Transcript 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.

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.
LTR 2004
Sudbury, 12-14 December 2004
Helenia Menghetti, Marco Selvi
Bologna University and INFN
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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
Positron spectrum
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.
LTR 2004
Sudbury, 12-14 December 2004
Helenia Menghetti, Marco Selvi
Bologna University and INFN
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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 g detection
External: more background…
Internal: better shielded
LTR 2004
Sudbury, 12-14 December 2004
Helenia Menghetti, Marco Selvi
Bologna University and INFN
3
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.
LTR 2004
Sudbury, 12-14 December 2004
Helenia Menghetti, Marco Selvi
Bologna University and INFN
4
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.
Neutron Signal
Random coincidences
background
LTR 2004
Sudbury, 12-14 December 2004
Helenia Menghetti, Marco Selvi
Bologna University and INFN
A=
B=
C=
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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
LTR 2004
Sudbury, 12-14 December 2004
Helenia Menghetti, Marco Selvi
Bologna University and INFN
6
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
LTR 2004
Sudbury, 12-14 December 2004
Helenia Menghetti, Marco Selvi
Bologna University and INFN
7
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
LTR 2004
Sudbury, 12-14 December 2004
Helenia Menghetti, Marco Selvi
Bologna University and INFN
8
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)
LTR 2004
Sudbury, 12-14 December 2004
Helenia Menghetti, Marco Selvi
Bologna University and INFN
9
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*10-3 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.
LTR 2004
Sudbury, 12-14 December 2004
Helenia Menghetti, Marco Selvi
Bologna University and INFN
10
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
LTR 2004
Sudbury, 12-14 December 2004
Helenia Menghetti, Marco Selvi
Bologna University and INFN
11
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.
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.....
LTR 2004
Sudbury, 12-14 December 2004
Helenia Menghetti, Marco Selvi
Bologna University and INFN
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