Large Volume Detector

1. Study of the muon-induced neutron background with the LVD detector at LNGS 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 m3 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. Helenia Menghetti, Marco Selvi Bologna University and INFN

2. Positron spectrum Inverse beta decay The detector is optimized for the observation of Supernova electron antineutrinos through the inverse beta decay: ne+ p n+e+ n + p D +g 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 ms. Helenia Menghetti, Marco Selvi Bologna University and INFN

3. Two different discrimination channel: • High Energy Threshold operatedat HET = 7 MeV for the external counter(43%), and at HET = 4 MeV for the inner ones (57%) better shielded from rock radioactivity • 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 g detection External: more background… Internal: better shielded Helenia Menghetti, Marco Selvi Bologna University and INFN

4. Neutron signal in LVD • 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 ms 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. Helenia Menghetti, Marco Selvi Bologna University and INFN

5. 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 185ms on the top of a flat behaviour due to accidental coincidences. A = B = C = Random coincidences background Neutron Signal Helenia Menghetti, Marco Selvi Bologna University and INFN

6. Analysis Study of the neutron production in the LVD detector in association with single muons events and multiple muon events • 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 Helenia Menghetti, Marco Selvi Bologna University and INFN

7. 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 Helenia Menghetti, Marco Selvi Bologna University and INFN

8. Neutron production vs distance from muon track 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 Helenia Menghetti, Marco Selvi Bologna University and INFN

9. Neutron production vs proton energy 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-a where a = (1.18 ± 0.02) Helenia Menghetti, Marco Selvi Bologna University and INFN

10. Neutron production vs muon track lenght “The production problem” 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*10-3neutron production in the rock p2=0,13*10-2increase 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. Helenia Menghetti, Marco Selvi Bologna University and INFN

11. PRELIMINARY 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 MORE THAN 164000 MULTIPLE MUON EVENTS Helenia Menghetti, Marco Selvi Bologna University and INFN

12. Comparison Multiple muon event – Single muon event Single:0.036 10-3 neutrons/muon/counter Multiple: 0.39 10-3 neutrons/event/counter. As the mean track multiplicity is 2.76, we obtain: 0.14 10-3 neutrons/muon/counter ..... UNDER STUDY..... 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. Helenia Menghetti, Marco Selvi Bologna University and INFN