${}^{242}\mathrm{Pu}$ neutron-induced fission cross-section measurement from 1 to 2 MeV neutron energy

Relative values of the neutron-induced fission cross section $\sigma (n,f)$ of ${}^{242}\mathrm{Pu}$ have been measured with respect to the standard ${}^1\mathrm{H}(n,p)$ elastic scattering cross section, at average energies of 1.0, 1.4, and $1.9\mathrm{MeV}$. The measurements are part of an international effort to reduce uncertainties and provide independent nuclear data relevant for fast-spectrum reactors. The shape of the measured cross section is in good agreement with data from Tovesson et al. [Phys. Rev. C 79, 014613 (2009)] and with the most recent data from Matei et al. [Phys. Rev. C 95, 024606 (2017)], but disagrees with the shapes of ENDF/B-VII.1 and JEFF3.2 evaluations. Absolute values of $\sigma (n,f)$, obtained under some assumptions, indicate an overestimation of $\sigma (n,f)$ in the evaluated libraries at 1.0 and $1.4\mathrm{MeV}$, while a good agreement is found with ENDF/B-VII.1 at $1.9\mathrm{MeV}$. A careful analysis of the impact of scattered neutrons and anisotropy of the fission fragment angular distribution has been performed. The measurement of the neutron flux by means of a proton-recoil detector is discussed. A comprehensive study of corrections applied to the data, of associated uncertainties, and of correlations between the measurements at the different energies is presented.

Figure 1

Main existing data sets (symbols) on ${}^{242}\mathrm{Pu}$ $\sigma (n,f)$ in the region between 0.8 and $2.6\mathrm{MeV}$ neutron energy (source EXFOR) and evaluations [4, 5] (lines).

Figure 2

Experimental setup.

Figure 3

Experimental pulse-height spectrum for ${}^{242}\mathrm{Pu}$ spontaneous fission measured with a photovoltaic cell. The energy threshold used to discriminate fission fragments and $\alpha$ background is indicated by the arrow.

Figure 4

Fission detector pulse-height spectra for ${}^{242}\mathrm{Pu}$ (a)–(d) and ${}^{240}\mathrm{Pu}$ (e)–(h) spontaneous fission at the beginning of the experiment (a),(e) and after 1 (b),(f), 5 (c),(g), and 7 days (d),(h).

Figure 5

Simulated fission rate as a function of neutron energy for a $1.1\mathrm{MeV}$ neutron source. Results for both the water-based (W.C.) and air-based (A.C.) cooling systems are presented. In the MCNP simulation the proton beam energy loss in the TiT target is not implemented.

Figure 6

Experimental setup as modeled in MCNP simulations. Only the back of the vacuum chamber is shown.

Figure 7

Spontaneous-fission efficiency with its statistical uncertainty as a function of run number. The full line is the mean value, the blue dashed lines indicate its statistical uncertainty (one standard deviation), the red dotted lines account for both statistical and systematic uncertainties.

Figure 8

(a) Proton pulse-height spectra obtained for standard (black) and background (red) measurements at $2\mathrm{MeV}$ nominal neutron energy. (b) Experimental proton pulse-height spectrum after background subtraction at $2\mathrm{MeV}$ nominal neutron energy (black), and simulated proton pulse-height spectra for a uniform $4\mu \mathrm{m}$ PP foil (blue) and for a non-uniform ${}^1\mathrm{H}$-contaminated PP foil (pink). Numbers indicate the different contributions to the proton pulse height spectrum (see text).

Figure 9

Comparison of experimental and simulated proton pulse-height spectra for 1.1 (a), 1.5 (b), and $2\mathrm{MeV}$ (c) nominal neutron energy. The spectra are simulated for a $4\mu \mathrm{m}$ uniform and a nonuniform PP foil (see text). The experimental spectra are corrected for the background and represent the whole statistics for each energy. The two vertical lines indicate the energy limits of integration of the proton peak.

Figure 10

Energy distributions of neutrons impinging on the ${}^{242}\mathrm{Pu}$ target for a nominal energy of $1.1\mathrm{MeV}$.

Figure 11

Comparison of the results of this experiment obtained with the air-based (red triangles) and water-based (red inverted triangles) cooling systems to the evaluations [4, 5] and the most relevant experiments. Our results are normalized to the ENDF-B/VII.1 $\sigma (n,f)$ value at $1.9\mathrm{MeV}$.