ICARUS comes of age
After several years of R&D, the ICARUS experiment, which acts as a sort of observatory for the study of neutrinos and the instability of matter, is starting to come together. In the summer of 2001, the first module of the ICARUS T600 detector passed a series of tests. The year 2004 will see the detector's installation and first data-taking at the Gran Sasso Laboratory. A 3000-ton version should be ready to receive the CNGS neutrino beam in 2006.
ICARUS is putting on weight to boost its chances of trapping neutrinos. Once it has grown to its full size, the experiment's 3000-tonne detector will be able to detect not only the neutrinos arriving from CERN but also those from the sky. The detector comprises 300-tonne semi-modules, which are stacked together like giant pieces of lego until the desired mass is achieved. Each semi-module is 3.9 m x 4.3 m and 19.6 metres in length. The semi-modules are built outside the tunnel at the Gran Sasso Laboratory and then transported inside "ready-to-fit". Each semi-module contains 250 000 litres of sensitive liquid argon cooled to -200°C in two chambers. To make them transportable, an individual semi-module cannot hold more than 350 tonnes of liquid argon.
An observatory to study rare events
The collaboration's aim is to assemble five 600-tonne modules in Hall B of the Gran Sasso underground laboratory to form a 3000-tonne detector for the study of rare events such as neutrino interactions and proton decays. In this way, as well as detecting the high-energy (17 GeV) neutrinos arriving from CERN, ICARUS T3000 will be able to see solar neutrinos (5-14 MeV), neutrinos produced by supernovas (10-100 MeV) as well as atmospheric neutrinos (1 GeV). It will also study the instability of matter by observing nucleon decays.
The detector is a new generation of bubble chambers capable of operating with a very large sensitive volume and thus offering such a broad spectrum of detection capabilities. It will be capable of providing a three-dimensional view of an event while at the same time recording the energy deposited per unit length by the particles produced at the moment of the event.
A new-generation detector
The first 600-tonne module was built at the University of Pavia and the first semi-module was tested with cosmic rays for 100 days from April to August 2001. On the basis of the data collected there is no reason to doubt that 2006 will see a bumper crop of data. Indeed one muon left an 18-m long track in the detector's liquid-argon time projection chambers. This type of chamber, the brainchild of Carlo Rubbia in 1977, offers the dual advantages of particle trajectory visibility (as in a bubble chamber) and electronic data acquisition flexibility.
Why choose liquid argon in this hunt for particles?
1- It's an excellent insulant, very weakly electronegative thus allowing the free electrons produced by ionisation to drift long distances;
2- It produces many electron-ion pairs allowing measurement of the energy deposited in the liquid;
3- It is a good scintillator, allowing measurement of the energy of the luminous flash produced by an event. This flash tells us when an event has taken place.
4- It is available in sufficient quantity: argon, in gaseous form, makes up some 1% of the earth's atmosphere.
When charged particles pass through liquid argon, ionisation electrons are produced in numbers proportional to the energy transferred to the liquid. These ionisation electrons drift towards an anode, made up of three read-out wire planes. Each wire in a read-out plane measures the energy deposited in a segment of the ionisation track. The combined spatial and calorimetric reconstructions allow precise measurement of the energy loss per unit length (dE/dx). This is where the ionising particle is identified.
For neutrinos (which, as their name suggests, have no electrical charge), the principle is the same. The only difference is that the neutrino interacts with liquid argon by producing one of the three leptons with which it is associated, namely an electron, muon or tau. These charged particles give birth to ionised secondary particles when they pass through liquid argon. By identifying the latter particles it is possible to reconstruct back to the incident particle and thus determine whether it was a muon, tau or electron neutrino.
«In the framework of the CNGS project ICARUS, like OPERA, will be able to study oscillations of muon neutrinos into tau neutrinos with at least as many tau events as OPERA,» explains André Rubbia of the ICARUS collaboration. Both detectors indeed boast the same level of sensitivity. Furthermore, ICARUS T3000 will be looking for electron neutrinos that might have appeared during the journey of the muon neutrinos from CERN to Gran Sasso. For it is possible that muon neutrinos might also oscillate into electron neutrinos as well as into tau neutrinos. ICARUS could provide evidence of this.
Spotlight on ICARUS modules
Each half-module (T300) is an independent unit housing an internal detector composed of two Time Projection Chambers (TPCs), a field-shaping system, monitors and probes, and two arrays of photomultipliers. Externally the cryostat is surrounded by a set of thermal insulation layers. The detector layout is completed by a cryogenic plant made up of a liquid-nitrogen cooling circuit to maintain a uniform liquid-argon (LAr) temperature, and of a system of LAr purifiers.
Each TPC is formed of three parallel planes of wires with gaps of 3 mm. A uniform electric drift field perpendicular to the wire planes is established in the LAr volume of each half-module by means of a high-voltage (HV) system that comprises a cathode plane parallel to the wire planes placed in the middle of the LAr volume of each half-module. The HV system is complemented by field-shaping electrodes to guarantee the uniformity of the field along the drift direction. The read-out of the signals induced by ionizing events in LAr on the wires of the TPCs provides a means for full 3D-image reconstruction of the event topology and for accurate measurement of the energy deposited.
Ionization events in LAr are accompanied by prompt scintillation light emission. The absolute time measurement of the event and an internal trigger signal are achieved by detecting this light with an array of PhotoMultiplier Tubes positioned behind the wire planes of each TPC.
Each of the wire planes of the TPC provides a two-dimensional projection of the event image, where one coordinate is given by the wire position and the other by the drift time. The 3D reconstruction of the event is obtained by correlating signals from at least two different planes with the help of the common drift-time coordinate.
