(Xe-133) are by-products of the Mo-99 production process and will be sufficiently available if Mo-99 is available.
These other medical isotopes are not being recovered for sale by all major Mo-99 producers because they can be more cheaply produced and purchased from other sources.3
Point 3 deserves additional elaboration. The fission of uranium-235 (U-235) produces a spectrum of fission products (see Figure 2.5) including Mo-99, I-131, and Xe-133. These fission products are produced in the same proportions to each other whether HEU or low enriched uranium (LEU) targets are used. All of these isotopes can be recovered when the targets are processed to obtain Mo-99.
The primary purpose of this chapter is to provide a brief overview of the production and use of Mo-99 in nuclear medicine and is intended primarily for nonexpert readers. Knowledgeable readers may wish to skip directly to Chapter 3.
The decay product of Mo-99, Tc-99m, is the workhorse isotope in nuclear medicine for diagnostic imaging. Tc-99m is used for the detection of disease and for the study of organ structure and function. Tc-99m is especially useful for nuclear medicine procedures because it can be chemically incorporated into small molecule ligands and proteins that concentrate in specific organs or tissues when injected into the body. The isotope has a half-life of about 6 hours and emits 140 keV photons when it decays to Tc-99, a radioactive isotope with about a 214,000-year half-life. This photon energy is ideally suited for efficient detection by scintillation instruments such as gamma cameras. The data collected by the camera are analyzed to produce detailed structural and functional images. A recent report of the National Research Council and Institute of Medicine (NAS and IOM, 2007) provides a description of the imaging process.
As will be described in more detail in the following section, Tc-99m is currently produced through a multistep process that begins with the neutron irradiation of fissile U-235 contained in HEU (see Sidebar 1.1) or LEU targets in a nuclear reactor. This irradiation causes U-235 to fission and produces Mo-99 and many other fission products, including I-131 and Xe-133. Following irradiation, the targets are chemically processed to separate Mo-99 from other fission products. If desired, these other fission products can be recovered separately. The separated Mo-99, which is con-