Fluorescence Efficiency Measured by FLASH at SLAC

-Preliminary Results-

J.N. Matthews for the FLASH Collaboration

FLuorescence in Air SHowers(FLASH)

T. Abu-Zayyad1, J. Belz2, D. Bergman5, Z. Cao1, F.Y. Chang4, P. Chen3*, C.C. Chen4, C.W. Chen4, C. Field3, P. Huentemeyer1, W-Y. P. Hwang4,

R. Iverson3, C.C.H. Jui1, G.-L. Lin4, E.C. Loh1, K. Martens1, J.N. Matthews1, J.S.T. Ng3, A. Odian3, K. Reil3, J.D. Smith1, P. Sokolsky1*, R.W. Springer1,

S.B. Thomas1, G.B. Thomson5, D. Walz3, A. Zech5

1University of Utah, Salt Lake City, Utah2University of Montana, Missoula, Montana

3Stanford Linear Accelerator Center, Stanford University, CA4Center for Cosmology and Particle Astrophysics (CosPA), Taiwan

5Rutgers University, Piscataway, New Jersey

* Collaboration Spokespersons

Outline

• Air Fluorescence and UHECRs

• “The Problem”

• FLASH

• The Detector

• Some Measurements

• Future

Air FluorescenceAGASA 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….

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

A HiRes Event

• HiRes 1

• HiRes 2

Air fluorescence generated by the EAS is collected enabling observation of shower development from beginning to end

Fluorescence Spectra

• Remarkable agreement between all the spectra measured by fluorescence detectors (including the “hybrid” HiRes-Prototype/MIA)

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

Energy Spectrum

AGASA energy scaled by 0.79

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%

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 λ4 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:

SLAC E-165: FLASHFLuorescence 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%.

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.

E-165 Experimental DesignThin Target Stage

• Opposing UV LED calibration source.

• Remotely controllable filter wheel.

• Post filter UV LED calibration sources (4)

• Signal PMT.• 2 orthogonal arms

E-165 Experimental DesignThin Target Stage

• Electron beam passes (5x107-5x109 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 LED

Pres

PM

T

e-

E-165 Experimental DesignThin 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

E-165 Experimental DesignThin Target Stage

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)

Beam Charge Monitoring

x 1010 e-

Beam Spot Monitor

PMT Stability to LED (2.2%)

HV Stability (0.04%).

PMTGAIN=eV

With 6.

E-165 September 2003 RunExperimental 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 <= 107 e- per pulse.

E-165 September 2003 RunPreliminary Results

• Full pressure sweep was taken for each narrow band filter.

E-165 September 2003 RunFluorescence Spectrum Using Filters

P = 750 torr

Do NOT expect thisto look like Bunnerspectrum.

E-165 September 2003 RunFluorescence Spectrum Using Filters

P > 200 torrNote: Error bars are statistical (tiny), range of background subtraction (small exceptfaint lines) and 10% for absolute in toroid (relative extremely small).

E-165 September 2003 RunEffect of Humidity

P > 200 torr

SLAC Air is ~1.3 % H2O.lower butwithin error.

Signals ~5% lower than dry

M. Fraga (Airlight Wkshop)gives at 94%for 1% H2O.

E-165 Spectrograph

PMT

32 anodes

6 nm resolution from grating

E-165 September 2003 RunSpectrum via Spectrograph

E-165 September 2003 RunSpectrum via Spectrograph

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.

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.

E-165 Experimental DesignThick Target Stage

• 107 e- showering at 30 GeV approximately reproduces a 3x1017 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 (Al2O3).

• Any effects from the change in e- energy distribution?

• Does the signal deviate from dE/dx?

• Do shower models correctly predict the fluorescence signal?

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)