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WHY DO SCIENTISTS USE THE ADVANCED LIGHT SOURCE
--A TOOL THE SIZE OF A FOOTBALL FIELD-- TO LEARN ABOUT STRUCTURES AS TINY AS ATOMS AND MOLECULES ? |
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What's going on inside this building?
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Why is the ALS so large? To produce light of the wavelengths and brightness that scientists want, the ALS designers had to create a large machine. Its largest component, the storage ring, has a diameter two-thirds the length of a football field. The storage ring is a tubular vacuum chamber made to:
For more information, see ALS Components. Fact: Light produced by machines that operate like the ALS is called "synchrotron radiation." |
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Why is light from the ALS a useful tool? The ALS produces light in the far ultraviolet and soft x-ray regions of the electromagnetic spectrum. This light has wavelengths from 0.0001 micrometer to 0.1 micrometer. Which object is closest in length to a micrometer
Since the ALS produces x rays, why couldn't scientists just use an x-ray tube as a dentist does, instead of the ALS? Here are some facts:
X-ray tubes are found in the laboratory as well as in dental offices and continue to be used for many experiments. But the ALS has advantages over x-ray tubes when it comes to investigating most materials. An obvious advantage is the length of time the x-ray beam lasts. A beam from the ALS continues for hours, while the beam from an x-ray tube is often limited. A scientist could not use the light generated by an x-ray tube for experiments that take much time, for example, scanning the surface of a material for impurities. Also, x rays from the ALS have the right energies to interact with many electrons in lighter atoms, which make up most common materials. Interaction must take place; otherwise, an experiment will not yield information. X-ray tubes produce photons with higher energy than those from the ALS--an advantage for imaging objects made of very heavy elements such as gold (Au). But these energetic photons would pass right through materials made up of light atoms and not interact at all. The greatest advantage of the ALS is its brightness. You could compare an x-ray beam from the ALS with a laser and one from an x-ray tube with a floodlight. While they both might deliver an equal number of photons per second, those from the ALS are concentrated on a small area, whereas those from the x-ray tube are widely scattered. A higher concentration of photons on a smaller area allows scientists to increase the specificity of their experiments. They can study smaller objects or choose more specific photon energies (down to tenths of electron volts) to study a very specific target. |
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Activity If you could use the ALS or an x-ray tube as your source of photons, which would you choose to solve the following problems? 1. I have a material suspected to be contaminated with small amounts
of copper (Cu). If it is contaminated, I need to know how the copper is
distributed in the material. Which x-ray source should I use, and why
2. I need photons with an energy of exactly 285.5 eV (electron volts).
Photons with this energy are absorbed by an aromatic group in a fiber
I am studying, and I want to make an image of the fiber's cross section.
Which x-ray source should I use, and why 3. Archaeologists have found a sealed urn and suspect that it contains
gold (Au) coins. Which x-ray source would I use to determine the presence
of the coins, and why 4. I'm a curator for a large art museum, and I need to know if this
urn is genuine before we purchase it from the art dealer. All of the art
historians say that the style of the urn and the designs painted on it
are from the same time period and location as that which the dealer claims.
However, I have reason to believe that the urn might be a reproduction
from another time period. The art conservation literature states that
a distinguishing characteristic from this time period is the presence
of manganese (Mn) in a layer of pigment. Which x-ray source should I use
to determine whether there is manganese in the pigment, and why 5. I need an x-ray source to determine how quickly a chemical compound
found in automobile exhaust is converted to a component of smog. It is
suspected that this compound is converted at very fast rates--in picoseconds.
(Picoseconds are trillionths of a second.) Which source of x rays should
I use to detect the change 6. I fell down, and I heard a crack. My arm hurts a LOT. Which x-ray
source should be used to tell if my arm is broken, and why |
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