1 | \section{Underground laboratory and detector} |
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2 | |
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3 | \subsection{Results of a feasibility study in the |
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4 | central region of the Fr\'ejus tunnels} |
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5 | \label{sec:undlab} |
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6 | The site located in the Fr\'ejus mountain in the Alps, |
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7 | which is crossed by a road-tunnel connecting France (Modane) |
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8 | to Italy (Bardonecchia), has a number of interesting characteristics |
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9 | making it a very good candidate for the installation of a megaton-scale |
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10 | detector in Europe, aimed both at non-accelerator and accelerator |
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11 | based physics. |
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12 | Its great depth (4800 mwe, see figure~\ref{muonflux}), |
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13 | the good quality of the rock, |
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14 | the fact that it offers horizontal access, its distance from CERN (130 km), |
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15 | the opportunity of the excavation of a second (``safety'') tunnel, |
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16 | the very easy access by train (TGV), by car (highways) |
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17 | and by plane (Geneva, Torino and Lyon airports), |
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18 | the strong support from the local authorities |
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19 | represent the most important of these characteristics. |
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20 | |
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21 | \begin{figure}[htb] |
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22 | \centerline{\epsfig{figure=./figures/muonflux_red.eps,width=8cm}} |
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23 | \caption{\it Muon flux as a function of overburden. |
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24 | The Frejus site is indicated by "LSM".} |
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25 | \label{muonflux} |
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26 | \end{figure} |
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27 | |
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28 | On the basis of these arguments, the DSM (CEA) and IN2P3 (CNRS) institutions |
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29 | decided to perform a feasibility study of a Large Underground Laboratory |
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30 | in the central region of the Fr\'ejus tunnel, near the already existing, |
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31 | but much smaller, LSM Laboratory. This preliminary study |
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32 | has been performed by the SETEC (French) and STONE (Italian) companies |
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33 | and is now completed. These companies already made the study and managed |
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34 | the realisation of the Fr\'ejus road tunnel and of the LSM |
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35 | (Laboratoire Souterain de Modane) Laboratory. |
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36 | A large number of precise and systematic measurements of the rock |
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37 | characteristics, performed at that time, have been used to make a |
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38 | pre-selection of the most favourable regions along the road tunnel |
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39 | and to constrain the simulations of the present pre-study for the |
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40 | Large Laboratory. |
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41 | |
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42 | \begin{figure}[htb] |
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43 | \centerline{\epsfig{figure=./figures/tunnel.eps,width=8cm,}} |
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44 | \caption{\it Possible layout of the Fr\'ejus underground laboratory.} |
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45 | \label{fig:tunnel} |
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46 | \end{figure} |
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47 | |
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48 | Three regions have been pre-selected : |
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49 | the central region and two other regions at about 3 km from |
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50 | each entrance of the tunnel. |
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51 | Two different shapes have been considered for the cavities to be excavated: |
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52 | the ``tunnel shape'' and the cylindrical ``shaft shape''. The main purpose |
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53 | was to determine the maximum possible size for each of them, |
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54 | the most sensitive dimension being the width (the so-called ``span'') |
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55 | of the cavities. |
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56 | |
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57 | The very interesting results of this preliminary study |
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58 | can be summarized as follows : |
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59 | \begin{enumerate} |
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60 | \item the best site (rock quality) is found in the middle of the mountain, |
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61 | at a depth of 4800 mwe; |
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62 | \item of the two considered shapes : ``tunnel'' and ``shaft'', |
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63 | the ``shaft shape'' is strongly preferred; |
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64 | \item cylindrical shafts are feasible up to a diameter $\Phi$ = 65 m |
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65 | and a full height h = 80 m ($\sim$ 250000 m$^3$); |
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66 | \item with ``egg shape'' or ``intermediate shape between |
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67 | cylinder and egg shapes'' the volume of the shafts could be still increased |
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68 | (see Fig.~\ref{fig:egg}); |
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69 | \item the estimated cost is $\sim$ 80 M Euro per shaft. |
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70 | \end{enumerate} |
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71 | |
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72 | \begin{figure}[htb] |
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73 | \centerline{\begin{tabular}{cc} |
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74 | \epsfig{figure=./figures/egg.eps,width=8.5cm} & |
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75 | \epsfig{figure=./figures/eggsim.eps,width=5.5cm} |
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76 | \end{tabular}} |
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77 | \caption{\it An example of ``egg shape'' simulation, |
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78 | constrained by the rock parameter measurements made during the road tunnel |
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79 | and the present laboratory excavation. |
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80 | The main feasibility criterium is that the significantly perturbated |
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81 | region around the cavity should not exceed a thickness of about 10 m.} |
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82 | \label{fig:egg} |
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83 | \end{figure} |
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84 | |
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85 | Fig.~\ref{fig:tunnel} shows a possible configuration for this |
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86 | large Laboratory, where up to five shafts, of about 250000 m$^3$ each, |
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87 | can be located between the road tunnel and the railway tunnel, |
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88 | in the central region of the Fr\'ejus mountain. |
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89 | |
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90 | Two possible scenarios for Water \v{C}erenkov detectors are, for instance: |
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91 | \begin{itemize} |
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92 | \item 3 shafts of 250000 m$^3$ each, with a fiducial mass of 440 kton |
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93 | (``UNO-like'' scenario). |
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94 | \item 4 shafts of 250000 m$^3$ each, with a fiducial mass of 580 kton. |
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95 | \end{itemize} |
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96 | In both scenarios one additional shaft could be excavated |
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97 | for a Liquid Argon and/or a liquid scintillator detector of about |
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98 | 100 kton total mass. |
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99 | |
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100 | The next step will be a Design Study for this Large Laboratory, |
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101 | performed in close connection with the Design Study of the detectors |
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102 | and considering the excavation of 3 to 5 ``shafts'' of about 250 000 m$^3$ |
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103 | each, the associated equipments and the mechanics of the detector modules. |
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104 | |
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105 | \subsection{Detector: general considerations} |
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106 | |
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107 | The 20 year long successful operation of the |
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108 | Super-Kamiokande detector has clearly |
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109 | demonstrated the capabilities and limitations of large water \v{C}erenkov |
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110 | detectors\,: |
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111 | \begin{itemize} |
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112 | \item This technique is by far the cheapest |
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113 | and the most stable to instrument a very large detector |
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114 | mass, as price is dominated by the photodetectors and their associated |
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115 | electronics (this price growing like the outer surface of the detector), |
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116 | while the active mass, made of water, is essentially free except for the |
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117 | purification system |
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118 | \item These detectors are mainly limited in size by the finite attenuation |
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119 | length of \v{C}erenkov light, found to be 80 meters at $\lambda = 400$~nm |
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120 | in Super-Kamiokande, and by the pressure of water on the photomultipliers at |
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121 | the bottom of the tank, which gives a practical limit of 80~m in height. |
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122 | At large depths, the maximal size of underground cavities actually limits |
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123 | relevant dimensions to about 70~m. |
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124 | \item The detection principle consists in measuring \v{C}erenkov rings produced |
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125 | by charged particles going faster than light in water. This has several |
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126 | consequences\,: |
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127 | \begin{enumerate} |
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128 | \item neutral particles and charged particles |
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129 | below \v{C}erenkov threshold are undetectable, so that some energy may |
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130 | be missing |
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131 | \item complicated topologies are difficult to handle, and in practice only |
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132 | events with less than 3 to 5 rings are efficiently reconstructed |
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133 | \item ring topology, based on their degree of fuzziness, allows to separate |
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134 | between electromagnetic (e, $\gamma$) rings and ($\mu$, $\pi$) rings |
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135 | \item the threshold in particle energy depends mainly on photocathode |
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136 | coverage and also on water purity (due to radioactive backgrounds, such as |
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137 | radon). Super-Kamiokande has achieved an energy threshold of 5 MeV with |
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138 | 40\% cathode coverage |
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139 | \item due to points 1 and 2, water \v{C}erenkov detectors are not suited to |
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140 | measure high energy neutrino interactions, as more rings and more undetectable |
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141 | particles are produced. A further limitation comes from the confusion between |
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142 | single electron or gamma rings and high energy $\pi^0$'s giving 2 overlapping |
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143 | rings. In practice water \v{C}erenkov's stay excellent neutrino detectors |
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144 | for energies below 1 (may be 2) GeV, when interactions are mostly quasi-elastic |
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145 | and the 2 rings from $\pi^0$ well separated. |
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146 | \end{enumerate} |
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147 | \end{itemize} |
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148 | |
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149 | \subsection{Detector design} |
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150 | |
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151 | Three detector designs are being carried out worldwide, |
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152 | namely Hyper-Kamiokande \cite{uno} in Japan, |
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153 | UNO \cite{hyperk} in the USA and the present project MEMPHYS in Europe. |
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154 | All of them are rather mild extrapolations of Super-Kamiokande, and rely on the |
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155 | expertise acquired after 20 years of operation of this detector. |
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156 | Their main characteristics are summarized in table~\ref{WC:tab-1}. |
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157 | \begin{sidewaystable} |
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158 | \centering |
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159 | % |
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160 | \begin{tabular}{rccc} |
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161 | \hline\noalign{\smallskip} |
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162 | Parameters & \textbf{UNO} (USA) & |
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163 | \textbf{HyperK} (Japan) & \textbf{MEMPHYS} (Europe)\\ |
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164 | \noalign{\smallskip}\hline\noalign{\smallskip} |
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165 | \multicolumn{4}{l}{\textbf{Underground laboratory}} \\ |
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166 | location & Henderson / Homestake & Tochibora & Fr\'ejus \\ |
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167 | depth (m.e.w.) & 4500/4800 & |
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168 | 1500 & 4800 \\ |
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169 | Long Base Line (km) & $1480\div2760$ / $1280\div2530$ & 290 & 130 \\ |
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170 | & FermiLab$\div$BNL & JAERI & CERN \\ |
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171 | \noalign{\smallskip}\hline\noalign{\smallskip} |
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172 | \multicolumn{4}{l}{\textbf{Detector dimensions}} \\ |
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173 | type & 3 cubic compartments & 2 twin tunnels & $3\div5$ |
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174 | shafts\\ |
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175 | & & 5 compartments & \\ |
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176 | dimensions & $3\times (60\times60\times60)\mathrm{m}^3$ |
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177 | & |
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178 | $2\times 5 \times (\phi=43\mathrm{m} \times L=50\mathrm{m})$ & |
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179 | $(3\div5)\times(\phi=65\mathrm{m} \times H=65\mathrm{m}) $ \\ |
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180 | fiducial mass (kt)& 440 & 550 |
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181 | & $440\div730$\\ |
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182 | \noalign{\smallskip}\hline\noalign{\smallskip} |
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183 | \multicolumn{4}{l}{\textbf{Photodetectors}} \\ |
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184 | type & 20" PMT & 13" H(A)PD & 12" PMT |
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185 | \\ |
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186 | number (internal detector) & 57,000 & 20,000 per compartment |
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187 | & 81,000 per shaft \\ |
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188 | surface coverage & 40\% (1/3) \& 10\% (2/3) & 40\% & 30\%\ |
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189 | |
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190 | \\ |
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191 | \noalign{\smallskip}\hline |
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192 | \end{tabular} |
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193 | % |
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194 | \caption{\label{WC:tab-1} |
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195 | \it Some basic parameters of the three Water \v{C}erenkov |
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196 | detector baseline designs} |
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197 | \end{sidewaystable} |
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198 | |
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199 | These 3 projects aim at a fiducial mass around half a megaton, taking into |
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200 | account the necessity to have a veto volume on the edge of the detector, 1 to 2 |
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201 | meters thick, plus a minimal distance of about 2 meters between photodetectors |
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202 | and interaction vertices, leaving some space for ring development. |
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203 | The main differences between the 3 projects lie in the geometry |
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204 | of the cavities (tunnel shape for Hyper-Kamiokande, shafts for MEMPHYS, |
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205 | intermediate with 3 cubic modules for UNO), and the photocathode coverage, |
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206 | similar to Super-Kamiokande for Hyper-Kamiokande |
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207 | and MEMPHYS, while UNO keeps this |
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208 | coverage on only 1 cubic detector, while the 2 others have only 10\% coverage |
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209 | for cost reasons. Another important parameter is the rock overburden, similar |
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210 | for UNO and MEMPHYS (4800~mwe), but smaller for Hyper-Kamiokande (1500~mwe), |
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211 | which might be a limiting factor for low energy physics, due to spallation |
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212 | products and fast neutrons produced by cosmic muons, more |
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213 | abundant by 2 orders of magnitude (see figure~\ref{muonflux}). |
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214 | |
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215 | The basic unit for MEMPHYS consists of a cylindrical detector module 65~meters |
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216 | in diameter and 65~meters high, which can be housed |
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217 | in a cylindrical cavity with 70~meter diameter and 80 meter height, as proven |
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218 | by the prestudy. This corresponds to a water mass of 215 kilotons, that is |
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219 | only 4 times the Super-Kamiokande detector. |
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220 | Conservatively substracting 2~m for the |
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221 | outer veto plus 2~m for the fiducial volume, this leaves us with a fiducial |
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222 | mass of 146 kilotons per module. The baseline design uses 3 modules, giving |
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223 | a total fiducial mass of 440 kilotons, like UNO, corresponding to factor |
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224 | 20 increase over Super-Kamiokande (4 modules would give 580 |
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225 | kiloton fiducial mass). The modular aspect is actually mandatory for |
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226 | maintenance reasons, so that at least 2 of the 3 modules would be active |
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227 | at any time, giving 100\% duty cycle for supernova explosions. Furthermore, |
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228 | it would offer the possibility to add Gadolinium in one of the modules, which |
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229 | has been advocated to improve diffuse supernova neutrino detection. |
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230 | We estimate an overall construction time of less than 10 years, and of course |
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231 | the first module could start physics during the completion of the two other |
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232 | modules. |
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233 | |
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234 | \subsection{Photodetection} |
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235 | |
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236 | The baseline photodetector choice is photomultipliers (PMT) as they have |
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237 | successfuly equipped the previous generation of large water \v{C}erenkov |
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238 | detectors and many other types of presently running detectors in HEP. The PMT |
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239 | density should be chosen to allow excellent sensitivity to a broad range |
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240 | of nucleon decays and neutrino physics while keeping the instrumentation costs |
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241 | under control. |
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242 | |
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243 | Our goal for MEMPHYS is to reach in the whole detector |
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244 | the same energy threshold as Super-Kamiokande, |
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245 | that is 5 MeV, important for solar neutrino studies, for the proton decay into |
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246 | $K^+ \nu$ using the 6~MeV tag from $^{15}$N desexcitation, and also very useful |
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247 | for SN explosions, since the measurement of the $\nu_{\mu}$ and $\nu_{\tau}$ |
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248 | fluxes could be achieved using the neutral current excitation of Oxygen. |
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249 | |
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250 | Our first approach was to consider 20" Hamamatsu tubes as used by |
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251 | Super-Kamiokande, but the cost for 40\% coverage becomes prohibitive, as these |
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252 | tubes are manually blown by specially trained people, which makes them very |
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253 | expensive. Following a suggestion presented at the NNN05 conference by Photonis |
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254 | company, we have considered the possibility of using instead 12" PMT's, which |
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255 | can be automatically manufactured and have better characteristics |
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256 | compared to 20" tubes\,: quantum efficiency (24\% vs 20\%), collection |
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257 | efficiency (70\% vs 60\%), risetime (5~ns vs 10~ns), jitter (2.4~ns vs 5.5~ns). |
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258 | Based on these numbers, 30\% coverage with 12" PMT's would give the same number |
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259 | of photoelectrons per MeV as a 40\% coverage with 20" tubes. Taking into |
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260 | account the ratio of photocathodes (615~cm$^2$ vs 1660~cm$^2$), this implies |
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261 | that going from 20" tubes to twice as many 12" tubes will give the same |
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262 | detected light, with a bonus on time resolution and on pixel locations. If the dark |
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263 | current of better photocathodes does not increase dramatically the trigger rate, |
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264 | we can expect MEMPHYS performances be at least as good as Super-Kamiokande. |
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265 | A GEANT4 based Monte Carlo is under development to quantify the effective |
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266 | gain. Pricewise, each 20" PMT costing 2500 Euros |
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267 | is replaced by 2 12" PMT's costing 800 Euros each. |
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268 | The only caveat is to make sure that the savings on PMT's are not |
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269 | cancelled by the doubling of electronic channels. An R\&D on electronics |
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270 | integration is presently underway (see Sec. \ref{sec:photo}). |
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271 | |
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272 | \subsection{Photomultiplier tests} |
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273 | |
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274 | A joint R\&D program between Photonis company and French laboratories has been |
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275 | launched to test the quality of the 12" PMTs in the foreseen conditions of |
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276 | deep water depth, and to make a realistic |
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277 | market model for the production of about 250,000 PMTs that would be necessary |
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278 | to get the 30\% geometrical coverage. |
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279 | |
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280 | In parallel, studies on new photo-sensors have been launched. |
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281 | The aim is to reduce cost, while improving production rate and performance, as |
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282 | it is essential |
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283 | to achieve the long term stability and reliability which is proven for PMTs. |
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284 | Hybrid photosensors (HPD) could be a solution\,: |
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285 | the principle has been proven by ICRR and Hamamatsu with a 5" HPD prototype. |
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286 | Successful results from tests of an 13" |
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287 | prototype operated with 12 kV are now available, |
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288 | showing a $3 \cdot 10^4$ gain, good single photon sensitivity, |
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289 | 0.8 ns time resolution and a satisfactory gain and |
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290 | timing uniformity over the photo-cathode area. |
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291 | The development of HPD has also been initiated in Europe, |
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292 | in collaboration with Photonis. |
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293 | |
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