\section{Introduction}
\label{sec:introduction}

In this paper, we present results from a test sensor
which was designed to investigate a novel approach
to sampling electromagnetic calorimeters (ECAL).
In this approach, the
readout of the sensitive layers of the ECAL are binary, 
meaning that each of the pixels which cover the
sensitive layers
return a yes/no result with no further amplitude 
information. With a fine enough pixel granularity, specifically
less than 100\,$\mu$m pitch such that
the probability of more than one charged particle hitting
a pixel is kept small, then the number of pixels reporting
a hit in a layer should be effectively a measure of the 
number of charged particles crossing that layer. In contrast,
an analogue readout ECAL measures the energy deposited by
the charged particles rather than counting them directly.
Hence, compared to the binary device,
the resolution of the analogue device will be degraded
by the fluctuations in the amount of energy deposited by
each charged particle. Therefore, a binary ECAL should
have an intrinsically better resolution for electromagnetic
showers.

Although the potential applications for a binary ECAL are
wide, the sensor discussed in this paper was designed to study
a specific application, namely the International Linear
Collider (ILC)~\cite{ref:introduction:ilc}. 
The work presented in this paper
has been done within the context of the
CALICE Collaboration~\cite{ref:introduction:calice},
which is studying calorimetry for future linear colliders.
For a recent status report on the collaboration activities,
see~\cite{ref:introduction:prc}.
One particular crucial aspect of ILC calorimetry is its
ability to support particle flow techniques~\cite{ref:introduction:pfa},
so as to obtain excellent hadronic jet energy resolution. These techniques
depend on spatial pattern recognition within the calorimeters.
A binary ECAL would have far superior spatial granularity (by
about two orders of magnitude) compared to an analogue ECAL
and so should also give better particle flow performance.

The basic motivation for a binary readout ECAL is
presented in~\cite{ref:introduction:decal} and a preliminary
comparison of an ECAL with a fixed sampling geometry but
with binary and analogue readout has been presented
recently~\cite{ref:introduction:ichep}. As pointed out in
the former, a critical limitation for any simulation
comparison of binary and analogue ECALs is that the
density of charged particles in the core of a high-energy
electromagnetic shower has not been measured at the
relevant granularities, namely less than 100\,$\mu$m.
Differences in the core density between data and simulation 
could have a significant impact on the achievable resolution of
the binary ECAL. Therefore, the sensor described in this
paper was developed for two purposes. Firstly, it allows a study
of the feasibility of implementing a MAPS sensor for a binary 
ECAL. Secondly, once a functional sensor has been developed, 
it can then be used to 
experimentally determine the electromagnetic shower core density at the
relevant granularity. This will
allow simulation to give an accurate and robust prediction of the 
performance of a full-scale binary ECAL.

This paper covers the first of these issues, namely the performance
of a MAPS sensor designed to study an architecture appropriate for a
binary ECAL.
In the rest of this paper,
the design and fabrication of the test sensor are 
presented in section~\ref{sec:design}.
Section~\ref{sec:simulation} describes the simulations
used for modelling the sensor response.
Sections~\ref{sec:testp} and \ref{sec:pixel} presents
results of individual pixel measurements, from test pixels
with analogue readout and bulk pixels with digital readout,
respectively.
Finally, section~\ref{sec:sensor} describes
results of sensor-wide measurements.
