Jose Bellido

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Fluorescence Detectors for the Study of High
Energy Cosmic Rays (lecture 2)
Jose Bellido
Second School of Cosmic Rays and Astrophysics
August 30th to September 8th , 2006 Puebla -
Mexico
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Description of Each Fluorescence Detectors
-Fly's Eye -HiRes -Auger (next lecture will be
dedicated to Auger)
Detector description Atmosphere
monitoring Reconstruction of the EAS
Observational Results
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The Utah Fly's Eye Fluorescence Detector
(1981 -1993)
Cassiday, Bergeson, Loh, Sokolsky et al.
R. M. Baltrusaitis etal., Nucl. Instrum. Methods.
Phys. Res. Sect. A 240, 410 (1985)
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The Utah Fly's Eye Fluorescence Detector
Location 40 N, 113 W, Atmospheric depth
860 gr/cm²
Fly's Eye I -67 spherical mirrors -The mirrors
were arranged so that the entire night sky is
imaged.
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Mirror Units
- 1.5m diameter - each with 12 or 14 PMT at the
focus. - each PMT viewing a hexagonal region of
the sky 5.5 in diameter.
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Fly's Eye II
-Started operations in 1986 -Located 3.4 km away
from Fly's Eye 1 -Consists of 36 mirrors of the
same design. -Fly's Eye II only views the half of
the night sky in the direction of Fly's Eye II
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Triggering an Event - The PMT threshold is
automatically adjusted to maintain a constant 50
Hz trigger rate. -A mirror is triggered when at
least 2 PMT are triggered within a certain time
window (8?s, 20?s and 40?s for the fast, medium
and slow channels respectively). -The event is
recorded when two or more mirrors are triggered
within the time window (8?s, 20?s and 40?s).
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Monitoring the Detector efficiency and
atmospheric conditions - The absolute tube gain
and mirror optical efficiency were calibrated
once or twice a year. - An optical pulser is
installed in each mirror to monitor the
efficiency of the whole system in a regular
basis. -Vertical flashers (28) were mounted
around Fly's Eye I and fire hourly to monitor the
atmospheric conditions and to cross-check the
tube and mirror efficiency.
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Reconstructing the EAS Geometry
t0 - Rp/(ctan?)
t0 time when the shower front crossed the Rp
point.
Field of view of PMTi
Rp /tan?
co ? ci
?
t0
Rp /sin?
Rp
EAS
?o
Detector
?i
ti t0 - Rp/(ctan?) Rp/(csin?) t0
Rp/c (1/sin? - 1/tan?) ti t0 Rp/c tan?/2
ti t0 Rp/c tan(?o - ?i)/2
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Reconstructing the EAS Energy
fraction of electrons with energy gt E
Accounting for the Cerenkov light -The Cerenkov
light contribution is estimated. - Angular
distribution of Cerenkov photons. (used to
estimate the direct Cerenkov light arriving to
the detector) - Cerenkov photons scattered out of
the Cerenkov beam toward the Fly's Eye at an
angle ? within a solid angle d?.
E threshold for Cerenkov production
Cerenkov light produced per unit of length at a
height 'h'
n refraction index Ho Scale height (7.3km) ?o
Value of ?? at ground
Rayleigh scattering
Mie scattering
?M 26.7
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Reconstructing the EAS Energy
Photons scattered out of the beam -Due to
Rayleigh scattering. - Due to Mie
Scattering. - Cerenkov photons scattered out
of the Cerenkov beam toward the Fly's Eye are
also scattered.
?o 0.00107 g/cm³ at 0 at
Fly's Eye altitude
LM 14 km (typical Mie scattering free path) HM
1.2 km (is the scale height)
R. M. Baltrusaitis etal., Nucl. Instrum. Methods.
Phys. Res. Sect. A 240, 410 (1985)
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Reconstructing the EAS Energy
Attenuation ( ... using previous slide
equations) -The Rayleigh transmission factor
(fraction of photons not Rayleigh scattered from
the beam).. - The Mie transmission
factor. - Total transmission factor.
xo 860 g/cm² at Fly's Eye altitude
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Reconstructing the EAS Energy
Attenuation ( ... using previous slide
equations) -The Rayleigh transmission factor
(fraction of photons not Rayleigh scattered from
the beam).. - The Mie transmission
factor. - Total transmission factor.
HM and LM are monitored using vertical flashers
that are vertically fired around the Fly's Eye
location.
xo 860 g/cm² at Fly's Eye altitude
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Reconstructing the EAS Energy
-Number of fluorescence photons at the shower is
transformed to Ne using knowledge of fluorescence
yields on air. -Ne(x) is fitted to the
Gaisser-Hilla function - Then - A 10
correction is applied to account for the shower
energy not deposited in the atmosphere.
?o critical energy (81 MeV) xo radiation
length in air (37.1 g/cm²) Then the energy loss
rate per electron ?o/xo 2.18 Mev/(g/cm²)
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Main Results from the Fly's Eye Detector
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A ?
J(E) 10 E
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Main Results from the Fly's Eye Detector
The Highest Energy Particle Observed
Peak size 2 x 1011 particles !
Flys Eye Experiment 1990 - 3.2 x 1020 eV
50 J
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Proton
Iron
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The High Resolution Fly's Eye
Located at Dugway, western of UtahCollecting
area in excess of 3000km2 at highest
energies Atmospheric Depth of 850 g/cm²
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HiRes-2 from the top
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HiRes-2 configuration
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HiRes Mirrors and Building
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Mirror Units

• a total of 67 mirror units at the two sites

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Mirror Units
The exact orientation of the camera is
obtained by observing star tracks along the
camera field of view. . .

• a total of 67 mirror units at the two sites

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Using Stars to determine the exact field of view
of the Camera
Xstar and Ystar are the estimated position of the
star light over the camera when the PMT's pulse
is at its peak value
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Mirror Units
Relative Calibration
Fiber Optics
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Event seen by both HiRes sites
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Atmosphere Monitoring
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vertical aerosol optical depth (VAOD)

NMOL can be estimated using clear
nights data or MC modeling.
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Horizontal Attenuation Length (HAL) and Aerosol
Phase Function
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Using Stereo events to crosscheck the aerosol
parameters (VAOD)
The shower size measured at a given point by
HiRes-1 and HiRes-2, should be the same. If
the VAOD (?A) used is not the correct one, we may
compute different signals in HiRes-1 and HiRes-2,
depending on the distance difference to the
shower (?r).
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HiRes Geometrical Resolution
SDP lt 0.3
18.5
18.5
18.5
18.5
eV
eV
eV
eV
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E lt 10
E lt 10
E lt 10
E lt 10
HiRes-1
18.5
18.5
7
eV
eV
E gt 10
Shower Axis
HiRes-2
3
0.3
Stereo
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Reconstruction of the Shower Energy
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Energy Spectrum of the Electrons at Xmax
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Value used in Fly's Eye
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Correction for missing energy used by Fly's Eye
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Main Results From HiRes
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Comparing MC and Real distributions for Rp
Aperture
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Systematic Uncertainties
Absolute calibration 10 of the
PMTs. Fluorescence Yield 10 Unobserved
Energy 5 correction. Total Energy Scale
15, 17 Uncertainty Systematic
uncertainty 27, 31 in the flux.
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