Specific Activity Converter: Bq/g, Ci/g, Bq/kg

Specific activity is the radioactivity per unit mass of a radioactive material, expressed in units such as becquerels per gram (Bq/g) or curies per gram (Ci/g). It quantifies how intensely radioactive a given mass of material is. A substance with a short half-life has a high specific activity because many atoms decay in a short time, whereas a long-lived isotope has low specific activity. Specific activity is important in nuclear medicine, radiation protection, radiochemistry, and environmental monitoring.

Specific activity (in Bq/g) is calculated using the formula: A = (ln2 / t_half) x (N_A / M), where t_half is the half-life in seconds, N_A is Avogadro's number (6.02214076 x 10^23 atoms per mole), and M is the molar mass in grams per mole. This formula follows from the radioactive decay law: activity equals the decay constant (ln2 / t_half) multiplied by the number of atoms per gram (N_A / M). A shorter half-life or lower molar mass both increase specific activity.

The curie was originally defined as the activity of one gram of radium-226 (Ra-226), the isotope Marie Curie studied extensively. When the unit was established in the early twentieth century, Ra-226 was the most precisely characterized radioactive substance available. Its measured activity of 3.7 x 10^10 disintegrations per second per gram became the definition of one curie. This definition was later adopted exactly by international agreement, giving Ra-226 a specific activity of exactly 1 Ci/g (3.7 x 10^10 Bq/g).

Specific activity is inversely proportional to half-life (for a given element): halving the half-life doubles the specific activity. Short-lived isotopes such as iodine-131 (half-life 8 days) have extremely high specific activity (around 4.6 x 10^15 Bq/g), making them useful in targeted medical therapy. Long-lived isotopes such as uranium-238 (half-life 4.47 billion years) have very low specific activity (about 12,440 Bq/g), so large masses are needed to achieve measurable activity levels.

Representative values: carbon-14 (C-14, half-life 5,730 years) has a specific activity of approximately 1.65 x 10^11 Bq/g (165 GBq/g); cobalt-60 (Co-60, half-life 5.27 years) is approximately 4.18 x 10^13 Bq/g (41.8 TBq/g); iodine-131 (I-131, half-life 8.02 days) is approximately 4.6 x 10^15 Bq/g; and radium-226 (Ra-226, half-life 1,600 years) is exactly 3.7 x 10^10 Bq/g, which defines the curie. Tritium (H-3, half-life 12.3 years) has a specific activity of about 3.59 x 10^14 Bq/g.