Total absorption spectroscopy study of the $\beta$ decay of ${}^{86}\mathrm{Br}$ and ${}^{91}\mathrm{Rb}$

The beta decays of ${}^{86}\mathrm{Br}$ and ${}^{91}\mathrm{Rb}$ have been studied using the total absorption spectroscopy technique. The radioactive nuclei were produced at the Ion Guide Isotope Separator On-Line facility in Jyväskylä and further purified using the JYFLTRAP. ${}^{86}\mathrm{Br}$ and ${}^{91}\mathrm{Rb}$ are considered to be major contributors to the decay heat in reactors. In addition, ${}^{91}\mathrm{Rb}$ was used as a normalization point in direct measurements of mean gamma energies released in the beta decay of fission products by Rudstam et al. assuming that this decay was well known from high-resolution measurements. Our results show that both decays were suffering from the Pandemonium effect and that the results of Rudstam et al. should be renormalized. The relative impact of the studied decays in the prediction of the decay heat and antineutrino spectrum from reactors has been evaluated.

Figure 1

Upper panel shows the relevant histograms for ${}^{91}\mathrm{Rb}$ decay: measured spectrum (square points), summing-pileup contribution (orange line), reconstructed spectrum response A (red line), reconstructed spectrum response B (blue line). Response A corresponds to the conventional analysis. Response B has additional optimization on the branching-ratio matrix to reproduce the measured $\gamma$ intensities in high-resolution experiments. The good reproduction of the experimental spectrum by both analyses makes the spectra almost indistinguishable from each other. Lower panel shows the relative residuals of the two reconstructed spectra with respect to the experimental one.

Figure 2

Comparison of the accumulated feeding distributions obtained in this work for the decay of ${}^{91}\mathrm{Rb}$ with the distributions from earlier high-resolution measurements [38] and with that obtained by Greenwood et al. [16].

Figure 3

Accumulated strength of the decay of ${}^{91}\mathrm{Rb}$ obtained for the two analyses presented in this work compared with QRPA calculations assuming oblate and prolate shapes for the ground state of ${}^{91}\mathrm{Rb}$.

Figure 4

Upper panel shows the relevant histograms for ${}^{86}\mathrm{Br}$ decay: measured spectrum (squares with errors), reconstructed spectrum response A (blue line), reconstructed spectrum response B (red line), summing-pileup contribution (orange line), background (green line). The good reproduction of the experimental spectrum by both analyses makes the spectra almost indistinguishable from each other. Lower panel shows the relative residuals of the two reconstructed spectra with respect to the experimental one.

Figure 5

Comparison of the accumulated feeding distributions obtained in this work for the decay of ${}^{86}\mathrm{Br}$ with the distributions from earlier high-resolution measurements [42].

Figure 6

Upper panel shows the differences between the mean energies reported in the work of Rudstam et al. [3] and the deduced mean $\gamma$ energies from the work of Greenwood et al. [16] and our recent data (in red) [13, 14, 47]. Lower panel shows the the same as the upper panel, but renormalizing the mean energies reported in Rudstam et al. [3] by the 1.14 value deduced in this work.

Figure 7

Comparison of the $\beta$ spectrum deduced from our TAGS measurements, Greenwood measurements, and from ENSDF, assuming allowed transitions, with the measurements of Tengblad et al. [4].

Figure 8

Comparison of the $\beta$ spectrum deduced from our TAGS measurements for both analyses presented in this work and from ENSDF, assuming allowed transitions, with the measurements of Tengblad et al. [4].

Figure 9

Impact of the new TAS data relative to the high-resolution data on the decay heat of ${}^{235}\mathrm{U}$. The continuous line represents the electromagnetic component, and the dotted line represents the light-particle component.

Figure 10

Impact of the new TAS data relative to the high-resolution data on the decay heat of ${}^{239}\mathrm{Pu}$ (for details see Fig. 9).

Figure 11

Impact of the new TAS data relative to the high-resolution data on the antineutrino spectrum of ${}^{235}\mathrm{U}$.

Figure 12

Impact of the new TAS data relative to the high-resolution data on the antineutrino spectrum of ${}^{239}\mathrm{Pu}$.