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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\footnote{University of Birmingham, 
Bristol University,
Imperial College London, University of Oxford, and Rutherford Appleton
  Laboratory} studying
monolithic CMOS pixel sensors for future colliders, in particular high
energy lepton colliders (ILC and CLIC).

The collaboration is studying both fundamental technologies for future
detector sensors and also novel applications of CMOS 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) and is exploring pixels with
four-transistor (``4T'') structures for particle physics.
The DECAL  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}. The 4T structure is a
new approach to monolithic pixels coming from the imaging area, which has a 
very low-noise
performance.
A prototype sensor has been produced and shows very promising
performance but requires further testing with beam.
The aim is to explore this low-noise design further in upcoming sensor
productions.

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 \cite{4ttwepp09} 
, is based on a 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 14 different 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 constrain significantly 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 give 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 or even 20\,GeV would be sufficient if reasonable
electron rates at 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. With an estimate 
of
around 1M events being needed, for each combination of
converter thickness and energy point,
to accurately measure the
shower core, then at least eight days will be needed to acquire the total
dataset.

\begin{thebibliography}{99}
\bibitem{SPIDERWEB}
\url{https://www.spider.ac.uk}.

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

\bibitem{SPIDERPRC}
The SPiDeR Collaboration,
``DESY PRC Report'', Oct 2009,
\url{http://www.hep.ph.imperial.ac.uk/calice/official/091105prc/spider.pdf}
%\url {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,
\url{https://twiki.cern.ch/twiki/pub/CALICE/CaliceCollaboration/CALICE\_PRC09.pdf}.

\bibitem{4ttwepp09} R.~Coath {\it et al.},
``Advanced Pixel Architectures for Scientific Image Sensors'',
presented at TWEPP '09, Paris, France, September 2009 and
to be published in the proceedings thereof.

\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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