[Paper Review] The Absolute, Relative and Multi-Wavelength Calibration of the Pierre Auger Observatory Fluorescence Detectors

R. Knapik, P. Bauleo+21 more|arXiv (Cornell University)|Aug 14, 2007

Calibration and Measurement Techniques3 references3 citations

TL;DR

This paper presents a comprehensive calibration framework for the Pierre Auger Observatory's fluorescence detectors, using a portable drum light source for absolute calibration at 375 nm, multi-wavelength measurements at 320–405 nm for spectral response, and a relative calibration system with optical fibers to track nightly and seasonal response changes. The key contribution is a 9.5% total uncertainty in absolute calibration, validated by independent laser cross-checks and improved through redundant systems and integration of relative calibration data.

ABSTRACT

Absolute calibration of the Pierre Auger Observatory fluorescence detectors uses a 375 nm light source at the telescope aperture. This end-to-end technique accounts for the combined effects of all detector components in a single measurement. The relative response has been measured at wavelengths of 320, 337, 355, 380 and 405 nm, defining a spectral response curve which has been normalized to the absolute calibration. Before and after each night of data taking a relative calibration of the phototubes is performed. This relative calibration is used to track both short and long term changes in the detector's response. A cross check of the calibration in some phototubes is performed using an independent laser technique. Overall uncertainties, current results and future plans are discussed.

Motivation & Objective

• To develop a reliable end-to-end calibration method for fluorescence detectors in the Pierre Auger Observatory that accounts for all detector components.
• To measure and correct for spectral response variations across 320–405 nm using multi-wavelength drum measurements.
• To track short-term (nightly) and long-term (seasonal) response changes in photomultiplier tubes using a relative calibration system with optical fibers.
• To reduce systematic uncertainties in absolute calibration through cross-checks with an independent laser-based technique.
• To integrate relative calibration data into nightly corrections, minimizing reliance on frequent drum calibrations and laser checks.

Proposed method

• A portable drum light source with UV LEDs and a xenon flasher is used to uniformly illuminate all 440 pixels of each FD camera, simulating a shower-like light distribution.
• Absolute calibration is performed using a 375 nm LED source, with intensity transferred to NIST-traceable Si photodiodes via a three-step intensity transfer process involving pulsed and continuous beams.
• Multi-wavelength relative response is measured at 320, 337, 355, 380, and 405 nm using the same reference PMT, with quantum efficiency data used to normalize the spectral curve to the 375 nm absolute calibration.
• A relative calibration system using optical fibers and a high-power LED illuminates all mirrors simultaneously before and after each data-taking night to monitor response drift.
• An independent laser cross-check system fires a known-energy laser 4 km in front of the FD, using Rayleigh scattering to predict photon flux and validate the drum calibration.
• Data from all calibration systems are combined, with epochs defined from December 2004 onward to update reference calibrations and correct for hardware changes and long-term trends.

Experimental results

Research questions

• RQ1 How can the absolute response of the Pierre Auger fluorescence detectors be calibrated with minimal systematic uncertainty across all components?
• RQ2 What is the spectral response of the FD cameras across the 320–405 nm range, and how can it be accurately normalized to the absolute calibration?
• RQ3 How do nightly and seasonal variations in PMT response affect shower reconstruction, and what method can track these changes effectively?
• RQ4 To what extent do independent laser cross-checks agree with the drum calibration, and what uncertainties remain?
• RQ5 Can integration of relative calibration data reduce the need for frequent drum and laser calibrations?

Key findings

• The absolute calibration of the fluorescence detectors achieves a total uncertainty of 9.5%, with the largest contributions from intensity transfer (6.0%) and temperature effects (3.5%).
• The multi-wavelength response curve, measured at 320, 337, 355, 380, and 405 nm, has a relative uncertainty of 4% at each wavelength and is normalized to the 375 nm absolute calibration.
• Nightly and seasonal response variations in the cameras are tracked with a relative calibration system, showing fluctuations at the 3–4% level within each epoch.
• Laser cross-checks of selected pixels show consistent agreement with drum calibrations over three years, with systematic uncertainties dominated by energy probe calibration and atmospheric corrections.
• The integration of relative calibration data into nightly corrections is underway, reducing the impact of response drift and decreasing reliance on infrequent drum and laser calibrations.
• A redundant lab calibration system with independent systematic uncertainties is being developed to cross-validate the current drum calibration procedure and further reduce overall uncertainty.