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\centerline{\Large \bf SPiDeR: Request for SPS beam time in 2010}
\bigskip
\centerline{\Large \bf The SPiDeR Collaboration}
\bigskip
\centerline{\Large \bf December 2009}

\section{Introduction}
The SPiDeR (\underline{S}ilicon \underline{Pi}xel \underline{De}tector 
\underline{R}\&D) collaboration~\cite{SPIDERWEB}
consists of five UK institutes studying
monolithic CMOS pixel sensors for future colliders, in particular high
energy lepton colliders (ILC, CLIC).

The collaboration is studying both fundamental technologies for future
detector sensors and also novel applications of the sensors. These could
be applied both to tracking and calorimetry. In particular, the
collaboration is currently doing preliminary tests towards a digital
electromagnetic calorimeter (DECAL). This would be a new approach to
EM calorimetry where the initial electron or photon energy is estimated
by counting particles rather than from the deposited energy. 
The collaboration aims to provide a proof-of-principle of the DECAL
technique over the next three years. This DECAL
work is being undertaken as part of the CALICE 
collaboration~\cite{CALICEWEB}.

Further information about the SPiDeR collaboration
and its aims can be found in a recent DESY PRC submission~\cite{SPIDERPRC}.
The DECAL technique and status is summarised in the CALICE submission
to the same meeting~\cite{CALICEPRC}.

\section{The SPiDeR sensors}
There are two sensors which the collaboration would like to
test in beam in 2010, namely the ``Fortis'' and ``TPAC'' sensors.
Both sensors are around $1\times 1$\,cm$^2$ in size.

The tracking-like sensor, Fortis, is based on a four-transistor
(``4T'') pixel circuit design, which is expected to give very low noise
and high efficiency for MIPs. It was designed as a test sensor,
implementing several
possible design technologies, and so has over ten pixel designs
within each sensor. The pixel sizes vary from 15 to 45\,$\mu$m.
In addition, the sensors 
were fabricated both with and without
deep p-well implants~\cite{DPW} and using both standard and high-resistivity
substrates. This means there are multiple variants which should be tested
to understand the sensor fully and this will require significant beam time.

The TPAC (Tera-Pixel Active Calorimeter) sensor is a study sensor
for the DECAL application. This has a uniform array of 
$50\times 50$\,$\mu$m$^2$ pixels with a readout architecture 
compatible with an ILC beam timing structure. While this
$1\times 1$\,cm$^2$ sensor is too small to contain a full EM
shower, it can be used to study the shower core densities which
dominate the EM resolution and set the pixel size requirement for
a realistic DECAL.
Such measurements of high energy EM shower densities at the 
sub-millimetre level have not previously been performed. These
measurements would help to significantly constrain the sensor
design for the large-scale DECAL sensor which SPiDeR intends
to design and fabricate within the next two years. First
measurements of the shower densities will be done with a low
energy electron beam at DESY in March 2010.


\section{SPS request}
The collaboration estimates that two weeks in the SPS test beam 
would allow a thorough test of all variants of the Fortis sensor
and sufficient statistics for the TPAC shower density studies.
Both detectors require an external tracking telescope and so
will need to have the EUDET telescope available. The DAQ system
has been tested and found to run at around 100\,Hz. This implies
a total data rate of up to 1000 events per spill and the
estimates below are based on that figure.

One Fortis sensor at a time will be mounted between the planes of the EUDET
telescope. These tests
need high energy (120\,GeV) pions to minimise
scattering and so give the most accurate interpolation from the
telescope to the sensor. Each sensor variant will be exposed
for around one day to get sufficient statistics for all the pixel
designs within the sensor, so these measurements will take much of
the first week. The pions are also useful for MIP calibration of the
TPAC sensors, which will be mounted downstream with no converter
material, so acting as a second tracking system.

Following the Fortis studies, then the TPAC response to electron
showers will be measured. Varying amounts of tungsten converter 
material will be added in front of the sensors to get measurements
of the shower density as a function of material depth.
The previous DESY beam test will give
information on showers in the 1 to 6\,GeV range, so the CERN beam
test will be used to extend this range dramatically. Ideally,
6\,GeV data would also be taken at CERN to provide a cross-check
although 10\,GeV would be sufficient if lower energies are not
feasible. The beam energy would be scanned from this lowest point
up to around 100\,GeV.
The EUDET telescope is needed to provide an accurate shower centre;
this is essential as the core density falls rapidly on the scale
of 1\,mm so that the centre needs to be known more accurately than
this. Several tungsten thicknesses will be installed and the electron
energy will be scanned for each. With an estimate of around 1M events
per thickness and energy needed to accurately measure the shower core,
then at least eight days will be needed to acquire the total dataset.

\begin{thebibliography}{99}
\bibitem{SPIDERWEB}
{\tt https://heplnm061.pp.rl.ac.uk/display/spider/Home}.

\bibitem{CALICEWEB}
{\tt https://twiki.cern.ch/twiki/bin/view/CALICE/WebHome}.

\bibitem{SPIDERPRC}
The SPiDeR Collaboration,
``DESY PRC Report'', Oct 2009,
{\tt https://heplnm061.pp.rl.ac.uk/download/attachments/1114156/desyprc.pdf?version=1}.

\bibitem{CALICEPRC}
The CALICE Collaboration,
``Report to the DESY PRC'', Oct 2009,
{\tt https://twiki.cern.ch/twiki/pub/CALICE/CaliceCollaboration/CALICE\_PRC09.pdf}.

\bibitem{DPW}
J.~A.~Ballin {\it et al.},
  ``Monolithic Active Pixel Sensors (MAPS) in a quadruple well technology for nearly 100\% fill factor and full CMOS pixels,'',
Sensors {\bf 8} (2008) 5336.

\end{thebibliography}

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