type of beta radiation, as well as gamma photons when the positrons annihilate with electrons.

The use of tungsten for shielding offers several advantages. It has a greater density than lead, allowing for thinner shield walls while still providing effective radiation safety. This is particularly beneficial in a clinical nuclear medicine setting where space may be limited, and the weight of shielding materials is a consideration. Additionally, tungsten is less prone to degradation over time compared to some other materials, which ensures consistent shielding properties.

Choosing the appropriate shielding material is crucial not only for the safety of healthcare workers but also for ensuring that the radionuclides can be handled with minimal risk. While lead is traditionally a common shielding material, tungsten's performance in the context of positron-emitting radionuclides makes it the superior choice in this scenario. Other materials like gold or steel do not offer the same level of effectiveness or practicality for this specific application compared to tungsten.