Decays of Exotic Nuclei

I’m interested in neutron-rich nuclei surrounding 78Ni. This region of the nuclear landscape provides critical information used as inputs to simulations. Studying these decays provide us insight into how elements heavier than iron formed. Understanding them in detail can lead to safer and more efficient nuclear reactors. Let’s look at some of these in a little more detail.

Fundamental Science

From the perspective of fundamental nuclear physics, this region holds interest due to the fact that the nuclear shell structure may change as nuclei become more neutron rich. This is illustrated by the change in the ground state spin-parity in odd-A Cu isotopes[efn_note]Lisetskiy, A. F., Brown, B. A. and Horoi, M.; Exotic nuclei near 78Ni in a shell model approach; Eur. Phys. J. A, 2005, Vol. 25, pp. 95-96 [/efn_note],[efn_note] Ilyushkin, et al.; β-decay studies of the transitional nucleus 75Cu and the structure of 75Zn; Phys. Rev. C, 2011, Vol. 83, pp. 014322 [/efn_note] and by the hypothesized emergence of sub-shell gaps (N = 58) as one crosses the N = 50 shell closure[efn_note] Winger, J. A., et al.; New subshell closure at N=58 emerging in neutron-rich nuclei beyond 78Ni; Phys. Rev. C, American Physical Society, 2010, Vol. 81, pp. 044303 [/efn_note].

Astrophysics

Schematic of the r-process
Schematic of the r-process

The astrophysical rapid neutron capture process (seen right) originates near 78Ni. Understanding the r-process requires detailed knowledge of nuclear data. The r-process occurs in supernovae and is thought to be one of the major sources of elements heavier than iron.

For nuclei at closed shells, the neutron capture cross section drops. These are called waiting point nuclei and the process must “wait” for beta decay until the neutron capture resumes[efn_note]Burbidge, E. M., Burbidge, G. R., Fowler, W. A. and Hoyle, F.; Synthesis of the Elements in Stars; Rev. Mod. Phys., 1957, Vol. 29, pp. 547 [/efn_note]. The abundance of some elements sharply increases due to the presence waiting points.

Many astrophysics calculations must rely on extrapolations of known data into the exotic regions. The behavior of exotic nuclei can be different than less exotic species. Accurate models of the r-process require experimentally determined neutron capture cross sections and beta-decay half-lives[efn_note]Madurga, M., et al.; New Half-lives of r-process Zn and Ga Isotopes Measured with Electromagnetic Separation; Phys. Rev. Lett., 2012, Vol. 109, pp. 112501 [/efn_note],[efn_note]Nishimura, S., et al.; β-Decay Half-Lives of Very Neutron-Rich Kr to Tc Isotopes on the Boundary of the r-Process Path: An Indication of Fast r-Matter Flow; Phys. Rev. Lett., 2011, Vol. 106, pp. 052502 [/efn_note].

Decay Heat Calculations

Lastly, the nuclei in this region are abundantly produced in the fission of 235U and their properties influence the operation of nuclear reactors[efn_note]OECD Nuclear Energy Agency; Assessment of Fission Product Decay Data for Decay Heat Calculations; Nichols, A. L. (ed.), 2007, Vol. 25 [/efn_note]. The design stages of a reactor must take into account the amount and energy distribution of neutrons emitted from the fission fragments. The modeling of transportation and storage of spent fuel require experimentally determined inputs. This means that there must be careful study of the beta decay half-lives and neutron cross sections for fission products. These data are necessary for inputs into the decay heat calculations. The decay half-lives will also have an effect on the reactor cooling time. This information is necessary for environmental and safety considerations.

Conclusions

As we can see beta-decay of exotic nuclei affects a lot of different fields. Understanding these exotic decays is the firs step in a much wider path. Next, let’s take a look at one of the more exciting decay modes: beta-delayed neutron emission!

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