The present physics programme of the ICARUS project, requiring a sensitive mass of liquid argon of the order of several kilotons, was submitted to the Gran Sasso Scientific Committee in early 1994. As a part of this programme small scale prototypes have been built and operated. The "bubble chamber" quality of ICARUS images can be easily demonstrated by a typical cosmic ray shower observed in the 3-ton protype at CERN (fig. 1). The best way to reach the sensitive mass needed to fulfil our scientific goals has been considered to go through an intermediate step, before the realization of the final multi-kiloton detector. This step-wise strategy should allow us to develop progressively at the Gran Sasso Laboratory:

• the infrastructure needed to build and operate a large detector,
• the in situ experience and
• to obtain a definitive and practical evaluation of the engineering choice for the final phase.

The most efficient and economical way to have operative a module of intermediate size is to build and test it outside the Laboratory and after to move it inside the underground laboratory for the final assembly. This implies that the module has to be transportable. The size of the module is therefore limited by this requirement and amounts to about 600 tons.
It turned out that an intermediate detector of this size would at the same time allow an important first step in the ICARUS scientific programme. In fact, this detector has a mass close to the old Kamiokande detector mass, but the higher efficiency and the much more detailed information which can be collected for each event should rapidly allow us to reach definite conclusions on the phenomena to be studied. In particular, the ICARUS technique can provide a background-free detection of neutrino events and proton decays.

Figure 1: Cosmic ray shower observed in the ICARUS 3-ton prototype at  CERN. Photon conversion into pairs as well as pion and muon decays are clearly visible in the electromagnetic showers.

Atmospheric and solar neutrinos are areas which could be deeply investigated. While a sensitive mass in excess of a few thousand tons of liquid argon is the only way to achieve, in a number of "standard" proton decay channels, the 10³⁴ years range in proton decay lifetime, many other "exotic" channels have only been poorly investigated so far or not at all, and would be easily covered in this first phase with the 600-ton detector. The same consideration holds for the study of neutrino oscillations with the long baseline neutrino beam from CERN. In this case the large mass of the final ICARUS detector is required to enhance the statistical significance of the experiment. Nevertheless, the use of the 600-ton detector is considered an important first phase of operation during the beam start-up, expected for the year 2006.
Finally, this intermediate step opens in addition the possibility to explore a new route towards larger detector volumes: the construction of a number of identical 600-ton detectors installed next to one another.
A proposal for the first 600 t module has been submitted for funding to INFN in 1995 and fully approved in 1996. The final project and designing of all the detector components, including cryogenics, read-out chambers and electronics, have been completed by mid 1998. The construction of the detector has been completed by the end of 2000 in Pavia (INFN site). The major industrial counterparts collaborating to the ralization of the ICARUS 600 t detector were AIR LIQUIDE, for the cryogenic system, and CAEN, for the electronics.  Transportation, final assembly and operation start-up at the Gran Sasso underground laboratory will take place by the end of the year 2007. A brief description of the detector and the physics programme which can be achieved with the first 600 t module at Gran Sasso is given below.

The ICARUS T600  module

The operating principle of the ICARUS liquid argon TPC is rather simple: any ionizing event (from particle interaction or decay) taking place in a volume of liquid argon, where a uniform electric field is applied, produces ion-electron pairs. A fraction of them, depending on the field intensity and on the density of ion pairs, will not recombine and will immediately start to drift parallel to the field in opposite directions. Only the motion of the much faster electrons induces a current on a number of parallel wire planes (the read-out chambers) located at the end of the sensitive volume.
The choice of the liquid was driven by the following considerations:

• it must be an excellent insulator and available at an extremely high purity level, so that free electrons produced by ionization can drift in the liquid over long distances;
• it must have a high electron-ion pair yield with respect to the energy deposited in the liquid;
• it must be easily available in large quantities, which is the case for argon, a natural component of the Earth's atmosphere (1%).

The main cryogenic container for the ICARUS 600-ton consists of two semi-independent and symmetric parallelepids of approximately 3.6 by 3.9 by 19.9 cubic meters. Its walls are made of aluminium honeycomb panels. The thermal insulation uses an innovative method requiring no vacuum and based on honeycomb insulating material with cold gas flowing through the cells.

The read-out chambers (two TPC for each half-vessel) are mounted on the internal walls with the cathode at the centre, to maximise the LAr sensitive volume (corresponding to about 480 ton in mass). A general layout of the 600 t detector is shown in Fig.2. The read-out chamber scheme consists of three parallel planes of wires (horizontal, +60 and -60 degrees). Information is read both by electric charge induction on the first two readout planes encountered by drifting electrons and by electric charge collection on the last readout plane. The signals from the three wire planes, together with measurement of the drift time, provide a (redundant) full 3-D image reconstruction of the event. The main features of this type of chamber is that there is no charge amplification in the chamber, to allow the drifting electrons to induce signals on different wire planes. This requires a high quality electronics to maintain a good signal over noise ratio.
The wire pitch, in the read-out chamber planes, is reduced to 3 mm, instead of the 5 mm foreseen for the multi-kiloton module, in order to allow for higher precision measurements. The total number of electronic read-out channels is about 55000. A suitable neutron shield is foreseen around the entire volume, in order to reduce as much as possible the natural radioactivity background, affecting detection of low energy solar neutrino interactions. The final assembly of the external insulation and the construction of the neutrons shield will be the only two major works that will be performed inside the underground laboratory. 

Figure 2: The ICARUS 600-ton detector. 

 Physics issues

Main physics issues of the ICARUS 600-ton experiment are the search of neutrino oscillation and the search of nucleon decay events. The neutrino oscillation study can be performed with atmospheric neutrinos, solar neutrinos and long baseline beam neutrinos produced at CERN.

Atmospheric neutrinos:
The atmospheric neutrino charged current events rate, expected without oscillations, in one year, for a sensitive mass of about 500 t, is of about 140 events, 80 from n_m and 60 from n_e interactions. These rates correspond to the number of events with a vertex within this sensitive mass. Atmospheric neutrino interactions, with a mean neutrino energy of the order of few hundreds MeV, produce low multiplicity events with a lepton (m or e) accompanied by a nucleon (p or n) and eventually by one (or two) pion(s). This kind of topologies can be perfectly reconstructed with the ICARUS detector, providing a complete information on the incoming neutrino kinematics.
Therefore the ICARUS experiment is well suited to perform, in a few years of exposure, the atmospheric neutrino study with sufficient sensitivity to cover the Super-Kamiokande allowed regions in the neutrino oscillation parameter space.

Solar neutrinos:
The ICARUS device is sensitive to the ⁸B part of the solar spectrum.
If the proposed MSW solution is relevant to explain the solar neutrino deficit, then not only the rate of ⁸B neutrinos is affected but the shape of the neutrino spectrum should be significantly distorted. Therefore, the ICARUS goal in the solar neutrino area is not only to confront the Standard Solar Model (absolute event rate), but also to provide a Solar Model independent measurements, by observing various independent processes differently affected by possible oscillation. ICARUS can detect solar neutrinos by observing the electron produced in the following two reactions:

• n_x + e ---> n_x + e, which occurs with all types of neutrino flavours and for both charged and neutral current exchange (about 1 event/ton/yr);
• n_e + Ar ---> K* + e, which only occurs with the electron neutrino (about 4 event/ton/yr, including Fermi and Gamow-Teller reactions).

The measurement of the ratio of rates for these two processes provides directly a measurement of the n_e oscillation probability.

Long baseline neutrinos:
A detailed feasibility study has shown that it is technically possible to derive a n_m beam pointing to Gran Sasso, 732 km far away in Italy, from a fast extracted proton beam accelerated by the CERN SPS ring. Proton accelerators provide essentially n_m beams from the decay of p's and K's, produced when the extracted proton beam hits a target. These "parent" particles are focused towards the detector and left to decay in a tunnel to produce muons and n_m . The muons and all remaining hadrons are dumped at the end of the decay tunnel leaving only the neutrinos travelling towards the detector target. The ICARUS 600 t sensitivity to long baseline neutrino oscillation is limited by statistics, in case the Super-Kamiokande parameters are confirmed. Therefore a complete study of the long baseline neutrino sector will be accomplished only with a larger detector mass of the order of several kilotons. Nevertheless the use of the ICARUS 600-ton detector will be able to provide preliminary fundamental results during the beam operation start-up.

Proton Decay:
Even with a relatively low mass, the ICARUS 600 t experiment can provide important contributions to the proton decay search, in particular for those decay modes (referred as exotic decay modes) with high multiplicities (3 or 4 particles in the final state) or, more generally, with signatures particularly difficult to identify with previously used detector techniques.
The ICARUS technique is particularly well suited to identify these decay modes. The detector techniques used so far, especially water Cherenkov detectors, do not allow to study these decay modes in an exclusive mode, as can be argued from the relatively modest present limits. For most of these decays the background in ICARUS is expected to be negligible, hence one single event could be sufficient for discovery.
With the 600 t module, in a few years, it will be possible to explore a lifetime region exceeding 10³² years for most of these "exotic" channels, with a considerable improvement (a factor 5 to 100) of the present limits. It is evident that with one single module we cannot reach the lifetime limit of 10³⁴ years, needed to fully test the minimal SUSY Gran Unification Theory. This is the final goal for the multi-kiloton detector. Nevertheless, we will satisfy completely the requirement for the first phase of our programme, namely to extend the present knowledge of the nucleon stability over the widest possible range of decay modes at the same level of the presently best studied channels.