Why does attenuation coefficient decrease with increased energy?

[amylase]

Hello. Not sure if this is the right board to post. This is not homework. I am just self-teaching quantum physics as I read a book on the topic of X-ray Physics. It has about 30 chapters in total and so far I find myself having about 3 questions per chapter. Hope you guys don't mind me asking here.

Why does attenuation coefficient decrease (general trend of decrease) with increased energy?

Diagram at the very end of this link shows attenuation coefficient (y axis) and energy in keV on X-axis.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/Physics/attenuationCoef.htm

Also diagrams here:

http://imageshack.us/photo/my-images/259/p13922459f57a1064518b4c.png/

My understanding is attenuation coefficient be it linear or mass, is a description of how good a material is at attenuating incidence beam.

For example linear attenuation coefficient is the log of the ratio of incidence intensity against exit intensity, per unit length of the material.
Mass attenuation coefficient is that divided by density, and is the same regardless of what state the substance is in (eg. vapour, liquid, ice all have same mass attenuation coefficient).

What I don't understand therefore is, how does increasing the intensity of incidence beam, decrease attenuation coefficient? I thought the coefficient is more like a constant for a given material, and should therefore be independent of the intensity of the incidence beam.

The question I am asking is: why does attenuation coefficient go down with increased energy? why isn't the coefficient a constant for that material regardless of energy intensity of the incidence beam?

Thanks.

 

[Drakkith]

As the energy of each photon increases, the wavelength decreases and the chances of a certain thickness of material absorbing it also decreases.

Also, I'm not sure how the term "intensity" is being used. It looks like from your post it refers to the energy of each photon, but I have seen it also used as an increase in the number of photons per second through a material. If you increase the intensity of the beam by simply increasing the number of photons, then the coefficient stays the same, as each photon has the same energy as it did before.

 

[amylase]

As the energy of each photon increases, the wavelength decreases and the chances of a certain thickness of material absorbing it also decreases.

Also, I'm not sure how the term "intensity" is being used. It looks like from your post it refers to the energy of each photon, but I have seen it also used as an increase in the number of photons per second through a material. If you increase the intensity of the beam by simply increasing the number of photons, then the coefficient stays the same, as each photon has the same energy as it did before.

Hi Drakkith, thanks for the reply.

Intensity the way I intended was energy content per photon.

Going back to your first sentence, Why would shorter wavelength decrease absorption?
I would have thought shorter wavelength is even more easily absorbed... :S

 

[smartCH]

Hi,

Intensity = number of photons (flux) is not the same as the photon energy.

The attenuation coefficient is the sum of different interactions taking place in matter. The probability of these interactions to occur depend on the photon energy as well as the atomic number of the absorber.
In these typical energy regions shown in the graph, several interactions are concurrent to each other:
- photo electric effect
- compton scattering
- pair production (only for energies > 1022 keV)

In the range you were looking at, the photo electric decreases while the compton scattering becomes more dominant. The overall result is still that the attenuation decreases. It will reach a minimum but will slightly increase again at higher energies as the pair production will become important. At very high energies photonuclear reactions and e.m. cascades will be dominant.

If I may suggest you an excellent book on radiation and radiation detection:
Knoll, Radiation Detection and Measurement

 

[amylase]

In the range you were looking at, the photo electric decreases while the compton scattering becomes more dominant. The overall result is still that the attenuation decreases. It will reach a minimum but will slightly increase again at higher energies as the pair production will become important. At very high energies photonuclear reactions and e.m. cascades will be dominant.

If I may suggest you an excellent book on radiation and radiation detection:
Knoll, Radiation Detection and Measurement

Hi smartCH

Thanks for clarifying a few things and recommended book.

I still don't understand though, why does attenuation decrease as intensity increases?

I can understand at low energy the effect is mainly from photoelectric effect, at medium energy (of hundrends keV) the effect is from Compton scatter, at high energy (thousands keV) we are looking at pair production, and at very high energy (MeV) we get photodisintegration. As energy increases, attenuation by photoelectric effect decreases, while Compton scattering increases. Then as energy further increases Compton scattering also decreases etc such that overall the total attenuation is on a downward trend the energy increases. I see this phenomenon, but my question is why? For example why does photoelectric effect decrease when energy increase? Why does Compton scattering decrease when we further increase energy? when energy is big enough, we get pair production. But why does attenuation from pair production decrease when we further increase intensity?

Would appreciate any comments from anyone. Thanks.

 

[smartCH]

Well, the probability for the photoelectric effect is difficult to calculate. Most is known from experimental results which show a relationship of about Z⁴/E^-3, not considering here the specific edges coming from the different shell levels.

For the Compton scattering cross section a relationship has been derived from QED (Klein-Nishina). For pair production there should be a similar explanation possible with QED.

 

[Drakkith]

This is mostly a guess, but if you look at Quantum Mechanics the wavelength of a particle gets smaller as the energy increases, similar to the wavelength of a photon getting smaller as it's energy increases as well. Perhaps that's the reason?