I Can Flexible Bonds Provide More Effective Neutron Radiation Shielding?

[Mzzed]

Hi all,

I've only just started studying nuclear physics so forgive me if this question makes no sense. I've read that the way neutron shielding works (in simple terms)is that the neutrons act as billiard balls by knocking into the shielding material atoms and being scattered like this until all their energy is eventually dissapated. Apparently this is the reason neutron shielding must be made from dense, thick shielding.

My question is: can a material be made from some sort of 'spring-like' molecules that absorb the large energies by slowing the neutrons down a bit more gradually instead of just acting like billiard balls. By spring-like molecules I mean a molecule that has slightly flexible bonds and if a collision occured, the bonds may be able to flex?

I have a feeling that if anything, the energy of most neutron radiation would be too large for any bonds to withstand.

 

[Drakkith]

Well, all molecular bonds are "flexible" to some degree. That's how things like sound waves can travel through a material. The compression and expansion of the lattice couldn't happen if the molecular bonds were entirely rigid. I don't know if a material having more flexible molecular bonds makes a good neutron absorber though, but I'd guess that the transfer of energy from the neutron to the ion is probably independent of the flexibility of the molecular bonds. Interesting question, and not something I'd thought of before.

 

[Vanadium 50]

Apparently this is the reason neutron shielding must be made from dense, thick shielding.

Except that it's not. Neutron shielding is often not dense at all: paraffin and borax for example.

 

[Mzzed]

Ah ok I never considered shielding based on converting radiation from one type into another which is easier to deal with

 

[mfb]

The time and energy scales don't work out. A 1 MeV neutron moves at 14,000 km/s. The nuclear interaction happens within 1 fm, or 10^-22 s, and the energy transfer is in the hundreds of keV range. Molecular bonds don't play a significant role at this time and energy scale. They can become important once the recoil energy is comparable to chemical energies, but then the neutron is nearly thermal already.

Hydrogen is great because it gets the most energy in an elastic collision. In addition, it can absorb neutrons without getting radioactive (for ¹H).

 

[Mzzed]

Thankyou! Yeah I had a feeling that would be the case.

 

[rare earth]

Just to add, the above discussion focused on the MeV range and lower energies (typical for nuclear reactors), where hydrogen is a good moderator for neutrons. However, at higher energies, tens/hundreds of MeVs and above, thick and dense shields are needed (steel for example), where the neutrons undergo spallation reactions, leading to the creation of secondary neutrons. Many of the secondaries will be in the MeV range, and then hydrogen can be used to slow the neutrons down. However, some of the secondaries can still have energies which can match the primary beam energy, which leads to the need for thick dense shields. At an accelerator-driven neutron source several meters of steel can be used to shield the target area, followed by a hydrogen containing material (typically concrete, even though it doesn't contain much hydrogen it's rather cheap).

 

[Vanadium 50]

Steel is almost never used as a shield against high energy neutrons. There is a dip in the cross section at a particular energy, so once neutrons hit that energy the iron is transparent and the neutrons stream out.

 

[rare earth]

Steel is almost never used as a shield against high energy neutrons. There is a dip in the cross section at a particular energy, so once neutrons hit that energy the iron is transparent and the neutrons stream out.

Steel is indeed not the best material for shielding high-energy neutrons, for the reason you mention. That being said, it is extensively used in high-power spallation sources in the target shielding. SINQ, SNS, and the upcoming ESS (just to name a few), all have significant steel shielding which is several meters thick in all cases. Concrete is then added after the steel shielding to help mitigate the streaming through the Fe windows.

 

[Vanadium 50]

But is the steel there to stop the neutrons, or is it there to stop the secondaries?

 

[rare earth]

The need for several meters thick of steel shielding is driven by the neutron dose requirements on the outer surface of the shield, related to the deeply penetrating high-energy neutrons and the secondary neutrons produced in the shield. Secondary gamma-rays also contribute to the dose, but the bulk thickness is due to neutron attenuation. Steel may not be the best choice from a neutron shielding point of view, but other factors related to engineering design or cost, for example, result in steel being used for this purpose in the end.