Fission-fragment properties in ${}^{238}\mathrm{U}(n,f)$ between 1 and 30 MeV

The fragment mass and kinetic energy in neutron-induced fission of ${}^{238}\mathrm{U}$ has been measured for incident energies from 1 to 30 MeV at the Los Alamos Neutron Science Center. The change in mass distributions over this energy range were studied, and the transition from highly asymmetric to more symmetric mass distributions is observed. A decrease in average total kinetic energy $\left(\overline{\mathrm{TKE}}\right)$ with increasing excitation energy is observed, consistent with previous experimental work. Additional structure at multichance fission thresholds is present in the $\overline{\mathrm{TKE}}$ data. The correlations between fragment masses and total kinetic energy and how that changes with excitation energy of the fissioning compound nucleus were also measured. The fission mass yields and average total kinetic energy are important for fission-based technologies such as nuclear reactors to understand nuclear waste generation and energy output when developing new and advanced concepts. The correlations between fragment mass and kinetic energy are needed both as input for theoretical calculations of the deexcitation process in fission fragments by prompt radiation emission and for validating advanced theoretical fission models describing the formation of the primordial fragments.

Figure 1

Schematic of the ionization chamber. Separation distances and voltages of the anodes (outer gray), grids (inner lined), and cathode (center gray) are shown. The beam direction is incident on the upstream sample side of the detector. The target's backing side is downstream. The orientation of the fission-fragment emission angles $\theta$ is indicated with respect to the beam axis.

Figure 2

Anode pulse height (in arbitrary digitizer units) as a function of $cos\theta$ after the angular correction. The two band features in the left pad are due to the light and heavy fission fragments. The scatter at low anode pulse heights are from $\alpha$ and charged particles. The angular distributions are also shown in the right pad for $E_n<10$ MeV .

Figure 3

The average energy of the sample (red) and the backing (blue) side fragments are plotted as a function of $1/(cos\theta )$ to determine the amount of energy loss in the target material. The data were fit between values of $1/(cos\theta )=1.0$–2.0 to determine the parameters $b_1$ and $b_2$, the slopes of the linear fit.

Figure 4

The calibrated anode pulse height distributions for the backing (blue) and the sample (red) sides of the detector for all incident neutron energies. The overlap of the distributions suggests that the corrections determined at $E_n=1.5$–2.0 MeV are also valid at high incident energies.

Figure 5

The average number of prompt neutrons emitted as a function of fragment mass, as simulated in gef for a variety of neutron energies. Features such as the dip at $A=132$ are due to nuclear shell structure and wash out for a system with high excitation energy.

Figure 6

The thermal scaled $\overline{\nu }\left(A\right)$ (dotted line) is compared with the gef calculations at $E_n=5$, 15, and 30 MeV. The quantity $\overline{\nu }\left(A\right)$ has proven difficult to measure experimentally and is one of the main sources of systematic uncertainty in calculating fragment mass.

Figure 7

Pulse height defect curve for P-10 as a function of fragment mass. The data were taken from Ref. [27].

Figure 8

The $\overline{\mathrm{TKE}}$ as a function of neutron energy for ${}^{238}\mathrm{U}$ pre-neutron emission along with the endf/b-vii.1-evaluated neutron-induced fission cross section [6]. Error bars are statistical. The measurement by Zöller [3] is also shown.

Figure 9

The $\overline{\mathrm{TKE}}$ as a function of neutron energy for ${}^{238}\mathrm{U}$ post-neutron emission. A confidence band around the Lestone model is shown in blue [5] with the Madland model shown in red [4].

Figure 10

The pre-neutron-emission $\overline{\mathrm{TKE}}$ obtained by correcting for prompt neutron emission is shown. The $\overline{\mathrm{TKE}}$ as a function of neutron energy for ${}^{238}\mathrm{U}$ is shown with other previous pre-neutron-emission $\overline{\mathrm{TKE}}$ data at the low end of the neutron energy range [1, 2, 3].

Figure 11

The variance in the $\overline{\mathrm{TKE}}$ as a function of neutron energy for ${}^{238}\mathrm{U}$ pre-neutron emission. Neutron-induced fission cross sections are also shown [6] along with previous data [3].

Figure 12

The variance in the $\overline{\mathrm{TKE}}$ as a function of neutron energy for ${}^{238}\mathrm{U}$ post-neutron emission with previously measured data [3].

Figure 13

The evolving mass landscape of ${}^{238}\mathrm{U}$ fission fragments with incident neutron energy, preprompt neutron emission. Yields are normalized to 200%. The color scale goes with the yield.

Figure 14

The pre-neutron-emission fission-fragment masses, calculated using the 2E method, are shown with a previous measurement at four ranges of neutron energies [3].

Figure 15

England and Rider [7] evaluations for independent yields are shown with the appropriate corresponding neutron energies. F denotes the fission neutron spectrum with additional pooling from $E_n=0.5$–2 MeV. HE denotes an energy bin from $E_n=14$ to 15 MeV with the majority of events at 14.7 MeV. [7].

Figure 16

The TKE with fragment mass is shown for a few incident energies. At around $E_n=10$ MeV, multichance effects and symmetric fission probabilities overtake shell effects as the dominent influence of $\overline{\mathrm{TKE}}$ and mass distributions.

Figure 17

The average TKE as a function of fragment mass is shown for a few incident energy ranges, confirming that more symmetric fragments have lower TKE, on average.