Nuclear accidents, have emitted toxic radioisotopes into the environment and residential areas, including cesium-137, strontium-90, and iodine-131. All of these particular radioisotopes undergo beta decay, with cesium-137 and strontium-90 undergoing β- decay. At high levels, exposure can lead to death, and even at low levels, radiation can cause cancer and other physiological consequences in the long term. In order of greatest to least, the destructiveness of exposure to radioactive emissions is usually: alpha > beta > gamma. All these can travel through air and to some degree are capable of penetrating human tissue. The depth of penetration (in log mm) for alpha particles is approximately (0.4), while for beta slightly more (0.54), and much higher for gamma rays. Gamma rays can penetrate lead up to nearly 3.0 log mm units while beta particles can penetrate to only roughly 1/3 that depth.
Technetium-99m is an example of a metastable isotope that can be intravenously injected and used in medical diagnostic procedures. 99mTc emits a gamma ray as its nucleus undergoes isomeric transition and rearranges to a lower energy state:
99mTc → 99Tc + γ
This decay is considered first-order, and using rate constant k, the rate can be solved using the equation:
N = N0e-kt
The half-life for this radioactive process is approximately 6 hours. Patients are usually injected with the substance 3 hours before the imaging, which allows time for their target organs to absorb the isotope. Specific types of organs can be targeted for study based on the ligand(s) selected for binding to the metal ion. To study the kidneys, liver, and various other internal organs, the ion is prepared with organic phosphonate complexes. To target an area like the brain, the ligand of choice is sodium pertechnetate-99m (Na99mTcO4).
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