Cosmic-Ray Detection at
the ARGO-YBJ observatory
P. Camarri
University of Roma “Tor Vergata”
INFN Roma Tor Vergata
TeV gamma-ray astronomy
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India - Dec 2011
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TeV γ-ray astronomy: science topics
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Physics targets for -ray astronomy
The gamma-ray spectrum
Galactic sources
 Supernova Remnants
 Plerions
 Shell type SNR
 Pulsars
 Diffuse emission from the galactic disk
 Unidentified Sources
Extragalactic sources
 Active Galactic Nuclei (blazars)
 Gamma Ray Bursts
Cosmological γ–ray Horizon
Cerenkov
Telescopes
EAS arrays
Satellites
106
1 MeV
109
1 GeV
HAFC EAS
arrays
1012
1 TeV
1015
1 PeV
1018 eV
1 EeV
 Probe of the Extragalactic Background
Light (EBL)
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-ray sources:
naturally multiwavelength
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TeV γ-rays: production processes
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TeV γ-rays: production processes
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Satellite vs Ground-based detectors
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Satellite:
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lower energy
primary detection
small effective area ~1m2
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lower sensitivity
large duty-cycle
large cost
low bkg
Ground based:
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higher energy
secondary detection
huge effective area ~104 m2
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higher sensitivity
Small/large duty-cycle
low cost
high bkg
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India - Dec 2011
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P. Camarri - WAPP 2011 - Darjeeling,
India - Dec 2011
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Statistical significance
Excess of events coming from the source over the estimated background
Signal
S  significan ce 
Background
Signal  ON - OFF, Background  OFF source
1
S  TON  Aeff   Q f

standard deviations
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The main drawback of ground-based measurements
…background showers induced by primary Cosmic Rays
CRAB( >1 TeV) 2 ·10-11 ph/cm2 ·s
bkg( >1 TeV) · (= 1 msr) 1.5 ·10-8 nuclei/cm2·s

 10 
3
signal
bkg
No possible veto with an anticoincidence shield as in satellite experiments
In addition…
Cosmic Ray showers  γ-ray showers
… fortunately, some difference does exist !!
Ground based -Ray Astronomy requires a
severe control and rejection of the BKG.
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India - Dec 2011
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Detecting Extensive Air Showers
Air Cherenkov Telescopes
detection of the Cherenkov light
from charged particles in the EAS
Classical EAS arrays
detection of the charged
particles in the shower
High energy threshold ( 50 TeV)
Very low energy threshold ( 60 GeV)
Moderate bkg rejection ( 50 %)
Good background rejection (99.7 %)
Modest sensitivity ( Fcrab)
-2
High sensitivity (< 10 Fcrab)
Modest energy resolution
Good energy resolution
High duty-cycle (> 90 %)
Low duty-cycle (~ 5-10 %)
Large field of view (~2 sr)
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Small field of view  < 4°
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The birth of TeV γ-ray astronomy
Discovery of the emission of photons with
E > 0.7 TeV coming from the Crab Nebula by
the Whipple Cherenkov telescope in 1989:
50 h per 5σ
HESS: 30 seconds !
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India - Dec 2011
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The TeV sky
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Why an EAS array ?
• Provides synoptic view of the sky
• Sees an entire hemisphere every day
• Large fov & high duty-cycle
 GRBs
 Transient astrophysics
 Extended objects
 New sources
Excellent complement to satellites
ACTs can monitor only a limited number of sources / year at
stated sensitivity
A sensitive EAS array is needed to extend the
FERMI/GLAST measurements at > 100 GeV.
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A new-generation EAS array
The Goal
• Low energy threshold < 500 GeV
• Increased sensitivity Φ  Φcrab  <10-1 Φcrab
N e 4300m   5  N e 2700m 
N  1MeV   7  N e  1MeV 
The Solution
• High-altitude operation
• Secondary-photon conversion
• Increase the sampling (~1%  100%)
Improves angular resolution
Lowers energy threshold
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India - Dec 2011
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The ARGO-YBJ experiment
• ARGO detects air-shower particles at ground level
• ARGO is a wide field of view gamma-ray telescope which
operates in “scanning mode”
• ARGO is optimized to work with showers induced by
primaries of energy
E > a few hundred GeV
This low energy threshold is achieved by:
 operating at very high altitude (4300 m asl)
 using a “full-coverage” detection surface
Excellent complement to AGILE/GLAST to
extend satellite measurements at > 100 GeV
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The ARGO-YBJ experiment
An Extensive Air Shower detector exploiting
the full-coverage approach at very high
altitude, with the goal of studying
 VHE -Ray Astronomy
 -Ray Burst Physics
 Cosmic-Ray Physics
Longitude 90° 31’ 50” East
Latitude 30° 06’ 38” North
90 Km North from Lhasa (Tibet)
4300 m above the sea level
The Yangbajing Cosmic Ray Laboratory
ARGO
Tibet ASγ
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12 RPC =1 Cluster
( 5.7  7.6 m2 )
8 Strips = 1 Pad
(56  62 cm2)
99 m
74 m
Central Carpet:
130 Clusters
1560 RPCs
124800 Strips
10 Pads = 1 RPC
(2.80  1.25 m2)
Gas Mixture: Ar/ Iso/TFE = 15/10/75, HV = 7200 V
78 m
111 m
BIG
PAD
Layer of RPC covering 5600
(  92% active surface)
(+ 0.5 cm lead converter)
+ sampling guard-ring P. Camarri - WAPP 2011 - Darjeeling,
RPC
m2
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ADC
Read-out of the
charge induced on
“Big-Pads”
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The ARGO-YBJ Resistive Plate
Chambers
Gas mixture:
C2H2F4/Ar/iC4H10 = 75/15/10
Operated in streamer mode
Time resolution ~ 1.5 ns
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Shower recostruction
time (ns)
meters
Fired pads on the carpet
Arrival time vs position
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Analog read-out
It is crucial to extend the dynamics of
the detector for E > 100 TeV, when the
strip read-out information starts to
become saturated.
Max fs: 6500 part/m2
40000
3500
0
3000
2500
2000
1500
1000
500
Fs: 4000 -> 1300/m2
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Detector Pixels
σt≈1 ns
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India - Dec 2011
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Operational Modes
INDEPENDENT DAQ

Shower
Detection of Extensive Air Showers (direction, size, core …)
Mode:
Coincidence of different detector units (pads) within 420 ns
Trigger : ≥ 20 fired pads on the central carpet (rate ~3.6 kHz)
Object:
• Cosmic Ray physics (above ~1 TeV)
• VHE γ-astronomy (above ~300 GeV)

Scaler Mode:
Recording the counting rates (Nhit ≥1, ≥2, ≥3, ≥4) for each cluster at
fixed time intervals (every 0.5 s) lowers the energy threshold down to
≈ 1 GeV. No information on the arrival direction and spatial
distribution of the detected particles.
Object:
• flaring phenomena (high energy tail of GRBs, solar
flares)
Camarri
- WAPP 2011 - Darjeeling,
• detectorP.and
environment
monitor
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The Moon Shadow
Cosmic rays are hampered by the Moon
•
Size of the deficit
•
Position of the deficit
Deficit of cosmic rays in
the direction of the Moon
Angular Resolution
Pointing Error
Geomagnetic Field: positively charged particles
deflected towards the West and negatively
charged particles towards the East.
Ion spectrometer
1.60
 
E (TeV )
The observation of the Moon shadow can
provide a direct check of the relation
between size and primary energy
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Moon diameter ~0.5 deg
Energy calibration
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-ray astronomy
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Crab Nebula
Mrk 421
MGRO 1908+06
Cygnus region
and more…
no γ/h discrimination applied so far
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γ/h discrimination
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Some algorithms developed based on
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2-D topology
Time profile
Time distribution
Q factor = 1.2 - 3 depending on the number
of fired pads
Very heavy, fine tuning needed
Many months for data reprocessing
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India - Dec 2011
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Cosmic-Ray Physics
•Spectrum of the light component (1-100
TeV)
•Medium and large scale anisotropies
•The anti-p/p ratio
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The Earth-Moon system as a spectrometer
The shadow of the Moon can be used to put
limits on antiparticle flux.
In fact, if proton are deflected towards West,
antiprotons are deflected towards East.
If the displacement is large and the angular
resolution small enough we can
distinguish between the 2 shadows.
If no event deficit on the antimatter side
is observed an upper limit on antiproton
content can be calculated.
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India - Dec 2011
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(under peer reviewing for publication on PRD)
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India - Dec 2011
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Conclusions (2)
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-ray astronomy in the energy range above ~300 GeV
can only be investigated by ground-based Cherenkov
and EAS detectors.
The ARGO-YBJ experiment, a full-coverage EAS array
at high altitude, is giving very nice results in TeV -ray
astronomy and cosmic-ray physics at E > 1 TeV. By
exploiting the analog read-out of its RPCs, it will be
possible to study the energy region around the “knee” up
to ~1016 eV.
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A few references
http://tevcat.uchicago.edu/reviews.html
G. Di Sciascio and L.Saggese, Towards a solution of the knee problem with
high altitude experiments
Invited contribution to the Book "Frontiers in Cosmic Ray Research", 2007
Nova Science Publishers, New York, Ed. I.N. Martsch, Chapter 3, pp. 83 - 130.
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