Fortunately, newer detectors can catch this and separate the bombs from the Brazil nuts. IANANP, but very sensitive germanium semiconductor detectors, and cert

Fortunately, newer detectors can catch this and separate the bombs from the Brazil nuts. IANANP, but very sensitive germanium semiconductor detectors, and certain scintillation detectors (sodium iodide (NaI), etc.) can be used to determine isotope type at a distance by gamma-ray spectroscopy. Fissile material (plutonium or uranium in particular) can also be very effectively detected through neutron emission measurements, but this depends on pricey helium-3-based detectors. In these detectors, thermal (low energy) neutrons from radiological material are captured by He3, causing the He3 to split into hydrogen and tritium (H3) and the release of a gamma photon. The gamma photons can then be detected by traditional means, and the photon counts can be related to neutron flux. Flux measurements can then be used to (hopefully) determine the type of neutron source. n + 3He → 3H + 1H + 0.764 MeV The problem with He3 detectors is that the gas has a very low natural abundance, and is accordingly expensive. Most of the current He3 stockpile originates from decaying tritium present in nuclear weapons (~5% of the tritium decays to He3 each year). It can also be manufactured by creating tritium in a reactor and waiting for it to decay. Tritium production is expensive though, as the process requires high-neutron flux bombardment of a boron, lithium, or nitrogen target.