Part I - Poor Little Kid 穷孩子

Types of interaction¶

Photoelectric, Compton, Rayleigh, Pair and Photonuclear¶

Here, I introduce photon interactions with matter the way I believe most accessible and understandable. The various interaction types are competing possibilities, so they should be introduced side by side — rather than introduced under separate headings. It is important to appreciate that a single photon interacting with matter typically gives rise to a sea of progeny (or secondary) particles of different type and energy — chances are we will see photoelectric absorption, Compton scattering, Rayleigh scattering and pair production many times over. That is, from a single photon only.

These pages will also demonstrate all four ways of expression interaction probability: cross section (barns/atom), mass attenuation coefficient ( $\mu/\rho$ ), linear attenuation coefficient ( $\mu$ ) and mean free path ( $\lambda$ ). The units are convertible to and fro each other. Numerical work examples are given. Differences in meaning are highlighted.

Photoneutron production¶

In the slides, photoneutron production (and photonuclear reaction in general) is put into context alongside photoelectric absorption, Compton scattering, Rayleigh scattering and pair production. The two sides are often discussed in isolation by textbooks; sometimes given attention by separate communities. Yet, when a photon enters a material, it stands a chance to undergo one of the possibilities: photoelectric absorption, Compton scattering, Rayleigh scattering, pair production or photonuclear reaction -- if the photon exceeds the threshold.

( $\gamma$ ,neutron) production thresholds are not that low after all. It is 2.22 MeV for ²H, 1.67 MeV for ⁹Be, 4.95 MeV for ¹³C, 10.55 MeV for ¹⁴N. The list goes on. We can either calculate the thresholds (Q values) ourselves or look them up from Brookhaven’s QCalc.