1. Cosmic Rays at Extreme Energies The Pierre Auger Observatory
Ezio Menichetti
Dip. di Fisica Sperimentale, Universita’ di Torino
INFN, Sez. di Torino
Lecture 2

2. EAS Detectors Long history, dating back to the ’60s AGASA HiRes
Volcano Ranch (USA) – Sampling
Haverah Park (UK) – Sampling
SUGAR (Australia) - Sampling
Yakutsk (Russia) - Sampling
Fly’s Eye (USA) – Fluorescence
AGASA
HiRes

3. AGASA Akeno Giant Air Shower Array
111 scintillators + 27 muon det.
Akeno, Japan
11 Super-GZK events
Small Scale Clustering
Constraints on Composition:
 protons at UHEs.
(“Classical” analysis based on Ne vs Nm)

4. HiRes - High Resolution Fly’s Eye
Pioneers of FluorescenceTechnique
Air fluorescence detectors
HiRes mirrors
HiRes mirrors
Dugway, Utah, USA
No Super-GZK flux
No Small Scale Clustering
Composition Change

5. EAS – Early History

6. Ground Array EAS sampled at ground by a number of detector stations
Shower front
Shower core
hard muons
EM shower

7. Particle Composition

8. Shower Development

9. Universality of EM Component
Emax=25 MeV

10. The Muon Component

11. Tank: Signal Components e 
1 GeV
10 MeV
1.2m Pure Water
240 MeV
10 MeV
~track ~ energy

12. Tank: Light Emission by Muons
photomultiplier
41°
Retro-diffusing
Walls
Water
When a particle travels faster than the velocity of light in water,
Cerenkov light is emitted

13. Pulse Shape

14. Pulse Height from Muons
Signal unit ~ 1 VEM ~ 94 ph.e.

15. Pulse Height for Calibration
Most frequent signals ~ atmospheric muons . .
A/D ratio - sliding average over 3 min
of signals > 512

16. EAS Direction from Timing
B.Rossi, 1953

17. Direction from Timing
cDt d

18. Timing

19. Ground Array - LDF
Lateral Distribution Function
LDF Parametrization

20. Constant Intensity Cut
Need to unfold the zenith angle dependence of the tank response to any given energy
Assume isotropic flux at high energy: Expect same flux at all angles at any given E
→Plot cumulative (i.e. N(S>S0) vs. S0 ) pulse height distribution for each q bin
Normalize
q=380
150 evts
Relative tank response vs angle

21. Ground Array - Energy
Measured signal density extrapolated to a reference point
Typ. S(600m) to S(1000m) to minimize fluctuations
Compare to Monte Carlo simulation
Good linearity with Eprimary
Results somewhat depending on :
Primary interaction (physics)
Composition (p vs. Iron)
E range fixed by detector spacing
Lot of fun for us accelerator physicists!
Get energy from lateral density…

22. Ground Array - Composition
Several tool available. All in all, looks like a tricky business…
Just as an example: Signal rise time
Iron nucleus:
Less cascade steps before reaching 
 More ’s.
 Less EM energy
 Smaller muon rise time

23. Atmospheric Fluorescence
Radiative
De-excitation
High energy electron
N2 Molecule
UV Photons
Excited
N2 Molecule
Collision
excitation
N2 Molecule +
Fluorescence light
E.Menichetti – Bari, Aprile 2007

24. Fluorescence Yield
Atmosphere working as a large scale calorimeter

25. A Recent Measurement

26. The Fly’s Eye Concept
Fluorescence Light
EAS Path
Sensor Array

27. Fluorescence Detector
Shower-detector plane
Each PM seeing one segment of the shower
 Sequence of fast pulses by adjacent PMs
Impact point
Photomultiplier array

28. Atmospheric Headaches - I
Effect of Molecular (Rayleigh) Scattering
~18km
h~7.5km
=h/~0.41

29. Atmospheric Headaches - II
Effect of Aerosol (Mie) Scattering
~20km
h~1.2km
=h/~0.06

30. Cherenkov Headaches Estimate of Cherenkov contamination
Time (100ns bins)
Photons (equiv 370nm)
Estimate of Cherenkov contamination
Total F(t)
Direct
Rayleigh
aerosol

31. Fluorescence Detector
Practical implementation of light collection (also in Auger)
Pixel camera
(Photomultipliers)
Spherical mirror
(Large ~ 10 m2!)

32. Fluorescence Detector
PM current
Signal formation:
One PM, sequence of time slices

33. Fluorescence Detector
Composition (Fly’s Eye)
E= eV (Fly’s Eye) p
Xmax Fe
Unique FD capability: Longitudinal profile

34. Comparison: GA vs. FD GA FD Sampling detector 100% duty cycle
Large number of simple, modular elements
Shower parameters from x-section
Need reliable MCarlo simulation FD
Integral calorimeter
10% duty cycle
Small number of complex stations
Shower parameters from full shower track
Need good atmospheric monitoring+calibration

35. The Hybrid Concept FD: Measuring Etot & Longitudinal Profile
SD: Sampling Lateral Profile at ground