  The uncertainty in the initial neutrino flux is the main limitation for a
precise determination of the absolute neutrino cross section. The ERC funded
ENUBET project (2016-2021) is studying a facility based on a narrow band beam
to produce an intense source of electron neutrinos with a ten-fold improvement
in accuracy. Since March 2019 ENUBET is also a Neutrino Platform experiment at
CERN: NP06/ENUBET. A key element of the project is the instrumentation of the
decay tunnel to monitor large angle positrons produced together with $\nu_e$ in
the three body decays of kaons ($K_{e3}$) and to discriminate them from neutral
and charged pions. The need for an efficient and high purity e/$\pi$ separation
over a length of several meters, and the requirements for fast response and
radiation hardness imposed by the harsh beam environment, suggested the
implementation of a longitudinally segmented Fe/scintillator calorimeter with a
readout based on WLS fibers and SiPM detectors. An extensive experimental
program through several test beam campaigns at the CERN-PS T9 beam line has
been pursued on calorimeter prototypes, both with a shashlik and a lateral
readout configuration. The latter, in which fibers collect the light from the
side of the scintillator tiles, allows to place the light sensors away from the
core of the calorimeter, thus reducing possible irradiation damages with
respect to the shashlik design. This contribution will present the achievements
of the prototyping activities carried out, together with irradiation tests made
on the Silicon Photo-Multipliers. The results achieved so far pin down the
technology of choice for the construction of the 3 m long demonstrator that
will take data in 2021.
