Mass yield distributions and fission modes in photofission of ${}^{238}\mathrm{U}$ below 20 MeV

Photon activation technique is used to obtain photofission mass distributions of ${}^{238}\mathrm{U}$ after irradiation in a bremsstrahlung beam with end-point energies 19.5, 29.1, 48.3, and 67.7 MeV. The weights of the symmetric (SL) and asymmetric (STI and STII) modes in the mass distributions of photofission of ${}^{238}\mathrm{U}$ and neutron-induced fission of ${}^{235,238}\text{U}$ at the average excitation energies of up to 20 MeV are calculated on the basis of the obtained results and other available data. The weight of the SL mode increases by about an order of magnitude as the average excitation energy increases from 8.5 to 17 MeV, while the weight of the STI mode decreases by two times, and the weight of the STII mode slightly increases. It is shown that the peak-to-valley ratios in photofission of ${}^{238}\mathrm{U}$ and neutron-induced fission of ${}^{235}\mathrm{U}$ are very close across almost the whole studied energy range. A similar conclusion is made concerning the behavior of asymmetric and symmetric fission modes in photofission of ${}^{238}\mathrm{U}$ and neutron-induced fission of ${}^{235}\mathrm{U}$ and ${}^{238}\mathrm{U}$.

Figure 1

$\gamma$-ray spectra of induced activity of uranium target after irradiation at $T=29.1$ MeV, summed over 10 h. From top to bottom: (1) measurement 1 h after irradiation; (2) measurement 3 days after irradiation.

Figure 2

Relative independent yield of the ${}_{53}^{134}\text{I}$ isotope as a function of the average excitation energy of ${}^{238}\mathrm{U}$: (dots) present work; Pommé 1993: [36]; De Frenne 1984: [37]; empty dots (Wahl 2002) represent relative independent yield of the ${}_{53}^{134}\text{I}$ isotope calculated from the systematics [16] for neutron- and proton-induced fission.

Figure 3

Evaluated cross section of the photofission reaction on ${}^{238}\mathrm{U}$ ([42], right axis) and simulated bremsstrahlung spectra at $T=19.5$, 29.1, 48.3, 67.7 MeV (left axis) used to calculate the average excitation energies of photofission.

Figure 4

Fraction mass yields $\left(Y_{\mathrm{FMY}}\right)$ of photofission of ${}^{238}\mathrm{U}$ and results of fitting the distributions with five Gaussian curves at bremsstrahlung end-point energies 19.5, 29.1, 48.3, and 67.7 MeV: experimental data (squares), asymmetric component STI (dashed curve), asymmetric component STII (dotted curve), symmetric component SL (solid curve), sum of the components (dashed-dotted curve).

Figure 5

Comparison of the mass yield distributions of photofission at bremsstrahlung end-point energies $T$ from 15 to 30 MeV. From top to bottom: (1) $T=30$ MeV, $\langle E^{*}\rangle =13.9$ MeV (Ref. [21]); (2) $T=29.1$ MeV, $\langle E^{*}\rangle =13.7$ MeV (this work); (3) $T=20$ MeV, $\langle E^{*}\rangle =12.9$ MeV (Ref. [21]); (4) $T=19.5$ MeV, $\langle E^{*}\rangle =11.9$ MeV (this work); (5) $T=15$ MeV, $\langle E^{*}\rangle =10.2$ MeV (Ref. [22]).

Figure 6

Peak-to-valley ratios ${(p/v)}_1$ (left) and ${(p/v)}_2$ (right) for photofission of ${}^{238}\mathrm{U}$ (dots, stars) and neutron-induced fission of ${}^{235}\mathrm{U}$ (squares) and ${}^{238}\mathrm{U}$ (triangles) as a function of the average excitation energy $〈E^{*}〉$. Photofission data from this work (dots) and Refs. [21, 22, 45, 46, 47, 48] (stars), neutron-induced fission for ${}^{238}\mathrm{U}$ from Refs. [25, 50, 51, 52, 53, 54, 55], for ${}^{235}\mathrm{U}$ from Refs. [24, 49].

Figure 7

Contributions (weights) of different fission modes in photofission of ${}^{238}\mathrm{U}$ and neutron-induced fission of ${}^{235}\mathrm{U}$ as a function of the average excitation energy $〈E^{*}〉$. Circles: contributions of the STII, STI, SL modes of photofission of ${}^{238}\mathrm{U}$ calculated from mass distributions obtained in the present work and in works [17, 18, 20, 21, 22]; stars: contributions of the STII, STI, SL modes of neutron-induced fission of ${}^{235}\mathrm{U}$ from [15, 23, 24]; squares: ${}^{238}\mathrm{U}$ [24, 25]. Mode yields are normalized per 100 fission reactions: $Y_{\mathrm{SL}}+Y_{\mathrm{STI}}+Y_{\mathrm{STII}}=200\%$.