The Fate of Photon Energy Upon Interaction with Matter

The energy of a photon when it strikes matter can be altered through various mechanisms. These interactions, ranging from the photoelectric effect to photonuclear reactions, influence the overall behavior and energy of the photon. Understanding these processes is crucial for various fields, including radiation health, physics, and materials science.

Key Mechanisms of Photon Interaction

The energy behavior of a photon upon interaction with matter can be summarized through several primary mechanisms:

1. Photoelectric Effect

The photoelectric effect is one of the most well-known interaction mechanisms. In this process, the photon is consumed, and a tightly bound electron is ejected. The kinetic energy of the ejected electron is the difference between the energy of the photon and the binding energy of the electron. This interaction predominantly occurs in the lower energy region, particularly with K shell electrons, though some also apply to higher shell electrons.

2. Compton Scatter

The Compton Scatter is a "glance" or "rebound" interaction involving a more loosely bound electron. Some refer to this as "incoherent scattering." Here, the incident photon bounces off the electron, leading to a scattered photon with lower energy. The energy loss in the Compton Scatter event is mathematically related to the scatter angle. The maximum energy transfer occurs at a 180-degree backscatter angle, limiting photon energy to approximately half an electron mass (~255 KeV).

3. Pair Production

In pair production, the photon interacts with the positively charged nucleus, converting into two elementary particles: an electron (positron) and an anti-electron (negatron). This interaction requires photons with at least ~1.02 MeV of energy. The particles then carry the remaining energy minus 1.02 MeV. The negatron eventually joins the electron population, while the positron annihilates with an electron, releasing its energy.

Other Interaction Mechanisms

Besides the primary interactions, there are several less frequent mechanisms:

4. Rayleigh Scattering

Rayleigh scattering, also referred to as "coherent scattering," refers to a process where the photon changes direction but retains its energy and wavelength. This interaction is common for low-energy photons and is generally not a concern for radiation health.

5. Thomson Scattering

Thomson scattering involves the scattering of a photon by a free electron. Despite the photon disappearing, it is re-emitted in another direction.

6. Photonuclear Reaction

Photonuclear reactions occur when the photon interacts with the positively charged nucleus, leading to the production of different particles such as alpha particles, neutrons, protons, or even initiating fission events. This process typically involves higher energy photons.

Total Interaction Probability

The following graph illustrates the total interaction probability of the various interaction types, highlighting the dominance of the photoelectric and Compton effects in lower and mid-energy regions, respectively. Pair production becomes increasingly likely with higher photon energies.

[Graph of total interaction probability]