ASNT

Since a proton remains after conversion of the neutron, the mass of the nucleus does not change but the atomic number increases. Gamma Gammas are high-energy electromagnetic radiation originating in the nucleus. The structure of the nucleus allows for certain transitions producing well-defined gamma energies from tens of keV to several MeV. Available radionuclide source activities and gamma energies define the uses for radiographic applications. Cobalt 60, iridium 192, and selenium 75 are a few that are commonly used for industrial radiography purposes (National Research Council of the National Academies 2008). Penetration of material objects by energetic X-rays (gammas) provides users with means for inspecting parts and recording images on film and pixelated solid-state detectors. Neutrons Neutrons can be produced by spontaneous fission and by combining an alpha emitter with beryllium, for industrial uses. Californium 252 is an example of fission neutron source material; it produces some 2 × 10 12 neutrons per second per gram. Americium 241 combined with beryllium (americium-berylium) produces neutrons at 3.4 Ci / gram (National Research Council of the National Academies 2008).

DECAY MODES Nuclei decay and emit four different particles— alphas, betas, gammas, and neutrons. Alpha Alpha particles are helium nuclei that have escaped the unstable nucleus but remain identifiable by the two proton and two neutron (helium) configura- tion. Their kinetic energy is in the MeV range, but the large mass (approximately four proton masses) is easily stopped by thin sheets of shielding matter such as paper or plastic and will range out over tens of millimeters in air at atmospheric pressure. After alpha emission, the remaining nuclei are poorer by two protons and two neutrons, so A (mass) and Z (atomic number) of the remaining nucleus will ref lect this ( α = He 4 2 ) . For example, the decay of americium 241, used commonly in residential smoke detectors, leaves behind a stable isotope of neptunium via Equation 13: Beta Beta particles are electrons ejected from nuclei with a wide range of energies and two different charge states; that is, β - (an electron) and β + (positron). In this decay of a neutron, emission of a neutrino (or antineutrino) is necessary, so the complete equation for the decay of carbon 14, used in age determination of organic archeological objects, is shown as Equation 14: (Eq. 13) Am 95 241 →α + Np 93 237

14 → N

7 14 + β − + neutrino

C 6

(Eq. 14)

CHAPTER 2

49

Part 3