Why does the effectiveness of low-Z shielding increase with photon energy?

In summary, the reason for needing less concrete to obtain the same lead equivalent for photon energy 500 keV compared to 200 keV is due to the properties of lead, specifically its increased cross-section per unit mass at higher photon energies. This can be seen by comparing the cross-section per unit mass of lead to that of concrete, which decreases as the photon energy increases. This is reflected in regulations for shieldings in PET departments, where a lower lead/concrete ratio is recommended for higher energy photon sources.
  • #1
taffer33
4
0
Concrete for example - you need less concrete to obtain the same lead equivalent for photon energy 500 keV than for 200 keV. What is the reason for this?
 
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  • #2
Ok, I did some thinking ;) Is it because for low Z materials, photons with higher energy interacts with absorber almost by Compton Effect only? And Compton is independant of atomic number.
 
  • #3
Can one show some calculations or data to support the assertion that "you need less concrete to obtain the same lead equivalent for photon energy 500 keV than for 200 keV." The mass attenuation coefficient continually decreases as a function of gamma energy, although the mass energy-absorption coefficient increases slightly between 0.2 to 0.5 MeV. But this is misleading, since the 500 keV gamma will scatter to a lower energy, and that photon will scatter, and so on.

https://physics.nist.gov/PhysRefData/XrayMassCoef/ComTab/concrete.html
 
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  • #4
https://archive.org/details/jresv38n6p665
it's in this article for example

I asked this question as I was reading about shieldings in PET departments, where they suggest lead/concrete ratio 12-15, while ratio for 150 keV X-Ray is 80... (These are example regulations from my country).
 
  • #5
taffer33 said:
https://archive.org/details/jresv38n6p665
it's in this article for example

I asked this question as I was reading about shieldings in PET departments, where they suggest lead/concrete ratio 12-15, while ratio for 150 keV X-Ray is 80... (These are example regulations from my country).
It's a property of lead rather than a property of the concrete.
Have a look at the cross-section per unit mass of lead below, and compare it to the same for concrete:

z82.gif


concrete.gif


Lead has an elemental absorption edge at around 88keV, so the cross-section for lead is massively increased when the photon energy, as the photon energy is further increased, the photo-ionisation cross-section due to that particular energy-level decreases, becoming more comparable to that of the concrete.
 

1. Why is low-Z shielding more effective at higher photon energies?

Low-Z shielding materials, such as aluminum or plastic, contain atoms with fewer protons and electrons compared to high-Z materials like lead. This means that at higher photon energies, the interaction between photons and low-Z atoms is less likely to produce secondary particles, making the shielding more effective.

2. How does the thickness of low-Z shielding affect its effectiveness at higher photon energies?

The effectiveness of low-Z shielding at higher photon energies is directly related to its thickness. Thicker shielding allows for more interactions between photons and low-Z atoms, resulting in a greater reduction of photon energy and a higher level of protection.

3. Can low-Z shielding be used for all types of radiation?

No, low-Z shielding is most effective for protecting against photon radiation, such as X-rays and gamma rays. It is less effective for stopping charged particles, such as alpha and beta particles, which require high-Z materials for effective shielding.

4. How does the composition of a low-Z shielding material affect its effectiveness?

The composition of a low-Z shielding material, specifically the type and amount of low-Z atoms present, can greatly impact its effectiveness at higher photon energies. Materials with a higher concentration of low-Z atoms, such as plastic, are more effective than those with a lower concentration, like wood.

5. Is low-Z shielding the most effective option for all radiation protection?

No, low-Z shielding is not always the most effective option for radiation protection. The type and energy of the radiation, as well as the specific application, should be considered when selecting the most appropriate shielding material. Low-Z shielding may be more cost-effective and suitable for certain situations, but high-Z materials may be necessary for maximum protection.

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