What happens during this workshop?

Students calibrate and use scintillation detectors to interpret the energy spectrum of a radioactive source. Students learn about coincidence measurements and how they are used in Positron-Emission-Tomography (PET) to locate a positron source (Na-22).

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

• positrons, annihilation, E=mc2, rest mass of electrons / positrons E=511 keV, positron emitters ("beta-plus decay")
• energy conservation, momentum conservation
• scintillators and photomultipliers (optional), energy spectra of radioactive sources, Compton scattering (optional)

Key concepts

1. Anti-particle: for every type of particle, there exists a corresponding type of anti-particle, which has the same mass but the opposite electric charge. The positron is the anti-particle of the electron.
2. Annihilation: Annihilation is the process that occurs when a particle interacts with its respective antiparticle. When an electron interacts with a positron they transform into two photons.
3. Energy and momentum conservation: In all processes in a closed system, the total energy and total momentum are constant.

Helpful material

• Watch this video by Imperial College London to find out: How does a PET scan work?
• Quiz: You can use the quiz questions below to find out if your students are well prepared for this experiment:

1. What is a positron? Select one:

a) The anti-particle of the proton.
b) A proton with a higher mass.
c) Positrons don’t exist.
d) The anti-particle of the electron.

2. What happens when a positron interacts with an electron? Select one:

a) They transform into a single photon.
b) They transform into a pair of photons.
c) They simply disappear.
d) They repel each other.

3. Imagine an object at rest, which then explodes into two parts. What happens? Select one:

a) The two parts split in opposite directions.
b) The two parts move in the same direction.
c) The two parts move apart at 90 degrees to each other.

Summary and link to CERN physics: 

PET scanners make use of the special properties of annihilation photons. By measuring annihilation photons, it is possible to locate a positron source in the human body. Therefore, radioactively marked substances help find tumours.

CERN's expertise focuses on particle accelerators, detectors and computing. However, many important diagnostic and therapeutic techniques have been developed from fundamental research results. One example of medical applications of particle physics (link is external) is Positron-Emission-Tomography.