AirFly: precise measurement of the absolute yield of fluorescence - - PowerPoint PPT Presentation

AirFly: precise measurement of the absolute yield

• f fluorescence photons in atmospheric gases

Frederick Kuehn for the AirFly collaboration ISVHECRI 2010

Purpose: Fluorescence energy determination

• Energy deposited in the atmosphere by the induced cosmic ray shower is related

to the fluorescence light emitted by excited Nitrogen molecules

• Need an absolute calibration to determine the Pierre Auger Observatory energy

scale

• Goal is to measure the fluorescence yield per MeV deposited to a few percent

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Nλ Ri A T(λ) ε(λ)

Bunner (1964)

34 band intensities measured (relative to the 337 nm line)

AIRFLY

• Astropart. Phys. 28 (2007) 41

FILTER

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AIRFLY measurements

313.6 nm 337.1 nm 353.7 nm

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Method & setup

• Compare fluorescence

yield to a well known process to eliminate photo-detector systematics

• The ratio of Fluorescence/

Cherenkov make many systematics cancel

• Also used a laser for

absolute calibration cross check

~ 3 fluorescence photoelectrons per 104 protons in air

Beam

Photon Detector Integrating sphere Diffuser for Cherenkov run Cherenkov dump Acceptance counter Cherenkov trigger rod

Veto counter Veto counter

Shutter Beam defining counter Gas/Vacuum container

N337(fluo)

• = Yfl

× Geomfluo

• × Tfilter × QE337
• × Np
• measured
• N337(cere)

=

known

Ycere ×

MC

• Geomcere ×

∼cancel

• Tfilter × QE337 ×

relative

• Np

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To preserve the optical geometry a second diffuser

• pens and closes a dummy

Cherenkov dump perpendicular to the beam

White Gore highly UV reflective material inside sphere

Diffuser actuator

vetos Beam defining counter signal PMT dummy Cherenkov dump Cherenkov rod hidden by dump

Experimental setup

Beam structure @ M-Test (120 GeV protons)

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4 s 19 ns 10 µs

Up to 70 bunches PMT

1 spill / min Several 105 particles/spill UV transparent acrylic rod

Spill train bunch

Summed traces

• ver a spill

using a 500 MHz FADC

Cherenkov rod spectrum

1p ped PMT glass

PMT Veto

Single particle triggering

• Require limit on max veto

signal

• Beam defining counter

(“finger”) signal in peak region

• Cherenkov rod signal in

peak

• Impose a time based

isolation window of ±80 ns

• Defines a “single particle”
• 25% of trains pass the

requirements

• ~1.1 single particles per

train

Photon counting

• Select single charged particles

passing through the chamber

• Search in (±80 ns) time window

defined by single particle trigger for a single photoelectron

• Require that single photoelectron

trigger occurs after single charged particle trigger

• Time distribution of single

photoelectrons used to determine yield

• Data taking conditions:
• Cherenkov/Fluorescence modes,

gas type (N, Ar, Air, He), primary particle (p,e-,π), pressure (vacuum→ atm), photon detector shutter position (closed/open)

Photon arrival time distribution Exponential plus constant fit function

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Time constant determined by sphere geometry Cherenkov & fluorescence photon are prompt

Backgrounds

• Measure fluorescence & Cherenkov signal in vacuum, helium (should have ‘zero’

signal), air, and argon (no 337nm excitation line), with the shutter open and closed

• Checked background from exit port and delta rays
• Background is about 3% of Nitrogen signal

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charged particles

(e.g., 120 GeV protons) Cherenkov port (open) integrating sphere delta rays fluorescence photon Cherenkov photon signal PMT shutter (open/closed)

Backgrounds

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charged particles

(e.g., 120 GeV protons) Cherenkov port (closed) integrating sphere delta rays fluorescence photon Cherenkov photon

• Measure fluorescence & Cherenkov signal in vacuum, helium (should have ‘zero’

signal), air, and argon (no 337nm excitation line), with the shutter open and closed

• Checked background from exit port and delta rays
• Background is about 3% of Nitrogen signal

signal PMT shutter (open/closed)

In situ laser calibration

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NIST Calibrated Photodetector (5% absolute scale)

Second integrating sphere plus collimator

Photon Detector

Integrating sphere Cherenko dump

337 nm Nitrogen laser

Independent calibration method

Efficiency of the second sphere measured to be εsph= 2.49 · 10-6

Second integrating sphere 337 nm laser

During the setup at M-Test

collimator

Analysis & preliminary results

• Current measurements of Fluorescence in Nitrogen give (preliminary numbers!)
• 19.48 ± 0.15 (stat) x 10-4 photons/proton
• 18.6 ± 0.51 (stat) x 10-4 photons/pion
• 18.95 ± 1.12 (stat) x 10-4 photons/electron
• Current measurements of Cherenkov yield in Nitrogen give (preliminary numbers!)
• 10.17 ± 0.22 (stat) x 10-4 photons/proton
• Laser calibration gives consistent results within 5%
• GEANT simulation needed to convert Cherenkov/fluorescence ratio to the yield ongoing
• ~5 photons/MeV consistent with other experiments

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Systematic uncertainty at the level of 4%

}

N337(fluo)

• = Yfl

× Geomfluo

• × Tfilter × QE337
• × Np
• measured
• N337(cere)

=

known

Ycere ×

MC

• Geomcere ×

∼cancel

• Tfilter × QE337 ×

relative

• Np

Summary

• Absolute yield measured to ~10% in early

data runs (June 2009)

• Two independent calibrations give

consistent results within 5%

• Currently analyzing data taken in December

2009 and January 2010

• Confirms earlier results with improved

statistical and systematic uncertainties

• Measurements with different particles

confirms that fluorescence is proportional to energy deposited

• Final uncertainty of a few percent is within

reach

• Expect final results by the end of the year

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