  The search for weakly interacting, light particles that couple to photons
received significant attention in recent years. When those particles are
produced at high energies, they lead to two, nearly collinear photons after
their decay and hence can be detected by an electromagnetic calorimeter system.
The typical dominant background in searches for those high energetic weakly
particles are single, high energetic photons, which leave similar signatures in
a standard calorimeter system. One promising approach to separate signal from
background events is to employ a dedicated pre-shower detector in front of the
calorimeter that can distinguish one- and two-photon signatures. In this work
we present a conceptual design of such detector which is able to separate one
from two collinear photon signatures with efficiencies between 20% to 80% for
two photons separated by 0.1mm to 2mm, respectively, with energies above 300
GeV and a background rejection of more than 90%. Our pre-shower detector design
has an active surface area of 10cm x 10cm, a depth of 230mm and is based on
Micromegas detectors, thus offering a cost effective solution.
