Why is it beneficial to have multiple types of shielding for gamma exposure?

Question

20 inches of lead in more effective than 20 inches of water, but the combination of 10 inches of each is more effective than either.

Why?

But, I'll bet that it works best only if the water is on the inside.

True

Hint: Which travels farther, light shining straight through a piece of glass, or light shining at an angle through the same thickness of glass.

http://en.wikipedia.org/wiki/Linear_attenuation_coefficient

Answer 1

Really interesting question.

>>>>>>>>>>>>>
Jack, it looks like you really want someone to solve this thing with all your hints. It is a great question.

There are three types of ways matter can slow and stop a gamma ray: Photoelectric effect, Comptom scattering and pair production.

Pair production is the primary way high-energy gamma rays (i.e. 10 Mev) are stopped. They transfer all of their energy to an electron-positron pair and when the positron anihilates itself, you get two 0.5Mev gamma rays. (The .5 Mev gamma rays are stopped fairly easily by lead) This interaction doesn't happen too often and you need alot of material for this to occur. The denser (i.e. lead) the better.

Comptom scattering is the primary mechanism for slowing down mid range gamma rays (2 - 10 mev). In this case, an electron is ejected, and a new gamma ray goes off with lower energy in a different direction. It also has the advantage of changing the gamma rays path so it will spend more time in the media. Comptom scattering is fairly independent of atomic mass. However, the Z/A ratio, which is fairly constant for most elements, does have an effect. Turns out Hydrogen, which is unique with a Z/A ratio of 1, is great for Comptom scattering. See http://www.fas.org/sgp/othergov/doe/lanl/lib-www/la-pubs/00326397.pdf , pg 39.

The photoelectric effect is great for stopping the less energetic gamma rays. Denser elements, with more electrons per unit volume, are the best for killing off these less energetic photons.

So, you put your ten inches of water to slow down and scatter the gamma rays (so they spend more time in the media, i.e., lead and water), and put your lead on the outside to absorb the leftovers.

Note: With really energetic gamma rays, you just want a lot of lead between you and the source.

Answer 2

Gamma rays are a form of electomagnetic wave as is light. The same principles apply to all electromagnet radiation, so I'll use light as an example as it is easier to visualize.

Imagine three types of material - say a mirror, glass, and wood.

Light can not pass through a mirror. The reason being that the all those free electrons on the thin sheet of silver in the mirror reflect visible light back. However, a mirror will not reflect say x-rays. They pass through the electron layer of the silver. This is because the distance between the electrons and the silver atom is such that only certain wave lengths of radiation are reflected.

Next, light passes through glass. The reason for this is that the atoms/molecules in the glass are arranged in a latice and are equally spaced. The wavelength of visibile light is such that it can pass through millions of atoms in the glass without ever actually hitting one. However, an x-ray can't pass through glass as easily, since it's wavelength causes it to eventually hit the atoms. This is also why when you hold clear glass at an angle, it acts as a mirror. Since the angle now causes the distance between atoms/molecules in the glass to be the percise distance where the light will always strike an atom/molecule and be deflected, like a mirror.

Finally, In the case of wood, no light passes through. This is because wood is make of so many different molecules and with various spacings that there is no wavelength which allows light to pass between all the molecules without hitting a molecule. It also explains color in objects in that certain wavelengths of light are relected at the surface and others make it some distance before being stopped. X-rays only make it through wood if they have enough energy to force molecules out of their way. Enough wood would stop them as easily as light.

Ok... so now to your example.

Gamma rays have various wavelengths (although much, much shorter than light). Lead has a heavy nucleus, a lot of electrons, and as a solid the atoms are fairly close. The atoms are also not evenly spaced (such as in glass) such that most wavelengths of radiation are likely to hit a lead atom. Water obviously isn't as good because the molecules are further apart, lighter, and fairly evenly spaced apart -- alowing most wavelengths of radiation to pass through without even hitting an atom or to knock a light water molecule out of the way.

However, lead isn't perfect. There are some wavelengths of which will allow the eletromagnetic wave to miss 'most' of the lead atoms and gamma rays have enough energy to push the rest out of the way. Thus, some wavelengths of radiation will pass through -- just like glass will allows light through but not x-rays. However, the gamma rays which make it through the lead will tend to be all of similiar wavelengths.

When this radition hits another substance, such as water, it is likely that these wavelengths will not be those which pass by the water molecules as easily.

Answer 3

Being that I'm not an expert in particle physics, my guess would be that the lead filters the particles moving/vibrating in a certain direction while the water filters out the particles moving/vibrating in another direction.

I bet my wife, the metallurgist, would know.