1. Villa Olmo 2009 1 Search for neutrino bursts from gravitational collapse of stars at the Baksan Underground Scintillation Telescope Yu.F. Novoseltsev, M.M. Boliev, I.M. Dzaparova, M.M. Kochkarov, S.P. Mikheyev, R.V. Novoseltseva, V.B. Petkov, P.S. Striganov, G.V. Volchenko, V.I. Volchenko, A.F. Yanin Institute for Nuclear Researsh of the Russian Academy of Sciences ννν

2. Villa Olmo 2009 2 BUST – the general view ( the effective depth 850 m of w.e.) The facility has dimensions 171711 m 3 and consists of four horizontal scintillation planes and four vertical ones. The total number of standard autonomic detectors is 3180. Each standard detector is 707030 cm 3 in size, and is filled with an organic liquid scintillator (on the basis of white spirit C n H 2n+2,n ≈9), viewed by one photomultiplier with a photocathode of 15 cm in diameter. The total mass of scintillator is 330 t. Three lower horizontal planes have 1200 standard detectors, and 130 t of scintillator.

3. Villa Olmo 2009 3 The operation threshold of detectors is 8 MeV (before 1992 this threshold was 10 MeV). The most probable energy deposition of a muon in a detector is 50 MeV. The timing accuracy is 0.2 ms. Synchronization of the clock is performed by GPS signal.The operation threshold of detectors is 8 MeV (before 1992 this threshold was 10 MeV). The most probable energy deposition of a muon in a detector is 50 MeV. The timing accuracy is 0.2 ms. Synchronization of the clock is performed by GPS signal. Until 2001 there was a system of selection of events which consisted of blocks of nanosecond electronics and logic circuits, generating triggering signals. Until 2001 there was a system of selection of events which consisted of blocks of nanosecond electronics and logic circuits, generating triggering signals. The information from the telescope was written down only in case of a generation of a master-pulse by a physical program. Since 2001 every event is written down. Since 2001 every event is written down. One of the important advantages of such a set of the information is the possibility to examine the operation of each detector for any time interval. Dead time of the BUST is ≈ 1 ms.

4. Villa Olmo 2009 4 The neutrino signal from a supernova explosion is recorded with the help of the reaction If the mean antineutrino energy is E ν = 12 – 15 MeV, the pass of e+ will be, as a rule, within the volume of one detector. In that case the signal from a supernova explosion will appear as a series of events from singly triggered detectors during the neutrino burst. The single triggered detector is one and only one detector from 3180. The search for a neutrino burst consists in recording a bunch of single events within the time interval of τ = 20 s.

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6. 6 Background events - radioactivity, ghost signals from detectors, ghost signals from detectors, cosmic ray muons if only one detector hit cosmic ray muons if only one detector hit The total count rate from background events is The total count rate from background events is n = 0.02 Hz in internal planes (three lower horizontal layers) n = 0.02 Hz in internal planes (three lower horizontal layers) n= 1 Hz in external planes. n= 1 Hz in external planes. Therefore three lower horizontal planes are used as the target (130 t). Background events can imitate a bunch of k single events Background events can imitate a bunch of k single events within the time interval τ with a count rate within the time interval τ with a count rate

7. Villa Olmo 2009 7 The number of bunches with k single events within the time interval of τ =20s. Squares are experimental data, the curve is the distribution according to the expression (P). (2001 – 2008 y; 236126 RUNs (RUN=900s), T= 2459.65 days)

8. Villa Olmo 2009 8 The search for a neutrino burst consists in recording of a bunch of single events within the time interval of = 20 s. If a distance from a star is 10 kpc and the total energy irradiated in neutrinos is ε tot = 3∙10 53 erg, the expected number of single events, under the assumption of e flux being equal to 1/6* ε tot,, from reaction (1) will be E (MeV) η 1 η 1 10 10 0.7 0.7 20 20 0.9 0.9 η 1 is the detection efficiency of e + in the reaction (1).

9. Villa Olmo 2009 9 If the scenario of 2-stage collapse (Imshennik V.S.,1995) is realized in Nature and the mean neutrino energy during the first stage is (30 – 40) MeV, the following reactions begin to work: The estimations of the expected number of events for reactions (2) and (3) are Reaction (2) allows us to measure the total neutrino flux with the energy E > 15,1 MeV. (3) (2)

10. Villa Olmo 2009 10 The lower part of the overlap between horizontal scintillation planes is the 8 mm iron layer. This can be used as the target in the reaction η 4 ≈ 0.4 is the detection efficiency of 26 MeV e − produced into the 8mm iron layer It should be noticed that, if E = 30-40 MeV a noticeable percentage of neutrino reactions (4) will cause triggering of two adjacent detectors. If the scenario of 2-stage collapse is realized in Nature, the total signal from collapse, i.e. the number of neutrino induced events, increases by ≈ 50%.

11. Villa Olmo 2009 11 The program of search for collapse neutrinos with the BUST started in the middle of 1980 th. The program of search for collapse neutrinos with the BUST started in the middle of 1980 th. The total observation time is T = 24.7 years. The probability of collapse absence during the time interval T is (according to the Poisson law) exp(-f col x T ). is (according to the Poisson law) exp(-f col x T ). (f col is the mean frequency of collapses) (f col is the mean frequency of collapses) The upper bound on the mean frequency of gravitational collapses in the Galaxy at 90%CL is The upper bound on the mean frequency of gravitational collapses in the Galaxy at 90%CL is

12. Villa Olmo 2009 12