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Isotopes and Nuclear Stability

Two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei.

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Nuclear Medicine

Nuclear Medicine

Credit: Myohan
Source: http://commons.wikimedia.org/wiki/File:Thyroid_scan.jpg
License: CC BY-NC 3.0

By using iodine-123, this individual's thyroid can be viewed using special cameras. By using radioactive isotopes doctors are able to diagnose otherwise unseen illnesses.

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Credit: Dnn87
Source: http://en.wikipedia.org/wiki/File:CsCrystals.JPG
License: CC BY-NC 3.0

Cesium is a common isotope used to identify and treat diseases [Figure2]

  • Radioactive substances, such as Iodine, Palladium or Cesium are some of the common isotopes that are used in nuclear medicine to help in the diagnosis and treatment of various diseases. These radioactive isotopes are combined with well known compounds to form compounds known as radiopharmaceuticals. When these compounds enter the human body, they localize in locations or regions depending upon which compound is used. By using gamma ray cameras to capture the radiation given off by the radiopharmaceuticals, doctors are able to see the extent of a disease in the body without having to rely on opening and collecting samples from the body.
  • Radioactive isotopes are unstable nucleuses that undergo radioactive decay via the three possible forms of radioactive emission: gamma ray emission and/or alpha or beta decay.
  • Gamma emission: When a nucleus is in an excited state, photons can be emitted to drop the nucleus to a lower energy level. Gamma decay usually produced along with alpha and/or beta decay is produced after the previously mentioned decay(s) occur. This is a result of after alpha or beta decay leaving the nucleus in an excited state.
  • Alpha decay: The excess energy in the nucleus is emitted via release of an alpha particle. The alpha particle released consists of 2 protons and 2 neutrons, which is a Helium atom. This process is best represented by the balanced equation

\begin{align*}(Z, N) \rightarrow \alpha + (Z -2, N - 2)\end{align*}

Therefore, it can easily be seen that the process of alpha decay causes a transformation of the original element into a new one.

  • Beta decay: Beta decay is a result of the nucleus having too many neutrons. This is identified by the ratio of \begin{align*}\frac{N}{Z}\end{align*}, where \begin{align*}N\end{align*} is the number of neutrons and \begin{align*}Z\end{align*} is the number of protons. The optimal ratio is reached via emission of a beta particle. Beta particles are either electrons \begin{align*}(\beta^-)\end{align*} or positrons \begin{align*}(\beta^+)\end{align*} that are emitted from an atomic nucleus. In each of the possible decay process, a transformation of the parent atom into a new element is achieved.

\begin{align*}& \beta^-: (Z, N) \rightarrow (Z + 1, N -1) + e^- + \overline{v} \\ & \beta^+: (Z, N) \rightarrow (Z - 1, N +1) + e^+ + v\end{align*}

where \begin{align*}v\end{align*} and \begin{align*}\overline{v}\end{align*} are neutrinos and antineutrinos respectively.

  • Learn more about the various forms of decay: 


  • See the different penetration capabilities of each form of decay: 


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Using the information provided above, answer the following questions.

  1. Which of the three forms of decay emit particles or waves that have the greatest penetration ability?
  2. If someone were to direct a stream of alpha particles at you, would your t-shirt be enough to protect you, or would you need a really thick wall of lead?
  3. What type of decay would cause uranium-238 to transform into thorium-234?

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Image Attributions

  1. [1]^ Credit: Myohan; Source: http://commons.wikimedia.org/wiki/File:Thyroid_scan.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: Dnn87; Source: http://en.wikipedia.org/wiki/File:CsCrystals.JPG; License: CC BY-NC 3.0

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