Electron microprobes are mystery solvers: They can help determine the origin of meteorites, find lost trade routes in the South Pacific and reconstruct movements of soldiers on 19th-century battlefields.

The University of Arizona Electron Microprobe Laboratory recently celebrated the arrival of a second electron microprobe. At first glance, it's an almost magical device – it can tell you the elemental composition of just about anything you stick inside it.

The probe awarded to the UA cost more than $1 million and was funded by grants from NASA and the National Science Foundation. Assembled by CAMECA, the French company that in 1956 produced the first electron microprobe, and painstakingly transported to the U.S., its arrival makes the UA one of a select few U.S. universities to have two working microprobes.

"It's very unusual for a school to actually get two microprobes," said Kenneth Domanik of the UA's Lunar and Planetary Lab. "Usually they're dealt out very sparingly. Most prestigious universities want to have one, and so to actually get two microprobes – a new state-of-the-art one plus one that actually has been kept up so well and has been used so much that it still can be running for several years yet – is a tribute to all of the work that the faculty and students have done here."

Said Michael Drake, regents' professor of cosmochemistry and geochemistry and head of the UA's Lunar and Planetary Lab: "The quality of the science being done was so high that it justified having a second machine."

A major benefit to researchers of using an electron microprobe is that it is non-destructive – it provides a way to analyze a material to know what elements it is made of without breaking it, boring a hole in it or chipping a piece off.

"As far as I know, there is no microprobe in the country that had the usage rate that our one has," Drake said, referring to the older probe, CAMECA SX50, which for the last 10 years has averaged 6,000 hours of usage per year.

Planetary scientists frequently use the UA microprobe to study the origin of meteorites. The probe reveals the types of atoms that make up a meteorite, giving scientists clues as to how it might have formed – whether it is a piece of an asteroid composed of dust particles from the early solar system, or a chunk of Mars that was blasted into space by another meteorite's impact, or something else entirely.

The second microprobe will see every bit as much action as the first. The new CAMECA SX100 probe can look at trace elements down to 10 parts per million, and with higher spatial resolution than the old machine. It also is equipped with five spectrometers, one more than the old probe. Spectrometers are instruments that detect the many signals that result from bombarding materials with a high-energy beam of electrons – which is exactly what an electron microprobe does.

Bullet shells and pottery shards

Much sought after in the sciences, the applications of electron microprobes extend to other, sometimes surprising disciplines.

One of the diverse applications of electron microscopy at the UA involved a 19th-century battleground. Using data collected with the UA's CAMECA SX50, researchers were able to reconstruct movements of individual soldiers on the battlefield of Little Bighorn.

"In those days you couldn't make rifles precisely enough that they would all look the same, and so each individual shell casing from a rifle would have a different impact pattern," said Drake. "Because each one was different, each one was a fingerprint for an individual soldier."

Researchers used the UA microprobe to match the microscopic impact patterns of shell casings from the battlefield with the rifles that fired them. By looking at where the characteristic shell casings had been discharged, they were able to retrace the steps of individual soldiers around the battlefield, generating important clues about the sequence of events during the battle.

Another unusual case for the UA's microprobe involved the finding of strange pottery in the Marquesas Islands in the South Pacific that closely resembled Japanese pottery from the 6th century C.E. With no evidence of Japan having far-ranging fleets at that time, the origin of the pottery is a mystery. One way to learn more is to use an electron microprobe to generate an atomic-scale map of the material, showing if the clay contains the same elements in the same proportions as the clay used to make 6^th century Japanese pottery. Determining the origin of historical trade items can lead to new hypotheses about the routes by which they came.

How does an electron microprobe work?

An electron is a tiny bundle of negatively charged energy smaller than an atom. Electrons can be represented mathematically as both a particle and as a wave, but to understand how an electron microprobe works, it is easiest to think of them as particles.

Electrons normally orbit the positively charged nuclei (centers) of atoms in what is called an electron shell – a bit like planets orbiting the sun. Atoms are surrounded by layers of electron shells containing up to several electrons in each shell. The number of shells, and of electrons in the shells, varies depending on the type of atom. Electrons in inner shells are closer to the nucleus and have lower energy than those in outer shells.

The microprobe works by firing a high-speed beam of electrons at a sample. The electrons hit the sample usually with enough energy to penetrate the outer shells and knock electrons in the inner shells out of orbit (you can think of rogue planets fired in from outside the solar system).

Because the positive charge of the nucleus draws negatively charged electrons in as close as possible (opposite charges attract), an outer shell electron will move in to take the place of a displaced inner shell electron. The extra electrons from the beam and the displaced electrons are conducted through the sample as electrical energy.

As the replacement electron changes from an outer to an inner shell, it loses energy. This energy is released in the form of an X-ray photon – a particle of X-ray light.

"The number of X-ray photons is proportional to the concentration of the element that the X-ray corresponds to in the sample," said Drake. In other words, more photons means there's more of element X present in the sample. Researchers use this data to know exactly how much of different kinds of elements make up a sample.

A crystal placed in the microprobe reflects the X-rays into an X-ray detector that counts the photons and determines the type of element by comparing the wavelength of X-rays produced by the sample with known values of standards kept at the lab.

The information is fed from the microprobe to a computer that displays it to the researcher. The entire process takes only a matter of minutes.

Backscattering, atomic mapping and secondary electrons

The interaction of an electron beam with electrons in a sample produces several useful signals besides X-rays. Some of the more useful of these are a process known as backscattering and the production of secondary electrons.

Sometimes, the combined energy of negatively charged electrons in the sample atoms overwhelms the energy of the incoming electrons in the beam (like charges repel one another). Instead of penetrating the shells and displacing an inner-shell electron, the beam electrons are turned around like a sling-shot, and reemerge from the surface of the sample – that is to say, they bounce back – usually at rather high speeds.

Detectors on the roof of the sample chamber pick up the signals of the rejected electrons and use the information to generate a nano-scale map of the sample surface.

The number of backscattered electrons strongly depends on the number of electrons in the sample atoms, which varies depending on the type of atom. Thus, the number of backscattered electrons helps scientists to determine what type of atom the beam hit. Among other things, backscattered electrons are used to produce a real-time "chemical map" of the sample showing where different elements are concentrated.

Other useful signals come when electrons transfer energy to outer shell electrons. Sometimes this extra energy is just enough to cause outer shell electrons to break free of orbit and head off in a different direction. Because these so-called secondary electrons come from outer shells that are close to the surface of a sample, their trajectories indicate the surface topography of the sample. Secondary electrons can be used to construct high-resolution images of the sample surface.