Detection of special nuclear material (SNM, Pu, U, etc.) is essential for nuclear nonproliferation and for securing weapons and nuclear materials. Nuclear weapons contain SNM, which emits radiation, gamma rays and neutrons (so they can help in the detection of SNM). In this paper, I shall endeavor to present nuclear resonance fluorescence (NRF) imaging technique to detect SNM. NRF imaging is used to identify materials based on the spectrum of gamma rays, emitted by a nucleus when struck by photons of a specific energy. When atoms of a given element are illuminated with photons above an energy threshold, unique to that element, their electrons absorb the photons' energy and move to a higher energy level. The electrons then drop back to their normal state, emitting photons that are slightly less energetic than in the inbound photons. This process is called fluorescence. Each isotope has a unique combination of numbers of protons and neutrons in its nucleus, so it vibrates at a unique frequency (resonant frequencies). When the nucleus is struck by photon at precisely that energy level, it will absorb the photon and move to an excited state. The nucleus then reverts to its initial state, giving off photons slightly less energetic than those it absorbed. This process is known as nuclear resonance fluorescence. NRF produces a gamma-ray spectrum unique to each isotope. Analyzing the spectrum of emitted photons identifies the element and isotope. In the case of chemical weapons, if the kind of chemical explosive and the mix of isotopes and impurities in SNM is known, then it can help in dismantlement or finding the source of the weapon.
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