Study of $\boldsymbolβ$ Decay Shape Factors in First-Forbidden Transitions with $\boldsymbol{ΔI^π= 0^-}$ for Reactor Antineutrino Spectra Predictions

TL;DR

This work addresses the accuracy of reactor antineutrino spectrum predictions by measuring and analyzing the beta spectra of the major contributing decays $^{92}$Rb and $^{142}$Cs using high-purity beams and novel $\Delta E$–$E$ detectors. It integrates detailed calibrations, Geant4-based detector-response simulations, contaminant assessments, and EM-based deconvolution to obtain deconvolved beta spectra, then tests beta-shape models including standard, Hayen_Allowed, Huber, and first-forbidden corrections for $\Delta I^{\pi}=0^-$ transitions. The key findings show that first-forbidden ground-state corrections for the $^{142}$Cs decay have negligible impact on the spectrum and that TAGS-based ground-state feedings provide better agreement with the deconvolved data than high-resolution data, underscoring the Pandemonium effect's role in shaping reactor predictions. The results validate the use of Pandemonium-free feedings in summation calculations and motivate extending measurements to first-forbidden transitions with $\Delta I>0$ to further reduce uncertainties in reactor antineutrino spectra.

Abstract

The electron spectra of the $β$ decays of $^{92}$Rb and $^{142}$Cs, key contributors to the reactor antineutrino spectrum, were measured at the IGISOL facility using radioactive beams of high isotopic purity. The shapes of the measured $β$ spectra were compared with various $β$ shape models, including first-forbidden correction factors for $ΔI^π= 0^-$ ground-state to ground-state transitions. Comparisons with previous experimental results are also provided. The shapes of the newly measured $β$ spectra are well reproduced employing feedings extracted from total absorption gamma spectroscopy measurements.

Figures (13)

• Figure 1: Mounted e-Shape detector before its installation inside the setup chamber.
• Figure 2: Scheme of the I233 experimental setup (side view). The radioactive beam (blue arrow) is extracted from JYFLTRAP (left side of the figure) and passes through the trap's beam tuning system and security valve to the I233 setup (right side of the figure). A magnetic tape is drawn from the tape station, located below the beam trajectory. The tape is guided to the chamber where the detectors are placed for beam implantation. A vacuum extension tube connects the trap to the setup. A beam collimator system is installed in the vacuum extension tube. The e-Shape chamber is connected to the vacuum extension and contains the e-Shape detectors inside. The HPGe detector and a turbo pump are placed below and above the chamber, respectively.
• Figure 3: Fit to the end-point channel of the $^{98}$Zr $\beta$ decay. Upper panel: The black line represents the plastic coincidence spectrum of the $^{98}$Zr$\rightarrow$$^{98}$Nb$\rightarrow$$^{98}$Mo $\beta$ decay chain measured with an e-Shape detector. The blue line corresponds to the fit of the $^{98}$Nb $\beta$ spectrum, which serves as the background function for the total fit of the spectrum, represented by the red line, from which the end-point channel of $^{98}$Zr is obtained. Lower panel: The relative difference between the total fit and the experimental spectrum is shown, together with the one-sigma uncertainty band in gray. The relative differences are defined as $\frac{model-exp}{exp}$.
• Figure 4: Fit to the E0 conversion electron peak in the $^{98}$Nb $\beta$ decay. Upper panel: The black line represents the plastic coincidence spectrum of the $^{98}$Zr$\rightarrow$$^{98}$Nb$\rightarrow$$^{98}$Mo $\beta$ decay chain measured with an e-Shape detector. The violet line corresponds to the fit of the $^{98}$Zr $\beta$ spectrum, which serves as the background function for the total fit of the spectrum, represented by the red line. The green and blue lines correspond to Gaussian fits with high- and low-energy exponential tails, associated with the conversion electrons emitted from the K and L1 atomic layers of $^{98}$Mo, respectively. The relative heights and channel separations correspond to the intensities and energy emissions provided by BrIcc KIBEDI_2008, respectively. Lower panel: The relative difference between the total fit and the experimental spectrum is shown, together with the one-sigma uncertainty band in gray.
• Figure 5: Upper panel: Comparison of the measured spectrum of the combined $^{114}$Pd$\rightarrow$$^{114}$Ag$\rightarrow$$^{114}$Cd $\beta$ decays with Monte Carlo simulations using different shape models. The background (Backg.) is added to the simulations. Fermi refers to the basic $\beta$ shape with the Fermi function. Hayen and Huber denote the basic $\beta$ shape with the corrections of Hayen et al.HAYEN_Allowed and Huber HUBER, respectively. The weak magnetism term of Huber HUBER is included in the comparison with the corrections of Hayen et al.HAYEN_Allowed. Lower panel: Relative differences between the experimental and simulated spectra. A one-sigma error band is shown in grey in the lower panel.