1 LTR 2004 Sudbury, December 2004 Helenia Menghetti, Marco Selvi Bologna University and INFN Large Volume Detector The Large Volume Detector (LVD) in the INFN Gran Sasso National Laboratory (3000 m.w.e., mean muon energy 270 GeV), Italy, consists of an array of 840 scintillator counters, 1.5 m 3 each. These are interleaved by streamer tubes, and arranged in a compact and modular way to maximize the livetime of the experiment. The active scintillator mass is M=1000 t. Study of the muon-induced neutron background with the LVD detector at LNGS

2 LTR 2004 Sudbury, December 2004 Helenia Menghetti, Marco Selvi Bologna University and INFN The detector is optimized for the observation of Supernova electron antineutrinos through the inverse beta decay:  e + p n + e + n + p D +  Positron spectrum Inverse beta decay Which originates in the liquid scintillator to 2 subsequent pulses: the prompt one, due to the positron, and the second one due to the 2.2 MeV gamma from the neutron capture, with a mean time delay of 185  s.

3 LTR 2004 Sudbury, December 2004 Helenia Menghetti, Marco Selvi Bologna University and INFN External: more background… Internal: better shielded Two different discrimination channel: 1) High Energy Threshold operated at HET = 7 MeV for the external counter (43%), and at HET = 4 MeV for the inner ones (57%) better shielded from rock radioactivity 2) All counters are equipped with an additional discrimination channel, set at a lower threshold, LET = 1 MeV, which is active for 1 ms after the HET pulse, for the  detection

4 LTR 2004 Sudbury, December 2004 Helenia Menghetti, Marco Selvi Bologna University and INFN Neutrons in liquid scintillator may have the same signature of the inverse beta decay. Infact their interactions on proton produce:  a prompt signal due to the proton recoil  a 2.2 MeV gamma from the neutron capture delayed with respect to the prompt one of 185  s Neutron candidates in LVD are then selected as high energy threshold signal followed by at least a low energy threshold signal within 1 ms in the same counter. Taking into account the energy transfer in the interaction between neutron and proton, the proton quenching and the value of the high energy threshold of the detector, the neutrons selected in this way have energies greater than about 20 MeV. Neutron signal in LVD

5 LTR 2004 Sudbury, December 2004 Helenia Menghetti, Marco Selvi Bologna University and INFN How can we discriminate the accidental coincidence of a high energy threshold with a following low one from a true coincidence due to neutron interaction? If we look at the time delay distribution between the HET signal (or the time of the muon) and the LET one we expect an exponential shape due to neutron capture with a mean lifetime of 185  s on the top of a flat behaviour due to accidental coincidences. A = B = C = Neutron Signal Random coincidences background

6 LTR 2004 Sudbury, December 2004 Helenia Menghetti, Marco Selvi Bologna University and INFN Analysis We perform the following measurement:  Neutron production as a function of the distance from the muon track  Neutron production as a function of the energy  Neutron production as a function of the muon path lenght in scintillator Study of the neutron production in the LVD detector in association with single muons events and multiple muon events

7 LTR 2004 Sudbury, December 2004 Helenia Menghetti, Marco Selvi Bologna University and INFN Single muon event Selection cuts:  Only one reconstructed track per event  At least three points in each projection  At least two high energy threshold signal from two different counters within 250 ns MORE THAN 7 MILLIONS SINGLE MUON EVENTS

8 LTR 2004 Sudbury, December 2004 Helenia Menghetti, Marco Selvi Bologna University and INFN The number of neutrons per counter per event has been evaluated as a function of the distance between the reconstructed muon track and the center of the counter where the neutron is detected. Neutron flux measured up to ~ 20 m Neutron production vs distance from muon track

9 LTR 2004 Sudbury, December 2004 Helenia Menghetti, Marco Selvi Bologna University and INFN Number of neutrons detected as a function of the recoiling proton energy as measured in the scintillator (without quenching correction). The data are well fitted by a power law spectrum: Y=A E -  where  = (1.18 ± 0.02) Neutron production vs proton energy

10 LTR 2004 Sudbury, December 2004 Helenia Menghetti, Marco Selvi Bologna University and INFN The mean number of neutrons per event has been evaluated as a function of the muon track lenght inside the liquid scintillator. y=p1+p2*x p1=0,13*10 -3 neutron production in the rock p2=0,13*10 -2 increase in the neutron production with the muon path lenght in the scintillator Comparing the two values we can conclude that the neutron production in the core of the experiment in mostly due to the interaction of muons with the detector nuclei. Neutron production vs muon track lenght “The production problem”

11 LTR 2004 Sudbury, December 2004 Helenia Menghetti, Marco Selvi Bologna University and INFN Multiple muon event Selection cuts:  At least two reconstructed tracks with three points in each projection  At least two high energy threshold signal from two different counters within 250 ns  Space angle between tracks less than 10º PRELIMINARY MORE THAN MULTIPLE MUON EVENTS

12 LTR 2004 Sudbury, December 2004 Helenia Menghetti, Marco Selvi Bologna University and INFN Single: neutrons/muon/counter Multiple: neutrons/event/counter. As the mean track multiplicity is 2.76, we obtain: neutrons/muon/counter..... UNDER STUDY..... Comparison Multiple muon event – Single muon event For the multiple muon events the distance is defined as the minimum one between each reconstructed track and the center of the counter where the neutron is detected.