Absolute measurement of the ${}^{242}\mathrm{Pu}$ neutron-capture cross section

The absolute neutron-capture cross section of ${}^{242}\mathrm{Pu}$ was measured at the Los Alamos Neutron Science Center using the Detector for Advanced Neutron-Capture Experiments array along with a compact parallel-plate avalanche counter for fission-fragment detection. The first direct measurement of the ${}^{242}\mathrm{Pu}(n,\gamma )$ cross section was made over the incident neutron energy range from thermal to $\approx 6$ keV, and the absolute scale of the $\left(n,\gamma \right)$ cross section was set according to the known ${}^{239}\mathrm{Pu}(n,f)$ resonance at $E_{n,R}=7.83$ eV. This was accomplished by adding a small quantity of ${}^{239}\mathrm{Pu}$ to the ${}^{242}\mathrm{Pu}$ sample. The relative scale of the cross section, with a range of four orders of magnitude, was determined for incident neutron energies from thermal to $\approx 40$ keV. Our data, in general, are in agreement with previous measurements and those reported in ENDF/B-VII.1; the ${}^{242}\mathrm{Pu}(n,\gamma )$ cross section at the $E_{n,R}=2.68$ eV resonance is within $2.4\%$ of the evaluated value. However, discrepancies exist at higher energies; our data are $\approx 30\%$ lower than the evaluated data at $E_n\approx 1$ keV and are approximately $2\sigma$ away from the previous measurement at $E_n\approx 20$ keV.

Figure 1

Production/destruction network of nuclear reactions in plutonium-containing nuclear fuel, which leads to the formation of americium (Am) and curium (Cm) isotopes [1]. The ${}^{242}\mathrm{Pu}\left(n,\gamma \right)$ reaction is among the key inputs to the series of reactions. Improved ${}^{242}\mathrm{Pu}\left(n,\gamma \right)$ cross sections would contribute to improved network calculations. (The unit ‘m’ for the half-life of ${}^{244}\mathrm{Am}$ denotes ‘minutes’.)

Figure 2

The DANCE summed $\gamma$-ray energy versus cluster multiplicity with a gate at $E_{n,R}=2.68$ eV. Events with $M_{\mathrm{cl}}=1\text{–}8$ and $E_{\mathrm{sum}}=0\text{–}14$ MeV are shown.

Figure 3

The background-subtracted, summed $\gamma$-ray energy spectrum for the ${}^{242}\mathrm{Pu}\left(n,\gamma \right)$ resonance at $E_{n,R}=2.68$ eV. The $E_{\mathrm{sum}}=3.5\text{–}4.5$ MeV region is shaded in black and is located on the lower side of the ${}^{242}\mathrm{Pu}\left(n,\gamma \right) Q$ value of 5.034 MeV. The energy region shaded in gray is $E_{\mathrm{sum}}=0.0\text{–}5.25$ MeV. Only cluster multiplicities 3 and 4 are shown.

Figure 4

The summed $\gamma$-ray spectra for incident neutron energies at (a) 2.68 eV, (b) 1.0 keV, and (c)10.0 keV with the requirement of $M_{\mathrm{cl}}=(3,4)$. The inclusive spectra are black, the backgrounds (data normalized over the area of $E_{\mathrm{sum}}=7\text{–}9$ MeV) are blue, and the subtracted spectra are red.

Figure 5

The cluster multiplicity $\left(M_{\mathrm{cl}}\right)$ spectrum for the ${}^{242}\mathrm{Pu}\left(n,\gamma \right)$ resonance at $E_{n,R}=2.68$ eV. The highlighted region corresponds to multiplicities $M_{\mathrm{cl}}=3$ and 4. The solid line is a Poisson distribution fit to $M_{\mathrm{cl}}=2\text{–}9$ to estimate multiplicities $M_{\mathrm{cl}}=0$ and 1.

Figure 6

The time difference between $\gamma$ rays detected by the DANCE array and charged particles detected in the PPAC for ${}^{239}\mathrm{Pu}(n,f)$. The time resolution is $\approx 2.8$ ns for the peak at $\approx -5$ ns. An $\approx 8$ ns gate was placed around this peak. The peak at 0 ns is an artifact of the timing algorithm and is not related to the event time-of-flight.

Figure 7

The measured cluster multiplicity versus total $\gamma$-ray energy for the neutron-induced fission of ${}^{239}\mathrm{Pu}$.

Figure 8

Comparison of the absolute ${}^{242}\mathrm{Pu}\left(n,\gamma \right)$ cross section between the current work (black circles) and the evaluated data in ENDF/B-VII.1 [29] (histogram) spanning (a) $E_n={10}^{-2}\text{–}20$ eV, (b) $E_n=10\text{–}{10}^3$ eV, and (c) $E_n={10}^3\text{–}{10}^6$ eV.