highspecificactivity

High specific activity is a characteristic of a radioactive substance describing a large amount of radioactivity per unit amount of substance. Specific activity is typically expressed as becquerels per gram (Bq/g) or becquerels per mole (Bq/mol). For a pure sample consisting entirely of a single radionuclide, the theoretical maximum specific activity can be approximated by (ln 2 / T_half) × (N_A / M), where T_half is the isotope’s half-life, N_A is Avogadro’s number, and M is the molar mass. This reflects the relationship that shorter half-lives and lighter molar masses yield higher activity per unit mass.

High specific activity is achieved when the radionuclide is produced in a carrier-free form (no stable isotopes accompanying it) and is isolated with high radiochemical purity. Production methods, including neutron or proton irradiation and subsequent chemical separation, influence the final specific activity. The presence of stable isotopes or carrier material reduces specific activity because the same total mass or mole amount carries less radioactivity.

In practical terms, high specific activity is important in radiopharmaceuticals and tracer studies because it allows labeling of biological molecules at very low mass, minimizing pharmacological effects while delivering the desired radiation dose. Isotopes used for medical imaging and therapy, such as certain PET isotopes (for example, fluorine-18) and others produced for high activity per quantity, benefit from high specific activity. However, the attainable level depends on the isotope’s chemistry, production route, and isotopic purity. Shorter half-lives enable high initial activity but impose rapid handling and use, while longer half-lives require different logistical considerations.