The basic element for neutrino detection is the liquid scintillation counter consisting of an iron tank of dimensions (1.5 x 1.0 x 1.0) m³, filled with white spirit (C_nH_2n+2 with a mean n value of 9.6 and density of 800 kg m^-3), doped with appropriate scintillating chemicals. Three photomultiplier tubes on the top of each counter watch the scintillating liquid. 8 liquid scintillation counters are assembled in independent modules: 35 of such modules, tightly arranged in five columns, constitute one tower. The whole LVD apparatus, consisting of three 'towers', is thus characterized by a very high modularity. The counters are set at different energy thresholds, depending on their position with respect to the cavern walls: the external ones (43% of the total), operate at threshold ˜ 7 MeV, while the inner ones (57% of the total), better shielded from rock radioactivity operate at 4 MeV. All counters are then equipped with an additional discrimination channel, set at a lower threshold, ˜ 1 MeV.

Table 1: main LVD characteristics.

Length	22.7 m
Width	13.2 m
Height	10. m
Steel mass (scint. containers and support structure)	1020 tons
Scintillator Volume	1260 m3
Scintillator Mass	1008 tons
Nb. of scintillator counters	840
Nb. of PMTs	2520
Nb. of L-shaped tracking modules	105
Nb. of limited streamer tubes	~ 9000

Concerning the detection principle, neutrinos can interact in the scintillator either with the free hydrogen protons or with the carbon nuclei. More in detail, the observable neutrino interactions are:

1. Charged current reaction of electron antineutrinos on protons to form a neutron and a positron
   
   v_e+p -> n + e⁺
   
   which is observed through a prompt signal from e⁺, followed by the emission of a detectable g of 2.2 MeV due to the neutron capture
   
   n + p -> d + g
   
   with a mean time delay of ˜ 180 ms.
   
2. Charged current interactions of electron neutrinos and antineutrinos with 12C nuclei:
   
   v_e + ¹²C -> ¹²B + e⁺
   v_e + ¹²C -> ¹²N + e^-
   
   which are observed through two signals: the prompt one due to the e⁺ (e^-) followed by the signal from the b^- (b⁺) decay of ¹²B (¹²N), with mean life time 15.9 ms (29.4 ms).
   
   
3. Neutral current interactions of all neutrino (antineutrino) species with ¹²C nuclei
   
   v_x + ¹²C -> v_x + ¹²C + g(15.1 MeV) ( x = e, m, t)
   
   whose signature is the monochromatic photon from carbon de-excitation.
   
   
4. Elastic scattering of neutrinos (antineutrinos) of all families on electrons
   
   v_x + e^- -> v_x + e^-
   
   which yields a single signal due to the recoil electron.

 

The LVD detector is furtherly equipped with a tracking system which consists of L-shaped detectors for each 8-counters module. Every element contains two staggered layers of 6.3 m long limited streamer tubes. The bidimensional read-out is made by means of 4 cm strips, parallel and perpendicular to the tubes, providing high detection efficiency and an angular resolution better than 4 mrad.

LVD physics

In spite of the lack of a ”standard” model of the gravitational collapse of a massive star, some features of its dynamics and, in particular, of the correlated neutrino emission appear to be well established. At the end of its burning phase a massive star (M > 8 solar mass) explodes into a supernova, originating a neutron star which cools emitting its binding energy ˜ 3 x 10⁵³ erg mostly in neutrinos. The largest part of this energy, almost equipartitioned among neutrino and antineutrino species, is emitted in the cooling phase. The energy spectra are approximatively a Fermi-Dirac distribution, but with different mean temperatures,
since n_e, n_e and n_t have different couplings (cross sections values) with the stellar matter: T_ne < T_ne < T_nx

Since the described features of stellar collapses are essentially common to all existing models and lead to rather model independent expectations for supernova neutrinos, the signal observable in LVD, in different reactions and due to different kinds of neutrinos, besides providing astrophysical information on the nature of the collapse, is sensitive to intrinsic n properties, as oscillation of massive neutrinos.

LVD has been continuosly monitoring the whole Galaxy in the search for neutrino bursts from gravitational stellar collapses since 1992. No stellar collapses in our Galaxy have been detected so far.

LVD is involved in the SNEWS (Supernova Early Warning System), which is an international collaboration including all of the current major supernova neutrino detectors. The goal of SNEWS is to provide the astronomical community with a prompt alert of the occurrence of a galactic core collapse event: the neutrino signal indeed will emerge promptly from a supernova while it may take hours for the first photons to be visible.

In addition to its major purpose, the LVD detector allows for other important physics studies, mainly related to cosmic rays. The tracking system design provides good detection efficiency even for tracks near the horizontal, allowing for the study of the muon intensity over a very large range of slant depths up to the plateau region. The combined information from the scintillation counters and the tracking system provides a good measurement of the energy loss of muons per unit path length which has allowed the study of the local muon underground energy spectrum and of characteristics of the deepest component of the penetrating cosmic radiation.