MoNA

From Michigan to California, undergraduate physics students are working on components of a new detector to enhance research at MSU’s National Superconducting Cyclotron Laboratory (NSCL). They expect to link their individual contributions in a Modular Neutron Array (MoNA) as early as next fall.

The recently completed coupling of the two cyclotrons at the NSCL significantly enhances the capabilities to study very neutron-rich exotic nuclei. These nuclei are not only important to understand the structure and interactions of nuclei but they also play a key role in the formation of the elements on the earth, the solar system and the whole universe. Exploring these exotic nuclei involves the efficient detection of neutrons which get stripped off during reactions in a target.

“We knew researchers wanted this kind of equipment,” says physics professor Michael Thoennessen. Collaborators at NSCL user group meetings said a high-efficiency neutron detector would enhance the new capabilities at the upgraded cyclotron. One of the collaborators, James Brown, assistant professor of physics at Millikin University in Decatur, Illinois, suggested that students could build the components. “It would be a wonderful project for a senior lab,” Thoennessen agreed.

NSCL visiting assistant professor Thomas Baumann turned the idea into a proposal that linked nine colleges and universities (see sidebar). The National Science Foundation (NSF) funded it last fall with $1 million. “NSF told us this project is a showcase for major research universities involving smaller schools in first rate research,” Thoennessen recounts. Each school will build one layer of the array; MSU will build the support structure and assemble the electronics and data acquisition for the detector.

Each of the nine layers will have 16 detector blocks stacked 1.6 meters high. The blocks are 2-meter-long plastic scintillators fitted on each end with photo-multiplier tubes that translate light into electrical pulses. To make the detector blocks, students will couple the ends of each scintillator to photo-multiplier tubes and install electronic components that measure activity within the detector.

And there’s a lot of activity, though it’s hard to detect. Neutrons moving at about half the speed of light strike the scintillator atoms causing their nuclei to bounce through the block. Along the way they excite other atoms, which emit a faint light. The photo-multipliers amplify the light by a factor of 30 million and record precisely when they see it. By comparing the difference in arrival time at each end of the detector block, researchers can determine within a few centimeters the point at which the light was emitted.

MoNA will increase the neutron detection efficiency by a factor of seven for single-neutron events and a factor of 50 for two-neutron events, according to simulations performed by Baumann: “We’ll be able to study very neutron-rich nuclei that can only be produced with small intensities and therefore are out of reach with our present neutron detection capability.”

Although the detector will support highly sophisticated research into the nature of atoms, the layers themselves aren’t very complicated, Thoennessen says. “Technically, not much can go wrong, even with widely separated groups working on it,” he adds. “We’ll establish stringent limits on tolerances so everything fits together.”

The greatest challenges are organizational. Bryan Luther, associate professor of physics at Concordia College in Moorhead, Minnesota, is using his sabbatical at the NSCL to coordinate the project. Students and their advisors will gather at MSU in late February for a workshop to learn more about the detector and how researchers will use it. After they’ve built their layers, they’ll return to fit the components together, test their product, and participate in experiments using the array. “We’ll select sophomores and juniors so they’ll be around to see MoNA work,” Thoennessen says.

Consortium members

Ball State University, Muncie, Indiana

Central Michigan University, Mount Pleasant, Michigan

Concordia College, Moorhead, Minnesota

Florida State University, Tallahassee, Florida

Hope College, Holland, Michigan

Indiana University at South Bend

Millikin University, Decatur, Illinois

Western Michigan University, Kalamazoo, Michigan

Westmont College, Santa Barbara, California