What's going on inside this building?

Why is the ALS so large?

The storage ring is a tubular vacuum chamber made to:

• Hold an electron beam travelling through it at nearly the speed of light.

• Maintain the high energy of the electron beam.

For more information, see ALS Components.

Fact: Light produced by machines that operate like the ALS is called "synchrotron radiation."

ALS floor diagram.

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

• a submarine
• an ant
• the diameter of a human hair
• a virus

Reason 1

Light from the ALS can penetrate materials. Just as your dentist uses x rays to see inside your gums, scientists use the light from the ALS to look inside materials.

Dental x ray.

Why do dental x rays penetrate your gums and not your teeth

Reason 2

It is impossible to "see" anything smaller than the wavelength of the light you are using. So to study atoms or molecules, you must use light waves about their size or smaller. The ALS produces light with wavelengths about the sizes of atoms, molecules, chemical bonds, and the distances between atomic planes in crystals.

Atoms, chemical bonds, and the distances between atomic planes in crystals all measure a few angstroms, about the same as the wavelengths of light from the ALS.

Reason 3

Photons (or particles of light) from the ALS have the right energies to interact with many electrons in atoms.

The diagram below shows what can happen when light shines on a material.

Electrons may absorb the photons' energy and escape from the material (as shown at the top of the diagram). Scientists in the late 19th century observed this phenomenon and called it the photoelectric effect.

OR

Electrons in the atoms of the material may absorb the photons' energy and jump to a higher energy level. When an electron does this, its atom is said to be "excited." Soon the electron loses the extra energy and returns to a lower level--a process called de-excitation. Often this lost energy escapes from the atom in the form of photons. Excitation and de-excitation are shown at the bottom of the diagram.

Also, you may observe no interaction. Can you guess why?

Scientists at the ALS detect and analyze the escaping electrons or photons to learn more about the structure and behavior of atoms and the materials in which they are found. Analyses like these serve many purposes, for example:

• Detecting the presence and quantity of trace elements from their unique emission patterns (see Selenium: A Window On Wetlands).
• Providing images that show the structure of materials (see Kevlar--the Wonder Material).

Reason 4

The ALSis one of America's brightest soft x-ray sources available for researchers. The x rays produced here are a hundred million times brighter than those from the most powerful x-ray tube, the source used in a dentist's machine. High brightness means that the x rays are highly concentrated. Many x-ray photons per second can be directed onto a tiny area of a material.

(left) Brighter; (right) not so bright.

Reason 5

Besides their brightness, x rays from the ALS have other useful characteristics such as tunability, near-coherence, pulsed nature, and polarization.

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:

 

ALS X Rays	X Rays Produced by an X-Ray Tube
Penetrate matter	Penetrate matter
Have wavelengths near the size of atoms and molecules	Have wavelengths smaller than the sizes of many atoms and molecules
Have the right energies to interact with electrons in light atoms such as carbon and oxygen	Have energies too high to interact with many electrons in light atoms but can interact with those in heavy atoms such as gold
ALS "soft" x-rays are brighter than any other x-ray source in America; similar to the light from a laser	Are not bright enough for high-resolution experiments; more like a floodlight than a laser
Are produced in tiny pulses constantly for 6 hours or more	Can be produced in a single short burst (i.e., dental x-ray tube)

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.

Activity

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