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In nuclear physics the term meta-stable state refers to an excited state with an unusually long half-life, which can sometimes be even longer than that of the corresponding ground state. As a result, these states are often considered to be nuclei in their own right and, together with the corresponding ground state nuclei, they are referred to as isomers. The yields of such isomers in fission significantly impact the yield evolution of the fission products, and hence also the criticality and decay heat calculations in nuclear reactors.
In fission, the Isomeric Yield Ratio (IYR) is the amount of fission products populating a certain nucleus, relative to the population all isomers of set nucleus. In the formation of isomers, a combination of shape, spin and spin projection of a nucleus, inhibits the decay, and results in excited states with long half-lives [1, 2, 3]. Consequently, the IYR of fission products are connected to the angular momentum and energy distributions of the initial fission fragments,before neutron and gamma emission. Hence, the reaction code TALYS, that describes this de-excitation procedure, provides an indirect way [4] to explore the angular momentum distribution of fission fragments.
With this in mind, a measurement of IYRs was conducted at the IGISOL facility. In the measurement, a 25 MeV proton beam was used to induce fission of $^{238}$U and produce fission products. A Penning trap utilizing the Phase-Imaging Ion-Cyclotron-Resonance (PI-ICR) technique was then used as a high resolution mass filter in order to separate the fission products. With this technique isomers down to a few tens of keV can be separated by mass and their yields measured through direct ion counting.
In total, 19 IYRs were measured: $^{102}$Nb,$^{126,128,129}$In, $^{129,131}$Sn, $^{129,132,134}$Sb, $^{129,131,133}$Te, $^{132,133,134,136}$I, $^{133,135}$Xe and $^{138}$Cs. A strong odd-even effect is ob-served in the IYRs of several of the isotopes. Further more, the proton numbers of isotones N = 77, 79 and 81 cross the magic proton number Z = 50; and the neutron numbers of isotopes Z = 51 and 53 cross the magic neutron number 82. In this way possible shell effects in population of isomers can be studied.
References:
[1] Philip Walker and George Dracoulis. Energy traps in atomic nuclei.Nature(London),399:35–40, 1999.
[2] G.D. Dracoulis. Isomers, nuclear structure and spectroscopy.Phys. Scr,2013:014015, 2013.
[3] G.D. Dracouliset al. Review of meta-stable states in heavy nuclei.Rep.Prog. Phys.,79:076301, 2016.
[4] V. Rakopoulos et al.First isomeric yield ratio measurements by direct ion counting and implications for the angular momentum of the primary fission fragments.Phys. Rev. C,98:024612, 2018.