What occurs at the end of a positron's range in the medium during pair production?

At the end of a positron's range in the medium, annihilation occurs, resulting in the production of two gamma photons, not protons. This process takes place when the positron, which is the antimatter counterpart of the electron, encounters an electron in the medium. During this encounter, they annihilate each other, and their mass is converted into energy, which is emitted in the form of two gamma photons moving in opposite directions. This is a fundamental interaction in particle physics and illustrates the principles of mass-energy equivalence.

The other choices are not representative of what happens during pair production involving a positron. The emission of a gamma photon refers to the outcome of the annihilation event itself, but specifically, it is the annihilation of the positron and electron that results in gamma photons, not their direct emission from a pair production process. The creation of a free electron is not relevant in this context, as free electrons are involved in different interactions. Lastly, absorption of energy by the nucleus typically relates to interactions like photonuclear reactions and does not describe the process that occurs between positrons and electrons in medium. Understanding the specific interactions and outcomes is crucial for comprehending the nuances of particle behavior in medical dosimetry and radiation physics.