Muon induced neutrons in LVD - University of Minnesota€¦ · LS = (4.2 ± 1.3) x 10-4 n/(g/cm2)...
Transcript of Muon induced neutrons in LVD - University of Minnesota€¦ · LS = (4.2 ± 1.3) x 10-4 n/(g/cm2)...
Muon induced neutrons in LVD
Rino Persiani
University & INFN Bologna
on behalf of the LVD collaboration
AARM, Minneapolis, June 23th 2012
Large Volume Detector
R. Persiani, AARM, Minneapolis, June 2012
LVD
LNGS
2
It is located in the hall A of the Gran Sasso
National Laboratories (LNGS) at a depth
of about 3600 m w.e.
The mean muon energy is about 280 GeV.
It is made by 1000 t of liquid scintillator
and by other 1000 t of iron for the
structure.
LVD is highly modular: 105 independent
modules
1 3 2
R. Persiani, AARM, Minneapolis, June 2012
The LVD detector
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• 8 counters in one module
• 3 identical towers in the detector
• 35 active modules in a tower
External dimensions: 13 x 23 x 10 m3
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The detector basic elements
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External dimensions: 1.5 x 1 x 1 m3
Scint. composition: CnH2n+2
<n>=9.6 +1 g/l PPO + 0.03 g/l POPOP
Scint. density: ~ 0.8 g/cm3
Attenuation lenght: > 15m @ l=420 nm
Flash point at: ~39oC
PMT: FEU-49B
Photocathode diameter: 15 cm
Quantum efficiency: 10-15%
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LVD Physics
● LVD is mainly dedicated to the study of neutrinos from
gravitational core collapse in the galaxy (SNEWS)
● Neutrino Physics
➢ Neutrinos in time correlation with GRB, solar flares
and gravitational waves
➢ Monitoring the CNGS beam
➢ Neutrino velocity
● Physics of the cosmic rays
➢ Muon annual modulation
➢ Muon coinciding with other experiment in the
laboratories
➢ Muon vertical intensity
➢ Muon-induced neutrons
R. Persiani, AARM, Minneapolis, June 2012
The inverse β-decay
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Neutrinos from supernovae are detected by the inverse β-decay.
The electronic antineutrinos interact with a proton in the liquid
scintillator with the following:
This interaction has a characteristic signature made by a prompt
signal due to the positron and a second delayed signal due to the
neutron capture on proton which gives a 2.2 MeV gamma.
νe + p n + e+
n + p d + γ
HET LET
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The trigger
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• The electronics is designed to detect both the product of the Inverse Beta Decay, keeping the signal to noise ratio to an acceptable level.
• The trigger has a HIGH threshold at
~4 MeV.
• When the trigger is satisfied, the threshold is LOWered to ~1 MeV, for about 1 ms, in all the counters close to the ‘high threshold’ one, in order to detect the gamma from the neutron capture.
R. Persiani, AARM, Minneapolis, June 2012
Muons in LVD
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In LVD a muon is defined requiring the
time coincidence (within 250 ns) of at
least two HET signals in two different
scintillation counters.
The mean energy released by a muon
in each counter is 185 MeV.
R. Persiani, AARM, Minneapolis, June 2012
Muon-induced neutrons in LVD
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After each muon detected in LVD we look for the signal of a neutron
capture in those counters where the threshold is lowered to 1 MeV,
in a time window <1 ms after the muon pulse
m
n
R. Persiani, AARM, Minneapolis, June 2012
What do we want to measure?
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LVD is well suited to detect both muons and
neutrons.
With LVD is possible to measure the number
of muon-induced neutrons in liquid
scintillator for muons with a mean energy of
280 GeV.
A full MC simulation can sort out the
discrepancy between LVD and other
Experimental value
LVD
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What do we want to measure?
LVD is well suited to detect both muons and
neutrons.
With LVD is possible to measure the number
of muon-induced neutrons in liquid
scintillator for muons with a mean energy of
280 GeV.
A full MC simulation can sort out the
discrepancy between LVD and other
Experimental value
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MC simulation
Monte Carlo simulation in geant4 (v 9.3)
with the geometrical description of the
whole LVD liquid scintillator and iron
structure, the hall A at the LNGS and the
rock and concrete around it.
QGSP_BIC_HP physics list
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Primary particle the muon intensity is approximately 1.1 μ/(m2 h)
Energy spectrum and angular direction are obtained with MUSIC and MUSUN
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Neutron Yield
where: Nn = number of detected neutrons Nμ = number of muons Sm = fraction of active internal counters fi = fraction of detected neutrons produced in the i-th material εi = detection efficiency λi = muon mean path length i-th material
where i = LS (liquid scintillator), Fe (iron)
DATA
MC
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Neutron estimation
Background
neutrons
The background rate at 1 MeV is very large in LVD: O(200 Hz). The number of neutron captures can be obtained fitting the time distribution of the LET signals, relative to the muon crossing time: The distribution has 2 components: Flat, due to uncorrelated background Exponential distribution due to neutron captures
Very important a good estimation of t !
t1 t2
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τ measured with a 252Cf source
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Neutron source outside the counter:
Neutron source in the center of the counter: t = (202 ± 2) ms
in agreement with the MC and the theoretical value (203 ms)
Neutrons uniformly generated in the whole counter, t = (182 ± 1) ms (from MC)
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τ from the data
τDATA = (240 ± 10) μs
τMC = (150 ± 2) μs
τDATA = (140 ± 27) μs
τMC = (140 ± 4) μs
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Test with LED
The effect observed in tank crossed by muons can be explained taking into account a derivative circuite between the PMTs and discriminator After a big energy deposition a small signal could be not detected
With a LED inside a counter we simulated the light emitted by the muon and we measure the time distribution of the events in LET
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Data sample and muon selection
We analyzed a data set with the detector in its final configuration (3 towers),
from January 2005 till December 2008; the total livetime is 34659 hours.
The mean energy released by a muon in each counter is 170 MeV and the average nb. of
crossed counters is 4-5.
We select only muons that gives (within 250 ns) at least 2 distinct HET signals in 2 different
scintillation counters (one of which has to be internal), with an energy deposited > 10 MeV.
Total number of muon events (single, multiple, cascades): 8.3 M
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Counter and event selection
Internal counters, not affected by hardware problems:
• Bad ADC and TDC • Time distribution in the gate not flat • Gate width shorter than 400 ms
253 internal counters satisfy the requirements (over 400). Sm = 253/400
Low energy signals: • Energy in [1, 5] MeV • Time difference from the muon pulse in [20, 400] ms • In counters NOT crossed by the muon
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Muon mean path length
Lμ(Fe) = 41.1 cm
From the MC simulation we get the muon path length in iron and in liquid scintillator
for all the selected muons.
RMS = 16.4 cm
σLμ
= 5.5 x 10-3 cm
Lμ(LS) = 463 cm
RMS = 153 cm
σLμ
= 5.1 x 10-2 cm
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Fraction of detected neutrons
We estimate the fraction of detected neutrons that were produced in iron or in
liquid scintillator:
We select the muon sample
We apply the event and counter
For each LET signal we look for a correspondence with a captured neutron
somewhere in te detector
Once we find it, we get the material were it was originated
NLET
74063
Ndet
(roccia) 2148 (3%)
Ndet
(Fe) 62571 (84.5%)
Ndet
(LS) 9092(12.3%)
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Detection efficiency
The detection efficiency is evaluated as the number of neutrons produced in a
specific material that are detected somewhere in LVD.
materiale Nn,det Nn,prod ε (%)
ferro 62571 3731732 1.68
scintillatore 9092 772928 1.18
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Neutron yield in LVD
Nn = 68300 ± 6.1% ± 30% neutroni
DATI
YFe
= (2.3 ± 0.7) x 10-3 n/(g/cm2)
YLS
= (4.2 ± 1.3) x 10-4 n/(g/cm2)
MC
YFe
= 1.29 x 10-3 n/(g/cm2)
YLS
= 2.44 x 10-4 n/(g/cm2)
The distribution of the time delay between the muon and the neutron signals in 4 years of LVD data is shown in figure 4. The fit is performed between 20 and 400 µs, where the τ value has been fixed by Monte Carlo simulation to a value of 143
We have all the ingredients to measure the neutron yield:
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Neutron yield in a homogeneous material
μ
MC
Yh,Fe
= 1.19 x 10-3 n/(g/cm2)
Yh,LS
= 1.95 x 10-4 n/(g/cm2)
Muon with LNGS energy spectrum Fired through a homogeneous block of material We cut the edges of the block to avid any kind of edge effects We evaluated the neutron yield
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Comparison block/LVD
Neutron yield in liquid scintillator (x10-4 n/(g/cm2))
MC DATI
blocco 1.95 X
LVD 2.44 4.2
Neutron yield in iron (x10-3 n/(g/cm2))
MC DATI
blocco 1.19 X
LVD 1.29 2.3
(2.1)
(3.4)
ΔLS
= 19.0%
ΔFe
= 8.5%
Summary of the results from data and from the MC simulation for homogeneous and not homogeneous detector
The ratio between the values in the MC columns estimate the difference in the neutron yield for both cases:
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Possible explanation
We introduced two possible explanation that can explain the difference:
LVD is not homogeneous
Geometrical effects
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Neutron yield event by event
Liquid scintillator
Iron
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Neutron yield in liquid scintillator
Other points show the results from experiments at: (A) 20 m w.e., (B) 25 m w.e., (C) 32 m w.e., (D) 316 m w.e., (E) 570 m w.e., (F) 2700 m w.e., (G) 3000 m w.e., and (H) 5200 m w.e..
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Conclusions
We studied the neutrons produced by cosmic muons in the LVD detector during 4 years (2005-2008), with a total number of 8.3 millions detected muons.
A full MonteCarlo simulation has been developed with Geant4.
We measured the neutron yield in LVD
• in LS: (4.2 ± 1.3) 10-4 • in iron: (2.3 ± 0.7) 10-3 (first measurement ever)
If we want to compare our result with homogeneous detectors we have to scale the neutron yield
• in LS: (3.4 ± 1.0) 10-4 • In iron: (2.1 ± 0.6) 10-3
... still work to do: neutron energy spectrum, distance from the muon track, ...
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backup
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1.Ricavo i coseni direttori del muone dalla
conoscenza degli angoli e
2.Genero un punto random su un cerchio con
centro nel punto centrale dell’apparato e diametro
uguale alla massima dimensione del rivelatore
3.Ruoto il cerchio in modo tale che sia
perpendicolare alla direzione del muone
Per trovare il punto di generazione del
muone, devo trovare il punto d’intersezione
tra la superficie esterna del World Volume e
la retta passante per il punto random preso
all’interno del cerchio e parallela alla
direzione del muone