\section{MEMPHYS detector} 
\label{sec:MEMPHYSdet}
\subsection{General considerations}
%
The 20 year long successful operation of the 
Super-Kamiokande detector has clearly
demonstrated the capabilities and limitations of large water \v{C}erenkov
detectors\,:
\begin{itemize}
\item This technique is by far the cheapest 
and the most stable to instrument a very large detector
mass, as price is dominated by the photodetectors and their associated
electronics (this price growing like the outer surface of the detector),
while the active mass, made of water, is essentially free except for the 
purification system
\item These detectors are mainly limited in size by the finite attenuation
length of \v{C}erenkov light, found to be 80 meters at $\lambda = 400$~nm
in Super-Kamiokande, and by the pressure of water on the photomultipliers at
the bottom of the tank, which gives a practical limit of 80~m in height.
At large depths, the maximal size of underground cavities actually limits
relevant dimensions to about 70~m.
\item The detection principle consists in measuring \v{C}erenkov rings produced
by charged particles going faster than light in water. This has several
consequences\,:
\begin{enumerate}
\item neutral particles and charged particles
below \v{C}erenkov threshold are undetectable, so that some energy may
be missing
\item complicated topologies are difficult to handle, and in practice only
events with less than 3 to 5 rings are efficiently reconstructed
\item ring topology, based on their degree of fuzziness, allows to separate
between electromagnetic (e, $\gamma$) rings and ($\mu$, $\pi$) rings
\item the threshold in particle energy depends mainly on photocathode
coverage and also on water purity (due to radioactive backgrounds, such as
radon). Super-Kamiokande has achieved an energy threshold of 5 MeV with
40\% cathode coverage
\item due to points 1 and 2, water \v{C}erenkov detectors are not suited to
measure high energy neutrino interactions, as more rings and more undetectable
particles are produced. A further limitation comes from the confusion between
single electron or gamma rings and high energy $\pi^0$'s giving 2 overlapping
rings. In practice water \v{C}erenkov's stay excellent neutrino detectors
for energies below 1 (may be 2) GeV, when interactions are mostly quasi-elastic 
and the 2 rings from $\pi^0$ well separated.
\end{enumerate}
\end{itemize}
%%%%%%%%%%%%%%%%%%%%%%%%%%%  
\subsection{Detector design}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Three detector designs are being carried out worldwide,
namely Hyper-Kamiokande \cite{uno} in Japan,
UNO \cite{hyperk} in the USA  and the present project MEMPHYS in Europe (Fig.~\ref{fig:Phys-MEMPHYSdetector}) \cite{deBellefon:2006vq}.
All of them are rather mild extrapolations of Super-Kamiokande, and rely on the 
expertise acquired after 20 years of operation of this detector.
Their main characteristics are summarized in table~\ref{WC:tab-1}.
%
%
\begin{figure}
\centering
\includegraphics[width=0.45\textwidth]{MEMPHYS.eps}
\caption{\label{fig:Phys-MEMPHYSdetector}Sketch of the MEMPHYS detector under the Fréjus mountain (Europe).}	
\end{figure}
%
\begin{sidewaystable}
\centering
%
\begin{tabular}{rccc}
\hline\noalign{\smallskip}
 Parameters                  &        \textbf{UNO} (USA)            &   
\textbf{HyperK} (Japan)          &      \textbf{MEMPHYS} (Europe)\\
\noalign{\smallskip}\hline\noalign{\smallskip}
\multicolumn{4}{l}{\textbf{Underground laboratory}}  \\ 
       location   &   Henderson / Homestake      &   Tochibora    & Fr\'ejus \\
                depth (m.e.w.)  &    4500/4800                 &    
1500                  &        4800  \\
Long Base Line (km)   & $1480\div2760$ / $1280\div2530$ & 290    &   130 \\
                       & FermiLab$\div$BNL       & JAERI         &   CERN \\
\noalign{\smallskip}\hline\noalign{\smallskip}
\multicolumn{4}{l}{\textbf{Detector dimensions}}          \\
type              & 3 cubic compartments   & 2 twin tunnels  & $3\div5$
shafts\\
                  &                        & 5 compartments  &           \\
dimensions            & $3\times (60\times60\times60)\mathrm{m}^3$ 
                                                                        &
$2\times 5 \times (\phi=43\mathrm{m} \times L=50\mathrm{m})$       &
$(3\div5)\times(\phi=65\mathrm{m} \times H=65\mathrm{m}) $ \\   
fiducial mass (kt)& 440                          &       550                  
& $440\div730$\\
\noalign{\smallskip}\hline\noalign{\smallskip}
\multicolumn{4}{l}{\textbf{Photodetectors}}          \\
           type   & 20" PMT              & 13" H(A)PD           & 12" PMT
\\
    number (internal detector)     & 57,000      & 20,000 per compartment
                                                & 81,000 per shaft (Inner Vol.) \\
 surface coverage & 40\% (1/3) \& 10\% (2/3) & 40\%  & 30\%\
                                            
\\
\noalign{\smallskip}\hline
\end{tabular}
%
\caption{\label{WC:tab-1}
\it Some basic parameters of the three Water \v{C}erenkov
detector baseline designs. An option is to fill one tank of MEMPHYS with Gadolinium (0.2\% Gd Cl$_3$).}
\end{sidewaystable}

These 3 projects aim at a fiducial mass around half a megaton, taking into
account the necessity to have a veto volume on the edge of the detector, 1 to 2
meters thick, plus a minimal distance of about 2 meters between photodetectors
and interaction vertices, leaving some space for ring development.
The main differences between the 3 projects lie in the geometry 
of the cavities (tunnel shape for Hyper-Kamiokande, shafts for MEMPHYS,
intermediate with 3 cubic modules for UNO), and the photocathode coverage,
similar to Super-Kamiokande for Hyper-Kamiokande 
and MEMPHYS, while UNO keeps this
coverage on only 1 cubic detector, while the 2 others have only 10\% coverage
for cost reasons. Another important parameter is the rock overburden, similar
for UNO and MEMPHYS (4800~mwe), but smaller for Hyper-Kamiokande (1500~mwe),
which might be a limiting factor for low energy physics, due to spallation
products and fast neutrons produced by cosmic muons, more
abundant by 2 orders of magnitude (see figure~\ref{muonflux}).

\begin{figure}[htb]
\centerline{\epsfig{figure=muonflux_red.eps,width=8cm}}
\caption{\it Muon flux as a function of overburden. 
The Frejus site is indicated by "LSM".}
\label{muonflux}
\end{figure}


The basic unit for MEMPHYS consists of a cylindrical detector module 65~meters
in diameter and 65~meters high, which can be housed
in a cylindrical cavity with 70~meter diameter and 80 meter height, as proven
by the prestudy. This corresponds to a water mass of 215 kilotons, that is
only 4 times the Super-Kamiokande detector. 
Conservatively substracting 2~m for the
outer veto plus 2~m for the fiducial volume, this leaves us with a fiducial
mass of 146 kilotons per module. The baseline design uses 3 modules, giving
a total fiducial mass of 440 kilotons, like UNO, corresponding to factor
20 increase over Super-Kamiokande (4 modules would give 580 
kiloton fiducial mass). The modular aspect is actually mandatory for
maintenance reasons, so that at least 2 of the 3 modules would be active
at any time, giving 100\% duty cycle for supernova explosions. Furthermore,
it would offer the possibility to add Gadolinium in one of the modules, which
has been advocated to improve diffuse supernova neutrino detection.
We estimate an overall construction time of less than 10 years, and of course
the first module could start physics during the completion of the two other
modules.
%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Photodetection}
%%%%%%%%%%%%%%%%%%%%%%%%%%%
The baseline photodetector choice is photomultipliers (PMT) as they have
successfuly equipped the previous generation of large water \v{C}erenkov
detectors and many other types of presently running detectors in HEP. The PMT
density should be chosen to allow excellent sensitivity to a broad range
of nucleon decays and neutrino physics while keeping the instrumentation costs
under control.

Our goal for MEMPHYS is to reach in the whole detector
the same energy threshold as Super-Kamiokande,
that is 5 MeV, important for solar neutrino studies, for the proton decay into
$K^+ \nu$ using the 6~MeV tag from $^{15}$N desexcitation, and also very useful
for SN explosions, since the measurement of the $\nu_{\mu}$ and $\nu_{\tau}$
fluxes could be achieved using the neutral current excitation of Oxygen.

Our first approach was to consider 20" Hamamatsu tubes as used by
Super-Kamiokande, but the cost for 40\% coverage becomes prohibitive, as these
tubes are manually blown by specially trained people, which makes them very
expensive. Following a suggestion presented at the NNN05 conference by Photonis
company, we have considered the possibility of using instead 12" PMT's, which
can be automatically manufactured and have better characteristics
compared to 20" tubes\,: quantum efficiency (24\% vs 20\%), collection
efficiency (70\% vs 60\%), risetime (5~ns vs 10~ns), jitter (2.4~ns vs 5.5~ns).
Based on these numbers, 30\% coverage with 12" PMT's would give the same number
of photoelectrons per MeV as a 40\% coverage with 20" tubes. Taking into 
account the ratio of photocathodes (615~cm$^2$ vs 1660~cm$^2$), this implies
that going from 20" tubes to twice as many 12" tubes will give the same
detected light, with a bonus on time resolution and on pixel locations. If the dark
current of better photocathodes does not increase dramatically the trigger rate, 
we can expect MEMPHYS performances be at least as good as Super-Kamiokande.
A GEANT4 based Monte Carlo is under development to quantify the effective
gain. Pricewise, each 20" PMT costing 2500 Euros 
is replaced by 2 12" PMT's costing 800 Euros each. 
The only caveat is to make sure that the savings on PMT's are not
cancelled by the doubling of electronic channels. An R\&D on electronics
integration is presently underway (see Sec. \ref{sec:photo}).
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\subsection{Photomultiplier tests}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
A joint R\&D program between Photonis company and French laboratories has been
launched to test the quality of the 12" PMTs in the foreseen conditions of 
deep water depth, and to make a realistic
market model for the production of about 250,000 PMTs that would be necessary
to get the 30\% geometrical coverage. 

In parallel, studies on new photo-sensors have been launched. 
The aim is to reduce cost, while improving production rate and performance, as
it is essential
to achieve the long term stability and reliability which is proven for PMTs. 
Hybrid photosensors (HPD) could be a solution\,:
the principle has been proven by ICRR and Hamamatsu with a 5" HPD prototype. 
Successful results from tests of an 13"
prototype operated with 12 kV are now available, 
showing a $3 \cdot 10^4$ gain, good single photon sensitivity, 
0.8 ns time resolution and a satisfactory gain and 
timing uniformity over the photo-cathode area.
The development of HPD has also been initiated in Europe, 
in collaboration with Photonis.
%%%%%%%%%%%%%%%%%%
\subsection{Smart-photodetector electronics}
\label{sec:photo}
%%%%%%%%%%%%%%%%%%%
The coverage of large areas (around 17,500 m$^2$ for MEMPHYS) with
photodetectors at lowest cost implies a readout integrated electronics
circuit (called ASIC). This makes it possible to integrate: high-speed
discriminator on the single photoelectron (pe), the
digitisation of the charge on 12 bits ADC to provide numerical signals
on a large dynamical range (200~pe), the digitisation of time on 12
bits TDC to provide time information with a precision of 1~ns, and
channel-to-channel gain adjustment to homogenize the response of the
photomultipliers and to thus use a common high voltage. Such an ASIC
for readout electronics allows moreover a strong reduction of the
costs, as well as external components (high-voltage units, cables of
great quality...) since the electronics and the High Voltage may be
put as close as possible to the PMTs and the generated numerical
signals are directly usable by trigger logical units and 
the data acquisition computers (Fig.~\ref{fig:MEMPHYSPMTS}). 
%

The main difficulty in associating very fast analog electronics and
digitization on a broad dynamic range does not make it possible yet to
integrate all these functions in only one integrated circuit, but
certain parts were already developed separately as for example in the
OPERA Read Out Channel \cite{Lucotte:2004mi}
(Fig.~\ref{fig:OPERAROC}). The evolution of integrated technologies,
in particular BiCMOS SiGe 0.35$\mu$m, now make it possible to consider
such circuits and has triggered a new campaign of research and
development. 

\begin{figure}[htb]
 \begin{minipage}[c]{0.44\textwidth}
\centering
\includegraphics[width=0.85\textwidth]{ModuleArriere.eps}
\caption{\label{fig:MEMPHYSPMTS}
\it Sketch of a possible photosensor basic module composed of a matrix
of $4\times4$ 12" PMTs with the electronic box containing the High
Voltage unit and the Readout chip.}	 
\end{minipage}
%
 \begin{minipage}[c]{0.04\textwidth}
~
\end{minipage}
%
 \begin{minipage}[c]{0.44\textwidth}
\centering
\includegraphics[width=\textwidth]{electronic.eps}
\caption{\label{fig:OPERAROC}
\it Sketch of the existing Read Out electronics developed for the
OPERA Target Tracker and that is intented to be extended for MEMPHYS
by integrating the ADC and TDC.}	 
\end{minipage}
\end{figure}
%



 
