YSM Issue 94.2

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WHY DO FRAGMENTS SPIN AFTER
A NUCLEAR FISSION SPLIT?
By Meili Gupta
In 1938, chemists Otto Hahn and Fritz Strassmann
bombarded a uranium nucleus with a neutron. To their
surprise, they discovered that barium and krypton were
produced. This division of heavy nuclei was termed “fission,”
and since its discovery it has deeply intrigued physicists.
For decades, physicists have puzzled over why the resulting
fragments spin after nuclear fission. A recent collaboration
between scientists from France, Germany, the UK, and other
European countries offers clear answers.
At the IJC Laboratory in Orsay, France, scientists used highresolution
spectroscopy to inspect the fission of 232 Th, 238 U,
and 252 Cf. They specifically used a method from the University
of Manchester to measure the average spin of the fission
fragments. They discovered that the fragment spin appeared to
depend only on the fragment mass, not the mass of the original
nucleus or the mass or charge of its partner fragment. Their
results deny hypotheses that the spin is generated before the
nucleus splits (pre-scission), as these hypotheses predict equal
magnitude spins for partner fragments. The large asymmetries
in the angular momentum between the spins of heavy and
light partner fragments support a post-scission hypothesis and
the idea that the fragments should be treated as independent
quantal—discrete—systems.
Beyond deepening a fundamental understanding of fission,
these findings have important applications for nuclear
energy, since fragment spins influence the heating effects of
reactor gamma rays. This research may help unlock the full
potential of nuclear fission. ■
Wilson, J.N., Thisse, D., Lebois, M. et al. Angular momentum
generation in nuclear fission. Nature 590, 566–570 (2021).
https://doi.org/10.1038/s41586-021-03304-w
4 Yale Scientific Magazine May 2021 www.yalescientific.org