Why does nuclear fission produce pear-shaped nuclei?

December 20, 2018, University of Tsukuba
Figure A: Evolution of the fissioning system as a function of time. The total time between the left to the right is 20.4 zeptosecond (1 zeptosecond = 10-21 s ). Credit: University of Tsukuba

Nuclear fission is a process in which a heavy nucleus split into two. Most of the actinides nuclei (plutonium, uranium, curium, etc) fission asymmetrically with one big fragment and one small. Empirically, the heavy fragment presents on average a xenon element (with charge number Z=54) independently from the initial fissioning nucleus. To understand the mechanism that determines the number of protons and neutrons in each of the two fragments has been a longstanding puzzle.

It was expected that the deformation of the fragments could play a role. Indeed, the can have different shapes depending on their internal structure. Some of them are spherical, most of them are deformed like a rugby ball and a few have a pear-shaped deformation. The internal structure of the nuclei varies as a function of the number of protons and neutrons composing the nuclei.

To describe dynamically the fission process, the state of the art of nuclear theory has been used by Guillaume Scamps (University of Tsukuba) and CĂ©dric Simenel (Australian National University). This simulation of the uses the quantum-mechanics to takes into account the motion of the nucleons in the nuclei and uses adequate simplifications to solve the many-body problem.

Using that model, in the case of the 240Pu, it has been found that the fission fragments are preferably formed with a pear-shaped deformation (see figure). This pear-shaped deformation is due to the strong Coulomb repulsion of the two fragments. This initial deformation favours nuclei which are pear-shaped in their . This is the case of the Xenon due to some effects associated with a number of proton Z=54.

This is strong enough to strongly influence the partition of nucleons in several fissioning systems. This mechanism has been found in simulations of the fission of 230Th, 234U, 236U, 246Cm and 250Cf in agreement with the experimental observations.

These findings may explain in future, surprising recent observations of asymmetric fission of lighter than lead nuclei, and improve predictions of fission properties of exotic which impact the abundance of elements produced in the astrophysical processes.

Explore further: Heavy barium nuclei prefer a pear shape

More information: Guillaume Scamps et al. Impact of pear-shaped fission fragments on mass-asymmetric fission in actinides, Nature (2018). DOI: 10.1038/s41586-018-0780-0

Related Stories

Heavy barium nuclei prefer a pear shape

June 7, 2016

Certain heavy barium nuclei have long been predicted to exhibit pear-like shapes. However, until recently, experimental confirmation had been impossible to achieve as these nuclei typically only live for a few seconds. The ...

Unresolved puzzles in exotic nuclei

March 27, 2018

Research into the origin of elements is still of great interest. Many unstable atomic nuclei live long enough to be able to serve as targets for further nuclear reactions—especially in hot environments like the interior ...

New type of nuclear fission discovered

December 6, 2010

(PhysOrg.com) -- Nuclear fission, or the splitting of a heavy nucleus, usually results in symmetrical fragments of the same mass. Physicists attribute the few known examples of fission that is asymmetric to the formation ...

Dynamics of nuclear fission at low excitation energy

August 25, 2015

The mechanisms of nuclear fission, especially the origin of asymmetric mass division in the low-excitation region of U and Pu, are still not clear. There are many conflicting arguments to explain the experimental data, making ...

New territory in nuclear fission explored with ISOLDE

January 17, 2011

An international collaboration led by the University of Leuven, Belgium, exploiting ISOLDE’s radioactive beams, has recently discovered an unexpected new type of asymmetric nuclear fission, which challenges current theories. ...

Recommended for you

New thermoelectric material delivers record performance

January 17, 2019

Taking advantage of recent advances in using theoretical calculations to predict the properties of new materials, researchers reported Thursday the discovery of a new class of half-Heusler thermoelectric compounds, including ...

Zirconium isotope a master at neutron capture

January 17, 2019

The probability that a nucleus will absorb a neutron is important to many areas of nuclear science, including the production of elements in the cosmos, reactor performance, nuclear medicine and defense applications.

Mechanism helps explain the ear's exquisite sensitivity

January 16, 2019

The human ear, like those of other mammals, is so extraordinarily sensitive that it can detect sound-wave-induced vibrations of the eardrum that move by less than the width of an atom. Now, researchers at MIT have discovered ...


Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Dec 22, 2018
Well, duh - that fission can go pear-shaped has been observed since the early sometimes deadly experiments to today's reactor failures!
not rated yet Dec 26, 2018
Well, duh - that fission can go pear-shaped has been observed since the early sometimes deadly experiments to today's reactor failures!

Erm, did you read a different article? This one's about investigating WHY it goes pear-shaped, not if.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.