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Physics Professor Probes the Structure of the Atom’s Heart

Study Funded by the U.S. Department of Energy for $5M over a 25-year Period

Partha Chowdhury in the lab Photo by Edwin L. Aguirre
Prof. Partha Chowdhury will use the neutron detector arrays that were designed and built in the university’s Radiation Laboratory on North Campus to study the structure of the atom’s nucleus.

By Edwin L. Aguirre

Ever since the nucleus of the atom was discovered in 1911, scientists have split, fused and smashed nuclei together to unravel its internal structure. Among the numerous researchers worldwide who are trying to better understand the inner workings and underlying physics of the atom’s heart is Prof. Partha Chowdhury of the Department of Physics and Applied Physics.

“The atomic nuclei form the core of all visible matter in the universe,” says Chowdhury. “They are also the fuel that powers the stars. Almost all ordinary matter – 99.95 percent – is in the form of protons and neutrons in the center of the atom. Our fundamental research probes the structure of the nuclei, which are held together in a delicate balance between strong subatomic forces that bind them and electromagnetic forces that want to tear them apart.”

This research has been funded continuously by the Office of Science of the U.S. Department of Energy (DOE) since 1995, when Chowdhury joined the UMass Lowell faculty. The latest grant renewal extends the funding to 2020 and brings the total support to $5.05 million. Chowdhury is the principal investigator (PI) for the project; his previous co-PI on the proposal was physics Prof. Christopher (Kim) Lister, who is now an emeritus faculty. Physics Asst. Prof. Andrew Rogers is the new co-PI for the current cycle.

Partha Chowdhury and Kim Lister in the lab Photo by Edwin L. Aguirre
Chowdhury with Prof. Christopher (Kim) Lister in the Radiation Lab.

While the DOE funding is for basic science, the advances in gamma-ray detector technology and detection techniques that occur as part of the team’s research can lead to future practical applications.

“For example, they can help improve imaging in nuclear medicine and homeland security,” says Chowdhury. “They can also help with calculations of the patient’s dosage for diagnostic and therapeutic medical isotopes, as well as for cancer patients undergoing radiation treatment.”

A Powerful ‘Microscope’

Chowdhury explains that nuclei are miniscule – about one-millionth the size of an average atom.

“The atoms themselves are about a nanometer, or one-millionth of a millimeter in size, so we can say that we study objects that are a million times smaller than what anyone else on our campus is studying,” he notes.

Andy Rogers in the lab Photo by Edwin L. Aguirre
Asst. Prof. Andrew Rogers poses next to the Radiation Lab’s high-resolution gamma-ray detector array, which is used by physics faculty and students for accelerator and reactor experiments.

To get a clearer picture of the structure of atomic nuclei, scientists use an accelerator that shoots a pencil-thin beam of heavy, electrically charged particles traveling at about a tenth of the speed of light onto a stationary target, like a thin metal foil, and then focus a powerful “microscope” on the byproducts of the collision.

“The violence of the collision produces short-lived, unstable exotic nuclei in excited quantum states,” says Chowdhury. “We are able to ‘see’ the nuclei we create through the gamma rays they emit, which get picked up by our array of sensitive detectors. Through this high-resolution gamma-ray spectroscopy technique, we can tell whether our nuclei resemble a sphere, a football or a pear. While astronomers point their telescope arrays out toward the heavens, we point our microscope arrays in toward the center of the atom.”

Although the atomic nucleus was discovered more than a century ago, he says scientists are still a long way from a detailed understanding of its complexity.

“Our research pushes at the edges of current knowledge, toward super-heavy nuclei on one hand and neutron- and proton-rich nuclei at the threshold of stability on the other,” says Chowdhury. “What limits the number of chemical elements in the universe? Do the rules of ordinary stable matter apply to exotic neutron-proton combinations? How are heavy elements formed in the stars? These are just some of the questions we are trying to answer.” 

Some of the team’s key experiments are performed at Argonne National Laboratory in Illinois and the National Superconducting Cyclotron Laboratory at Michigan State University. Preparatory tests are conducted at UML campus facilities.

“The unique combination of a 5.5-megavolt Van de Graaff accelerator and a 1-megawatt research reactor at the university’s Radiation Laboratory serves a critical role for staging test runs for detector and hardware development as well as providing in-house, hands-on training for both our graduate and undergraduate students.”

To date, the research has produced about 150 papers in peer-reviewed journals. Ten postdoctoral researchers, about 17 graduate students and more than 30 undergraduate students have worked on the project over the years.

“We now have, on average, one to two postdocs, a half dozen grad students and a similar number of undergrads in our research group,” says Chowdhury. “With new faculty joining and taking charge, our research directions are expanding, the reputation of the UMass Lowell group has grown and the future of nuclear structure and nuclear astrophysics research on campus looks very promising.”

He adds: “Prof. Rogers is spearheading the lab’s efforts in nuclear astrophysics. Under the current grant renewal, he will study unstable proton-rich nuclei that have direct relevance to astrophysics and the physics of X-ray bursts, which occur in X-ray binary star systems.”