RPCs in the ARGO-YBJ
experiment
P. Camarri (University of Roma “Tor Vergata” and INFN Roma 2)
for the ARGO Collaboration
Workshop on Physics with Atmospheric Neutrinos and Neutrinos from
Muon Storage Rings
Mumbai, August 1-2, 2005
The ARGO-YBJ Collaboration
Collaboration Institutes:
 Chinese Academy of Science (CAS)
 Istituto Nazionale di Fisica Nucleare (INFN)
Spokesman: B. D’Ettorre Piazzoli
INFN and Dpt. di Fisica Università, Lecce
INFN and Dpt. di Fisica Universita’, Napoli
INFN and Dpt. di Fisica Universita’, Pavia
INFN and Dpt di Fisica Università “Roma Tre”, Roma
INFN and Dpt. di Fisica Università “Tor Vergata”, Roma
IFSI/CNR and INFN, Torino
IFCAI/CNR, Palermo and INFN, Catania
Spokesman: Z. Cao
IHEP, Beijing
Shandong University, Jinan
South West Jiaotong University, Chengdu
Tibet University, Lhasa
Yunnan University, Kunming
Zhenghou University, Henan
The YangBaJing High Altitude
Cosmic Ray Laboratory
Longitude 90° 31’ 50” East
Latitude 30° 06’ 38” North
4300 m above the sea level
90 Km North from Lhasa (Tibet)
Astrophysical Radiation
with
Ground-based Observatory
The ARGO-YBJ site
Outline
 Introduction
Ground based g-ray astronomy
The ARGO-YBJ experiment
Detector layout and RPC details
Physics goals and sensitivity
Present status and first measurements
 Conclusions
Why ground-based detectors ?
Satellite measurements are limited by the E- g (g = 2 ÷ 3) law
for g-ray flux
CRAB (>500 GeV)  6 · 10-11 photons/(cm2 s)
1 m2 detector needs  5 · 104 hours of
observation to collect 100 photons
CRAB (>1 TeV)  2 · 10-11 photons/(cm2 s)
 1.4· 105 hours
VHE g-astronomy possible only by ground-based
detectors exploiting the amplification effect of the
Extensive Air Showers (EAS)
Detecting Extensive Air Showers
Air Cherenkov Telescopes
EAS arrays
Very low energy threshold ( 60 GeV)
Good background rejection (99.7 %)
High sensitivity (< 10-2 crab)
Good energy resolution
Low duty-cycle (~ 5-10 %)
Small field of view D < 4°- 5°
High energy threshold ( 50 TeV)
Moderate bkg rejection ( 50 %)
Modest sensitivity ( crab)
Modest energy resolution
High duty-cycle (> 90 %)
Large field of view (~ 1-2 sr)
A new generation of EAS arrays
The Goal
• Low energy threshold < 500 GeV
• Increase sensitivity Φ  Φcrab  10-1 Φcrab
N e 4300m   5  N e 2700m 
Ng  1MeV   7  N e  1MeV 
The Solution
• High altitude operation
• Secondary photon conversion
• Increase the sampling (~1%  100%)
Improves angular resolution
Lowers energy threshold
ARGO-YBJ Physics Goals
 g-ray astronomy
Search for point-like galactic and extra-galactic sources at few hundreds
GeV energy threshold
 Diffuse g-rays
from the galactic plane and SNRs
 GRB physics (full GeV / TeV energy range)
 Cosmic ray physics
•
p  p ratio at TeV energy
• Spectrum and composition around the “knee” (E > 10 TeV)
 Sun and heliosphere physics (E > 10 GeV)
The ARGO detector: bakelite Resistive Plate Chambers
operated in streamer mode
Graphite layer
Bakelite plate
Bakelite plate
Gas gap
Graphite layer
PET spacer
thickness of the gas volume : 2mm
Gas mixture: Ar/ i-C4H10 /C2H2F4 = 15/10/75
Operating voltage = 7.2 kV (10.2 kV at sea level)
Single RPC absorption current @ 7.2 kV = 3- mA
Single RPC count rate @ 7.2 kV = 4 kHz
ARGO RPC details (1)
Bakelite plate
Read-out strip panel
Front-end board
ARGO RPC details (2)
High-voltage connection
Closed ARGO chamber
Low-voltage connection
RPC performance in the ARGO preliminary test
• Efficiency
• Altitude effect
TFE/ iBUT=97/3
TFE/Ar/ iBUT=75/15/10
• Time resolution
Gas mixture: Ar/ i-C4H10 /C2H2F4 = 15/10/75
Operating voltage = 7.2 kV (10.2 kV at sea level)
Single RPC absorption current @ 7.2 kV = 3-4 mA
Single RPC count rate @ 7.2 kV = 4 kHz
Detector Layout
99 m
74 m
12 RPC =1 Cluster
( 5.7  7.6 m2 )
78
Clusters
8 Strips = 1 Pad
(56  62 cm2)
10 Pads = 1 RPC
(2.80  1.25 m2)
78 m
111 m
Layer of RPCs covering 5600
(  92% active surface)
+ 0.5 cm lead converter
+ sampling guard ring
m2
Central Carpet:
130 Clusters, 1560 RPCs, 124800 Strips
time resolution ~ 1 ns
space resolution = 6.5  62 cm2 (1 strip)
ARGO-YBJ Experimental Hall
Cluster
RPC chamber
Trigger and Data Acquisition
 Shower mode
a minimum Pad multiplicity is required on the central detector,
with space/time consistency as for a shower front
Local Station
(basic unit of
distributed
DAQ System)
Pad Multiplicity info
Central Station
DATA
Trigger
• Trigger
• Data storage
 Scaler mode
measurement of the Pad rate from each Cluster
(integration time: 0.5 s)
Aim - detection of unexpected increases in CR flux (GRB, Solar flares …)
Detector Control System (DCS)
and Analog Charge readout
DCS
 High voltage control and monitoring
 Monitoring of environmental parameters (indoor and outdoor
temperature, atmospheric pressure)
 HV fine tuning (to be implemented soon)
 RPC current monitoring
 RPC counting rate (for detailed diagnostics: to be added soon)
The DCS is crucial for detecting anomalous detector behaviours and
performing the required actions to protect the system.
Analog Charge Readout
BIG
PAD
ADC
RPC
Read-out
of the charge
induced on
“Big Pads”
Sensitivity to the Crab and angular resolution
Minimum Detectable Flux (5  in 1 y)
Opening angle
CRAB Whipple E-2.49
Glast
Zenith angle
 < 40°
Milagro
 4.3 h/day
ARGO
Whipple
0.55 TeV
 ≈ ψ / 1.58
Hegra
Veritas
~ 1 TeV
N (>1 TeV) ~ 10
~ 2 TeV
~ 5 TeV
T5 (>1 TeV) ~ 3 months
ARGO: without any g/h discrimination !
Af = 80  80 m2
ARGO can observe, in 1 year, a Crablike source of intensity 0.7 Crab units at
energies E > 0.5 TeV, with a significance
of 4 standard deviations.
g-hadron discrimination
 Development of an effective off-line procedure
 Multiscale image analysis has been showed to provide
an efficient tool for gamma/hadron discrimination
 Results are encouraging and allow to nearly double the
detector sensitivity.
 The best response is obtained in the few TeV range.
 The study is now being extended to all event categories
 The measurement of the muon content of the shower
allows hadron background rejection at higher energies
Summary of the main detector features and
performance
 Resistive
Plate Chambers (RPC) as active elements
 Space information from Strip (6.5 × 62 cm2 )
 Time information from 8-strip pads (resolution 1 ns)
 Large area ( 10000 m2 ) and full coverage (5600 m2 )
 High altitude (4300 m a.s.l.)
 pointing resolution (≤ 0.5 °)
 detailed space-time image of the shower front
 detection of small showers (low threshold energy)
 large fov and high “duty-cycle”
 continuous sky monitoring (-10° <  < 70°)
Status of the experiment
 16 clusters (~ 700 m2) in stable data taking for 10
months (Jan 2004 till October 2004)
 gas mixture optimization
 fine tuning of electronics parameters
 long term test of the input-stage protection of the FE electronics, necessary to avoid
damages due to high energy showers (tests at Roma 2 and in Tibet): fully successful
 monitoring of RPC efficiency
 time calibration operations
 check of the reconstruction algorithms
 42 clusters (~ 1900 m2) in data taking since the end of
2004
 detecting area large enough for Solar Flare and GRB searches.
 100-110 clusters (~ 4500 m2) in data taking at the end
of 2005
 Completion of the central carpet in spring 2006
Trigger rates (threshold N > 60 pads)
Shower Front
on
42 Clusters
(41 x 46 m2)
Event reconstruction with 42 clusters
(PRELIMINARY)
Zenith angle distribution
Direction cosine
distributions
<l> = -0.016
<m> = 0.025
DCS: HV monitoring (16 clusters, 10/02/2005)
DCS: RPC current monitoring (16 clusters, August 2004)
• Average Total RPC current
• Average barometric pressure
• Average hall temperature
Counting rate as a function of time
4 Clusters during 3.5 days
single pad
doubles
All Clusters react homogeneously to external changes
Analog Charge Readout: event on 4 Clusters
(180 m2) at YBJ (PRELIMINARY)
~30 part/m2
ADC Counts on each big-pad
Full scale = 4000 ADC counts = 300 mV
Graphical elaboration
1 m.i.p = 2 mV
Some events…
4000 ADC counts ~ 90 p/m2
Very big shower !!
More events…
Conclusions
 The detector performance is turning out to be as good as
expected
 All the subsystems (DAQ, DCS, ACR) are fully operational;
further improvements are foreseen on the DCS for
redundancy
 The analysis of the data collected on a ~ 1900 m2 carpet is
in progress: early results are going to be presented at
ICRC 2005
 The installation is in progress and will be completed in
2006
 Most important, a stand-alone RPC apparatus is turning out
to be a crucial tool for cosmic-ray astrophysics, apart
from its already established applications as a muon-trigger
detector in experiments at colliders