Total absorption $\gamma$-ray spectroscopy of the $\beta$ decays of ${}^{96\text{gs},m}\mathrm{Y}$

The $\beta$ decays of the ground state (gs) and isomeric state $\left(m\right)$ of ${}^{96}\mathrm{Y}$ have been studied with the total absorption $\gamma$-ray spectroscopy technique at the Ion Guide Isotope Separator On-Line facility. The separation of the $8^{+}$ isomeric state from the $0^{-}$ ground state was achieved thanks to the purification capabilities of the JYFLTRAP double Penning trap system. The $\beta$-intensity distributions of both decays have been independently determined. In the analyses the deexcitation of the 1581.6 keV level in ${}^{96}\mathrm{Zr}$, in which conversion electron emission competes with pair production, has been carefully considered and found to have significant impact on the $\beta$-detector efficiency, influencing the $\beta$-intensity distribution obtained. Our results for ${}^{96\text{gs}}\mathrm{Y}$ $\left(0^{-}\right)$ confirm the large ground state to ground state $\beta$-intensity probability, although a slightly larger value than reported in previous studies was obtained, amounting to $96.6_{-2.1}^{+0.3}\%$ of the total $\beta$ intensity. Given that the decay of ${}^{96\text{gs}}\mathrm{Y}$ is the second most important contributor to the reactor antineutrino spectrum between 5 and 7 MeV, the impact of the present results on reactor antineutrino summation calculations has been evaluated. In the decay of ${}^{96m}\mathrm{Y}$ $\left(8^{+}\right)$, previously undetected $\beta$ intensity in transitions to states above 6 MeV has been observed. This shows the importance of total absorption $\gamma$-ray spectroscopy measurements of $\beta$ decays with highly fragmented deexcitation patterns. ${}^{96m}\mathrm{Y}$ $\left(8^{+}\right)$ is a major contributor to reactor decay heat in uranium-plutonium and thorium-uranium fuels around 10 s after fission pulses, and the newly measured average $\beta$ and $\gamma$ energies differ significantly from the previous values in evaluated databases. The discrepancy is far above the previously quoted uncertainties. Finally, we also report on the successful implementation of an innovative total absorption $\gamma$-ray spectroscopy analysis of the module-multiplicity gated spectra, as a first proof of principle to distinguish between decaying states with very different spin-parity values.

Figure 1

JYFLTRAP purification trap mass scans for $A=96$. The frequency is selected to extract the isobar of interest from the trap.

Figure 2

Experimental $\beta$-gated spectra of the measurements for ${}^{96\text{gs}}\mathrm{Y}$ (black) and ${}^{96m}\mathrm{Y}$ (gray) free of contaminants. The sum peak of the two 511 keV $\gamma$ rays emitted in the positron annihilation from the pair emission of the 1581.6 keV level in ${}^{96}\mathrm{Zr}$ is indicated (see text for details).

Figure 3

Scheme of the possible $E0$ deexcitations of the level at 1581.6 keV excitation energy in ${}^{96}\mathrm{Zr}$. Pair production occurs with a probability $P_{e^{-}{e}^{+}}$.

Figure 4

MC response for the 1581.6 keV level of the $\beta$-gated TAGS spectrum. The solid gray line shows the normal penetration of $\beta$ particles in the spectrometer, whereas the solid black line shows the effect of also taking into account conversion electrons and the pair production for the deexcitation of this level. For more detail we add the dotted blue line that corresponds to considering only conversion electrons together with $\beta$ particles, and the red dashed line showing the contribution of pair production and $\beta$ electrons.

Figure 5

Efficiency of the $\beta$ detector as a function of the excitation energy in the daughter nucleus (${}^{96}\mathrm{Zr}$). The efficiency considering only $\beta$ particles (gray line) is compared with that obtained taking conversion electrons and pair production into account as well (black squares). The upper and lower panels show the comparisons for ${}^{96\text{gs}}\mathrm{Y}$ and ${}^{96m}\mathrm{Y}$, respectively. See text for more details.

Figure 6

(Top panel) Experimental $\beta$-gated spectrum summing-pileup subtracted for ${}^{96\text{gs}}\mathrm{Y}$ (gray) and reconstructed spectrum (red). The MC responses of each level fed in the daughter nucleus are shown with thinner lines and the 1022 keV peak due to the positron annihilation is highlighted. The relative deviations between experimental and reconstructed spectra are shown. (Bottom panel) $\beta$ intensities for the present TAGS results (red dots with error bars) and high-resolution $\gamma$-spectroscopy data from ENSDF (green line).

Figure 7

Module-multiplicity gated TAGS spectra for $M_m=1$–8. The experimental spectra free of contaminants and the MC spectra obtained with the results of the TAGS analysis are compared for the decay of ${}^{96\text{gs}}\mathrm{Y}$ (experiment: light gray, MC: red) and the decay of ${}^{96m}\mathrm{Y}$ (experiment: gray, MC: black).

Figure 8

(Top panel) Experimental $\beta$-gated spectrum summing-pileup subtracted for ${}^{96m}\mathrm{Y}$ (gray) and reconstructed spectrum (red). The MC responses of each level fed in the daughter nucleus are shown with thinner lines. The dominant level fed is shown as a blue line. The relative deviations between experimental and reconstructed spectra are shown. (Bottom panel) $\beta$ intensity for the present TAGS results (red dots with error bars) and high-resolution $\gamma$-spectroscopy data from ENSDF (green line).

Figure 9

Experimental $\beta$-gated TAGS spectra (gray line) with conditions in $M_m=5$ (top) and $M_m=6$ (bottom) for a measurement of ${}^{96m}\mathrm{Y}$ with a contamination of ${}^{96\text{gs}}\mathrm{Y}$. The experimental spectra are summing-pileup subtracted. The reconstructed spectra with the results of the corresponding TAGS analyses are shown by the red line. The relative deviations between experimental and reconstructed spectra are shown in both cases.

Figure 10

$\beta$-intensity distribution determined in the normal TAGS analysis (gray dots with error bars) compared with those obtained in the TAGS analyses of the $M_m=5$ (solid black) and $M_m=6$ (dotted red) TAGS spectra.

Figure 11

Ratio of reactor antineutrino spectra for ${}^{235}\mathrm{U}$, with and without the present new data, as a function of energy when the results obtained in the present work replace Rudstam's [76] data for ${}^{96\text{gs}}\mathrm{Y}$ and JEFF-3.3 data for ${}^{96m}\mathrm{Y}$. The effect of ${}^{96\text{gs}}\mathrm{Y}$ (solid line) and ${}^{96m}\mathrm{Y}$ (dotted line) are presented separately. The spike observed for ${}^{96\text{gs}}\mathrm{Y}$ is due to a difference in the $Q_{\beta }$ value between the original calculation and the current value.

Figure 12

Electron spectrum for the decay of ${}^{96\text{gs}}\mathrm{Y}$ from a MC simulation with the $\beta$-intensity distributions obtained in this work (solid blue line) compared with the experimental data of Tengblad et al. [77] (black points with error bars). The spike corresponds to the $E0$ transition from the level at 1581.6 keV. Note that we do not apply any experimental resolution to the MC $\beta$ spectra. Additional calculations without conversion electrons (dotted red) and assuming first forbidden unique $\beta$ shapes (dashed green) are also included for comparison. All spectra are normalized to 1. The relative deviations between calculated and experimental spectra are shown.