Effects of the nuclear structure of fission fragments on the high-energy prompt fission $\gamma$-ray spectrum in ${}^{235}\mathrm{U}(n_{\mathrm{th}},f)$

The prompt fission $\gamma$-ray energy spectrum for cold-neutron-induced fission of ${}^{235}\mathrm{U}$ was measured in the energy range $E_{\gamma }=0.8–20\mathrm{MeV}$, by gaining a factor of about ${10}^5$ in statistics compared to the measurements performed so far. The spectrum exhibits local bump structures at $E_{\gamma }\approx 4\mathrm{MeV}$ and $\approx 6\mathrm{MeV}$, and also a broad one at $\approx 15\mathrm{MeV}$. In order to understand the origins of these bumps, the $\gamma \text{−}\mathrm{ray}$ spectra were calculated using a statistical Hauser-Feshbach model, taking into account the deexcitation of all the possible primary fission fragments. It is shown that the bump at $\approx 4\mathrm{MeV}$ is created by the transitions between the discrete levels in the fragments around ${}^{132}\mathrm{Sn}$, and the bump at $\approx 6\mathrm{MeV}$ mostly comes from the complementary light fragments. It is also indicated that a limited number of nuclides, which have high-spin states at low excitation energies, can contribute to the bump structure around $E_{\gamma }\approx 15\mathrm{MeV}$, induced by the transition feeding into the low-lying high-spin states.

Figure 1

(a) Fission-fragment mass distribution for ${}^{235}\mathrm{U}(n_{\mathrm{th}},f)$ obtained from the measurement of the time-of-flight difference. (b) Recorded events on the $T_{\gamma }$ vs pulse height plane from the ${\mathrm{LaBr}}_3\left(\mathrm{Ce}\right)$ scintillator under the condition that fragments contained in the region A of panel (a) are moving to the scintillator of interest. (c) Same as (b), but for the fragments in B. The areas enclosed by the dashed lines in (b) and (c) indicate the gate position used to select the prompt $\gamma \text{−}\mathrm{ray}$ events.

Figure 2

Prompt $\gamma \text{−}\mathrm{ray}$ spectra obtained in the present study (filled triangles) are compared to those by Peelle et al. (open diamonds) [7], Verbinski et al. (filled circles) [9], and Oberstedt et al. (open rectangles) [4]. The spectra are compared with the calculation by the ${\mathrm{CoH}}_3$ code [44] (solid and dashed lines); see text for details. Expansion of the spectra in the energy range $E_{\gamma }=1–7\mathrm{MeV}$ is shown in the inset (a). The inset (b) shows the $\gamma \text{−}\mathrm{ray}$ spectra for ${}^{86}\mathrm{Br},{}^{102,103}\mathrm{Nb}$, and ${}^{109}\mathrm{Tc}$ with the ${\mathrm{CoH}}_3$ code [44] (dashed-dotted lines).

Figure 3

(a) PFGS obtained for fragment mass splits with $97\le A_{\mathrm{L}}\le 116$ or $120\le A_{\mathrm{H}}\le 139$ (filled triangles) and $81\le A_{\mathrm{L}}\le 96$ or $140\le A_{\mathrm{H}}\le 155$ (filled diamonds) are compared to the corresponding ${\mathrm{CoH}}_3$ calculations (dashed-dotted line and dashed line). Selected spectra from the calculation having the largest contribution to the bump at $E_{\gamma }=4.0\mathrm{MeV}$ are shown $({}^{104}\mathrm{Mo}+{}^{132}\mathrm{Sn},{}^{103}\mathrm{Mo}+{}^{133}\mathrm{Sn},{}^{103}\mathrm{Nb}+{}^{133}\mathrm{Sb}$, and ${}^{102}\mathrm{Nb}+{}^{134}\mathrm{Sb})$. (b) Calculated $\gamma \text{−}\mathrm{ray}$ yield in the energy range of $5.3\le E_{\gamma }\le 6.5\mathrm{MeV}$ as a function of the fragment mass (solid line). The mass yield distribution of Ref. [46] is given by the dashed line.

Figure 4

(a) Average prompt neutron multiplicity $\overline{\nu }\left(A\right)$ and (b) $\gamma \text{−}\mathrm{ray}$ multiplicity ${\overline{M}}_{\gamma }\left(A\right)$ for ${}^{235}\mathrm{U}(n_{\mathrm{th}},f)$ from each FF calculated by the ${\mathrm{CoH}}_3$ code (solid lines). The $\overline{\nu }\left(A\right)$ are compared with the experimental data by Maslin et al. (circles) [60], Nishio et al. (rectangles) [43], and Batenkov et al. (triangles) [61]. The ${\overline{M}}_{\gamma }\left(A\right)$ are compared with those by Albinsson et al. (circles) [59] and Pleasonton et al. (rectangles) [8].