Fast neutrons (E of 1.5 MeV) have a low probability of interaction with other nuclei. They are thermalized or slowed down (0.025 eV) to interact with other nuclei. Moderators (low MW materials like heavy water, beryllium or graphite) are distributed in spaces between fuel rods
University of Missouri Research Reactor (MURR) Columbia, MO
Nuclear Fission 235 U 144 + n 236 U 144 144 Ba 88 + 89 Kr 53 + 3n 99 Mo 42 + 135 Sn 50 + 2n 236 U unstable - undergoes fission immediately wide range of fission products - usually 1/3 and 2/3 split of the mass number
Fission products useful in nuclear medicine include: 99 Mo, 131 I, 133 Xe, 137 Cs and 90 Sr Mo-99 I-131
Reactor-Produced Radionuclides: Thermal Neutron Reactions (n, reaction: formed by reactions between targets and thermalized neutrons Y A z + n Y+1 A z + A=target; A=isotope produced same atomic number, different mass (n, reaction –not carrier-free, since target and product are same –radioisotopic purity can be high if cross section is sufficiently large (e.g. 176 Lu(n, ) 177 Lu)
Reactor-Produced Radionuclides: Thermal Neutron Reactions, cont’d (n, p reaction: formed by reactions between targets and thermalized neutrons Y A z + n Y B z-1 + p A=target; B=isotope produced different atomic number, same mass (n, p reaction –carrier-free, since target and product are different –example: 64 Zn(n,p) 64 Cu
Fisson/Reactor ProductsCyclotron Products Generally decay by - emission because of excess neutrons Not many are useful for diagnostic imaging, but several are useful for radiotherapy Generally decay by + emission or electron capture because of excess protons Many are useful for diagnostic imaging (gamma scintigraphy or positron emission tomography)
Photoelectric Effect: The energy of an incoming gamma ray is completely absorbed by the atom, and the energy absorbed is used to eject an electron from the atom.