Stability and behavior of the outer array of small water Cherenkov detectors, outriggers, in the HAWC observatory

TL;DR

This paper evaluates the stability and performance of the outrigger array of small water Cherenkov detectors in the HAWC observatory, focusing on leak light testing and charge analysis to enhance high-energy gamma-ray detection.

Contribution

It introduces the stability assessment and performance analysis of the outrigger detectors, a novel upgrade to improve high-energy event reconstruction in HAWC.

Findings

Leak light testing confirms detector stability.

Charge analysis shows effective detection performance.

Outrigger array enhances high-energy gamma-ray reconstruction.

Abstract

The High-Altitude Water Cherenkov (HAWC) Observatory is used for detecting TeV gamma rays. HAWC is operating at 4,100 meters above level sea on the slope of the Sierra Negra Volcano in the State of Puebla, Mexico, and consists of an array of 300 water Cherenkov detectors (WCDs) covering an area of 22,000 $m^2$. Each WCD is equipped with four photomultiplier tubes (PMTs) to detect Cherenkov emission in the water from secondary particles of extensive air-shower (EAS) that are produced in the interactions of primary particles (gamma rays or charged cosmic rays) in the atmosphere. HAWC is able to reconstruct the EAS in the 0.5 to 100 TeV energy range. In order to improve the core determination for events with high energy ($>10$ TeV) when the events arrive outside of the HAWC array, the Outrigger upgrade project is adding 350 small WCDs around the main array. These outrigger tanks each have one PMT in a 1.5 meter diameter cylindrical polyethylene tank, covering a total area four times larger than that of the HAWC array. In this work we present leak light testing to identify the stability of the detector and an analysis of deposited charges to understand the detector performance.