High-precision prompt-$\gamma$-ray spectral data from the reaction ${}^{241}\mathrm{Pu}$($n_{\mathrm{th}}$, $f$)

In this paper we present results from the first high-precision prompt-$\gamma$-ray spectral measurements from the reaction ${}^{241}\mathrm{Pu}$($n_{\mathrm{th}}$, $f$). Apart from one recent experiment, no data are reported in the literature for this fissioning system, which motivated a new dedicated experiment. We have measured prompt-fission $\gamma$ rays with three cerium-doped $\mathrm{La}{\mathrm{Br}}_3$ (two $5.08\text{cm}\times 5.08$ cm and one $7.62\text{cm}\times 7.62$ cm) and one $\mathrm{Ce}{\mathrm{Br}}_3$ ($5.08\text{cm}\times 5.08$ cm) scintillation detectors, which all exhibit excellent timing and good energy resolution. The average $\gamma$-ray multiplicity was determined to be ${\overline{\nu }}_{\gamma }=(8.21\pm 0.09)$ per fission, the average energy to be ${\epsilon }_{\gamma }=(0.78\pm 0.01)$ MeV, and the total energy to be $E_{\gamma ,\text{tot}}=(6.41\pm 0.06)$ MeV as the weighted average from all detectors. Since the results from all detectors are in excellent agreement, and the total released $\gamma$ energy is modestly higher than the one in the present evaluated nuclear data files, we suspect that the underestimation of the prompt-$\gamma$ heating in nuclear reactors is due to fast-neutron-induced fission on ${}^{238}\mathrm{U}$ or rather from fission induced by $\gamma$ rays from neutron capture in the construction material.

Figure 1

Sketch of the prompt-fission $\gamma$-ray-measurement setup: two $\mathrm{La}{\mathrm{Br}}_3$:Ce detectors and one $\mathrm{Ce}{\mathrm{Br}}_3$ detector, all of size (diameter $\times$ length) $5.08\text{cm}\times 5.08\text{cm}$, were placed at 90 degrees relative to the neutron-beam axis at a distance of 40 cm from the plutonium sample. The fourth detector, a $\mathrm{La}{\mathrm{Br}}_3$:Ce with dimensions $7.62\text{cm}\times 7.62$ cm, was placed at about 30 degrees relative to the neutron-beam axis and 60 cm away from the fission source.

Figure 2

Time-of-flight (TOF) spectra for different $\gamma$-ray energies, (a) $E_{\gamma }=200$ (5) keV and (b) $E_{\gamma }=843$ (21) keV. The FWHM of the prompt-fission $\gamma$-ray peak is 1.65 (2) and 1.06 (2) ns, respectively. The dashed arrows indicate from which part of the TOF spectrum the prompt-fission $\gamma$-ray spectra were constructed. The full (red) lines illustrate the sum of the different components. For higher TOF values the presence of $\gamma$ rays from isomer decay in fission fragments and inelastic scattering of prompt-fission neutrons in the detector material (${}^{27}\mathrm{Al}$, $E_{\gamma }=843$ keV) and the ionization chamber (IC; ${}^{56}\mathrm{Fe}$, $E_{\gamma }=846$ keV) is visible (dotted and dashed lines, respectively).

Figure 3

(a) The prompt-fission $\gamma$-ray emission spectrum for the thermal-neutron-induced fission on ${}^{241}\mathrm{Pu}$ taken with four lanthanide-halide detectors. All spectra agree very well with each other. Here, the point-wise uncertainty is given only for detector Q489 for the sake of clarity. (b) The low-energy part of the spectrum reveals the differences to the recent ENDF/B-VII.1 evaluation [13].

Figure 4

Overview of results for the measurement of prompt-$\gamma$-ray emission for the neutron-induced fission of ${}^{241}\mathrm{Pu}$: (a) Average photon multiplicity, (b) mean photon energy per fission, and (c) total released photon energy from this work are compared with values from ENDF/B-VII.1 [13] as well as to recent data from Ref. [25] Values averaged over the results obtained with all four detectors and their uncertainties are displayed as full drawn and dashed lines, respectively.