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Villa Olmo 2009Villa Olmo 2009 11
Search for neutrino bursts from Search for neutrino bursts from gravitationalgravitational collapse of stars at collapse of stars at
the Baksan Underground the Baksan Underground Scintillation TelescopeScintillation Telescope
Yu.F. Novoseltsev, M.M. Boliev, I.M. Yu.F. Novoseltsev, M.M. Boliev, I.M. Dzaparova, M.M. Kochkarov, S.P. Dzaparova, M.M. Kochkarov, S.P.
Mikheyev,Mikheyev,RR..VV. Novoseltsev. Novoseltsevaa, V.B. Petkov, P.S. , V.B. Petkov, P.S.
Striganov, G.V. Volchenko, V.I. Striganov, G.V. Volchenko, V.I. Volchenko, A.F. YaninVolchenko, A.F. Yanin
Institute for Nuclear Researsh of Institute for Nuclear Researsh of the Russian Academy of Sciencesthe Russian Academy of Sciences
ννν
Villa Olmo 2009Villa Olmo 2009 22
BUST – the general viewBUST – the general view
((the effective depth 850 m of w.e.the effective depth 850 m of w.e.))
The facility has dimensions 17•17•11 m3 and consists of four horizontal scintillation planes and four vertical ones. The total number of standard autonomic detectors is 3180.
Each standard detector is 70•70•30 cm3 in size, andis filled with an organic liquid scintillator (on the basis of white spirit CnH2n+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.
Villa Olmo 2009Villa Olmo 2009 33
•The The operation threshold operation threshold of detectors is 8of detectors is 8 MeV MeV ((before before 19919922 this threshold this threshold was 10 was 10 MeVMeV). ). TThe mosthe most probable energy deposition of a muon in a detector is 50 probable energy deposition of a muon in a detector is 50 MMeVeV..
•• The timing accuracy is 0.2 ms. The timing accuracy is 0.2 ms.Synchronization of the clock is performed by GPS signal.Synchronization of the clock is performed by GPS signal.
Until 2001 there was a system of selection of events Until 2001 there was a system of selection of events which consisted of blocks of nanosecond electronics and which consisted of blocks of nanosecond electronics and logic circuits, generating triggering signals. logic circuits, generating triggering signals.
The information from the telescope was written down The information from the telescope was written down only in case of a generation of a master-pulse by a physical only in case of a generation of a master-pulse by a physical program.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 One of the important advantages of such a set of the
information is the possibility to examine the operation of information is the possibility to examine the operation of each detector for any time interval.each detector for any time interval.
DDead time of the BUST is ead time of the BUST is ≈≈ 1 ms. 1 ms.
Villa Olmo 2009Villa Olmo 2009 44
)1( enpe
The neutrino signal from a supernova explosion is recorded with the help of the reaction
If the mean antineutrino energy is Eν = 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.
E
Villa Olmo 2009Villa Olmo 2009 55
e p n e
Villa Olmo 2009Villa Olmo 2009 66
Background events - radioactivity,Background events - radioactivity, ghost signals from detectors,ghost signals from detectors, cosmic ray muons if only one detector hitcosmic 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).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 within the time interval ττ with a count rate with a count rate
)()!1(
)()exp()(
1
Pk
nnnkp
k
Villa Olmo 2009Villa Olmo 2009 77
The number of bunches with The number of bunches with k k single eventssingle events within within the the time interval of time interval of ττ=20=20s. Squares are experimentals. Squares are experimental d data, the curve is ata, the curve is the distribution the distribution according to according to the the expression (P)expression (P).. (2001 – 2008 y; 236126 RUNs (RUN=900s), T= 2459.65 days) (2001 – 2008 y; 236126 RUNs (RUN=900s), T= 2459.65 days)
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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 εtot = 3∙1053 erg, the expected number of single events, under
the assumption of e flux being equal to 1/6* εtot , , from reaction (1)
will be
1381
HevN E (MeV)E (MeV) ηη11
1010 0.70.7
2020 0.90.9
)1( enpe
η1 is the detection efficiency of e+ in the reaction (1).
Villa Olmo 2009Villa Olmo 2009 99
If the scenario of 2-stage collapse If the scenario of 2-stage collapse (Imshennik V.S.,1995)(Imshennik V.S.,1995) is realizedis realized in in Nature and the mean neutrino energy during the firstNature and the mean neutrino energy during the first stage isstage is (30 – 40) MeV, (30 – 40) MeV, the following reactionsthe following reactions begin to work:begin to work:
MeVECCei
MeVECC thii
1,15,,,
1,15,
12*
*1212
12
mseCNMeVEeNC
e
the
9,15,34,17,
1212
1212
7.05.0)30(302.0)15(16
33
22
3
2
MeVENMeVEN
Cev
Cev
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)
Villa Olmo 2009Villa Olmo 2009 1010
The lowThe lowerer part of the overlap between part of the overlap between horizontal scintillationhorizontal scintillation planes isplanes is the 8 mm the 8 mm iiron layer. This can be usedron layer. This can be used as as the target the target iin the reactionn the reaction
)4(056.4,*5656 MeVEeCoFe the
43,64
FeevN
η4 ≈ 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%.
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The program of search for collapse neutrinos with The program of search for collapse neutrinos with the BUST started in the middle of 1980the BUST started in the middle of 1980thth..
The The total ototal observation time is bservation time is T T = 24.7 years. = 24.7 years.
CLyfcol %90,093.0 1
The probability of collapse absence during the time interval The probability of collapse absence during the time interval T T
is (according to the Poisson law) exp(-fis (according to the Poisson law) exp(-fcolcol x x TT)) . .
(f(fcolcol
is the mean frequency of collapses) is the mean frequency of collapses)
TheThe upper bound on the mean upper bound on the mean frequency offrequency of gravitational collapses in the Galaxy at 90%CL gravitational collapses in the Galaxy at 90%CL is is
Villa Olmo 2009Villa Olmo 2009 1212
k exp calc3 216884 2241224 31564 298835 3167 29886 293 2397 28 168 2 0,91
T=236126 Runs