Essentials of Nuclear Medicine Physics, Instrumentation, and Radiation Biology. Rachel A. Powsner

Figure 3.2 Transient equilibrium.

Figure 3.3 Transient equilibrium in a ⁹⁹Mo–^99mTc generator.

A deflector is used to direct the particles out through a window of the cyclotron into a target. Some of the particles and kinetic energy from these particles are incorporated into the nuclei of the atoms of the target. These energized (excited) nuclei are unstable.

Figure 3.4 Secular equilibrium.

Figure 3.5 Cyclotron.

      Indium‐111 (¹¹¹In) is produced in a cyclotron. The accelerated (bombarding) particles are protons. The target atoms are cadmium‐112 (¹¹²Cd). When a proton enters the nucleus of a ¹¹²Cd atom, the ¹¹²Cd is transformed into ¹¹¹In by discharging two neutrons. This reaction can be written as:

      or

      Other examples of cyclotron reactions include ¹²¹Sb(α,2n)¹²³I, ⁶⁸Zn(d,n)⁶⁷Ga, and ¹⁰B(d,n)¹¹C, where the symbols α and d denote alpha particles and deuterons (proton plus neutron) respectively.

Reactors

      Radionuclides for nuclear medicine are also produced in nuclear reactors. Some examples include ¹³¹I, ¹³³Xe, and ⁹⁹Mo.

      Reactor basics

A general schematic of a reactor is seen in Figure 3.6. A reactor is composed of fuel rods that contain large atoms (typically Uranium‐235, Uranium‐238, or Plutonium‐239) that are inherently unstable. These atoms undergo fission, (see Figure 1.14). Two or three neutrons and approximately 200 MeV of heat energy are emitted during this process. These neutrons leave the nucleus with moderately high kinetic energy and are referred to as fast neutrons. The neutrons are slowed with a moderator such as graphite, water, or heavy water. These “very slow” or thermal neutrons, and to a lesser extent the fast neutrons, in turn impact other fissionable atoms causing their fission, and so forth (Figure 3.7). If this chain reaction were to grow unchecked, the mass would explode. To maintain control, cadmium control rods are inserted to absorb the neutrons in the reactor. They can be further inserted or withdrawn to control the speed of the reaction. Medical nuclides are made in reactors by the processes of fission or neutron capture.

      Kinetic energy

      Kinetic means “motion.” The form of energy attributable to the motion of an object is its kinetic energy. Kinetic energy is related to both the mass (m) and velocity (v) of the object, specifically ½ mv². A moving car has kinetic energy, a parked car does not. A speeding car contains a great deal of kinetic energy that can be dissipated rapidly as heat, noise, and the destruction of metal in a collision.

Figure 3.6 Schematic of a nuclear reactor.

Figure 3.7 Chain reaction involving ²³⁵U and slow neutrons.

      Fission

      In this process, the desired radionuclide is one of the fission fragments of a heavy element (Z > 92), either the fuel atom itself or the atoms of a target placed inside the reactor. The by‐product is chemically separated from the other fission fragments. The fission reaction is denoted as

      For example, the formation of iodine‐131 and molybdenum‐99 are written as

      Neutron capture

In neutron capture the target atom captures a neutron. The new atom is radioactive and emits gamma photons or charged particles to produce the daughter nuclide (Figure 3.8). A gamma photon is emitted following capture of a thermal (slow) neutron. This reaction is written as

      For example:

      When the target atom captures a fast neutron a proton can be emitted. This capture reaction is sometimes referred to as transmutation and is symbolized as

      For example:

      A