Mushy Zones, X-ray Vision & a Quantum Identity Crisis

New members of the physics department are doing cutting-edge experimental research

By Allison Chisolm

There’s a buzz in the physics labs over in Haberlin, and it’s not just the computer hard drives. In the past five years, the physics department has hired three tenure-track faculty. Atomic physicist Timothy Roach arrived from Harvard in 1999 and established a state-of-the-art laboratory to study wave properties of cooled atoms. Matthew Koss, a condensed matter materials physicist from Rensselaer Polytechnic Institute, brought his NASA-funded dendritic growth laboratory to Haberlin in 2000. The newest member of the department, Tom Narita, is an astrophysicist from the Harvard-Smithsonian Center for Astrophysics; he designs and builds X-ray detectors.

The recent hires join a department of three theorists (Randy Ross in astrophysics, Janine Shertzer in atomic physics, and Robert Garvey in cosmology) and one experimentalist (De-Ping Yang in condensed matter).

Meandering through the Mushy Zone
It’s not a solid. It’s not a liquid. It’s in-between, and it’s called “the mushy zone.” While it may not sound very scientific, the mushy zone is the transition phase of solidifying materials that Assistant Professor Matthew Koss wants to understand. He’s studied this zone both on Earth and in space, in experiments conducted (from ground control) on the space shuttle in 1994, 1996 and 1997.

As materials like iron, steel or aluminum move from a liquid to a solid state, they produce dendrites—finely branched crystals resembling tiny trees. Think snowflakes: dendrites that form as water freezes. In his experiments, Koss works with succinonitrile (SCN), a transparent, organic plastic material with growth characteristics similar to metals and alloys. Understanding a material’s microstructure and the dynamics of dendrite growth, he explains, will help future metallurgists create desired properties in industrial materials, making them hard, soft, springy or stiff as needed for a particular application.

Why study dendrites in outer space? The gravitational forces on Earth contribute enough heat transfer to affect the growth process of dendrites, as they release energy when they solidify. In apparent microgravity conditions, Koss and other researchers on his team found that dendrites grow more slowly and their tips grow larger than on Earth, providing an important clue as to how metals
solidify.

Data collected from the three space flights is “absolutely a scientific gold mine,” says Koss. “I can’t see the end of interesting issues to explore.” But despite the project’s potential, funding for continued research in orbit has been discontinued due to budget cuts at NASA. However, back on Earth, Koss is examining how pressure changes affect dendrite growth to learn more about the dynamic process of the mushy zone.

In his three years at Holy Cross, Koss has already worked with a dozen students. His line of inquiry clearly remains appealing to future physicists who wish to enter “the mushy zone.”

His X-ray Vision Finds Dying Stars and Black Holes
Assistant Professor Tom Narita can’t be accused of being starry-eyed. That’s because the stars he studies aren’t always visible. He develops and uses X-ray telescopes to detect the radiation given off by dying stars across our galaxy and beyond.

While Narita has remained grounded in Worcester since 2001, his work can’t take place on Earth. That’s because our atmosphere, which protects us from the effects of intense radiation from outer space, blocks the X-rays. To get an X-ray picture of distant galaxies or stars, the telescopes have to work above our atmosphere. One option is to launch a telescope into space, as with the X-ray satellite Chandra, launched in 1999. A less-expensive method involves sending a computer-controlled telescope on a balloon up about 120,000 feet to the edge of our atmosphere to scan the universe. NASA has specialized launch sites for these high-altitude balloons in Texas and New Mexico.

“It’s often mistaken for a UFO,” says Narita, as the balloon is 100 meters across—the size of a football field—and is made of a reflective white material filled with helium gas. The balloon only lasts for a day or two before it loses altitude and parachutes back to Earth. Then a NASA truck has to chase after it to recover the million-dollar telescope. If it lands in water, it’s ruined. If it lands in the mountains, a special helicopter must retrieve it.

Because X-rays penetrate matter much more than ordinary visible light, the detector must be made with a dense semiconductor. Narita and Francis Niestemski ’03 have worked to develop the electronic circuit that will measure the energy of X-rays and gamma-rays received by a massive, 2,500 square-centimeter cadmium zinc telluride (CZT) detector in an imaging X-ray telescope called EXITE3. EXITE3 is scheduled to be launched on a balloon in 2005.

Once collected, interpreting X-ray data requires a process of deduction. Variations in brightness or X-rays of certain energy can be detected. Narita studies binary stars, which offer a useful method of locating the most peculiar objects in the sky.

“Most stars occur in pairs,” Narita explains. “Our sun is an exception.” When one of the pair dies (runs out of fuel), its core collapses and compresses to become a neutron star. If the compression is extremely powerful, it may collapse to a “black hole”—where the gravitational pull into that core is so strong, no light can escape.

“We know black holes exist because we see stars that are clearly circling an unseen companion,” says Narita. The X-ray telescope can detect radiation from that invisible companion star as it strips and sucks the gas from its binary partner. And “there are plenty of X-ray sources to look at,” he adds, noting our galaxy has hundreds of them.

Slow Down and Wave
Freeze it, zap it, trap it and let it fall. That's the approach Assistant Professor Timothy Roach takes to slow an atom to a crawl and then make it dance. When atoms slow down, they behave more like waves than particles. This dual nature has been predicted since the early 1900s, but the technology to demonstrate it has only recently caught up with the theory.

"We’re still investigating the fundamental properties of quantum physics," Roach says. Physicists demonstrated the wave/particle duality of electrons 75 years ago, but atoms, due to their greater mass, are harder to slow down. They have to be cooled to a much greater degree. The 1997 and 2001 Nobel prizes in physics were awarded for the discovery and application of methods to slow down atoms using laser light.

As Roach explains, at room temperature, a hydrogen atom will zoom around at 5,000 miles per hour. Freeze it to within an inch of its life—or a few millionths of a degree above absolute zero (that’s minus 273 degrees Celsius)—and it will slow to less than one mile per hour.

Roach chose to work with the element rubidium (Rb) because it has a simple internal structure, which makes it easy for laser light to slow the atom. This same structure makes rubidium a key component of certain atomic clocks.

Students working in his lab use a custom-built “magneto-optic trap.” This apparatus chills the atoms, traps them in a laser beam tuned to rubidium's specific wavelength, and then scatters them off a magnetic surface to view their diffraction patterns—in other words, to see them behave like waves. The magnetic surface in this case is a naked Zip® disk, programmed with a pattern of alternating magnetic polarities. The rubidium atoms actually bounce off the magnetic field generated just above the zip disk's surface, doing the curious dance of quantum physics.

“It’s unusual to find such sophisticated experimental laboratories at a small liberal arts college,” says department chair Janine Shertzer. “The research opportunities that we can offer undergraduates is an important factor in recruiting new students. With over 30 majors, Holy Cross has one of the largest physics departments among the liberal arts colleges. Of the past two graduating classes, over 75 percent of the physics majors entered graduate programs in physics-related fields; others pursued careers in physics education and patent law.

“Our graduates are using their physics degrees and that is how we measure the success of our program,” says Shertzer.

Allison Chisolm is a free-lance writer from Worcester.