222Rn daughters influence on scaler mode of ARGO-YBJ detector Irene Bolognino, University of Pavia and INFN E. Giroletti,C. Cattaneo,G. Liguori,P. Salvini,P. Vallania,C. Vigorito on behalf of the ARGO-YBJ Collaboration 32nd ICRC, Beijing August 13th 2011

ARGO-YBJ detector Detector layout (5,800 m 2 ) Active area 93% Strip = spatial pixel Pad = time pixel Time resolution ~1 ns 10 Pads (56 x 62 cm 2 ) for each RPC 8 Strips (6.5 x 62 cm 2 ) for each Pad 78 m 111 m 99 m74 m (  43 m 2 ) 1 CLUSTER = 12 RPC RPC + Analog charge read-out on “Big Pads”

Scaler mode Cluster counting over 4 channels: N hit ≥1,≥2, ≥3, ≥4, every 0.5 s, no timing or spatial distribution (E th > 1 GeV).  detector monitor: influence of meteorological effect, mainly pressure and gas temperature  flaring phenomena (gamma ray bursts, solar flares) Operation modes Shower mode Inclusive Trigger: N pad >20 within 420ns on the central carpet  rate ~ 3.6 kHz ( ~220 GBytes/day) Detection of Extensive Air Showers (direction, size, core …)  cosmic-ray Physics (threshold ~ 1 TeV)  VHE  -astronomy (threshold ~ 300 GeV)  gamma-ray bursts

Work focused on Scaler mode

Correlation with the environmental parameters  and  were calculated for different clusters and periods C ≥ 2, C ≥ 3, and C ≥ 4:  = % mbar -1  = % °C -1 C ≥ 1:  = % mbar -1  = % °C -1 Barometric, and thermal coefficient depend on the cluster considered and the specifical experimental conditions. Aielli et al., Astropart. Phys. 30 (2008) 85-95

Radioactive family: Uranium Radon-222 Complex time variations in a open building with ventilation conditions varying during the day In a stationary state: C Rn (t)= Radon concentration (Bq/m 3 ) E Rn (t)=Radon emission in volume V (Bq/s) Rn = Radon decay constant (2.1  /s) I vent = ventilation in (air exchange/s)

Montecarlo simulations in air E  keV  Simulation with FLUKA using graphics user interface (GUI) FLAIR. 6·10 6 gammas launched for each energy Equilibrium factor = 0.7  0.8 Hz per Bq per m 3 Efficiencies check ( 137 Cs, 60 Co) Air Cluster 43 m 2 Width (23 m) Length (17 m) Deep (m)

Radon enters the Argo hall from soil and cracks (north side) and exits through doors and windows with an ease dependent on ventilation and atmospheric conditions Bq/m 3 Julian day - offset 30 days, May2010 Radon measurements in air (Lucas Cell) Average trend of Rn concentration

1st analysis method: the LINEARIZATION We evaluated: time series (Radon, Scaler1, Pressure, Temperature) at different seasons of 2010 clusters at North, middle, and South side of the experimental hall normalized time series Our goal: C1 RESIDUE (t) = k C Rn Quantify the influence of natural radioactivity on C1 counts

Radon Concentration vs Time Scaler1 vs Time Pressure vs Time Gas Temperature vs Time From 2nd to 15th June 2010, central cluster

C1(t) – [ P(t) T(t)] = C1 RESIDUE (t) correl.coeff.(C1 RESIDUE,C Rn ) = 0.93 MJD, 2010 June from 2nd to 15th C1 RESIDUE (t)

North South Middle Corr coeff is better at the center and worse at the North and South sides. Worst correlations are obtained in high electric field variation periods (see “Observation of the Effect of the atmospheric electric fields on the EAS with the ARGO-YBJ experiment”, poster of this Conference). cosmic radon electric field background Influence is about 1-3% for radon concentration of 500 Bq/m 3

2nd analysis method:PROPORTIONAL Hp: P, and T influence the cosmic rays contribution  1, and the detector response with the same proportion as in  2,  3, and  4. k n =, n=2,3,4. North South Middle Influence is about 1-3% for radon concentration of 500 Bq/m 3 ~(20±5) kHz

Conclusions Natural radioactivity in air influences the ARGO-YBJ single counting rates at the level of about Hz per Bq/m 3 of 222 Rn concentration. The average radon (500 Bq/m 3 ) influence is ~1-3% of C1 counts according to Fluka simulations. In general the correlation of the clusters located at the center with C Rn is higher than the North and the South ones. The same methods was applied to C2, C3, and C4 and we didn’t find any radon influence as expected. Radon gas concentration is monitored in order to perform the best possible correction at the ARGO-YBJ lowest energy threshold.

THANK YOU!

Backup slides

The ARGO-YBJ experiment Collaboration between:  Istituto Nazionale di Fisica Nucleare (INFN)  Accademia Cinese delle Scienze (CAS) Site: Observatory for Cosmic Rays of Yangbajing (Tibet), China High Altitude Cosmic Ray YangBaJing Site altitude: 4,300 m a.s.l., ~ 600 g/cm 2 Coordinates: longitude 90° 31’ 50” E, latitude 30° 06’ 38” N ARGO-YBJ (Astrophysical Radiation Ground-based Observatory)

Shower mode Space pixel: single strip ( 7×62cm 2 ) Time pixel: pad (56×62 cm 2 ) is the OR of 8 strips, with a resolution of ~ 1 ns The high granularity, the time resolution and the full coverage allow reconstruction with unprecedent details. The detailed shower topology is a possible tool for gamma/hadron discrimination

222 Rn and daughters

Radioactive families secular equilibrium

Complete agreement between simulated and measured efficiencies : 1% for E  ≈1.25 MeV ( 60 Co source) and 0.5% for E  of 0.6 MeV ( 137 Cs source) Result of volumetric simulation: