Watching Neutrons Flow

Like water, neutrons seek their own level, and watching how they flow may teach us about how the chemical elements were made.

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Collisions of nuclei with more neutrons (blue) than protons (red) produced extremely deformed shapes with uneven distribution of neutrons. The flow of the excess neutrons is observed as the deformed system evolves and breaks apart.

The Science

Gases are easy to compress, not so liquids and solids. How much the volume of something changes under pressure, and how much it heats up when compressed, are described by its equation of state.  Nuclei can also be compressed. The nuclear equation of state describes how hard it is to squeeze a nucleus, how hot it gets when you squeeze it, and how both of these change when more neutrons are added to the nucleus. Large numbers of excess neutrons can be added to nuclei in certain types of stellar explosions or in the extreme environment of a neutron star. Understanding how the resulting neutron-rich matter behaves is an active area of investigation. Researchers at the Texas A&M University Cyclotron Institute used a new technique, equilibration chronometry, to monitor the transfer of excess neutrons over time. The transfer happens in less than a zeptosecond (10-21 second). Scientists took a series of “snapshots” that show the neutrons flowing from a small region of high concentration and expanding into a larger area of lower concentration.

The Impact

The nuclear equation of state is a stunning example of the connections that exist between vastly different aspects of science. The equation links the behavior of the nuclei of atoms to astrophysical extremes. A more accurate equation of state could provide new answers about stars and the formation of chemical elements.


By colliding 70Zn projectiles moving at one quarter the speed of light with stationary 70Zn nuclei, nuclear physics researchers at Texas A&M University created extremely deformed nuclei. Specifically, these elongated shapes had a smaller end that is neutron rich and a larger end that is relatively neutron poor. These deformed nuclei were spinning as they broke into two fragments, and the decay occurred before one full rotation was reached. By measuring the rotation rate, the final orientation of the two resulting fragments provides a measure of the time scale of the decay, which is less than 5x10-22 seconds on average. Before these two fragments part ways, the excess neutrons from the neutron-rich end migrate toward the relatively neutron-poor end. This equilibration ceases when the deformed nucleus breaks in two. The neutron excesses of the two fragments are initially far from each other, and approach each other exponentially as a function of time.


Alan B. McIntosh
Texas A&M University


The U.S. Department of Energy, Office of Science, Office of Nuclear Physics (NP), and the Welch Foundation funded this research.


A. Jedele, A.B. McIntosh, K. Hagel, M. Huang, L. Heilborn, Z. Kohley, L.W. May, E. McCleskey, M. Youngs, A. Zarrella, and S.J. Yennello, “Characterizing neutron-proton equilibration in nuclear reactions with subzeptosecond resolutionExternal link.” Physical Review Letters 118, 062501 (2017). [DOI: 10.1103/PhysRevLett.118.062501]

Related Links

Texas A&M University: Cyclotron InstituteExternal link 

Highlight Categories

Program: NP

Performer/Facility: University

Last modified: 1/25/2018 4:57:44 PM