What happens during this workshop?

Students learn more about high-energy photons and their interaction with matter. They understand how the X-ray machine works and observe the absorption of high-energy photons in different material. Students use pixel detectors to observe background radiation and to take radiographs.

Before taking part in this workshop, students should already know about:

• the properties of photons, especially the link between different photon energies and the electromagnetic spectrum
• how X-rays machines work, Bremsstrahlung (optional)
• semiconductor detectors (pixel detectors)
• energy levels of electrons in atoms (optional)

Key concepts

1. Energy spectrum: Different physics phenomena such as visible light, X-rays, and radio waves can be described as electromagnetic waves with different wavelengths. In particle physics this corresponds to photons with different energies.
2. Absorption of photons: Electrons can absorb photons. If they absorb a photon, they take up their energy. If the absorbed energy is high enough, the electron will leave its atom (ionisation).

Helpful material

• Watch this video by the educational YouTube channel Kurzgesagt to find out What is light
• Quiz: You can use the quiz questions below to find out if your students are well prepared for this experiment:

1. Which kinds of particle can describe X-rays?

   a) Proton
   b) Electron
   c) Photon
   d) X-rays cannot be described by particles

2. Can electrons interact with photons?

   a) No, electrons and photons don’t interact.
   b) Yes, electrons can emit photons.
   c) Yes, electrons can absorb photons.
   d) Yes, electrons repel photons.

Summary and link to CERN physics: 

High-energy photons produced by X-ray machines interact with matter (dominant effect: photo effect), as a result some of them are absorbed and we see a “shadow” behind strongly absorbing objects. Depending on their energy, photons interact differently with matter. For X-Ray photons, the electron density of the material correlates with the absorption rate. With the pixel detector one can measure particles all the time even without turning on the X-ray source – the so called background radiation (see cloud chambers).

Pixel detectors are used in the tracking systems of  LHC experiments, such as ATLAS. They can precisely measure the track of an electrically charged particle. Within the Medipix collaboration at CERN, several applications of this technology have been studied, for example, in medical imaging.