The Detector

        
The LVD experiment is one of those working in the Gran Sasso national underground facilities in Italy, operated by the Italian Institute of Nuclear Physics (INFN). The facilities are 3 big laboratory halls and service rooms located aside and more or less in the middle of a highway tunnel, 10.6 km long, under the Gran Sasso massif, nearly at 120 km east of Rome. There are also external facilities at ground level and a mountain observatory (EAS-TOP) at Campo Imperatore (~2000 m above sea level), almost on the vertical of the underground halls.
         The undergound complex is at 900 m above sea level. The mean rock width above the labs is 1300 m, equal to 3500 hg/cm2 of standard rock, or 3600 m of water equivalent. The rock is mainly CaCO3, with mean density 2.6 g/cm3.
         The main aim of LVD is the detection of neutrino bursts from gravitational collapses inside our Galaxy or in Magellanic Clouds.
         LVD is an array of stainless steel tanks, 1.0x1.0x1.5 m3, filled with liquid scintillator (Cn H2n+2, <n> = 9.6, with r = 0.8 g/ cm3 ), rise time 5 ns, attenuation lenght 15 m (l = 420 nm), with the activator POP (1g/l) and the wavelenght shifter POPOP (0.03 g/l). The tanks have their inner surfaces covered with mylar.
         In its final configuration, the experiment has 840 tanks, with an active mass of ~1.0 kton. The main detection channel for neutrinos is the reaction:

           `ne + p ® n + e+
                                          ® n + p® D + g
      (Eg = 2.23 MeV)
         The light emitted by the scintillator when occurs one of these reactions inside the tank is collected by three photomultipliers (FEU-49B, made in the former Soviet Union), photocatode of 15 cm and optically connected with the scintillator through a plexiglass window. For a released energy of 1 MeV, the light generated in the photocatode will produced ~15 photoelectrons.
         Each group of eight tank (two rows of four) is mounted on a iron structure called "porta tank", whose bottom and one side are shielded by streamer tubes modules for detection and tracking of charged particles with better angular resolution.
        Table I resumes the main characteristics of the detector:

Dimensions (m3) 13. x 23. x 10.
Number of tanks 840
Number of inner tanks 480
Number of photomultipliers 2520
Number of streamer tubes 20,000
Mass of scintilator (ton) 983
Central mass (ton) 562
Number of free protons 9.4 x 1031
Number of electrons 3.5 x 1032
Number of 12C nuclei 4.2 x 1031
Energy threshold (MeV) » 4
Energy resolution » (10-15)%
Spatial resolution for tracking (mrad) <4


         In the Table, central mass means the total mass of scintillator in the inner tanks, those without direct contact with the rock. The ratio between the central mass and the total mass of scintillator gives about half of 1 kton of scintillator with energy threshold lower than that of the outer tanks, where the noise due to the radiactivity of the rock cannot be neglected.

3.        Neutrino interactions inside the scintillator
         An important feature of LVD is the possibility to detect neutrinos in several channels. This fact allows a more carefull study of the energy characteristics of the neutrino burst from collapsing stars. The interaction cross sections of Table II have different dependences from the neutrino energy, and the relation among the numbers of events in the different channels (and the correspondent energy spectra) allows more detailed informations about the spectrum of neutrino emission.
         Table II shows the interaction channels inside the scintillator. The most importan one is the first.

Charged current interactions Neutral current interactions
1. `ne + p ® n + e+ 4. n(e,m,t) + e- ® n(e,m,t) + e-
2. ne + 12C® 12N + e- 5. `n(e,m,t) + e-® `n(e,m,t) + e-
3. `ne + 12C® 12B + e+ 6. n(e,m,t) + 12C ® 12C* + n(e,m,t)
  7. `n(e,m,t) + 12C® 12C* + `n(e,m,t)


         The scintillator can show also the gammas, allowing the detection of both products of the interaction 1 through the channels:
                      e+ + e-® 2 g
                      n + p ® D + g

         In the second reaction, the time between the emission and the capture of the neutron (approximately its thermalization) has an exponential distribution, with a decay constant of the order of 200
ms. The energy of gamma is: Eg = 2,2 MeV. The electronics was developed and tested to allow the identification both of the positron and gamma, giving a good signature of the reaction.
         The interactions 2, 3, 6 and 7 have also good signatures. In 2 and 3 the   e-   and the   e+  emitted have energies and emission time with well known distributions; in 6 and 7 the deexcitation of Carbon occurs with the emission of one gamma with energy = 15.11 MeV in 90% of the cases, giving a very precise way to identify these interactions. The elastic scattering of  
ne with   e-   (interaction number 4 and 5) produces a lower number of events, but allows the characterization of the dynamics of the collapse, since in its initial stage (neutronization) almost all neutrinos emitted are ne.

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Informations: selvi@bo.infn.it