Half-Life
Definition
The time required for half the radioactive atoms in a sample to decay. Set by the decay constant lambda through t1/2 = ln 2 / lambda. Carbon-14 sits at 5,730 years, which is what makes it the right clock for archaeology and questioned-document work covering the last fifty thousand years. Activity at any time t is N0 exp(minus lambda t).
- Carbon-14 half-life
- 5,730 years
- Dating range for C-14
- Up to 50,000 years
- Core principle
- Time for half of radioactive atoms to decay
Common questions
What does half-life mean in radioactivity?+
Half-life is the time it takes for exactly half of the radioactive atoms in a sample to decay. It's a fixed property of each radioactive element determined by its decay constant. This predictable timing makes half-life the key to radioactive dating and identification.
Why is carbon-14 useful in forensic investigation?+
Carbon-14 has a half-life of 5,730 years, which makes it perfect for dating materials from the last fifty thousand years. This window covers everything from ancient artifacts to questioned documents, making it a reliable clock for forensic and archaeological work.
How does half-life help in neutron activation analysis?+
Neutron activation analysis uses half-life timing to separate different radioactive isotopes in a sample. Short-lived isotopes are counted immediately, while long-lived ones are measured after a cooling period, allowing forensic experts to identify what elements are present.
Related terms
- HPGe Detector
- A high-purity germanium semiconductor crystal cooled with liquid nitrogen to about 77 kelvin. Resolution at the 1,332 keV gamma line of cobalt-60...
- Accelerator Mass Spectrometry
- A mass spectrometer fronted by a high-energy particle accelerator (typically a 500 keV to 6 MeV tandem). Counts individual carbon-14 atoms directly...
- BARC Trombay
- Bhabha Atomic Research Centre, Mumbai. Hosts the Dhruva and Apsara-U research reactors that have historically supplied neutron flux for Indian NAA work.
- Becquerel and Curie
- The becquerel (Bq) is one nuclear disintegration per second, the SI unit of activity. The older curie (Ci) is 3.7 × 10^10...
- Gamma Spectrometry
- Energy-resolving detection of gamma rays. Each radioisotope emits a characteristic gamma at a known energy, which is how NAA identifies elements.
- Geiger-Muller Tube
- A gas-filled cylindrical tube with a thin entrance window and a high-voltage central wire. An ionising particle entering the tube triggers a...
- INAA
- Instrumental Neutron Activation Analysis. The whole sample is irradiated, cooled, and counted directly. No wet chemistry. Non-destructive.
- Neutron Capture (N, Gamma)
- The reaction in which a stable nucleus absorbs a neutron and becomes a radioactive isotope of the same element, emitting a prompt...
- RNAA
- Radiochemical Neutron Activation Analysis. After irradiation, the analyst chemically separates the isotope of interest to remove interfering activity. More sensitive, destructive.
- Scintillation Detector
- A crystal or liquid (sodium iodide doped with thallium for gamma, an organic cocktail for low-energy beta) that emits visible-light flashes when...
- Thermal Neutron
- A low-energy neutron (around 0.025 eV) that has been slowed by a moderator. The workhorse particle of NAA because slow neutrons are...
Explained in these topics
- Neutron Activation Analysis (NAA) in Forensic ScienceTime for half the radioactive atoms to decay. NAA exploits half-life by timing the count: short-lived isotopes are read first, long-lived ones after a cooling...
- Radiochemistry: Radioactivity, Carbon Dating, Neutron ActivationThe time required for half the radioactive atoms in a sample to decay. Set by the decay constant lambda through t1/2 = ln 2 / lambda. Carbon-14 sits at 5,730 y...