Post-Detonation Nuclear Forensics
Dunlop, William, Smith, Harold, Arms Control Today
As responsible governments want to locate nuclear weapons in the hands of terrorists before they are detonated, they have tended to focus on improving methods to detect fissile material (pre-detonation) more than using forensic techniques to determine the products generated by fission (post-detonation). Predetonation technology includes x-ray machines that may show the presence of a nuclear device and gamma-ray detectors that indicate the presence of fissile material, hi post-detonation forensics, the arcane field of radiochemistry plays the major role.
In the event of a nuclear explosion, radiochemists would seek to obtain minute quantities of debris from the nuclear device near ground zero and/or in the atmosphere. They would first separate the atoms into groups of chemically similar elements and then measure the radioactivity of each group. To do so, scientists often employ gamma-ray spectroscopy to measure the time of emission and the energy of each detectable gamma ray, electromagnetic radiation produced by radioactive decay.
The energy of the detected gamma ray is unique to each isotope of a specific element, thereby indicating its presence in the debris. Furthermore, the rate at which that isotope emits its signature gamma ray decays in time according to its unique half-life, thereby providing a second identifier of the isotope. By knowing the chemistry of elements that have been separated, the energy of the gamma rays of any radioactive atoms in that chemical group, and the rate at which the emission of the gamma rays at each particular energy level decays over time, scientists can obtain an accurate measurement of many of the isotopes of the chemical elements in the debris. Because there is always experimental uncertainty, particularly with small samples, all three processes (separation, energy measurement, and tune dependence) may be used. …