What is it, that makes an absorber a good radiation absorber?

[Explorer_99]

In a nuclear power plant, absorbers are used to control the fusion process.
To be more precise, absorbers slow down the process.
Some materials qualify as absorbers, one I remember is silver.

What I would like to know is:

What makes a material a good radiation absorber?

How can the "strength" of absorption be measured?

What quality - possibly in terms of (e.g. atomic or molecular) structure - of a
material makes one absorber material a stronger absorber than another?

How can absorbers and their strength be found by computation?

 

[mfb]

Absorber for which type of radiation?
Alpha radiation? Everything works, just make sure you get some milligram/cm^2.
Beta radiation? Everything works, nuclei with few protons are better to reduce secondary radiation. You need more material here.
Gamma radiation? Absorbers with many protons per nucleus are better.
Neutron radiation? Absorbers with small nuclei are better.

In addition, you might have to consider other issues - stability, chemical reactions with other stuff, toxicity, price, ...

How can the "strength" of absorption be measured?

Depends on the radiation type. For photons (and high-energetic electrons), you can measure the mean free path length. For other high-energetic particles, you can measure the mean energy loss per distance.

How can absorbers and their strength be found by computation?

Depends on the radiation type.

In a nuclear power plant, absorbers are used to control the fusion process.

Fusion? Fission?

 

[Explorer_99]

Thank you, mfb!
With my question, I'm looking mainly at Beta and Gamma Radiation and then at Alpha Radiation.
You are right, it should have said fission not fusion.
I was hoping to find one material that's best for all three.
With what you've said so far it sure looks more involved.

 

[mfb]

Well, any good absorber for gamma radiation will absorb beta and alpha radiation (with typical nuclear energies, a few MeV), too.

 

[QuantumPion]

Based on the first part of your question, it sounds like you are asking about neutron absorption, as it is neutron absorption which is important for controlling the fission process.

What determines whether a material is good at absorbing neutrons is its neutron absorption cross section. Boron, silver, indium, cadmium, hafnium, and gadolinium are the primary materials used for neutron absorption because they have very high cross sections compared to most other materials. The neutron absorption cross section of a material depends on the structure of its nucleus and are a function of neutron energy. Generally these values are measured and quantified in laboratory. If you want to know how to estimate nuclear cross sections mathematically, you might want to go ask in the high energy/nuclear physics forum.

 

[Explorer_99]

Thank you, QuantumPion!
I like your explanation. When you say "neutron absorption cross section" then I take it to mean cross section at the absorber atom level, not thickness of absorber material.
I envision a sponge-like function for neutrons in an absorber. There may be more room for extra neutrons in some absorbers than in others. And I would presume that there will be a point of saturation when no additional neutrons can be accommodated. In this case absorption will degrade as saturation is approached, I take it. I imagine there is an absorption function per absorber type, which I'd like to learn more about. Please correct me, when I'm wrong.
In this process we are looking at particles, coming from a fission process which need to go somewhere. I phrased it "need" to go, because that's what I understand. I wonder, whether this need really exists. As a gedanken-experiment, I could easily accept a particle transformation, say annihilation. I'm sure not the first to think about alternatives and I would love to get to know the latest advances in this respect.

 

[mfb]

"more room"?

And I would presume that there will be a point of saturation when no additional neutrons can be accommodated.

I don't think you will reach that level soon with any reasonable setup. In addition, some atoms which caught neutrons might undergo beta decay, and be able to accumulate even more neutrons afterwards. In some stars, that is a common process to generate heavy nuclei.

The cross-section is a bit like a target for darts: If the neutrons hit a disk of that size, they get absorbed in that nucleus. That is a very classical picture and not really true, but it gives the correct prediction in terms of absorbed neutrons. The cross-section depends on the neutron energy, and usually increases with lower energy. Therefore, you want to slow neutrons first.

 

[QuantumPion]

Thank you, QuantumPion!
I like your explanation. When you say "neutron absorption cross section" then I take it to mean cross section at the absorber atom level, not thickness of absorber material.

Correct.

I envision a sponge-like function for neutrons in an absorber. There may be more room for extra neutrons in some absorbers than in others. And I would presume that there will be a point of saturation when no additional neutrons can be accommodated. In this case absorption will degrade as saturation is approached, I take it.

This analogy is not accurate. Nuclei are not like sponges, they do not have a capacity for absorbing and releasing neutrons in that way. A better analogy would be to chemistry. The elements which are one electron short of a full orbital, fluorine, chlorine, etc, react vigorously because they very strongly want that last electron. But once they get it, they are stable (e.g. Teflon)

 

[Explorer_99]

Thank you, mfb!
When you say: "some atoms which caught neutrons might undergo beta decay" I wonder about "some" and "might". Are there some that are likely to and others that will never? Do we know, which is which? And the "might" is it more a surprise or do we know the exact conditions that will start their decay and the strength of this decay?

 

[Explorer_99]

Thank you, QuantumPion!
I'm aware of the chemistry picture you're drawing. However , I'm not aware of an analog structure in the kernel. That looks like the perfect answer to my question on absorbing neutrons. Then anybody can look at the structure of atoms and easily identify good, better and best absorbers. Great. Can you point me to a source that explains this in more detail?

 

[Astronuc]

Thank you, QuantumPion!
I'm aware of the chemistry picture you're drawing. However , I'm not aware of an analog structure in the kernel. That looks like the perfect answer to my question on absorbing neutrons. Then anybody can look at the structure of atoms and easily identify good, better and best absorbers. Great. Can you point me to a source that explains this in more detail?

Various organizations have established/mapped a chart of nuclides, which provide basic data on decay modes and neutron absorption.

For example, http://www.nndc.bnl.gov/chart/ - see Half-life, Decay Mode, and σ(n,γ). Some nuclides decay by electron capture rather than positron emission. σ(n,γ) give the thermal neutron cross-section.

For more details, one can use - http://www.nndc.bnl.gov/sigma/

The most effective 'absorber' of radiations, β, γ, are dense, high Z materials. The β and γ radiations interact primarily with atomic electrons in matter. Alpha particles are stopped by thin materials, but beta and gamma are more penetrating into matter.

Most source neutrons are fast neutrons, so they must be slowed down to resonance or thermal energies. Hydrogen, deuterium, beryllium, boron-11 and carbon are effective moderators.

Boron-10 is a common neutron absorber in light water reactors. PWR reactors uses a compound of silver-indium-cadmium in control rods to absorb neutrons. Hf and Dy have also been used. In fuel, a boron coating in the form of ZrB2 may be used, otherwise, gadolinia (Gd-155, 157) or erbia are used as 'burable' absorbers, in the sense they are depleted with time.

 

[mfb]

Thank you, mfb!
When you say: "some atoms which caught neutrons might undergo beta decay" I wonder about "some" and "might". Are there some that are likely to and others that will never? Do we know, which is which? And the "might" is it more a surprise or do we know the exact conditions that will start their decay and the strength of this decay?

To study that in detail, you need the isotopic composition of the material. For each isotope, you can check what neutron absorption does in the chart of nuclides: Is the resulting nucleus stable? Is it radioactive - and if yes, which decay modes and half life does it have?

 

[wizwom]

OK, for ion radiation, like alphas and fission products, the ability to be ionized per linear length determines the "strength" of the slowing the particle receives. So you want dense materials with a high ionization energy.

For Beta particles, which are essentially fast electrons, the electron needs to interact with the electron shells, OR shed energy by cherenkov radiation. So, you want a dense electron cloud and a low speed of light.

For gamma radiation, you have pair production for the highest energies (so you need to stop the beta-, the beta+ will annihilate with any electron), compton scattering (which you need a dense electron cloud, preferably not localized) and excitation of inner electrons, so high Z.

For Neutrons, you need to get into quantum mechanics of the nucleus to predict which neuclides will be strong absorbers.