Fission Fragments Weighed

The fissioning of uranium results in a variety of unstable neutron-rich nuclei. A team of scientists from the University of Jyväskylä in Finland has for the first time made high-precision mass measurements of a number of isotopes produced in proton-induced fission reactions of uranium, including strontium, zirconium, and molybdenum.

These so-called refractory elements are hard to study as ionized beams because of their high boiling points. Instead, the researchers reach a high level of precision by coaxing the nuclei into a Penning trap, which employs a combination of a strong magnetic field and a static quadrupole field to trap ions. In this kind of device, the particle's mass can be deduced from the observed cyclotron motion -- that is, from the particle's looping orbit in a strong magnetic field.

The reason for wanting better isotope masses is that they provide information about nuclear binding energies. The mass of the simplest compound nucleus, the deuteron, for instance, is several million electron volts less than the sum of the masses of its constituent proton and neutron. The difference is the net binding energy.

In the case of the new studies, the isotope masses are determined with a precision of thousands of electron volts. By measuring the mass of several zirconium isotopes of increasing neutron numbers, one can see subtle effects in the complex structures of these nuclei. Astrophysicists, who consider how larger nuclei are built inside stars or novas also will be interested in knowing how nuclear mass increases with neutron number.

Hager et al., Physical Review Letters, upcoming article
Contact Ari Jokinen, ari.jokinen@phys.jyu.fi