Problem 74 - Chemistry: An Atoms-Focused Approach

Question

Several isotopes of arsenic are used in medical imaging. Which isotope, \(^{72}\) As or \(^{77}\) As, is more likely to be useful for PET imaging?

Step-by-Step Solution

Answer

Answer: \(^{72}\)As

Step 1: Understand PET imaging and positron emission

Positron Emission Tomography (PET) is a nuclear medicine imaging technique that makes use of positron-emitting radionuclides. During the decay process, a positron is emitted, which is the antiparticle of an electron. When a positron encounters an electron, both particles annihilate, producing gamma photons. These gamma photons can then be detected by the PET scanner for imaging purposes.

Step 2: Determine the decay modes of the isotopes

To determine which isotope would be more suitable for PET imaging, we need to find the decay modes for \(^{72}\)As and \(^{77}\)As. A reference such as a nuclear data table can be used to find this information. You may access online resources or your textbook for this purpose.

Step 3: Compare the decay modes of the isotopes

From the nuclear data table, we can see that: - \(^{72}\)As decays by beta-plus decay (positron emission) with a decay energy of 6.61 MeV and a half-life of 26 hours. - \(^{77}\)As decays by electron capture with a decay energy of 1.50 MeV and a half-life of 38.8 hours.

Step 4: Select the more suitable isotope for PET imaging

Based on the decay modes, \(^{72}\)As is the more suitable isotope for PET imaging, as it undergoes beta-plus decay (positron emission), which is required for PET imaging. On the other hand, \(^{77}\)As decays by electron capture, which does not involve the emission of positrons and therefore, is not suitable for PET imaging. In conclusion, \(^{72}\)As would be more likely to be useful for PET imaging, since it undergoes positron emission, which is the required decay mode for PET scan detection.