Speaker
Description
Prompt fission $\gamma$ rays (PFG) account for 40% of the total $\gamma$ heating in the core of a typical fast reactor, which in turn constitutes about 10% of the total energy release in fission [1]. Therefore, precise knowledge about multiplicities and $\gamma$-ray energies is important. Prompt fission $\gamma$ rays are commonly defined as those emitted in coincidence with fission fragments, i.e. prior to $\beta$ decay. To assess all those $\gamma$ rays in an experiment is difficult, for two reasons: $\textit{i)}$ nuclear level lifetimes may reach up to microseconds, while a chosen prompt timing window usually is of the order of tens of nanoseconds at most, $\textit{ii)}$ extending the prompt window will include contributions from delayed $\gamma$ rays. Fortunately, the latter effect is negligible.
Not very long ago, the time dependence of PFG spectral characteristics, in particular the multiplicity, was already studied experimentally, however only during the first 10 ns after the spontaneous fission of $^{252}$Cf [2]. In a recent experiment with VESPA (VErsatile $\gamma$ SPectrometer Array) at JRC Geel, we managed to extend this time region considerably, including late $\gamma$-ray emission up to 100 $\mu$s after the instance of fission. Multiplicities were determined for different time bins, covering the entire time range. The observed time dependence is compared to results from calculations [3] with the Monte Carlo Hauser-Feshbach code CGMF [4, 5]. We will give details on the recent experiment, present the obtained results, and discuss them in terms of nuclear lifetimes.
References
[1] G. Rimpault, D. Bernard, D. Blanchet, C. Vaglio-Gaudard, S. Ravaux, A. Santamarina, Phys. Procedia $\textbf{31}$, 3 (2012).
[2] A. Oberstedt, A. Gatera, A. Göök, and S. Oberstedt, Eur. Phys. J. A $\textbf{56}$ 196 (2020).
[3] P. Talou, T. Kawano, I. Stetcu, J. P. Lestone, E. McKigney, and M. B. Chadwick, Phys. Rev. C $\textbf{94}$, 064613 (2016).
[4] T. Kawano, P. Talou, M. B. Chadwick and T. Watanabe, J. Nucl. Sci. Tech. $\textbf{47}$, 462 (2010).
[5] P. Talou, B. Becker, T. Kawano, M. B. Chadwick, and Y. Danon, Phys. Rev. C $\textbf{83}$, 064612 (2011).