1. J.N. Matthews, ICRR 2/2004 Fluorescence Efficiency Measured by FLASH at SLAC -Preliminary Results- J.N. Matthews for the FLASH Collaboration

2. J.N. Matthews, ICRR 2/2004 FLuorescence in Air SHowers (FLASH) T. Abu-Zayyad 1, J. Belz 2, D. Bergman 5, Z. Cao 1, F.Y. Chang 4, P. Chen 3*, C.C. Chen 4, C.W. Chen 4, C. Field 3, P. Huentemeyer 1, W-Y. P. Hwang 4, R. Iverson 3, C.C.H. Jui 1, G.-L. Lin 4, E.C. Loh 1, K. Martens 1, J.N. Matthews 1, J.S.T. Ng 3, A. Odian 3, K. Reil 3, J.D. Smith 1, P. Sokolsky 1*, R.W. Springer 1, S.B. Thomas 1, G.B. Thomson 5, D. Walz 3, A. Zech 5 1 University of Utah, Salt Lake City, Utah 2 University of Montana, Missoula, Montana 3 Stanford Linear Accelerator Center, Stanford University, CA 4 Center for Cosmology and Particle Astrophysics (CosPA), Taiwan 5 Rutgers University, Piscataway, New Jersey * Collaboration Spokespersons

3. J.N. Matthews, ICRR 2/2004 Outline Air Fluorescence and UHECRs “The Problem” FLASH The Detector Some Measurements Future

4. J.N. Matthews, ICRR 2/2004 Air Fluorescence AGASA and other ground array experiments which sample the distribution of charged particles on the Earth’s surface HiRes (and now Auger) makes use of the atmosphere as its calorimeter a) The primary cosmic ray particle collides with air nucleus leading to b) a cascade of secondary particles, which in turn c) have more collisions producing a shower of a billion or more particles. As the charged particles pass through the atmosphere, they excite the gas causing it to fluoresce. An ultra-violet track develops at the speed of light….

5. J.N. Matthews, ICRR 2/2004 Air Fluorescence Technique Particle cascade dissipates much of its energy exciting and ionizing air molecules Fluorescence light emission is emitted isotropically Exited nitrogen molecules fluoresce in the near UV with emission line spectrum (roughly 80% of light is emitted between 300 and 450 nm) Intensity is proportional to the number of charged particles

6. J.N. Matthews, ICRR 2/2004 A HiRes Event HiRes 1 HiRes 2 Air fluorescence generated by the EAS is collected enabling observation of shower development from beginning to end

7. J.N. Matthews, ICRR 2/2004 Fluorescence Spectra Remarkable agreement between all the spectra measured by fluorescence detectors (including the “hybrid” HiRes-Prototype/MIA)

8. J.N. Matthews, ICRR 2/2004 The Problem: However, between the two experiments with the greatest exposures at the highest energies HiRes: fluorescence and AGAGA: ground array there is an offset Perhaps - at least partly - due to energy scale

9. J.N. Matthews, ICRR 2/2004 Energy Spectrum  AGASA energy scaled by 0.79

10. J.N. Matthews, ICRR 2/2004 HiRes Systematic Uncertainties PMT calibration: 10% Fluorescence yield: 10% Unobserved energy: 5% Atmospheric absorption: most sensitive to vertical aerosol optical depth (VAOD) –Mean VAOD = 0.04 –VAOD RMS = 0.02 –VAOD systematic is smaller. –Modify MC and analysis programs to use VAOD = 0.02 and 0.06, reanalyze. –J(E) changes by 15% Total systematic uncertainty = 21%

11. J.N. Matthews, ICRR 2/2004 Fluorescence efficiency is the foundation for our belief that we are measuring “energy” Bunner (1967), Kakimoto et al.(1995) Nagano et al.( 2003) indicates ~10-15% systematic errors in overall yield and larger errors in individual spectral lines. Non-linear effects possible due to λ 4 dependence of atmospheric attenuation: at 30 km, event energy can change by 25% if 390 nm line intensity changes by 40%. Pressure dependence not well measured esp. P<100 Torr Previous measurements show Y proportional to dE/dx, but no measurements 100 keV – 1 MeV Current Understanding:

12. J.N. Matthews, ICRR 2/2004 E-165 Motivation At large distances which are typical of the highest energy events seen in a fluorescence detector, knowing the spectral distribution of the emitted light becomes essential due to the λ -4 Rayleigh scattering At 30 km, event energy can change by 25% if the 390 nm line intensity changes by 40%. Bunner (1967)

13. J.N. Matthews, ICRR 2/2004 SLAC E-165: FLASH FLuorescence in Air SHowers Motivation: –Reduce the systematic uncertainty in energy reconstruction of UHECRs for detectors using the fluorescence technique. –Shed some light on the discrepancy between fluorescence and ground array experiments. Goals: –Building on the work of Bunner, Kakimoto and Nagano, we wish to further reduce the uncertainties in this measurement. –Measure the total fluorescence yield and resolve the spectral shape to better then 10%.

14. J.N. Matthews, ICRR 2/2004 E-165: Experiment Plan A two stage experiment: –thin target - to study gas composition and pressure effects –thick target – to study effects of particle energy and shower age A thin target test run was held in 2002 and a first data run was collected in the fall of 2003 The thick target stage is expected to run summer 2004. A third run is approved allowing us to address any systematic issues which arise.

15. J.N. Matthews, ICRR 2/2004 E-165 Experimental Design Thin Target Stage Opposing UV LED calibration source. Remotely controllable filter wheel. Post filter UV LED calibration sources (4) Signal PMT. 2 orthogonal arms

16. J.N. Matthews, ICRR 2/2004 E-165 Experimental Design Thin Target Stage Electron beam passes (5x10 7 -5x10 9 e - /pulse) through a gas chamber. 1x1 – 2x2 mm beam spot. 1 cm gap well defined by interior tubes. Interior blackened and baffled. HiRes PMTs used to measure the fluorescence signal. LED Pres PMT e-e-

17. J.N. Matthews, ICRR 2/2004 E-165 Experimental Design Thin Target Stage 15 positions of the filter wheel were used. –HiRes filter glass (band pass 300-400 nm). –Open and black. –337, 355, 390, 380, 395, 400, 315, 375, 330/325, 370, 425 and 296 nm narrow band (10 nm) filters. –425 nm (20 nm FWHM) filter

18. J.N. Matthews, ICRR 2/2004 E-165 Experimental Design Thin Target Stage

19. J.N. Matthews, ICRR 2/2004 Monitoring Measurements Measured –Beam charge with a torroid – monitored for linearity with yield –Beam position and size (transition radiator and CCD) –Vessel pressure and temperature –Gas composition –Background levels (blind PMTs and black filter)

20. J.N. Matthews, ICRR 2/2004 Beam Charge Monitoring x 10 10 e -

21. J.N. Matthews, ICRR 2/2004 Beam Spot Monitor

22. J.N. Matthews, ICRR 2/2004 PMT Stability to LED (2.2%)

23. J.N. Matthews, ICRR 2/2004 HV Stability (0.04%). PMT GAIN =e  V  With  6.

24. J.N. Matthews, ICRR 2/2004 E-165 September 2003 Run Experimental Program Measure the fluorescence yield for pure N2, dry air, and humid (SLAC) air. Repeat the measurement for each filter. Measurements were made at several pressures (10, 25, 50, 100, 250, 500, and 750 torr). Measure spectrum of N2, dry, and humid air using spectrograph. Confirm linearity with beam charge and perform tests relevant to future runs, such as ability to run with <= 10 7 e- per pulse.

25. J.N. Matthews, ICRR 2/2004 E-165 September 2003 Run Preliminary Results Full pressure sweep was taken for each narrow band filter.

26. J.N. Matthews, ICRR 2/2004 E-165 September 2003 Run Fluorescence Spectrum Using Filters P = 750 torr Do NOT expect this to look like Bunner spectrum.

27. J.N. Matthews, ICRR 2/2004 E-165 September 2003 Run Fluorescence Spectrum Using Filters P > 200 torr Note: Error bars are statistical (tiny), range of background subtraction (small except faint lines) and 10% for absolute in toroid (relative extremely small).

28. J.N. Matthews, ICRR 2/2004 E-165 September 2003 Run Effect of Humidity P > 200 torr SLAC Air is ~1.3 % H 2 O. lower but within error. Signals ~5% lower than dry M. Fraga (Airlight Wkshop) gives at 94% for 1% H 2 O.

29. J.N. Matthews, ICRR 2/2004 E-165 Spectrograph PMT 32 anodes 6 nm resolution from grating

30. J.N. Matthews, ICRR 2/2004 E-165 September 2003 Run Spectrum via Spectrograph

31. J.N. Matthews, ICRR 2/2004 E-165 September 2003 Run Spectrum via Spectrograph

32. J.N. Matthews, ICRR 2/2004 E-165 September 2003 Run “To Do” List Apply additional data analysis filters based on the Beam Spot Monitor. Calibration of detector arms. Absolute toroid calibration. Correction to gains vs time based on LED tracking. Spectrograph data normalized to beam charge. Full study of systematic errors.

33. J.N. Matthews, ICRR 2/2004 E-165 Future Runs We have two more runs scheduled for summer of 2004. The first run will be our thick target run. The third run may be a simultaneous run of thin target, thick target and spectrograph system.

34. J.N. Matthews, ICRR 2/2004 E-165 Experimental Design Thick Target Stage 10 7 e - showering at 30 GeV approximately reproduces a 3x10 17 eV UHECR shower (near shower max). Shower the FFTB beam in a range (1, 3, 7, & 11 rad lengths) of shower depths in air “equivalent” material (Al 2 O 3 ). Any effects from the change in e - energy distribution? Does the signal deviate from dE/dx? Do shower models correctly predict the fluorescence signal?

35. J.N. Matthews, ICRR 2/2004 Conclusions We have measured the spectrum and yield of air fluorescence. The shape and yield are in the right ball park. We expect to resolve the spectral shape very well with our combined method of narrow band filters and spectrograph. Work on calibration and systematics is ongoing. (The Hard Part)

36. J.N. Matthews, ICRR 2/2004