Energy dependence of fission product yields in the second-chance fission region

An extensive dataset of cumulative fission product yields has been generated under a joint collaboration between Los Alamos National Laboratory (LANL), Lawrence Livermore National Laboratory (LLNL), and the Triangle Universities Nuclear Laboratory (TUNL). The energy dependence of the cumulative yield for a select number of high-yield fission products has been measured using quasimonoenergetic neutrons with energies between 5.5 and 11.0 MeV. This is in addition to previously published data covering 0.5–4.5 and 14.8 MeV. The absolute number of fissions was determined during the irradiation period using dual-fission ionization chambers, and the fission products were measured postactivation by whole target $\gamma$-ray spectroscopy. This paper presents the absolute cumulative fission product yields as a function of incident neutron energy from the neutron-induced fission of ${}^{235}\mathrm{U}, {}^{238}\mathrm{U}$, and ${}^{239}\mathrm{Pu}$ isotopes at four incident energies in the second-chance fission region and compares them with existing literature values. Corrections relevant to this collaboration's previously published data are also discussed.

Figure 1

Existing low-energy fission product yield data for the fission product ${}^{147}\mathrm{Nd}$ from ${}^{239}\mathrm{Pu}(n,f)$ as of 2011. Figure taken from Ref. [5].

Figure 2

Schematic of the FPY experimental setup. The bottom-left panel shows the measured $n\mathrm{TOF}$ spectrum, the bottom-middle panel, the DFC pulse-height spectrum, and the right panel shows the time evolution of the $\gamma$-ray spectra.

Figure 3

The left panel shows a three-dimensional rendering of the neutron production gas cell and DFC used in this work. The right panel shows the simulated neutron flux in the vicinity of the gas cell and DFC. See text for dimensions.

Figure 4

Simulated neutron flux ratios between the downstream (FC1), upstream (FC2), and center (activation) foils for the ${}^{238}\mathrm{U}$ DFC.

Figure 5

Fission event detection efficiency as a function of incident neutron energy for the upstream and downstream FCs loaded with all three actinide targets. The ${}^{239}\mathrm{Pu}$ targets have less material for the fragments to pass through, and thus have an overall higher detection efficiency compared with the uranium foils.

Figure 6

The correction factor $C_{Att}$ accounts for $\gamma$-ray self-absorption and geometrical differences from the detector calibration source as a function of $\gamma$-ray energy for ${}^{235}\mathrm{U}$ (black), ${}^{238}\mathrm{U}$ (red), and ${}^{239}\mathrm{Pu}$ (green).

Figure 7

(left panel) $n\mathrm{TOF}$ spectrum for 11 MeV (top) and 9 MeV (bottom) as measured by a liquid scintillator detector for the ${}^2\mathrm{H}$(${d,n)}^3\mathrm{He}$ reaction. Plots are unnormalized. See text for description of features in the spectra. (right panel) Plot of the $n\mathrm{TOF}$ spectra from the left panel converted to energy and folded with the fission cross section [22] at 9 MeV (bottom) and 11 MeV (top). The spectrum is divided into regions of off-energy neutrons below $E_C$ and monoenergetic neutrons between $E_C$ and $E_{\mathrm{Max}}$ (shaded region in the right panel).

Figure 8

Results for the cumulative fission yield of ${}^{147}\mathrm{Nd}$ from ${}^{235}\mathrm{U}, {}^{238}\mathrm{U}$, and ${}^{239}\mathrm{Pu}$ compared with existing data in the present energy region.

Figure 9

Results for the cumulative fission yield of ${}^{140}\mathrm{Ba}$ from ${}^{235}\mathrm{U}, {}^{238}\mathrm{U}$, and ${}^{239}\mathrm{Pu}$ compared with existing data in the present energy region.

Figure 10

Results for the cumulative fission yield of ${}^{99}\mathrm{Mo}$ from ${}^{235}\mathrm{U}, {}^{238}\mathrm{U}$, and ${}^{239}\mathrm{Pu}$ compared with existing data in the present energy region.