The Absolute Calibration of the HiRes Detectors J.N. Matthews, S.B. Thomas, N. Manago, L. Perera, G. Burt, and R. Snow For the HiRes Collaboration

RXF Calibration The width/mean calibration Calibration of the Standard Candle Comparison of the two techniques Conclusion

Energy Spectrum - HiRes & AGASA AGASA energy scaled by 0.79

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%

PMT Clusters/Cameras 16 x 16 PMT Array 2 Sites – 12 km 34 Buildings 64 Cameras 16,384 PMTs

PMT Calibration Roving Xenon Flasher (RXF) as a Standard Candle RXF sits at the center of the mirror and illuminates the entire cluster Can take it to each PMT cluster at both sites Very stable (< 2% over a night)

Current Statistical Method The measured QDC distribution is used to calculate the number of photo-electrons for each PMT: Npe = α (μ/σ) 2 Applying the quantum efficiency then gives us the gain of the PMT or the number of ADC counts per 

Applied Calibration Result Sigma

Problems with the Current Method The excess noise factor, α, is not well known. The value was determined by a small number of single photo-electrons measurements and atmospheric measurements (molecular edge). It is consistent with values reported in the literature and discussions with Photonis. The QE( ) is not well understood.

Calibration of the Standard Candle A two step approach using NIST calibrated silicon photo-diodes and hybrid photo- diodes (HPD) Use the NIST Si photo-diodes to determine the response of the HPD by measuring the single photo-electron peak Then use the HPD to measure the luminosity of the RXF (#  /mm 2 delivered at the mirror-cluster separation distance)

SPE DistributionP(0) Mean: ; Events : P(1) ; Events: P(1) P(0) A Gaussian fit is applied to P(1) to obtain the mean.

RXF Measurement with HPD Mean: Events: 2275 Mean: 8412 Events: 3203

Calculations (RXF signal – RXF background) (RXF signal – RXF background) 0.183* 0.68 *(SPE Peak – background)*Area 0.183* 0.68 *(SPE Peak – background)*Area HPD Calibration Constants is the HPD efficiency is the HPD efficiency 0.68 is a distribution correction factor for the HPD. It is the ratio of the SPE mean over the SPE peak is a distribution correction factor for the HPD. It is the ratio of the SPE mean over the SPE peak. γ mm 2 =

HPD Systematics Si diode current: I = 0.192*10 -9 A ± 1% ± 2.6% Si diode responsivity: β = A.s/J ± 5% ± 0% Si diode area: A = 100mm 2 ± 1% ± 0% HPD count rate: μ = 16.22*10 3 counts/sec ± 5% ± 0% Counting Efficiency: ε = PE/count ± 3.5% ± 0% Neutral density filter attenuation: η = 1.36*10 4 ± 1.5% ± 0% Geometrical and other effects: ±5% ± 0% HPD Efficiency = 11.2 ± 9.6% Syst ± 2.6% Stat (#  /pe/mm 2 )

A Test: Measure and compare the light output of the RXF via:  The HiRes detector width/mean method  The HPD traceable to NIST The Plan: Measure RXF with the HPD in the University of Utah lab Take RXF to HiRes to calibrate the detector. Used 20 cameras at HiRes-I for this test. Ten minutes (about 900 shots) of data are recorded at each detector. Return to the U and remeasure RXF in the lab.

Result: Statistical Method: 8.97  /mm2 ± 10% (Syst) HPD Method: 9.55  /mm2 ± 9.6% (Syst) ± 2.6% (Stat) Comparison percentage is 6.5%. Well within expected values.

Conclusions The NIST Si-PD and HPD calibration will allow us to absolutely calibrate the RXF and thus the HiRes PMT Clusters. Good agreement with the statistical method. We need to work on the uncertainties to get them down. Should be able to get to the ~5% level.

The High Resolution Fly’s Eye (HiRes) Experiment University of Utah Rutgers University Columbia University University of Montana University of New Mexico University of Adelaide Los Alamos National Laboratory (LANL) University of Tokyo

High Resolution Fly’s Eye Dugway Proving Ground, Utah 112 W, 40 N 2 sites / stereo 64 cameras ~17,000 PMTs

Comparison of Absolute Calibration and HiRes-I Absolute Calibration (HPD) HiRes-I Uncertainty: HPD: ±9.9% HiRes: ±10%