1. Fluorescence from Air in Showers (FLASH) J. Belz 1, Z. Cao 2, P. Chen 3*, C. Field 3, P. Huentemeyer 2, W-Y. P. Hwang 4, R. Iverson 3, C.C.H. Jui 2, T. Kamae 3, G.-L. Lin 4, E.C. Loh 2, K. Martens 2, J.N. Matthews 2, W.R. Nelson 3, J.S.T. Ng 3, A. Odian 3, K. Reil 3, J.D. Smith 2, P. Sokolsky 2*, R.W. Springer 2, S.B. Thomas 2, G.B. Thomson 5, D. Walz 3 1 University of Montana, Missoula, Montana 2 University of Utah, Salt Lake City, Utah 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 SLAC E-165

2. MOTIVATIONS Discrepancy in the UHECR spectrum Future experiments: Auger, OWL, EUSO, etc. AGASA HiRes

3. FLUORESCENCE ISSUES Detailed shape of the fluorescence spectrum –Photon scattering during propagation sensitive to wavelength –Fluorescence spectrum exhibits structures on the order of 10nm Pressure dependence of the fluorescence yield –Detailed exploration below 100 torr (or above 15 km) important to space- based detectors such as EUSO and OWL Effects of impurities on fluorescence yield –Impurity composition, e.g., CO 2, Ar and H 2 O, above 20 km is neither stable nor well known. –There may even be surprises in the fluorescence yield at high altitude Effects of electron energy distribution on yield –A significant fraction of energy in an EAS is carried by energy <1 MeV –Fluorescence efficiency inthis energy range is poorly known

4. MEASURING FLUORESCENCE AT SLAC Extensive Air Showers (EAS) are predominantly a superposition of EM sub- showers. Important N 2 transition (2P) not accessible by proton excitation; only e-beam can do it. FFTB beam-line provides energy equivalent showers from ~10 15 to ~10 20 eV. –10 8 -10 10 electrons/pulse at 28.5 GeV. –2% of electron pulse bremsstrahlung option.

5. OBJECTIVES Spectrally resolved measurement of fluorescence yield to better than 10%. Investigate effects of electron energy. Study effects of atmospheric impurities. Observe showering of electron pulses in air equivalent substance (Al 2 O 3 ) with energy equivalents around 10 18 eV.

6. PROGRAM Gas Composition –N 2 /O 2 dependence, and Ar, CO 2, H 2 O impurities Pressure Dependence –Yield versus Pressure down to 10 torr Energy Dependence –Yield versus electron energy distribution down to 100keV Fluorescence Spectrum –Resolve individual bands using narrow band filters or spectrometer. Pulse Width –Pressure dependence of fluorescence decay time for each spectral band

7. THIN TARGET STAGE Pass electron beam through a thin-windowed air chamber. –Measure the total fluorescence yield in air. –Measure the yield over wide range of pressures at and below atmospheric. –Measure emission spectrum using narrow band filters or spectrometer. –Effects of N 2 concentration. Pure N 2 to air. Also H 2 O, CO 2, Ar, etc.

8. THICK TARGET STAGE Pass electron beam through varying amounts of showering material (Al 2 O 3 ). –Is fluorescence proportional to dE/dx? –What are the contributions of low-energy (<1 MeV) electrons? –Can existing shower models (EGS, GEANT, CORSIKA) correctly predict fluorescence light? –How does the fluorescence yield in an air shower track the shower development?

9. THICK TARGET SHOWER DEVELOPMENT

10. BREMSSTRAHLUNG BEAM OPTION

11. CORSIKA AIR SHOWERS

12. SYSTEMATIC UNCERTAINTIES Defining the yield as Y = n  /(x  n e ) –n   Number of photons –x  Distance of travel –n e  Number of e – (and e + ) We intend to measure Y and dY/d to better than 10%.

13. SYSTEMATIC UNCERTAINTIES Beam charge should be measurable by the beam toroids to better than 2%. –When showering the beam, the beam energy will also affect the number of particles in the shower. This should be determined to better than 0.5%. –If a bremsstrahlung beam is used the contribution of the converter foil thickness uncertainty should be less than 1%.

14. SYSTEMATIC UNCERTAINTIES The uncertainties in showering 3%. –Uncertainty in simulations and transition effects from dense target to air 2%. –Uncertainty in amount of showering material of 1-1.5%.

15. SYSTEMATIC UNCERTAINTIES Detector systematic uncertainties of 5.4%. –PMT calibration uncertainty of 5%. –Cable and ADC uncertainty of 2%. Detector Optics 4% (thin) 6.5 % (thick). –Wide band filters and mirrors (1%). –Narrow band filter transmission (3%).

16. SYSTEMATIC UNCERTAINTIES Beam charge should be measurable by the beam toroids to better than 2%. The uncertainties in showering 3%. Detector systematic uncertainties of 5.4%. Detector Optics 4% (thin) 6.5 % (thick). Total systematic uncertainty of 7-9%.

17. SYSTEMATIC UNCERTAINTIES Thin TargetThick Target Beam2%2.2% Showering-3% Detector System 5.4% Optical System 4%6.5% Total7%9.2%

18. THIN TARGET SETUP Thin target stage requires the 28.5 GeV beam to deliver 10 8 -10 9 e - per pulse. Located in the FFTB tunnel.

19. THIN TARGET AIR CHAMBER LEDs PMTs LED PMT

20. THICK TARGET SETUP Thick target stage requires the 28.5 GeV beam to deliver 10 7 -10 8 e - per pulse. Alternatively, the FFTB bremsstrahlung beam can be used to select 2% of a larger pulse beam. The layers of Al 2 O 3 must be remotely moveable to allow changing of shower depth observed.

21. THICK TARGET SHOWER WIDTH

22. THICK TARGET SETUP

23. E-165 PLAN SUMMARY Thin Target –Pressure variations –Fluorescence Spectrum –Linearity of fluorescence with beam charge. –Impurities.

24. E-165 PLAN SUMMARY Thick Target –Shower depth variations. –N 2 /O 2 variations. –Fluorescence spectrum.

25. CONCLUSION FLASH aims to achieve an accuracy of 10% in the total fluorescence yield and individual spectral lines. Verify energy dependence of yield down to ~100keV. Both thin target and thick target approaches will be invoked. Dependence of yield and spectrum on pressure and atmospheric impurities will be measured. Shower developments equivalent to ~10 18 eV will be measured at various depths and compared with codes. We hope that FLASH will help to shed light on the apparent differences between HiRes and AGASA, and provide reliable information for future fluorescence- based UHECR experiments.