Thorotrast: Analysis of the time evolution of its α activity concentration, in the 70 years following the chemical purification of Thorium

We simulate the α-activity of the Thorium series elements present in the contrast medium named Thorotrast, used until 1960 and cause of certified deaths until today. Assuming, as active components at t=0t=0, 232Th and 228Th in the same relative concentration they have in nature, α-activity oscillates for some decades before reaching a stationary value that in absence of biological depletion would be ASTAST = 24000 Bq/g. Our Montecarlo code generates the nuclear decays of the Thorium series with and without in-vivo biological depletion, arriving to three kinds of results for the activity:1)Theoretical activity concentration (no biological depletion). Our result is fitted by: A(t)A(t) = AST⋅{[1−exp(−t/10)]+[exp(−t/tB)(1−0.8exp(−t/tA))]} , with t   in years, tAtA = 1.07⋅10−21.07⋅10−2 years, and tBtB = 2.38 years.2)Weak biological depletion (228Ra/232Th equilibrium activity ratio 0.6, 224Ra/228Ra e.a.r. 0.9, 10% excretion for 220Rn). The ratio of the activity concentration to the theoretical activity concentration is fitted by: Aweak(t)/A(t)Aweak(t)/A(t) = 0.61 + 0.29 exp[−(t/15)2]exp[−(t/15)2] (t in years).3)Strong biological depletion (228Ra/232Th e.a.r. 0.4, 224Ra/228Ra e.a.r. 0.8, 10% excretion for 220Rn). The ratio of the activity concentration to the theoretical activity concentration is fitted by Astrong(t)/A(t)Astrong(t)/A(t) = 0.44 + 0.4 exp[−(t/13)2]exp[−(t/13)2] (t in years).We also report fluctuation calculation for two cases where standard statistical behavior is not expected.