Prompt fission neutron spectra in fast-neutron-induced fission of ${}^{238}\mathrm{U}$

Prompt fission neutron spectrum (PFNS) measurements for the neutron-induced fission of ${}^{238}\mathrm{U}$ are carried out at incident neutron energies of 2.0, 2.5, and 3.0 MeV, respectively. The time-of-flight technique is employed to determine the energy of fission neutrons. The prompt fission neutron energy spectra so obtained are analyzed using Watt parametrization to derive the neutron multiplicity and average prompt fission neutron energy. The present experimental PFNS data are compared with the evaluated spectra taken from the ENDF/B-VII.1 library and the predictive calculations carried out using the empire-3.2 (Malta) code with built-in Los Alamos (LA) and Kornilov PFNS models. The sensitivity of the empire-3.2 LA model–calculated PFNS to the nuclear level density parameter of the average fission fragment and to the total kinetic energy is investigated. empire-3.2 LA model PFNS calculations that use Madland 2006–recommended values [D. G. Madland, Nucl. Phys. A 772, 113 (2006)] of the total kinetic energy and the level density parameter $a=A/(10\pm 0.5)$ compare very well to measured data at all incident neutron incident energies.

Figure 1

Schematic of the experimental setup.

Figure 2

Fission fragment (FF) pulse height spectrum.

Figure 3

Measured efficiency for the EJ301 neutron detectors.

Figure 4

(a) Typical two-dimensional pulse shape discrimination (PSD) vs time-of-flight (TOF) plot; (b) neutron TOF spectrum.

Figure 5

Comparison of prompt fission neutron spectra among the present experimental data, Watt spectrum (solid line), ENDF/B-VII.1 evaluation (short-dashed line), empire-3.2 Los Alamos model (dotted line), and Kornilov model (long-dashed line) at incident neutron energies of (a) 2.0 MeV, (b) 2.5 MeV, and (c) 3.0 MeV.

Figure 6

(a) Average neutron multiplicity: present data (filled circles) and other experimental data (open circles) taken from EXFOR and ENDF/B-VII.1 evaluation (solid line). (b) Prompt fission neutron spectrum average energy $\left(\overline{E}\right)$ for neutrons emitted from neutron-induced fission of ${}^{238}\mathrm{U}$. Data are compared with previously reported experimental data and also with predictions of empire-3.2 Los Alamos model calculations.

Figure 7

Experimental (open circles) and calculated [ENDF/B-VII.1 evaluation (solid line), empire-3.2 Los Alamos model (short-dashed line), and Kornilov model (dotted line)] prompt fission neutron energy spectra, normalized to the Maxwellian distribution at an incident neutron energy of (a) 2.0 MeV, (b) 2.5 MeV, and (c) 3.0 MeV.

Figure 8

Experimental (open circles) and empire-3.2 LA model–calculated PFNS spectra obtained using the $\langle T_f^{\mathrm{tot}}\rangle$ values taken from the Madland systematic (solid line), normalized to the Maxwellian distribution at an incident neutron energy of 2.0, 2.5, and 3.0 MeV. The effect of changing the level density parameter value is also shown.