The Experimental Program of the “FLASH” Experiment

Fluorescence efficiency is the foundation for our belief that we are measuring “energy” How well is it know ( for an ionizing particle)? Is it linear with particle number (size) in an extensive air shower? Can it be affected by “accidental” conditions? Impurities, etc. How do we determine answers to these issues with sufficient accuracy?

Second Knee, showing correlation between knee energy and spectral normalization

Second Knee Spectrum, Shifted to make knee come out at same energy

Second Knee, cont. All experiments agree when a scale shift is applied. But what is the actual energy of the second knee? Fluorescence method should be very reliable ( nearby events, little atmospheric attenuation ). Position comes primarily from our knowledge of air fluorescence efficiency.

Bunner Air-fluorescence spectrum

Current Understanding Bunner (1967), Kakimoto et al. (1995), Nagano et al. ( 2002, unpublished) indicates ~15% systematic errors in overall yield and larger errors in individual spectral lines. Ground based experiments – non-linear effects possible due to λ dependence of atmospheric attenuation. At 30 km, event energy can change by 25% if 390 nm line intensity changes by 40%.

Photon yields between 300 and 406nm from Nagano, Kakimoto( HiRes) and Bunner

Relative Contributions of Different Spectral Lines at Different Horizontal Distances

Importance to Euso and OWL (space-based) experiments Path-lengths from shower to detector almost constant – small λ dependence (~10% integral variation over different fluorescence models) Most showers detected over oceans – effect of water-vapor and other impurities may be important. Some evidence for H2O quenching already exists.

Fluorescence Pressure Dependence τ= lifetime P=pressure T=temp.

Reference Pressure

Fluorescence Yield Y = photons per meter-ionizing particle P, P’ = pressure and reference pressure C = spectral line intensity at zero pressure

Dependence of Air Fluorescence ( from Nagano et al (personal communication) at.85 MeV

Life time vs Pressure (Air) from Nagano et al.(p.c.)-at.85 MeV

Air fluorescence lifetimes ~ 25 nsec Bunner quotes lifetimes near 40 nsec Large uncertainties remain.

Dependence on electron energy ( Kakimoto et al.NIM, 1995)

Dependence on electron energy – Kakimoto et al. and T461

SLAC test run results Two week run in June 2002 Prototype thin target setup Measured pressure dependence integrated over nm. Measured average lifetime over nm. Confirmed linear behaviour of Y with respect to beam current below 10 9 ppb

T461 Setup LEDs PMTs

SLAC test result on linearity

Effect was enhanced at low pressures and was reduced near sea- level pressure

Simple model for non-linearity –Beam pulse passes in ~ 3 picoseconds –Treat pulse as a uniform ball of charge and estimate impulse Δp it imparts on an electron (and hence the kinetic energy gained ΔE K : Δp ~ Ne 2 /4  0 r 2 ּ Δt (r ~ 1 mm) ΔE K =(Δp) 2 /2m ~ 700 eV for N=10 10 IF the “ejected” electron does not suffer collisions while being accelerated…this is enough energy to cause “secondary ionization” Important quantity here is the “Mean Free Path” λ.

Mean Free Path At room temperature in air/nitrogen, we have λ = 2x10 -4 /P λ in meter, P in torr

Mean Energy Gained (before collision) Estimate the energy gained by an electron before collision: ΔE K ~ Ne 2 /4  0 × [ 1/r - 1/(r+λ) ]

SLAC test result- comparison of N2 and Air efficiency

N 2 Decay Time Measurement

SLAC test N 2 Decay Time Measurement

N 2 (in air) Decay Time Measurement

SLAC Test and Beyond Test clearly established ability to detect air fluorescence in FFTB beams. Test showed that we can measure the pressure dependence and fluorescence lifetime integrated over total spectrum What is needed, however, is spectrally resolved pressure and lifetime measurement. Test only measured Y at 28.5 GeV. Energy dependence over realistic shower energies is required.

OBJECTIVES OF E-165 Spectrally resolved measurement of fluorescence yield to better than 10%. Investigate dependence on electron energy. Study effects of atmospheric impurities. Observe showering of electron pulses in air equivalent substance (Al 2 O 3 ) with energy equivalents around eV.

Proposed 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

THIN TARGET STAGE Pass electron beam through a thin-windowed air chamber. –Measure the total fluorescence yield in air at ~30 GeV. –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.

T461 Setup LEDs PMT Beam Gas Flow

General Requirements for thin target run FFTB downstream of last magnets <1% R.L. in beam – no radiation problem Require 10 8 – 10 9 e/pulse for linear operation. Require improved torroid sensitivity to monitor beam at this intensity (or equivalent cross-calibrated measurement).

THICK TARGET STAGE Pass electron beam through varying amounts of ~ air equivalent showering material (Al 2 O 3 ). Measure light yield as a function of depth in the shower ( sample light from a wide range of electron energies). –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?

THICK TARGET SETUP

CORSIKA AIR SHOWERS

BREMSSTRAHLUNG BEAM OPTION

THICK TARGET SHOWER DEVELOPMENT

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%.

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

T461 PMT Stability (2.2%).

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%.

T461 Beam Stability (~2%) x e -

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%.

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%).

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.