Nuclear forensicsImproving plutonium identification
Researchers have developed a new kind of sensor that can be used to investigate the telltale isotopic composition of plutonium samples — a critical measurement for nuclear non-proliferation efforts and related forensics, as well as environmental monitoring, medical assays, and industrial safety. The novel device, based on “transition edge” sensor technology developed at NIST, is capable of ten times better resolution than all but the most expensive and time-consuming of current methods, and reduces the time needed for sample analysis from several days to one day.
A collaboration between NIST scientists and colleagues at Los Alamos National Laboratory (LANL) has resulted in a new kind of sensor that can be used to investigate the telltale isotopic composition of plutonium samples — a critical measurement for nuclear non-proliferation efforts and related forensics, as well as environmental monitoring, medical assays, and industrial safety.
The novel device, based on “transition edge” sensor technology developed at NIST (TESs are used in a variety of applications —see, for example, New High-Resolution X-ray Spectrometer for Beam Lines, and Adding Up Photons with a TES), is capable of ten times better resolution than all but the most expensive and time-consuming of current methods, and reduces the time needed for sample analysis from several days to one day. Researchers from NIST and LANL describe the new design and its results in the journal Analytical Chemistry.
NIST says that plutonium (Pu), highly radioactive and extensively employed in nuclear weapons and reactors, has many isotopes, and any trace sample contains slightly different proportions. Pu-239 is the main constituent of both weapons-grade and power reactor-grade plutonium; Pu-240 is the principal minority isotope (Pu-240 is present in low concentrations in nuclear weapons owing to its tendency to fission spontaneously. Concentrations are higher in reactor fuel and other uses).
Analyzing the exact mass ratio of the two in a sample reveals important information about the material’s origin, processing history, safety, and possible intended use. A key potential application in nuclear forensics is the attribution of a radiological event.
Until now, high-resolution mass-ratio results have only been possible by using mass spectrometry, a complicated procedure in which atoms of different masses travel different trajectories through a magnetic field. The more common, but less precise, method is to use solid-state detectors which absorb the alpha particles (when plutonium breaks down by fission into lighter elements, it emits alpha particles [two neutrons and two protons bound together, equivalent to a helium nucleus] as well as electrons and radiation) emitted by the radioactive sample and record their energy. Each isotope emits alphas with slightly different specific energies. So measuring the number of alpha particles at each energy level reveals the content of the sample.
“That sounds simple, but the issue is complicated by the fact that each isotope can have multiple decay modes that can lead to two, three, even four or more alpha particle energies per isotope,” says Joel Ullom, leader of NIST’s Quantum
