Experiment of the week: neutron activation used for material analysis

Welcome to SciTech’s new “Experiment of the Week” section. The science staff at The Tartan has noticed that a lot of exciting science occurs at CMU and around the world that no one hears about. Our hard-working staffwriters hope to change that by featuring exciting experiments or research techniques that you’re probably not familiar with, from zero-gravity research aboard the “Vomit Comet” to NASA’s Gravity Probe B to the Mars Science Laboratory’s neutron activation. In fact, neutron activation is the first topic in this series.

Deep in the bowels of Doherty Hall there is a small locked room. In this room is a barrel that contains a small amount of plutonium. Plutonium may be the stuff of nuclear bombs, but it can also be used for neutron activation. This elegant physics technique is based on the idea that when atoms in any material are artificially made radioactive, photons of specific energies are released. Since the photons that an element is likely to emit are already known, neutron activation can be used to determine the composition of an unknown material.

So how does it work? To make a material radioactive, a source of neutrons is needed. Since neutrons have no electric charge, they are hard to obtain, but physicists have developed an ingenious way around this problem. When plutonium decays into uranium, it releases a type of radioactivity called an alpha particle. If this alpha particle is absorbed by the element beryllium, the two combine to make an isotope of carbon. And as this unstable carbon isotope decays, it releases a high-energy neutron. We have our neutron source!

Unfortunately, high-energy neutrons are fairly useless, since they are moving too fast for a target material to absorb them. To slow them down, the neutrons are passed through graphite, water, or wax, all of which have lots of protons. The neutrons bounce around among the protons, eventually slowing down enough to be absorbed.

When a target nucleus absorbs these “thermal” neutrons, the target becomes radioactive and decays, emitting photons of various energies in the process. Scientists have become very good at gauging the energy of photons and, using the measured “spectrum” that the radioactive target emits, they can predict the composition of the target material.

So what is neutron activation used for? NASA is going to use it to look for water on Mars. The new rover planned for 2009 will carry a neutron activation apparatus built by the Russian Federal Space Agency to look for hydrogen. The device will be used to detect water, which will be indicated by high hydrogen readings.

In 1979, Luis Alvarez and his son Walter Alvarez used neutron activation to claim that a 10-km asteroid had collided with Earth 65 million years ago. The element iridium is very rare on Earth, but the Alvarezes had detected unusually large quantities of the metal in the Cretaceous-Tertiary boundary of the Earth’s crust. When Alvarez realized that the high concentration could be due to a huge dust cloud from an asteroid impact, he predicted that iridium would be found in similar concentrations across the globe. Experiments at over 100 sites across the globe confirmed this, and the cause of dinosaur extinction was found!