Technetium metastable isotope

Making metastable technetium-99 is an expensive business. A cheaper, common alternative tracer is iodine-131, which emits a gamma ray when it decays. But the iodine isotope also releases beta particles that can damage tissues, making it less attractive as an imaging agent. 

The technetium isotope produced is the metastable "mTc, which is in a nuclear excited state. The eventual fall to the ground state has a half-life of 6 hours, and is accompanied by the emission of a gamma-ray photon. The gamma-ray photons have energies sufficiently low not to harm... 

Among the technetium isotopes, only " Tc, a pure emitter ( ,nax=0.29 McV) with a half-life of 213 000 a, can be obtained (Sect. 5.1) in amounts adequate for studying the technical applicability of the element and its compounds. However, the widespread use of technetium is necessarily restricted by its radioactivity. The application of the metastable isotope Tc in nuclear medicine has quite superior significance and will be described in detail in Part B. 

Radioactive elements have no stable isotopes. Among the natural elements, polonium (atomic number 84) and heavier ones are radioactive. Technetium with its atomic number 43 is the lightest of all elements having no stable isotopes. Another light such dementis 61 promethium. Technetium has 26 unstable and 11 metastable (isomer) isotopes, all radioactive [28.9]. Details of the three most long-lived isotopes and two metastable (m) isotopes, important for tracer work, are collected in Table 28.3. 

There are no stable isotopes of technetium. The element is obtained from fission reactions rather than from natural sources. The most commonly encountered isotopes are Tc, a weak 292-keV emitter with a half-life of 2.1 x 10 yr, and Tc, a metastable form that decays to Tc with the emission of a 140-keV y photon with a half-life of ca 6 h. The coordination chemistry [1, 2] and electrochemistry [3] of technetium have been reviewed on several occasions. 

One radioactive isotope of molybdenum is commonly used in medicine, molybdenum-99m. (The m in this instance stands for metastable, which means the isotope does not last very long.) This isotope is not used directly, however. Instead, it is used in hospitals to make another radioactive isotope, technetium-99m. This isotope of technetium (atomic number 43) is widely used as a tracer for diagnostic studies of the brain, liver, spleen, heart, and other organs and body systems. 

The m in the symbol tells you that this is a metastable element, which is one form of an unstable isotope. Technetium 99m gives off a gamma ray to become a more stable form of the same isotope, with no change in either atomic or mass number. 

The isotope of technetium is actually a special isotope of Tc-99 called Tc-99m, where the m indicates a so-called metastable isotope. 

Table 28.3 Long-lived isotopes and tracer isotopes of technetium. The designation m stands for metastable . Metastable (or isomeric) refers to an excited state of an isotope that lives a longer time than expected before decaying.

A patient is given 0.050 mg of technetium-99m (where m means metastable— an unstable but long-lived state), a radioactive isotope with a half-life of about 6.0 hours. How long until the radioactive... 

The radionuclide needs to be a gamma emitter (long-range ionizing radiation) that can be monitored from outside the patient s body. It should have a relatively short half-life to limit the radiation dose to the patient. The radionuclide most commonly used is technetium-99m, Tc. The m indicates that this is a metastable isotope, that is, that its half-life is considerably longer than most isotopes that undergo gamma decay. 

The m means that it is a metastable isotope. It is not completely stable, but it does have a somewhat longer half-life than would be expected.) Technetium is unusual in that it is one of only two elements lighter than lead (the other being promethium) that has no stable isotopes. Therefore, technetium has to be produced in nuclear reactors. Technetium is produced from molybdenum. Since Tc-99m has a half-life of only 6.0 hours, supplies must be produced continuously and then transported quickly to hospitals around the United States and Canada to be used before too much of it has decayed. 

Pure gamma emitters are rare, although gamma radiation accompanies most alpha and beta radiation. In radiology, one of the most commonly used gamma emitters is technetium (Tc). The unstable isotope of technetium is written as the metastable (symbol m)... 

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