Impact of low-energy photons on the characteristics of prompt fission $\gamma$-ray spectra

In this paper we report on a new study of prompt $\gamma$-rays from the spontaneous fission of ${}^{252}\mathrm{Cf}$. Photons were measured in coincidence with fission fragments by employing four different lanthanide halide scintillation detectors. Together with results from a previous work of ours, we determined characteristic parameters with high precision, such as the average $\gamma$-ray multiplicity ${\overline{\nu }}_{\gamma }=(8.29\pm 0.13)$, the average energy per photon ${\epsilon }_{\gamma }=(0.80\pm 0.02)$ MeV, and the total $\gamma$-ray energy release per fission $E_{\gamma ,\text{tot}}=(6.65\pm 0.10)$ MeV. The excellent agreement between the individual results obtained in all six measurements proves the good repeatability of the applied experimental technique. The impact of low-energy photons, i.e., below 500 keV, on prompt fission $\gamma$-ray spectra characteristics has been investigated as well by comparing our results with those taken with the DANCE detector system, which appears to suffer from absorption effects in the low-energy region. Correction factors for this effect were estimated, giving results comparable to ours as well as to historical ones. From this we demonstrate that the different techniques of determining the average $\gamma$-ray multiplicity, either from a properly measured and normalized spectrum or a measured multiplicity distribution, give equivalent and consistent results.

Figure 1

(a) Unfolded prompt fission $\gamma$-ray emission spectra from the spontaneous fission of ${}^{252}\mathrm{Cf}$, taken with four different lanthanide halide scintillation detectors: three ${\mathrm{LaBr}}_3$:Ce detectors and the ${\mathrm{LaCl}}_3$:Ce detector used in this work (see text for details). (b) Comparison between one of the experimental spectra from this work with the ones of model calculations from Refs. [14, 15] and data from the evaluated library ENDF/B-VII.1 [16]. Apart from the energy range above 4 MeV, the slopes of all spectra are in rather good agreement, i.e., within the error bars from our measurement, which for the sake of clarity are shown for the first mentioned detector only.

Figure 2

Overview of results for the measurement of prompt $\gamma$-ray emission from the spontaneous fission of ${}^{252}\mathrm{Cf}$: (a) Average photon multiplicity, (b) mean photon energy per fission, and (c) total released photon energy. Values from both this work and our previous work [9] are shown in full (red) circles and compared to other experimental results from the early 1970s [2, 18] and from a recent work by Chyzh et al. [17]. Also shown are data from ENDF/B-VII.1 [16] and results from recent Monte Carlo Hauser-Feshbach calculations [14, 15]. Values averaged over our results obtained in six measurements with five different detectors and their uncertainties are displayed as full drawn and dashed (red) lines, respectively.

Figure 3

Comparison of prompt fission $\gamma$-ray multiplicity distributions: as (red) histogram the calculated one from this work based on the average experimental value and a negative binomial distribution, and the measured one from Ref. [17] as open black dots.

Figure 4

Comparison of PFGS from ${}^{252}\mathrm{Cf}$(sf), obtained with one of the ${\mathrm{LaBr}}_3$:Ce detectors and the one obtained with the DANCE detector system [17], depicted as full (red) and dashed (black) lines, respectively. The (red) hatched areas below 500 keV and above 4 MeV indicate the missing photon yield in the DANCE spectrum (see text for details).