Activity 2.2 - Interaction with matter

• To introduce the students into the field of X-ray imaging
• To interpret an X-ray image
• To design experiments to determine how the degree of absorption of radiation depends on the material used and on the thickness of the absorbing material
• To define the law of attenuation and the concept Half Value Layer (HVL) and apply these to human tissues

• A computer with internet connection
• gamma source e.g. Cobalt-60. This isotope emits photons with energies 1332 keV and 1173 keV. In comparison to an X-ray generator, cobalt-60 produces energies comparable to a 1250 keV X-ray system.
• a device to detect the radiation e.g. a Geiger-Mueller tube or a radiation sensor with computer and data collection software (e.g. Coach 6)
• a set of absorption plates of the same thickness made from different materials e.g. Plexiglas, aluminium, steel, lead.
• a set of absorption plates of the same thickness made from lead.

Activity 1 engages students into the field of X-ray images and their interpretations. Let the students answer the given questions and discuss their answers with the whole class. Act as facilitator of the discussion.

In activity 2 students explore the process of attenuation. They have to design experiments to answer two research questions:
a) How does the degree of absorption depend on the material?
b) How does the degree of absorption depend on the thickness of absorber?

Experiment A.

In order to demonstrate how the degree of absorption depends on the material, 2 mm plates of Plexiglas, aluminium, steel and lead can be used.

Exemplary data: Co-60 gamma radiation, measurement time: t = 10 s

The measurements show that substances having a higher atomic number (“heavy elements”) are better able to attenuate gamma radiation than substances with a lower number. It can be seen that lead shields gamma radiation the best but it is able to penetrate even a 2 mm layer of lead but only a small part of the radiation is attenuated.

Experiment B.

The thicker the material, the more the gamma radiation is attenuated. This effect may be demonstrated by using lead plates, since the thickness of lead needed for a half-value layer can be easily achieved in an experiment.

Exemplary data: Co-60 gamma radiation; t = 10 s

The resulting graph (blue: measurement, red: function-fit) shows the exponential decrease.
The half-value layer for lead when used to shield Co-60 gamma radiation can be found. During a nuclear decay, Co-60 emits two consecutive photons having energies of 1332 keV and 1173 keV respectively. This makes it impossible to determine an exact curve describing the absorption behaviour of lead or to accurately determine the half-value layer.

The HVL for lead is approximately 12 mm.

Activity 3 explains the law of attenuation and gives definition of the concept Half Value Layer.

Activity 4 encourages students to apply the learned concepts to human tissues.

Calculated HVLs for soft tissue and bone at 30 and 60 keV are:

• Which material is the best absorber of gamma radiation?
• What is the function, which describes the relationship between the intensity of the radiation and the thickness of the absorber for lead?
• What is the thickness of lead for which radiation is reduced to 50% of the original radiation?
• What would you expect if you would use X-rays instead of gamma rays?