Prompt-fission $\gamma$-ray spectral characteristics from ${}^{239}\mathrm{Pu}(n_{\mathrm{th}},f)$

In this paper we present new results for prompt fission $\gamma$-ray spectral characteristics from the thermal neutron induced fission of ${}^{240}\mathrm{Pu}^{*}$. The measured spectra were unfolded by using the detectors' response functions, simulated with geant4. We obtained in average per fission a $\gamma$-ray multiplicity ${\overline{M}}_{\gamma }=(7.35\pm 0.12)$, a mean photon energy ${\overline{\epsilon }}_{\gamma }=(0.85\pm 0.02)$ MeV, and an average total energy released in fission ${\overline{E}}_{\gamma ,\mathrm{tot}}=(6.27\pm 0.11)$ MeV. Our results are in good agreement with historical data measured in the 1970s by Verbinski et al. and results from recent calculations in the framework of Monte Carlo Hauser–Feshbach models. Our measured average total energy is slightly smaller than the one deduced previously and present in evaluated data. From this we conclude that the ${}^{239}\mathrm{Pu}(n_{\mathrm{th}},f)$ reaction may be ruled out as possible source of $\gamma$ heating underestimation, when compared with benchmark calculations based on existing nuclear data.

Figure 1

Picture of the prompt-fission $\gamma$-ray measurement setup in Budapest: Four ${\text{LaBr}}_3$:Ce detectors 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 36 cm from the center of the chamber where the plutonium sample was mounted in the center of the cathode.

Figure 2

Fission-fragment distribution (a) before and (b) after applying $\alpha$ and pileup rejection to the raw data. The anode pulse height is proportional to the fragment's energy and the grid pulse height to its emission angle. In the upper plot (a) we can see multiple $\alpha$-particle pileups to the left, $\alpha$-fission-fragment pileups that are obvious in the center of the two fission fragments, blurring the separation of the heavy and the light fragments, and to the right an extra fission-fission pileup distribution that resembles the original one. In the bottom plot (b), all the $\alpha$-particle pileups and fission-fission pileups are properly filtered. A fraction of $\alpha$-fission-fragment pileups remains in the data, but they do not affect the total fission count.

Figure 3

Time-of-flight (TOF) spectra for different $\gamma$-ray energies. Time 0 is the fission trigger timing. The region on the far left before the fission trigger is pure background and is subtracted from the rest of the spectrum. The FWHM of the prompt-fission $\gamma$-ray peak is around 1.20 ns, and we consider most of prompt $\gamma$ rays to be included in the interval $[-3,3]$ ns which stops right before the neutrons reach the detector. A cut on this region after background subtraction constitutes our measured prompt-fission $\gamma$-ray spectrum. To the right of the prompt peak, we can see a few $(n,n^{\prime }\gamma )$ lines from all the materials surrounding the detectors. The iron from the cathode is too close to the target and when looking closely at the distribution around 847 keV, some of its neutron-induced $\gamma$ rays are slightly in the prompt window (easier to see in Fig. 4). This yield is removed during the unfolding process.

Figure 4

PFGS as measured before the unfolding process for the three lanthanum bromide detectors, which are labeled by their serial numbers. LaBr5415 has a slightly higher yield compared with the two other detectors. It could be explained by a shift in distance to target of 2 mm, which was the estimated precision of our distance measurements.

Figure 5

Overview of PFGS from the reaction ${}^{239}\mathrm{Pu}(n,f)$: the spectrum obtained in this work represents an average from three detectors after unfolding the response functions. Above $\approx 300$ keV the agreement with the measured data from Verbinski [9] and Chyzh [14] and with the calculated spectra by Litaize [15] and Talou [16] is rather good considering the error bars, below the spectrum from Ref. [14] deviates, which has been observed earlier [3]. The inset shows focus on the low-energy structure and shows how well the different calculations are able to predict it.