Boron Neutron Capture Therapy
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BSkin10B atom10B atom10B atom10B atomEpithermal neutronThermal neutronsThermal neutronsThermal neutronsThermal neutrons1H atoms1H atoms1H atoms1H atomsNeutrons and their reactionsheeirea⁷Li¹⁰Bnth⁴He⁴HeCompoundnucleusEmissionEmissionFission4(n, x)(n, x)Nuclear transmutation reactions2(n, n’)(n, n’)Inelastic scattering reactionsIn inelastic scattering, momentum is conserved, but kinetic energy is not. These reactions are excited state reactions in which the internal energy of colliding atomic nuclei increas-es, and with high-atomic-number substances, they mostly occur with neutrons of MeV-order or greater energy. While they can be useful insofar as they make it possible to shield neutrons of MeV-order or greater energy from high-atom-ic-number substances, they have no direct medical use.In a nuclear transmutation reaction, of which they are a variety of types, a compound nucleus formed by colliding neutrons and an isotope is split into a combination of isotopes that differ from the pre-reaction state. Of these, the most important reactions for BNCT are 10B (n, α) 7Li and 14N (n, p) 14C.It is from the former that the name for BNCT is derived. Figure 2 diagrams the process by which the epithermal neutrons that are used in BNCT, which have energy of about 0.5 eV to 40 keV (the typical energy of a thermal neutron is 0.0254 eV), are irradiated, encounter boron-10 (10B) in the body, and decay into 7Li while emitting alpha particles.The latter reaction of 14N (n, p) 14C gives off protons with energy of 0.58 MeV, which gives them a high cell-killing potential. Since large numbers of nitrogen atoms are present in both tumor cells and healthy cells, the reaction has a not insignificant impact on both. In particular, it is important to emphasize this latter reaction for its relation-ship with the development of issues (complications) in normal tissue.Figure 2. Epithermal Neutron Scattering Process and 10B Nuclear Fission ProcessNeutron thermalization processNuclear ssion processThe 10B atoms that encountered and absorbed the thermal neutrons become compound nuclei and ssion into 7Li and 4He. The 7Li and 4He are emitted in opposite directions, and the total distance traveled by both is 14μm, which corresponds to roughly the size of a cell.3(n, γ)(n, γ)Neutron capture reactions Unlike inelastic scattering reactions, colliding neutrons are absorbed by atomic nuclei in neutron capture reactions. Consequently, each nucleus becomes an isotope with a mass number that is one greater. In most elements, the binding energy of nuclear particles is about 8 MeV. The formed nuclei enter an excited state of about that level of energy, which then decays upon emitting gamma rays according to the excitation level. This reaction involves only the emission of gamma rays and is not desirable for BNCT, but it is used as a technique for prompt gamma-ray analy-sis (PGA) since 10B concentrations in blood or other biologi-cal specimens must be measured to obtain basic data for treatment. This reaction can also be described as one type of nuclear transmutation reaction (4).Upon irradiation, epithermal neutrons lose their kinetic energy primarily in a 1H (n, n) 1H reaction, undergoing neutron thermalization and encountering 10B atoms (boron-10 atoms).12

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