Transition strength calculation accounts for complexity of quantum mechanics

NSCL researchers have performed a precise calculation about low-energy transition strengths, a fundamental property of atomic nuclei. Their result, published in Physical Letters B, takes advantage of high performance computing to accurately account for the vagaries of quantum mechanics.

Like molecules and atoms, atomic nuclei can exist in various energy states. When a given nucleus is at its lowest energy, it's said to be at its ground state. If its protons and neutrons gain energy, the nucleus can move to higher-energy, excited states.

Efforts to understand how transitions occur between energy states are broadly relevant within rare isotope research to study the limits of nuclear stability and astrophysical processes.

While a postdoc at NSCL, Neil Summers worked on enhancements to Fresco, a core application used by nuclear physicists since the 1980s to model and understand transition strengths. Fresco is used to extract specific information related to energy transitions from a sea of data generated in the sort of low energy nuclear reactions produced at NSCL.

Using Fresco provides an alternative to direct observation, which entails detecting an excited nucleus and measuring its lifetime. Direct observation lifetime measurements yield precise information about the probability of moving between states, however the method can only be applied to a limited number of nuclear states.

Much of Summers effort was spent updating and parallelizing the Fresco code to better describe the effects of quantum mechanics, the powerful theoretical framework used by physicists to explain phenomena at the atomic and subatomic scale. Largely because of limited computing power, previous versions of the code relied on rough approximations of quantum phenomena.

The updated code takes the mathematical complexity associated with quantum mechanics and splits it into 54 manageable computational processes that run in parallel. Completing the calculations takes roughly 30 minutes, a vast improvement over the many days or weeks that would be required to perform the calculations on a single computer.

Working with NSCL assistant professor Filomena Nunes and other collaborators, Summers tested his code on data collected in several experiments related to the beryllium-11 isotope since the mid 1990s. Their results closely agreed with well-established data on beryllium-11transition strengths gleaned from direct observation lifetime measurements.

Summers, lead author on the paper that includes coauthors from more than 10 institutions, has accepted a job with Lawrence Livermore National Laboratory and will be leaving NSCL this summer. In the meantime, he is focused on continuing to validate and improve his code using chromium-49 data collected in a recent NSCL experiment.

The research was supported in part by the National Science Foundation and the Department of Energy.

NSCL is a world leading facility for rare isotope research and nuclear science education.

The paper is available on the arXiv.org preprint server at: http://arxiv.org/abs/nucl-th/0703055.

- Geoff Koch, June 6, 2007