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The beta particle is an ordinary electron or positron ejected from the nucleus of a beta-unstable radioactive atom. The beta has a single negative or positive electrical charge and a very small mass. The interaction of a beta particle and an orbital electron leads to electrical excitation and ionization of the orbital electron. These interactions cause the beta particle to lose energy in overcoming the electrical forces of the orbital electron. The electrical forces act over long distances; therefore, the two particles do not have to come into direct contact for ionization to occur. The amount of energy lost by the beta particle depends upon both its distance of approach to the electron and its kinetic energy. Beta particles and orbital electrons have the same mass; therefore, they are easily deflected by collision. Because of this fact, the beta particle follows a tortuous path as it passes through absorbing material. The specific ionization of a beta particle is low due to its small mass, small charge, and relatively high speed of travel. Gamma Ray The gamma ray is a photon of electromagnetic radiation with a very short wavelength and high energy. It is emitted from an unstable atomic nucleus and has high penetrating power. There are three methods of attenuating (reducing the energy level of) gamma-rays: photoelectric effect, compton scattering, and pair production. The photoelectric effect occurs when a low energy gamma strikes an orbital electron, as shown in Figure 2. The total energy of the gamma is expended in ejecting the electron from its orbit. The result is ionization of the atom and expulsion of a high energy electron.
Figure 2 Photoelectric Effect The photoelectric effect is most predominant with low energy gammas and rarely occurs with gammas having an energy above 1 MeV (million electron volts). Compton scattering is an elastic collision between an electron and a photon, as shown in Figure 3. In this case, the photon has more energy than is required to eject the electron from orbit, or it cannot give up all of its energy in a collision with a free electron. Since all of the energy from the photon cannot be transferred, the photon must be scattered; the scattered photon must have less energy, or a longer wavelength. The result is ionization of the atom, a high energy beta, and a gamma at a lower energy level than the original.
Figure 3 Compton Scattering Compton scattering is most predominant with gammas at an energy level in the 1.0 to 2.0 MeV range. At higher energy levels, pair production is predominate. When a high energy gamma passes close enough to a heavy nucleus, the gamma disappears, and its energy reappears in the form of an electron and a positron (same mass as an electron, but has a positive charge), as shown in Figure 4. This transformation of energy into mass must take place near a particle, such as a nucleus, to conserve momentum. The kinetic energy of the recoiling nucleus is very small; therefore, all of the photon's energy that is in excess of that needed to supply the mass of the pair appears as kinetic energy of the pair. For this reaction to take place, the original gamma must have at least 1.02 MeV energy.
Figure 4 Pair Production The electron loses energy by ionization. The positron interacts with other electrons and loses energy by ionizing them. If the energy of the positron is low enough, it will combine with an electron (mutual annihilation occurs), and the energy is released as a gamma. The probability of pair production increases significantly for higher energy gammas. Gamma radiation has a very high penetrating power. A small fraction of the original stream will pass through several feet of concrete or several meters of air. The specific ionization of a gamma is low compared to that of an alpha particle, but is higher than that of a beta particle. Neutron Neutrons have no electrical charge and have nearly the same mass as a proton (a hydrogen atom nucleus). A neutron is hundreds of times larger than an electron, but one quarter the size of an alpha particle. The source of neutrons is primarily nuclear reactions, such as fission, but they are also produced from the decay of radioactive elements. Because of its size and lack of charge, the neutron is fairly difficult to stop, and has a relatively high penetrating power. Neutrons may collide with nuclei causing one of the following reactions: inelastic scattering, elastic scattering, radiative capture, or fission. Inelastic scattering causes some of the neutron's kinetic energy to be transferred to the target nucleus in the form of kinetic energy and some internal energy. This transfer of energy slows the neutron, but leaves the nucleus in an excited state. The excitation energy is emitted as a gamma ray photon. The interaction between the neutron and the nucleus is best described by the compound nucleus mode; the neutron is captured, then re-emitted from the nucleus along with a gamma ray photon. This re-emission is considered the threshold phenomenon. The neutron threshold energy varies from infinity for hydrogen, (inelastic scatter cannot occur) to about 6 MeV for oxygen, to less than 1 MeV for uranium. Elastic scattering is the most likely interaction between fast neutrons and low atomic mass number absorbers. The interaction is sometimes referred to as the "billiard ball effect." The neutron shares its kinetic energy with the target nucleus without exciting the nucleus. Radiative capture (n, The fission process for uranium The distance that a fast neutron will travel, between its introduction into the slowing-down medium (moderator) and thermalization, is dependent on the number of collisions and the distance between collisions. Though the actual path of the neutron slowing down is tortuous because of collisions, the average straight-line distance can be determined; this distance is called the fast diffusion length or slowing-down length. The distance traveled, once thermalized, until the neutron is absorbed, is called the thermal diffusion length. Fast neutrons rapidly degrade in energy by elastic collisions when they interact with low atomic number materials. As neutrons reach thermal energy, or near thermal energies, the likelihood of capture increases. In present day reactor facilities the thermalized neutron continues to scatter elastically with the moderator until it is absorbed by fuel or non-fuel material, or until it leaks from the core. Secondary ionization caused by the capture of neutrons is important in the detection of neutrons. Neutrons will interact with B-10 to produce Li-7 and He-4.
The lithium and alpha particles share the energy and produce "secondary ionizations" which are easily detectable. Summary Alpha, beta, gamma, and neutron radiation are summarized below. Radiation Types Summary Alpha particles The alpha particle is a helium nucleus produced from the radioactive decay of heavy metals and some nuclear reactions. The high positive charge of an alpha particle causes electrical excitation and ionization of surrounding atoms. Beta particles The beta particle is an ordinary electron or positron ejected from the nucleus of a beta-unstable radioactive atom. The interaction of a beta particle and an orbital electron leads to electrical excitation and ionization of the orbital electron. Gamma rays The gamma ray is a photon of electromagnetic radiation with a very short wavelength and high energy. The three methods of attenuating gamma-rays are: photoelectric effect, compton scattering, and pair production. Neutrons Neutrons have no electrical charge and have nearly the same mass as a proton (a hydrogen atom nucleus). Neutrons collide with nuclei, causing one of the following reactions: inelastic scattering, elastic scattering, radiative capture, or fission.
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takes place when a neutron is absorbed to produce an excited nucleus. The
excited nucleus regains stability by emitting a gamma ray.
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