The elements of life
The origin of the elements that make life on Earth possible is a daunting puzzle. Tracing back the source of the calcium in your bones goes far beyond the milk in your cereal; the iron in your blood didn’t come from the last steak you ate; and the oxygen in the air you breathe was around long before there were any trees on Earth. Some elements were produced just minutes after the Big Bang, while others came much later as a result of nuclear reactions within stars and their eventual demise.
A star’s energy is fuelled by the nuclear reactions inside its core, which produce heavier elements from lighter ones throughout the star’s lifespan. When a star runs out of fuel, it dies, releasing the elements it produced back into space. Some stars go out with a bang like a supernova, while others die more peacefully. “Out of these elements, new stars and planets are born, leading ultimately to human beings on planet Earth in the here and now,” said Alan Chen, associate professor of nuclear astrophysics. “In other words, we are literally made of stardust, or less poetically, of nuclear waste.”
The rate at which these nuclear reactions occur influences how much energy the star generates, the types and quantities of elements it produces, and how long the star will live. High-mass stars use up their nuclear fuel and burn out more quickly than low-mass stars. The aim of Alan’s research is to recreate these stellar nuclear reactions in a laboratory setting. “I use particle accelerators to produce beams of ions that are then collided with a target in order to reproduce the reactions that are happening in the stars,” he explained. “We then use detectors to measure the properties of the reaction products.” These measurements provide detailed information about the reactions, which are then used in computer models of stars to explore energy production and nucleosynthesis.
Past stars determine the composition of future stellar generations. Much like parents pass on their genes to their children, older stars pass on their elements to younger stars in a process that recycles stellar material. When a star dies, it releases its elements into the interstellar medium, making it available for the creation of future stars.
Since the stars Alan studies are so diverse, he and his students use accelerators in labs around the world that are specifically suited to studying certain types of stars. Some labs are better suited for studying supernovae, novae and x-ray bursts, which are extremely hot, while other labs are more suited for studying stars like the sun, which are cooler.
“Because these stars are so hot, sometimes the critical nuclear reactions involve unstable, exotic elements instead of the garden-variety stable ones,” Alan explained. “This poses a technical challenge for our experiments because, generally speaking, it is difficult to produce an accelerated beam of unstable ions.” As a result, Alan’s research has taken him to TRIUMF in Vancouver, the National Superconducting Cyclotron Laboratory in Michigan, and RIKEN in Japan.
Working in the Department of Physics and Astronomy at McMaster University has enabled Alan to push the limits of his research in a friendly and supportive environment. “I like the combination of the high level of research intensity and the medium size of the department,” he said. “With the former, I'm motivated to keep pushing hard against the boundaries of my research area. With the latter, I get to enjoy the friendliness and collegial atmosphere of the place.”
Experimental nuclear astrophysics; explosive stellar nucleosynthesis; measurement of reaction cross sections and relevant nuclear level parameters; direct measurements with radioactive ion beams; indirect approaches with transfer reactions and in-beam gamma-ray spectroscopy; nuclear reactions; group leader for experiments planned for, and carried out at, TRIUMF, NSCL, Yale University, University of Tokyo (CNS), Technical University of Munich, Argonne National Laboratory, University of Tsukuba; evaluation and dissemination of stellar reaction rates.
Department of Physics & Astronomy
Dear Prospective Graduate Student,
Thanks for stopping by. My research interests lie in nuclear astrophysics and, in the following, I'd like to share with you a bit about the field to give you a general sense for the research that I do.
We now know that stars, through the course of their evolution, generate energy and create almost all of the chemical elements through nuclear processes in their interiors. The field of nuclear astrophysics was born through the desire to establish and understand this connection between the subatomic and the astronomical. There are open questions in all stages of stellar evolution, with many interesting ones found in the study of exploding stars, like supernovae, novae and x-ray bursts. In these explosions, extreme physical environments lead to nucleosynthesis far different from that found in other stars, like our sun for example. In fact, the key nuclear processes in these scenarios involve isotopes that are extremely short-lived. Further progress in our understanding of these events is contingent on developments in the theoretical modeling, in observations with telescopes, and our ability to study the key nuclear processes in the lab.
My primary research interest is in the last of these fronts. Once the important, often poorly understood, nuclear processes have been identified through results discovered in the other two areas, then the experimental nuclear astrophysicist faces the challenge of measuring the properties of nuclei and nuclear processes to the precision required. One needs then to understand the physics of atomic nuclei, how protons and neutrons are bound together in a nucleus, and what happens when two nuclei collide together. One also needs to design, build, and use the equipment (from ion sources to accelerators to particle detectors) that will be best suited for measuring these reactions and nuclear properties. Lastly, after carefully performing the experiment, one needs to understand how to interpret the data, and recast the information in a way that can be introduced into the stellar models.
Part of my research program is presently carried out, in collaboration with other scientists, at the TRIUMF accelerator laboratory, which is located in Vancouver and serves as Canada's national lab for subatomic physics. TRIUMF has a new facility for the study of nuclear astrophysics with unstable ions (the ISAC facility), and is one of the premier facilities in the world for research in the field. Our group has been involved in several experiments that have now been successfully performed at ISAC, and we're looking forward to more in the near future. Students in my group can expect to travel to Vancouver to participate in these experiments, and carry out their own. More recently, we have also established collaborations at Argonne National Laboratory and with the National Superconducting Cyclotron Laboratory (USA), where the production methods for the beams and the kinds of experiments that can be done are complementary to those at TRIUMF. We also will be carrying out experiments at other laboratories in the near future. In addition, we have a detector development laboratory here at McMaster, where students can gain hands-on experience with state-of-the-art detector systems and the associated signal-processing electronics.
If you're simply curious about the field and would like to find out more, or if you're potentially interested in joining my group, then feel free to contact me. I can be reached by email at email@example.com .