19.5
Dégradation radioactive et datation radiométrique
View the full transcript and gain access to JoVE Core videos
Q1: What is radioactivity and why does it occur?
Radioactivity is the spontaneous transformation of an unstable nuclide, resulting in radiation emission. This random process occurs because all nuclei in a sample have the same probability of decaying, regardless of sample size or conditions. The decay is an intrinsic property of unstable atoms seeking greater stability.
Q2: How is radioactive decay activity measured and what are its units?
Activity measures the number of disintegrations per unit time and is directly proportional to the number of radioactive nuclei present. The SI unit is the becquerel, equal to one disintegration per second. The curie, still used for large-scale applications, equals 37 billion becquerels.
Q3: What is half-life and how does it relate to decay constant?
Half-life is the average time required for a sample's activity to decrease to half its original value. It is inversely proportional to the decay constant, an intrinsic property unique to each radionuclide. Half-lives vary dramatically: radon-220 has a one-minute half-life, while thorium-232 has a 14-billion-year half-life.
Q4: How does carbon-14 dating determine the age of archaeological specimens?
Living organisms maintain a carbon-14 to carbon-12 ratio matching the atmosphere through continuous carbon replenishment. After death, carbon-14 decays by emitting beta particles while carbon-12 remains stable. By measuring the current ratio and comparing it to the atmospheric ratio, scientists calculate how many half-lives have elapsed, determining the specimen's age.
Q5: Why is carbon-14 dating limited to approximately 50,000 years?
Carbon-14 dating accuracy depends on detecting measurable carbon-14 levels in samples. With a half-life of 5,730 years, after about nine half-lives (approximately 50,000 years), carbon-14 levels become too low for reliable detection using standard methods. Beyond this timeframe, the remaining radioactive carbon is insufficient for precise age determination.
Q6: How do scientists use uranium-238 to date ancient rocks?
Uranium-238 decays into lead-206 over 4.5 billion years (its half-life). By measuring the uranium-238 to lead-206 ratio in rocks that contained no lead when formed, scientists can calculate the rock's age. This method works for much older geological samples than carbon-14 dating because of uranium-238's extremely long half-life.
Q7: What does the decay constant represent in radioactive decay calculations?
The decay constant is the average probability of decay per nucleus per unit time. It determines how quickly a radionuclide decays and is inversely proportional to half-life. Different radionuclides have different decay constants, reflecting their unique nuclear stability and decay rates.