1. University and INFN of Naples
Size spectrum and Lateral Distribution of air showers measuread by ARGO-YBJ at high energies (＞ 100TeV).
Michele Iacovacci
University and INFN of Naples
ICRC - August 2011, Beijing

2. Cluster = DAQ unit = 12 RPCs Pad =TIME PIXEL, 62 x 56 cm2, 15600
σt≈1.8 ns
Strip =SPACE PIXEL, 62 x 7 cm2,
BigPad =CHARGE readout PIXEL, 123 x 139 cm2, 3120 (central carpet)
BigPad
RPC
78 x 74 m2
110 x 100 m2
Pad
Strip
BP Amplitude :
mV to many Volts

3. Calibration and Stability
Analog readout system
The system can be operated with different full scale (f.s) namely 0.33, 0.66, 1.3, 2.5, 5, 10, 20 and 40 V.
The analog data acquisition starts when the cluster particle multiplicity gets higher than a specific threshold, namely ≥ 16,≥ 32, ≥ 64 and ≥ 73 hits.
Since December 2009 Analog Readout in operation on the central carpet. (5800 m2) with a local multiplicity threshold of ≥ 73.
Scale of operation:
December June 2010 , 330 mV ;
July to August 2010, V f.s.;
since middle August 2010, set to 20 V.
Total BP number 3120, dead channels in the order of 3%.
Spectrum of the particle number of a cluster obtained by using both digital and analog info at the low scale.
ADC count
Gain (mV/part)
Gain vs time
After calibration, the detector shows a quite good stability, evaluated in 2-4%, and homogeneity among channels estimated at 5% level.
(see poster HE and also HE )

4. RUN_95724_ev_51_PMax_1052
nxBP
nyBP
Npart
r (m-2)
r (m)
2.5 V fs

5. RUN_98177_ev_20_PMax_4998
nxBP
nyBP
Npart
r (m-2)
r (m)
20 V fs

6. RUN_98175_ev_5_PMax_7110
nxBP
nyBP
Npart
r (m-2)
r (m)
20 V fs

7. Lateral Distribution Data selection:
1. at least one BP with ADC count > 50;
2. cluster with maximum ADC count inside
the internal 54 (6 x 9) central clusters;
3. zenith angle θ ≤ 15◦.
4. ρmax ≥ 130m−2 and particle number > 1000
in the 8 BP nearest to BPmax
a) 20 V f.s., ADC count of BPmax in
b) 20 V f.s., ADC count of BPmax in
c) 2,5 V f.s., ADC count of BPmax in
d) 2,5 V f.s., ADC count of BPmax in
MC simulation:
CORSIKA/QGSjet;
quasi-vertical showers (q< 15%);
core in a fiducial areaof about 260 m2;
average efficiency of 95% both for detector and trigger;
the carpet geometry fully simulated.

8. Npart vs rmax
ARGO-YBJ Data
rmax (m-2)

9. rmax spectrum ρmax = 300m−2 normalization point
Slopes: 2.23+/-0.01 (2.5 V) and 2.23+/-0.02 (20V).

10. Spectrum of Npart(r<10,15,20)
ARGO-YBJ Data
ARGO-YBJ Data
rmax>30
rmax>300
2.5 V fs
20 V fs

11. rmax vs Npart(r<10) 3 f. s.
ARGO-YBJ Data
MC simulation:
CORSIKA/QGSjet;
quasi-vertical showers (q< 15°);
full detector simulation:
Ep=100,300,500, 700, 1000, 2000, 3000 TeV
EFe=100,300,500, 700, 1000, 2000, 3000 TeV

12. rmax vs Npart(r<10) : data+MC (1)
ARGO-YBJ Data
3. 104

13. rmax vs Npart(r<10) : data+MC (2)
ARGO-YBJ Data
3. 104

14. rmax vs Npart(r<10) : data+MC (3)
ARGO-YBJ Data
3. 104

15. rmax vs Npart(r<10) : data+MC (4)
ARGO-YBJ Data
3. 104

16. rmax vs Npart(r<10) : data+MC (5)
ARGO-YBJ Data
3. 104

17. rmax vs Npart(r<10) : data+MC (6)
ARGO-YBJ Data
3. 104

18. rmax vs Npart(r<10) : data+MC (7)
ARGO-YBJ Data
3. 104

19. rmax vs Npart(r<10) : data+MC (8)
ARGO-YBJ Data
3. 104

20. Conclusions
Since December 2009 the analog readout of the ARGO-YBJ detector is in operation on the central carpet (5800 m2), so extending the energy range of the detector above 100 TeV.
The core region is measured with unprecedented detail. Preliminary lateral distributions and maximum density distribution have been shown and compared with MC expectations.
By using specific combination of shower quantities, just belonging to the core region, it has been shown how promisingly we can approach the problem of the primary particle identification.