Precise Measurement of the Absolute Yield of Fluorescence Photons in Atmospheric Gases

Transcription

1 Precise Measurement of the Absolute Yield of Fluorescence Photons in Atmospheric Gases Paolo Privitera 5th Fluorescence Workshop 7 th Air Fluorescence El Escorial Workshop - Madrid, Spain September 22-24, September 2007 Coimbra, Portugal The Enrico Fermi Institute The Kavli Institute for Cosmological Physics for the AIRFLY Collab.

2 UHECR Fluorescence Detection and Fluorescence Yield Fluorescence Spectrum p, T and humidity dependence h N T(λ) R i ε(λ) A Proportionality to energy deposit Absolute yield E/E = 22% in Auger - largest syst. is 14% from absolute yield

3 AIRFLY strategy Precise measurement of fluorescence band intensities dependence on p, T, and H (relative to 337 nm band) Measurement of the absolute yield of 337 nm band Absolute yield of 337 nm line (photons/mev) Band intensity relative to the 337 nm line Quenching reference pressure Temperature dependence

4 + Fermilab (C. Hojvat, F. Kuehn)

5 AIRFLY beams Beam Test Facility INFN FRASCATI ( MeV) Chemistry Division Van de Graaff (0.5-3 MeV) ARGONNE ANL Advanced Photon Source (6-30 KeV) HEP Division Advanced Wakefield Accelerator (3 MeV-15 MeV) Meson Test Beam (Up to 120 GeV) 5

6 Air fluorescence spectrum Bunner (1964) Astropart. Phys. 28 (2007) band intensities measured (relative to the 337 nm line)

7 Pressure dependence Solved previous discrepancies due to neglecting secondary electrons escaping the field of view (~all the fluorescence emission is produced by secondary electrons) High pressure Low pressure Humidity dependence Temperature dependence RANGE [-40,30] 0 C up to 10% effect! 20% water vapour partial pressure NIM A597 (2008) 50 (assumed) 7

8 VdG+AWA Argonne MeV Frascati MeV APS Argonne 6-30 kev Energy dependence NIM A597 (2008) 46 Proportionality of fluorescence yield to E dep at few % 8

9 Absolute yield of the 337 nm line Main systematic comes from absolute QE of photon detector AIRFLY: normalize to a well known process (Cherenkov emission) to cancel detector systematics N 337 (fluor.) = FLY x Geom fluor x T filter x QE 337. measured MC ~cancel! known measured N 337 (Cher.) = CHY x Geom cher x T filter x QE 337. x e mirr 337 filter PMT Fluorescence run 337 filter PMT Cherenkov run 45 0 mirror First test performed in Frascati

10 AIRFLY at the Fermilab Test Beam Photon Detector 337 nm filter Diffuser to be placed for Cherenkov run Integrating sphere Cherenkov dump Acceptance counter Particle beam Veto counter High energy up to 120 GeV Well defined beam: single particle trigger and geometry Wide range of particles type and intensity (p, e, μ, π) Absolute calibration with two independent methods: Cherenkov and laser light

11 Cher. dump 11

12 Veto Downst. Signal PMT Veto Upstream Acceptance counter 12

13 Integrating Sphere 13

14 Why use an Integrating Sphere? Lambertian light output independent of input light distribution Fluorescence isotropic Depolarize light Cherenkov highly beamed Fluorescence unpolarized Cherenkov polarized Highly diffusive PTFE no syst. from angular or polarization dependent efficiency of PMT+ 337 nm filter Collect photons over ~4 Improve signal/bkg ratio 14

15 4 s Single particle triggering 1 spill / min Several 10 5 particles/spill 10 μs 20 ns 20 ns Up to 70 bunches Need single particle resolution, fast detector Cherenkov, FADC 500 MHz PMT UV transparent acrylic rod

16 Very clean single particle selection Single particle triggering Cherenkov rod Cherenkov rod spectrum PMT glass ped 1p Veto 1 2p Veto Veto 2 PMT

17 p.e./2 ns /10^4 protons Fluorescence Measurement in N GeV protons Single p.e. spectrum p.e. timing (wrt. proton) Signal 1000 mbar Background: vacuum Integrating sphere S FL (N 2 )= ( ) 10-4 p.e./proton Photon Detector S FL (vac) = ( ) 10-4 p.e./proton Counting experiment: select (single, clean) protons and then count the photons they produced. Potential background from secondaries produced in gas and hitting walls 17

18 Fluorescence Measurement in N 2 and air S Fl (N 2 )- S Fl (air ) = (Fl N2 + Bkg) (Fl air +Bkg) = Fl N2 Fl air = Fl N2 (1-1/r) (1-1/R) r = Measured at AWA Argonne Same background from secondaries produced in air and N 2 r = measured in the Fermilab apparatus with an alpha source Fl N2 = (19.44 Bkg = ( ) 10-4 p.e./proton 0.08) 10-4 p.e./proton 1% statistical unc. consistent with vacuum bkg only 3% of signal

19 p.e./2 ns /10^4 protons Cherenkov Measurement in N 2 and air Signal 1000 mb Integrating sphere Background: vacuum Photon Detector Diffuser S Ch (N 2 ) = ( ) 10-4 p.e./proton = Ch N2 +Fl N2 + Bkg + Bkg Ch Additional bkg from Cherenkov of beam in DRP (teflon) Cherenkov dump Bkg Ch = ( ) 10-4 p.e./beam particle Bkg ~10% of Fl+Ch Measured in vacuum, He Ch N2 = (10.27 Ch air = ( ) 10-4 p.e./beam particle 0.25) 10-4 p.e./beam particle 2% statistical unc. Air ~ N 2 19

20 Absolute Yield of 337 nm line Fluorescence to Cherenkov ratio R N2 = Fl N2 = Ch N2 Y air ( R N2 / r ) = (Y air ) MC ( R air ) MC Currently used in Auger: 5.05 photons/mev Geant4 MC simulation of the experiment. All individual components simulated according to measurement: wavelength and angular dependence of 337 nm filter transmittance, sphere transmission, PMT QE, relative intensities of fluorescence bands, etc. Given an absolute fluorescence yield in the simulation, (Y air ) MC, we obtain the expected Fluorescence/Cherenkov ratio, (R air ) MC = stat photons 337 /MeV 2.4 % statistical unc., systematic discussed later

21 A few of many checks Measurement reproducibility: 2 test beam periods, each 2 weeks. Apparatus taken apart and mounted again. Very stable results. Gas purity stable. Background estimation from He not fluorescing in principle. Helium runs continuously taken to measure bkg. A small amount of fluorescence observed, due to residual bottle contamination Confirmed by a He-N 2 mixture. 21

22 However S Ch (He )-S FL (He) = Bkg Ch thus He can be used to measure bkg from Cherenkov in DRP plug Is DRP scintillating? Checked with alphas hitting the sphere under vacuum. No scintillation observed. Bkg from secondaries produced in the DRP port in Cherenkov mode? We found S Ch (N )-S Ch (air) = S Fl (N )-S Fl (air), thus no bkg from secondaries produced in the DRP port 22

23 Fluorescence/Cherenkov stability Eg. with respect to time integration of signal 23

24 Cross-Check of Fl/Ch Use an aluminized mylar mirror to bounce Cherenkov light back into the sphere. Eliminate bkg from beam Cherenkov in the diffuse Bkg Ch = 0 Photon Detector Aluminized mylar mirror Cherenkov dump Independent Y air measurement, consistent within 2% (4% stat. uncertainty)

25 Absolute calibration with 337 mn laser Photon Detector Integrating sphere Cherenkov dump NIST calibrated probe (5%) Light from a 337 nm laser (absolute calibration at 5%), is attenuated in a second integrating sphere of know transmission. A known n. of photons enter the AIRFLY integrating sphere, and single p.e. are measured in the PMT Integr. sphere n. 2 (efficiency measured independently) Pulsed 337 nm laser Eff = ( ) 10-4 p.e./photon 337

26 Fluorescence PMT 337 nm laser Integrating sphere known transmission Trigger PMT NIST Calibrated Probe (5%) attenuators

27 Absolute Yield of 337 nm line Laser Calibration L N2 = Fl N2 = ( ) photons Eff 337 /proton A Geant4 MC simulation of the laser calibration is performed and (Eff) MC obtained is Y air ( L N2 / r ) = (Y air ) MC ( L air ) MC = stat photons 337 /MeV 1.3 % statistical unc. Consistent with the Fluorescence to Cherenkov ratio measurement NOTE: In situ laser calibration during test beam. Laser calibration reproduced in the lab at 1% level Fluorescence data sample independent of the Fl/Ch measurement 337 nm laser line vs Cherenkov light integrated over a 10 nm FWHM interference filter. Different systematic

28 Systematic Uncertainties Fluorescence to Cherenkov ratio Reflectivity of the sphere ~ 1% - PMT quantum efficiency ~ 1.5% - Monte Carlo statistics ~ 1% - N 2 /Air ratio ~ 1% - Geometry ~ 1% - Filter transmittance ~ 2% - Background subtraction ~ 1% - E dep model in MC (% level, large sphere) -. Other sources of uncertainties are under investigation Total systematic uncertainty is expected < 5%

29 Systematic Uncertainties Laser Calibration Laser probe calibration ~ 5% - Transmission sphere n. 2 ~ 2% - Monte Carlo statistics ~ 1% - N 2 /Air ratio ~ 1% - Geometry ~ 1% - Background subtraction ~ 1% -Edep model in MC (% level, large sphere) -.. Other sources of uncertainties are under investigation NOTE: Fluorescence produce by secondary electrons: high energy beam particle not important. We confirmed by measuring fluorescence with 32 GeV pions and 8 GeV electrons.

30 Summary AIRFLY has performed a precise measurement of the absolute fluorescence yield of the 337 nm line. Two independent calibration methods have been used, Cherenkov and laser light, giving compatible results. The preliminary results are consistent within the larger uncertainties of other experiments. The study of systematic uncertainties is being completed. We expect a final measurement with a total uncertainty of ~ 5%. Together with AIRFLY measurements of the air fluorescence spectrum and its pressure, temperature and humidity dependence, the total uncertainty on the energy scale of UHECR will be reduced to ~ 5%.