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 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 IHEP, Beijing Shandong University, Jinan South West Jiaotong University, Chengdu Tibet University, Lhasa Yunnan University, Kunming Zhenghou University, Henan Spokesman: B. D’Ettorre Piazzoli Spokesman: Z. Cao Collaboration Institutes: Chinese Academy of Science (CAS) Istituto Nazionale di Fisica Nucleare (INFN)

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) A strophysical R adiation with G round-based O bservatory

The ARGO-YBJ site

Outline  Introduction  The ARGO-YBJ experiment  Conclusions Ground based  -ray astronomy Detector layout and RPC details Physics goals and sensitivity Present status and first measurements

Why ground-based detectors ? Satellite measurements are limited by the E -  (  = 2 ÷ 3) law for  -ray flux  CRAB (>500 GeV)  6 · photons/(cm 2 s) 1 m 2 detector needs  5 · 10 4 hours of observation to collect 100 photons  CRAB (>1 TeV)  2 · photons/(cm 2 s)  1.4· 10 5 hours VHE  -astronomy possible only by ground-based detectors exploiting the amplification effect of the Extensive Air Showers (EAS)

Detecting Extensive Air Showers EAS arrays 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) Air Cherenkov Telescopes Very low energy threshold (  60 GeV) Good background rejection (99.7 %) High sensitivity (<  crab ) Good energy resolution Low duty-cycle (~ 5-10 %) Small field of view  < 4°- 5°

A new generation of EAS arrays Low energy threshold < 500 GeV Increase sensitivity Φ  Φ crab  Φ crab The Goal High altitude operation Secondary photon conversion Increase the sampling (~1%  100%) The Solution Improves angular resolution Lowers energy threshold

ARGO-YBJ Physics Goals   -ray astronomy Search for point-like galactic and extra-galactic sources at few hundreds GeV energy threshold  Diffuse  -rays from the galactic plane and SNRs  GRB physics (full GeV / TeV energy range)  Cosmic ray physics 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 thickness of the gas volume : 2mm Gas mixture: Ar/ i-C 4 H 10 /C 2 H 2 F 4 = 15/10/75 Operating voltage = 7.2 kV (10.2 kV at sea level) Single RPC absorption 7.2 kV = 3-  A Single RPC count 7.2 kV = 4 kHz Gas gap Bakelite plate Graphite layer Bakelite plate Graphite layer PET spacer

ARGO RPC details (1) Bakelite plate Read-out strip panel Front-end board

ARGO RPC details (2) Closed ARGO chamber High-voltage connection Low-voltage connection

RPC performance in the ARGO preliminary test Efficiency Time resolution Altitude effect TFE/ iBUT=97/3 TFE/Ar/ iBUT=75/15/10 Gas mixture: Ar/ i-C4H10 /C2H2F4 = 15/10/75 Operating voltage = 7.2 kV (10.2 kV at sea level) Single RPC absorption 7.2 kV = 3-4  A Single RPC count 7.2 kV = 4 kHz

78 m 99 m74 m 111 m Detector Layout 10 Pads = 1 RPC (2.80  1.25 m 2 ) 12 RPC =1 Cluster ( 5.7  7.6 m 2 ) 8 Strips = 1 Pad (56  62 cm 2 ) Layer of RPCs covering  5600 m 2 (  92% active surface) cm lead converter + sampling guard ring time resolution ~ 1 ns space resolution = 6.5  62 cm 2 (1 strip) 78 Clusters Central Carpet: 130 Clusters, 1560 RPCs, Strips

ARGO-YBJ Experimental Hall RPC chamber Cluster

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  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 …) Local Station (basic unit of distributed DAQ System) Central Station Trigger Data storage Trigger Pad Multiplicity info DATA

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) ARGO can observe, in 1 year, a Crab- like source of intensity 0.7 Crab units at energies E > 0.5 TeV, with a significance of 4 standard deviations. ARGO Veritas Glast Hegra Whipple Milagro CRAB Whipple E Opening angle Zenith angle  < 40°  4.3 h/day   ≈ ψ  / 1.58 ~ 1 TeV ~ 2 TeV~ 5 TeV 0.55 TeV N  (>1 TeV) ~ 10 ARGO: without any  /h discrimination ! A f = 80  80 m 2 T 5  (>1 TeV) ~ 3 months

 -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  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°) Resistive Plate Chambers (RPC) as active elements Space information from Strip (6.5 × 62 cm 2 ) Time information from 8-strip pads (resolution  1 ns) Large area (  m 2 ) and full coverage (5600 m 2 ) High altitude (4300 m a.s.l.)

Status of the experiment 16 clusters (~ 700 m 2 ) 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 m 2 ) in data taking since the end of 2004  detecting area large enough for Solar Flare and GRB searches clusters (~ 4500 m 2 ) 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 m 2 )

Event reconstruction with 42 clusters (PRELIMINARY) = = Zenith angle distribution Direction cosine distributions

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 doubles single pad 4 Clusters during 3.5 days All Clusters react homogeneously to external changes

Analog Charge Readout: event on 4 Clusters (180 m 2 ) at YBJ (PRELIMINARY) Graphical elaboration ADC Counts on each big-pad ~30 part/m 2 1 m.i.p = 2 mV Full scale = 4000 ADC counts = 300 mV

Some events… Very big shower !! 4000 ADC counts ~ 90 p/m 2

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 m 2 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