Measurement of relative isotopic yield distribution of even-even fission fragments from ${}^{235}\mathrm{U}$($n_{\text{th}},f$) following $\gamma$-ray spectroscopy

A detailed investigation on the relative isotopic distributions has been carried out for the first time in case of even-even correlated fission fragments for the ${}^{235}\mathrm{U}(n_{\text{th}},f$) fission reaction. High-statistics data were obtained in a prompt $\gamma$-ray spectroscopy measurement during the EXILL campaign at ILL, Grenoble, France. The extensive off-line analysis of the coincidence data have been carried out using four different coincidence methods. Combining the results from two-dimensional $\gamma \text{−}\gamma$ and three-dimensional $\gamma \text{−}\gamma \text{−}\gamma$ coincidence analysis, a comprehensive picture of the relative isotopic yield distributions of the even-even neutron-rich fission fragments has emerged. The experimentally observed results have been substantiated by the theoretical calculations based on a novel approach of isospin conservation, and a reasonable agreement has been obtained. The calculations following the semiempirical GEF model have also been carried out. The results from the GEF model calculations are found to be in fair agreement with the experimental results.

Figure 1

A top-view of the experimental EXILL setup at the PF1B neutron beam line area of the high-flux reactor facility at the Institut Laue-Langevin (ILL), Grenoble, France.

Figure 2

Partial level scheme of (a) ${}^{100}\mathrm{Zr}$ and (b) ${}^{134}\mathrm{Te}$, respectively. All the relevant spectroscopic information have been taken from the ENSDF database [32].

Figure 3

Representative coincidence spectra obtained by Methods 1, 2, 3, and 4 are shown in panels (a), (b), (c), and (d) respectively. For panel (a), the gate is set on 1279.0 ($2_1^{+}\to 0_1^{+}$)-keV transition of ${}^{134}\mathrm{Te}$. For panel (b), the gates are set on 1279.0 ($2_1^{+}\to 0_1^{+}$)-keV transition of ${}^{134}\mathrm{Te}$ and 351.9 ($4_1^{+}\to 2_1^{+}$)-keV transition of ${}^{100}\mathrm{Zr}$. For panel (c), the gates are set on 1279.0 ($2_1^{+}\to 0_1^{+}$)- and 297.0 ($4_1^{+}\to 2_1^{+}$)-keV transition of ${}^{134}\mathrm{Te}$. For panel (d), the gates are set on 1279.0 ($2_1^{+}\to 0_1^{+}$)- and 115.0 ($6_1^{+}\to 4_1^{+}$)-keV transition of ${}^{134}\mathrm{Te}$.

Figure 4

Raw isotopic yield distribution of the complementary even-even fission fragment pairs extracted from $\gamma \text{−}\gamma$ coincidence analysis through (a) Method 1, (b) Method 2, and (c) Method 3. See text for details.

Figure 5

Comparison of relative yield distribution for the complementary even-even fission fragment pairs extracted from the analysis of Methods 1, 2, and 3. The predicted results from GEF and ISCF calculations are also shown. The sum of the relative yields of all the isotopes of a particular fragment has been normalized to 100.

Figure 6

Comparison of the relative isotopic yield distribution of the complementary fission fragment nuclei obtained from the present investigation (by using the analysis procedure of Method 2) with those from the previous investigations. The Gaussian fit to the data has also been shown. Note the difference in the scale for the left and right ordinates. The scale of the left ordinate has been used for representing the relative yields extracted from the present $\gamma$-ray spectroscopic analysis. The right ordinate scale has been used for representing the yield data obtained from the previous measurements using recoil mass separators [21, 23]. See text for details.

Figure 7

Depiction of isospin ($T$ or $T_3$) versus mass number (A) for the fragment nuclei produced in the ${}^{235}\mathrm{U}(n_{\text{th}},f$) reaction after neutron emissions. The open squares on the zigzag line represent the isospin values ($T$) that have been assigned to each mass number. The other symbols represent the $T_3$ values for other fragment masses.