  The $\beta$ decays of the ground state (gs) and isomeric state (m) of
$^{96}$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 de-excitation of the 1581.6 keV level in
$^{96}$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 $^{96\text{gs}}$Y (0$^+$) 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
$^{96\text{gs}}$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
$^{96\text{m}}$Y (8$^{+}$), 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 de-excitation patterns. $^{96\text{m}}$Y (8$^{+}$) 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 (...)
