New prompt spectral $\gamma$-ray data from the reaction ${}^{252}$Cf(sf) and its implication on present evaluated nuclear data files

We present results from new spectral prompt $\gamma$-ray measurements from the spontaneous fission of ${}^{252}$Cf. Apart from one recent experiment, about four decades have passed since the last dedicated experiments were reported in the literature. Hence, there was a need for a revision. We have measured prompt fission $\gamma$ rays with both cerium-doped LaBr${}_3$ and CeBr${}_3$ scintillation detectors, both of which exhibit excellent timing and good energy resolution. The emission yield was determined to be ${\nu }_{\gamma }=(8.30\pm 0.08)$/fission and ${\nu }_{\gamma }=(8.31\pm 0.10)$/fission, with the average energy of ${\epsilon }_{\gamma }=(0.80\pm 0.01)$ MeV and ${\epsilon }_{\gamma }=(0.80\pm 0.01)$ MeV and total energy of $E_{\gamma ,\mathrm{tot}}=(6.64\pm 0.08)$ MeV and $E_{\gamma ,\mathrm{tot}}=(6.65\pm 0.12)$ MeV, with the LaBr${}_3$ and CeBr${}_3$ detectors, respectively. Since the results from the two detectors are in excellent agreement and confirm the historical data, but not those in the present evaluated nuclear data files, we strongly recommend an update.

Figure 1

Energy resolution (FWHM) in percent as a function of $\gamma$-ray energy for a 2 in. $\times$ 2 in. LaBr${}_3$:Ce detector (circles) and a 1 in. $\times$ 2 in. CeBr${}_3$ detector (squares); the data are compared to a standard NaI detector as well as a BGO detectors [30].

Figure 2

Intrinsic peak efficiency as a function of $\gamma$-ray energy for a 2 in. $\times$ 2 in. large LaBr${}_3$:Ce detector (circles) and a 1 in. $\times$ 2 in. CeBr${}_3$ detector (squares) together with the corresponding results from Monte Carlo simulations (full lines). For comparison the efficiency of both a NaI detector (dotted line) and a BGO detector (dashed line), both of size 1.5 in. $\times$ 1.5 in., are shown as well [30].

Figure 3

Timing resolution (FWHM) as a function of $\gamma$-ray energy for a 2 in. $\times$ 2 in. LaBr${}_3$:Ce detector (circles) and a 1 in. $\times$ 2 in. CeBr${}_3$ detector (squares); see text for details. For comparison the timing resolution for different NaI and BGO detectors, used in previous studies for spectral measurements, is shown as well [5, 6, 30].

Figure 4

Experimental setup with an ionization chamber containing a ${}^{252}$Cf source, which had a source strength of $8600\pm 100$ fissions/s. The distance between the fission source and the $\gamma$-ray detector was 63 cm. More details may be found in the text.

Figure 5

(a) The integral simulated spectrum (red line) adjusted to the experimental one (black line) from the measurement with a 2 in. $\times$ 2 in. LaBr${}_3$:Ce detector. (b) The relative difference between the measured spectrum and the simulation. (c) The unfolded prompt fission $\gamma$-ray emission spectrum taken with a 2 in. $\times$ 2 in. LaBr${}_3$:Ce detector (full red line) and a 1 in. $\times$ 2 in. CeBr${}_3$ detector (dashed blue line). The inset focuses on $\gamma$-ray energies below 1 MeV and demonstrates the very good agreement between the results obtained with the two detectors used in this work.

Figure 6

(a) The prompt fission $\gamma$-ray emission spectrum from this work taken with a 2 in. $\times$ 2 in. LaBr${}_3$:Ce detector (full red line) shown together with data from Refs. [5, 27] as well as from ENDF/B-VII for comparison. In the high-energy range, all spectra agree rather well with each other, but the data from both ENDF/B-VII.0 [33] and Ref. [27] lack structure in the low-energy range. This is even more obvious in (b), which focuses on $\gamma$-ray energies below 0.8 MeV, and demonstrates on the other hand the very good reproduction of the historical data from Ref. [5] (see text for details).