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Neutron

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Neutron �thunder’ accompanying an extensive air shower
Erlykin A.D.
P.N.Lebedev Physical Institute, Moscow, Russia
PeV energy region
Findings ( Antonova V.A. et al., 2002, J.Phys.G, 28, 251 ):
1. There are a lot of neutrons delayed by hundreds of ms
after the main shower front (�the neutron thunder’).
Their temporal distribution is different from the
standard one in neutron monitors.
2. Distortions of the temporal distributions seem to have a
threshold and begin in the PeV (�knee’) region.
3. Multiplicity of such neutrons is very high compared with
EAS model expectations.
4. These neutrons are concentrated in the EAS core
region.
5. Delayed neutrons are accompanied by delayed gammaquanta and electrons.
Structure of the neutron monitor
Neutron Monitor counting rate, I
I, ms-1
0
1000
2000
Delay t, ms
3000
Neutron monitor counting rate, II
I, ms-1
0
1000
2000
Delay t, ms
3000
Neutron monitor counting rate, III
I, ms-1
Delay t, ms
Saturation level
Imax≈ N/τ ≈
6/2Ојs = 3Ојs-1
Threshold of the distortions
Standard temporal distribution in the neutron monitor:
пѓ© 0 . 72
пѓ№
0 . 28
t
t
I пЂЅ Mf ( t ) пЂЅ M пѓЄ
exp( пЂ­
)пЂ«
exp( пЂ­
)пѓє
t1
t2
t
t
2
пѓ« 1
пѓ»
Imaxп‚»N/tп‚»
t 1 пЂЅ 250 пЂ­ 300 m s , t 2 пЂЅ 600 пЂ­ 650 m s
Threshold multiplicity:
пЂ­1
3ms
I max пЂЅ M thr f ( 0 ) пѓћ M thr пЂЅ
пЂЅ 900 пЂ­ 1060 .
f (0)
log M thr пЂЅ 2 . 9 пЂ­ 3 . 0
Distribution of the attenuation coefficient
I ~TYU~
exp(-lt)
1000
100
10
l, ms-1
Concentration of neutrons in the EAS core region
Layout of monitor and eg modules
Simulations
• Primary proton
• E0 = 1 PeV
• Zenith angle � = 0o
• Observation level: 3340 m a.s.l. ( 687 gcm-2)
• Interaction model QGSJET-II + Gheisha 2002d
• Electromagnetic component: NKG
• Energy thresholds: 50 MeV for h,m, 1 MeV for e,g
Lateral distribution of protons, pions
and neutrons
Neutrons are
the most
abundant in
total and at
large distances
from the core
among all EAS
hadrons
Energy spectrum of neutrons
total
inside
monitor
LogE, GeV
Most of neutrons
have low energies,
but in the core
there are TeV and
tens TeV neutrons.
The energy
spectrum in the
whole shower~E-2,
In the core ~E-1.
Hadrons in EAS and in the monitor
Total
in EAS
Monitor
at R = 0
Monitor
at R=4m
N
h
3445
55
9.25
E , GeV
h
72610
37680
823
N
n
1993
2.15
0.6
E , GeV
n
3389
1115
25.1
Application of the calibration
results
M = 35 Eh0.5 applied for Eh = 40 TeV gives
M ≈ 0.7 ×10
4
Delayed gamma-quanta and electrons
Electron counting rate
e
n
e
n
Conclusions for PeV energies
1. The bulk of observed neutrons are not born in the
shower. They are produced inside the neutron monitor.
2. Their temporal distribution is the standard monitor
distribution, distorted by the saturation of the counting
rate at high neutron fluxes.
3. The distortions start at the threshold when the counting
rate reaches the saturation level.
4. The very high multiplicity of produced neutrons and
their concentration near the EAS core are due to the
narrow lateral distribution of EAS hadrons and their
energy around the core.
5. Delayed gamma-quanta and electrons have also a
secondary origin – they are produced by delayed
neutrons in the detector environment.
This interpretation of �the neutron thunder’,
based on the analysis of the Chubenko et al.
experiment at Tien-Shan and Monte Carlo
simulations, coincides with that of Stenkin
et al., based on their own experimental data
in Mexico City and Baksan.
EeV energy region
Problems:
1. Delayed signals in large EAS were observed long ago
( scintillators, neutron monitors ) and their possible
origin from neutrons was discussed by Greisen K.,
Linsley J., Watson A. et al.
2. Previous simulations showed that low energy neutrons
spread out up to km-long distances from the EAS core.
Could these neutrons contribute to the signals in water
cherenkov detectors ?
Simulations
• Primary proton
• E0 = 1 EeV
• Zenith angle � = 0o
• Observation level: 1400 m a.s.l. ( 875 gcm-2)
• Interaction model QGSJET-II + Gheisha 2002d
-5
• Electromagnetic component: EGS4, thinning: 10
• Energy thresholds: 50 MeV for h,m, 1 MeV for e,g
Lateral distribution of eg, m and n
particles
energy
Neutrons can
contribute up to
10% of the signal
at km-long
distances from
the EAS core
Correlations between different
characteristics of EAS neutrons
Q-R
T-R
E-R
T-E
Interestingly
EAS neutrons
appear as two
well separated
groups of low
(recoil) and
high energy
(secondary)
neutrons
highest energy
neutrons
Temporal distribution of eg, m and n
at different EAS core distances
R<10m
R=100m
R=1000m
Time, ms or ms
After 5ms
at the core
distance of
1km neutrons
are the
dominant
component
of the shower
Conclusions for EeV energies
1. Neutrons are a dominant EAS component at Km-long
distances from the core and at ms-delays after the main
shower front. These distances and times are typical for
Pierre Auger experiment.
2. However, neutrons are neutral and at these distances –
non-relativistic, therefore they cannot give signals in the
water cherenkov detectors.
3. Experiments in PeV region showed that neutrons are
accompanied by gamma-quanta and electrons, which
in principle could give cherenkov light.
Sensitivity of water cherenkov detectors to neutrons
should be tested.
General remarks
1. The discovery of �the neutron thunder’ by Chubenko
A.P. with his colleagues is an outstanding achievement .
If our interpretation of it is correct, this phenomenon
extends our understanding of the EAS development and
its interaction with detectors and their environment.
The study of EAS neutrons is complementary to the
study of other EAS components by Geiger counters,
scintillators, ionization calorimeters, gamma-telescopes,
X-ray films etc. and all together they could give the full
picture of atmospheric shower.
2. The phenomenon of �the neutron thunder’ emphasizes the
role of detectors and their environment in the observed signals.
The surrounding materials containing water could increase the
neutron scattering, moderation, and production of secondary
gamma-quanta and electrons. In this aspect, mountain studies
can be particularly vulnerable. Mind that the EAS core at
mountains as the neutron generator carries much bigger
energy than at sea level. As for the environment, a good part of
the year mountain stations ( viz. Tien-Shan, Aragats, Antarctic
etc.) are covered by snow sometimes of meters thick.
As for the Tien-Shan station, there might be an additional
factor emphasizing the role of neutrons – its ground is a
permafrost containing a good fraction of ice.
There might be effects at shallow depths underground
connected with the propagation of neutrons produced
when the EAS core strikes the ground.
EAS-TOP in winter
Aragats in the spring
Aragats,
May 2006
3. The water cherenkov detectors are particularly worth of
attention. First of all water as hydrogen containing stuff
is a moderator like a polyethilene of the neutron monitor.
Secondly it might be sensitive to secondary electrons and
gamma-quanta, produced by neutrons interacting with
water ( mind the discovery of cherenkov radiation itself ).
Since water and ice cherenkov detectors are wide spread
all over the world ( MILAGRO, NEVOD, ICE-TOP ) and in
particular used in Pierre Auger Observatory, the
contribution of neutrons at large core distances and ms
delays might be noticable and needs a special study.
4. The same remark is relevant for large EAS, based on
hydrogen containing plastic scintillators ( Yakutsk,
Telescope Array ).
In any case the phenomenon of �neutron thunder’
complements our knowledge of the EAS
development and it is certainly worth of further
experimental and theoretical study
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