TOPIC 3: Shielding

Let’s look in the radiation protection technique of Shielding.

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Shielding is the protection concept of placing something between you and the radiation source. to minimize the amount of radiation dose that you receive.  Again, think of the fire example.  If the fire got hot, it wasn’t as hot when standing behind someone or something. The same is true with radiation.  Place something between you and the radiation source and you will get a lower dose.  Everyone thinks of lead as a good shield.  Lead is a very dense material providing excellent shielding against gamma and x-rays. But some radiations are quite weak and do not require much shielding.  Any shielding will provide some sort of protection. Lead is a great shield, but not very practical if you have to carry it around with you.

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The “shield” for the gamma and X-rays are the electrons that are orbiting around the nucleus in an atom.  The higher the “Z” number the more protons in the nucleus, which also means there are more orbital electrons.  So, with more electrons present in the path of the gamma or X-ray, the chance the gamma or x-ray will interact with an electron increases.  This is called “shielding”.  We can also call it attenuation because we attenuate, or reduce, the number of gamma or x-rays that interact with us.

Different types of radiation require different shielding.  Is it possible to shield betas using lead?  Yes.  But is cost effective? No! We have already learned about bremsstrahlung and that shielding high-energy betas with heavy metals causes the formation of bremsstrahlung X-rays.  Additional dense shielding would then be required to shield those X-rays.  To eliminate the problem, shield the betas with plastic.  This slows down the electrons without releasing the energy as a x-ray.

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As the size of the particle decreases, denser material is required to shield.

○  For alpha particles, paper is sufficient for shielding.

○  For beta particles, paper can shield some “soft” betas, but something slightly thicker is required.  Plastic works best since it can stop the betas but not create x-rays.

○  For gamma-rays and x-rays, lead being a high “Z” material, is one of the densest material that is readily available.

○  For neutrons, the reaction of the shielding material to the neutrons and the creation of secondary interactions make the monitoring and protection from neutrons more difficult.  Concrete and paraffin are generally used as the shielding materials against neutron emissions because the number of electrons is immaterial as the neutron isn’t repelled by the electron. Concrete and paraffin slow down the neutrons without interacting with it.

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For a given reduction in radiation intensity, the amount of shielding required is dependent on the shielding material’s density.  The higher the “Z” number, the more effectively it will shield gamma and X-rays.  Since lead is more dense than concrete, it will take less thickness to provide the same shielding protection as concrete.  It may take thicker quantities of concrete to provide the same amount of shielding as lead, but it may be less expensive to do so. Whatever the material used to shield, the amount that cuts the radiation level to 50% of its original level is considered a half-value layer.

Key points:

• The half-value layer (HVL) is the thickness of a specific material that reduces radiation intensity to one-half of its original value

ohalf-value layer Applies to a specific radionuclide source or photon energy

ohalf-value layer Applies to a specific shielding material and density

Just like we used the concept of half-life to describe when half of the activity of a source has been reduced by physical decay, we can use the concept of half-value layer (HVL) to describe the situation when half of the intensity of a gamma or x-ray beam has been reduced. The use of HVL allows rapid calculation and practical deployment of the approximate shielding necessary to reduce exposure to gamma or x-rays. If one HVL will reduce the gamma radiation intensity to one-half, then what intensity remains after 2 HVLs?  Please don’t say zero.  It will reduce it to one-fourth (half, then half of a half) and so on (1/2, 1/4, 1/8, 1/16, 1/32…).

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Lets do an example using Half-Value Layers.  We have a Cobalt-60 source emitting 500 mR/hr.  We want to put in shielding to reduce that to 125 mR/hr.  If it takes 100 inches of a particular density of substance to reduce the radiation level from 500 to 250 mR/hour, by adding another 100 inches, it will reduce the level from 250 to 125 mR/hour.

So, to calculate the number of HVLs we need to get to 125 mR/hr, we take 500 X ½ X ½ = 125 mR/hour.  So, it takes 2 Half Value Layers of our substance.  2 X 100 inches = 200 inches of our substance.

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Similarly, just like the half value layer, we can use the concept of tenth-value layer (TVL) to describe the situation when the intensity of a gamma or x-ray beam needs to be reduced to one-tenth of its initial intensity. Again, the use of TVL allows rapid calculation and practical deployment of the approximate shielding necessary to reduce personnel exposure to gamma or x-rays. If one tenth-value layer  will reduce the gamma radiation intensity to one-tenth, then 2 tenth value layers will reduce it to one-hundredth (one-tenth of one-tenth) and so on (1/10, 1/100, 1/1,000, 1/10,000, 1/100,000…or 0.1, 0.01, 0.001, 0.0001, 0.00001). TVL is more useful for very intense gamma radiation beams.

Key points:

• The tenth value layer (TVL) is the thickness of a specific material that reduces radiation intensity to one-tenth (or 0.1) of its value

otenth value layer Applies to a specific radionuclide source or photon energy

otenth value layer Applies to a specific shielding material and density

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Here are a problem to solve:

A Cs-137 source emits 300 mR/hr at 4 feet from the source. The TVL for Cs-137 using concrete is 6.2 inches. If we put about 2 feet of concrete between the source and the measurement point, what will be the approximate new reduced exposure rate?

2 feet = 24 inches

24 inches / 6.2 inches per TVL = 3.87 TVLs so it is close enough to assume 4 TVLs

4 TVLs reduces the intensity to one-tenth four times in succession

0.1 x 0.1 x 0.1 x 0.1 = 0.0001

So, finally we have, 0.0001 x 300 mR/hr = 0.03 mR/hr which is the approximate new reduced exposure rate.

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Anything that can absorb the energy from a gamma-ray, x-ray, beta, or alpha can be used to shield that radionuclide. For convenience, the amount of shielding to reduce the exposure to one-half its strength was determined for various radionuclides using different materials.  The amount of shielding to reduce the exposure to one-tenth was also determined.

Using the Half Value Layer and the Tenth Value Layer, we can quickly determine how thick our shielding would need to be to keep workers safe.