Statistical properties of ${}^{243}\text{Pu}$, and ${}^{242}\text{Pu}(n,\gamma )$ cross section calculation

The level density and $\gamma$-ray strength function ($\gamma \mathrm{SF}$) of ${}^{243}\text{Pu}$ have been measured in the quasicontinuum using the Oslo method. Excited states in ${}^{243}\text{Pu}$ were populated using the ${}^{242}\text{Pu}(d,p)$ reaction. The level density closely follows the constant-temperature level density formula for excitation energies above the pairing gap. The $\gamma \mathrm{SF}$ displays a double-humped resonance at low energy as also seen in previous investigations of actinide isotopes. The structure is interpreted as the scissors resonance and has a centroid of ${\omega }_{\mathrm{SR}}=2.42\left(5\right)$ MeV and a total strength of $B_{\mathrm{SR}}=10.1\left(15\right){\mu }_N^2$, which is in excellent agreement with sum-rule estimates. The measured level density and $\gamma \mathrm{SF}$ were used to calculate the ${}^{242}\text{Pu}(n,\gamma )$ cross section in a neutron energy range for which there were previously no measured data.

Figure 1

Initial excitation energy $E_{\mathrm{x}}$ versus $\gamma$-ray energy $E_{\gamma }$ from particle-$\gamma$ coincidences recorded from the ${}^{242}\text{Pu}{(d,p\gamma )}^{243}\mathrm{Pu}$ reaction. The raw $\gamma$-ray spectra (a) are first unfolded (b) by the NaI response function. The primary or first-generation $\gamma$-ray spectra (c) are extracted as function of excitation energy $E_{\mathrm{x}}$.

Figure 2

$\gamma$-ray multiplicity as function of excitation energy $E_x$ in ${}^{243}\text{Pu}$. Only $\gamma$ rays with $E_{\gamma }>0.4$ MeV are taken into account.

Figure 3

Level density for ${}^{243}\text{Pu}$. At low excitation energy $E_x$, the experimental data are normalized to the level density of known discrete levels (red solid line). At the neutron separation energy $S_n=5.038$ MeV, the normalization is done using the level density extracted from known neutron resonance spacings $D_0$. The connection between $\rho \left(S_n\right)$ (the upper right data point) and the experimental data is made with a constant-temperature formula with $T_{\mathrm{CT}}=0.40\left(1\right)$ MeV.

Figure 4

Experimental $\gamma \mathrm{SF}$ from the present ${(d,p\gamma )}^{243}\mathrm{Pu}$ experiment (black filled squares) and ($\gamma , x$) data (black filled triangles, empty triangles, and green dots) taken, respectively, from Berman et al. [30], Gurevitch et al. [31], and Moraes et al. [32]. The red curve represents the estimated underlying $E1$ part of the $\gamma \mathrm{SF}$. The structure in the present work at $E_{\gamma }=1–3.5$ MeV is interpreted as the scissors resonance.

Figure 5

The extracted $\gamma \mathrm{SF}$ for the scissors resonance in the quasicontinuum of ${}^{243}\text{Pu}$.

Figure 6

Measured level density (black filled squares) compared to the level density used in ENDF/B-VII.1 [43] and JENDL-4.0 [44] (red continuous curve), and TENDL2014 [42] (blue dotted-dashed curve) calculations. The measured level density was normalized to the level density of known levels (black line) and to the level density extracted from known neutron resonance spacings $D_0$ [22] (empty square).

Figure 7

Comparison between the $\gamma \mathrm{SF}$ extracted in the present work (red curve) from measured data (black filled squares) and the one used in ENDF/B-VII.1 and JENDL-4.0 (dashed black curve), TENDL2014 (blue dashed-dotted curve). The ($\gamma , x$) data (black filled triangles, empty triangles, and green dots) are taken, respectively, from Berman et al. [30], Gurevitch et al. [31], and Moraes et al. [32].

Figure 8

Calculated ${}^{242}\text{Pu}(n,\gamma )$ cross section using level density and $\gamma \mathrm{SF}$ parameters obtained in the present work, including the $M1$ scissors mode (continuous red curve with blue error-band) and without it (dashed red curve with red dots error-band). A zoom in the energy region 0.5 to 2 MeV, where the impact of the SR is the most important, is shown in the inset. The predictions are compared at low energy with measured data from Hockenbury et al. [46] (black triangles), Wisshak and Käppeler [47] (empty squares), and evaluations from ENDF/B-VII.1 (black curve), JENDL-4.0 (dashed grey curve), TENDL2014 (blue dotted-dashed curve).