  Background: $\beta$-delayed multiple neutron emission has been observed for
some nuclei with A$\leq$100, with $^{100}$Rb being the heaviest $\beta$2n
emitter measured to date. So far, only 25 P$_{2n}$ values have been determined
for the $\sim$300 nuclei that may decay in this way. Accordingly, it is of
interest to measure P$_{2n}$ values for the other possible multiple neutron
emitters throughout the chart of the nuclides. It is of particular interest to
make such measurement for nuclei with A$>$100 to test the predictions of
theoretical models and simulation tools for the decays of heavy nuclei in the
region of very neutron-rich nuclei. In addition, the decay properties of these
nuclei are fundamental for the understanding of astrophysical nucleosynthesis
processes such as the $r$-process, and safety inputs for nuclear reactors.
Purpose: To determine for the first time the two neutron branching ratio,
P$_{2n}$ value, for $^{136}$Sb through a direct neutron measurement, and to
provide precise P$_{1n}$ values for $^{136}$Sb and $^{136}$Te. Method: Pure
beams were provided by the JYFLTRAP at the IGISOL facility of the University of
Jyv\"askyl\"a, Finland. The purified ions were implanted into a moving tape at
the end of the beam line. The detection setup consisted of a plastic
scintillator placed right behind the implantation point, and the BELEN
detector, based on neutron counters embedded in a polyethylene matrix. The
analysis was based on the study of the $\beta$- and neutron- growth-and-decay
curves and the $\beta$-one-neutron and $\beta$-two-neutron time correlations.
Results: The P$_{2n}$ value of $^{136}$Sb was found to be 0.14(3)\% and the
measured P$_{1n}$ values for $^{136}$Sb and $^{136}$Te were found to be
32.2(15)\% and 1.47(6)\%, respectively. The measured P$_{2n}$ value is a factor
44 smaller than predicted by the FRDM+QRPA model used for $r$-process
calculations.
