Study of beta spectrum shapes relevant to the prediction of reactor antineutrino spectra

TL;DR

This work addresses reactor antineutrino spectrum predictions by providing direct measurements of beta-spectrum shapes for two key decays, $^{92}$Rb and $^{142}$Cs. The team uses trap-assisted beta shape spectroscopy with isotopically pure beams from IGISOL and JYFLTRAP, implanting the isotopes on tape and detecting betas with two deltaE-E telescopes in vacuum, complemented by HPGe and CeBr3 gamma detectors and an optimized data acquisition. The true beta spectra are extracted by solving the inverse problem with the response matrix R_ij and maximum-entropy/expectation-maximization deconvolution, with Geant4-based MC validated simulations. The deduced spectra are then compared to predictions built from TAGS feedings, including shape corrections proposed by Hayen et al. and Huber, and the results show good agreement, reinforcing the validity of these corrections for $0^-$ to $0^+$ decays; first-forbidden corrections were found to have negligible impact on the spectra in this energy range. The work provides experimental beta-spectrum data free from the Pandemonium effect, enabling independent checks of antineutrino-spectrum calculations and guiding future measurements on other relevant decays.

Abstract

The shapes of the beta spectra of 92Rb and 142Cs, two of the beta decays most relevant for the prediction of the antineutrino spectrum in reactors, have been measured. A new setup composed of two dE-E telescopes has been used. High purity radioactive beams of the isotopes of interest were provided by the IGISOL facility using the JYFLTRAP double Penning trap. The resulting beta spectra have been compared with model predictions using beta decay feedings from total absorption gamma spectroscopy measurements and shape corrections employed in the calculation of the antineutrino spectrum, validating both further. The procedure can be extended to other relevant nuclei in the future, providing solid ground for the prediction of the antineutrino spectrum in reactors.

Figures (6)

• Figure 1: Geometry of the e-shape setup implemented in Geant4 simulations. The upper right inset shows the vacuum chamber of the setup fully implemented in the simulations. The figure depicts the geometry of the relevant detectors: the e-shape telescopes placed inside the vacuum chamber, an HPGe below the implantation point, and a CeBr3 scintillator placed at the right side of the implantation point. These two detectors were placed outside the vacuum chamber and used for beta delay gamma identification. Please note that for the sake of simplicity, only the active volume of the gamma detectors is shown.
• Figure 2: Upper panel. Comparison of the measured spectrum of the combined $^{114}$Pd $\rightarrow$$^{114}$Ag $\rightarrow$$^{114}$Cd beta decays with Monte Carlo simulations employing different corrections. The background has been added to the simulations for the comparison. Std. means the classical Fermi approximation. Hayen Corr. and Huber Corr. stand for the Fermi approach plus the corrections introduced by Hayen et al.HAYEN_Allowed and Huber HUBER (see the End Matter section for more details). The weak magnetism term of Huber HUBER was included in the corrections of Hayen et al.HAYEN_Allowed. Lower panel. Relative differences between the experimental and simulated spectra. In the insert, a frequency plot of the differences is presented for the full energy range.
• Figure 3: Comparison of the measured $^{92}$Rb experimental spectrum (Exp., corresponding to $D_i$ in formula (\ref{['eq_Response']})) with the reproduction of the experiment obtained from the result of the expectation-maximization deconvolution algorithm (Rep., corresponding to $\sum_j\mathcal{R}_{ij}O_j$, on right side of formula (\ref{['eq_Response']})), obtained after the analysis. The relative differences are presented in the lower panel.
• Figure 4: Comparison of the deduced (or true) beta spectrum of the ${}^{92}\text{Rb} \rightarrow {}^{92}\text{Sr}$ beta decay ($^{92}$Rb Decay) with various beta decay feedings and beta shape correction models. Ref. corresponds to the prediction obtained using ENSDF high-resolution feedings ENSDF_92Rb, employing only the Fermi function. Zakari I and II refer to predictions based on Zakari-Issoufou et al. TAGS feedings ZAKARI. For Zakari I, the allowed shape corrections from Hayen et al.HAYEN_Allowed, the weak magnetism term from Huber HUBER, and the ground state to ground state first-forbidden shape correction from Hayen et al.HAYEN_1F were applied. For Zakari II, the allowed shape corrections from Huber HUBER were used. Rasco I and II denote predictions based on Rasco et al. TAGS feedings RASCO, with the same respective shape corrections applied.
• Figure 5: Comparison of the deduced (or true) beta spectrum of the $^{142}\text{Cs}\rightarrow{}^{142}\text{Ba}$ beta decay ($^{142}$Cs Decv.) with various beta decay feedings and beta shape correction models. Ref. corresponds to the prediction obtained using ENSDF high-resolution feedings ENSDF_142Cs, employing only the Fermi function. Wolinska I and II refer to predictions based on Wolińska-Cichocka et al. TAGS feedings WOLINSKA. For Wolinska I, the allowed shape corrections from Hayen et al.HAYEN_Allowed, the weak magnetism term from Huber HUBER, and the ground state to ground state first-forbidden shape correction from Hayen et al.HAYEN_1F were applied. For Wolinska II, the allowed shape corrections from Huber HUBER were used.