Measuring Optical Water Quality in HAWC and other Future Water Cherenkov Detectors (Or, How Dirty is Your Water?) Stoian Borissov, Georgia Tech School of Physics, Atlanta, GA HAWC Observatory, located in Pico de Orizaba near Puebla, Mexico

Overview Introduction Investigation Future Work and Summary HAWC Gamma Ray Bursts and Particle Showers Cherenkov Effect Investigation Problem Description Research Method and Detector Design Future Work and Summary

Introduction: HAWC HAWC Observatory: High Altitude Water Cherenkov (Gamma-Ray) Observatory Located in Pico de Orizaba near Puebla, Mexico High altitude (4 km) aids observations Composed of large array of water tanks, each of which acts as a particle detector Goal: Help astronomers look for supernovae and active galactic nuclei by observing gamma ray bursts and cosmic rays Scientists standing inside a tank at HAWC

Introduction: Gamma-Ray Bursts Discovered in 1967, during the Cold War, by the U.S. Vela nuclear test detection satellites. Velas were looking for evidence of secret Soviet nuclear testing behind the Moon after the 1963 Nuclear Test Ban Treaty. High energy events: 100 GeV to 100 TeV Sources: Supernovae and active galactic nuclei Artist Rendition of a gamma-ray burst One of the Vela Satellites

Introduction: Particle Showers Gamma-ray bursts induce particle shower when they hit Earth’s atmosphere Illustrations of particle showers

Introduction: Cherenkov Effect Similar to idea of a supersonic body moving through air and creating shockwaves Occurs when charged particles move faster than the speed of light in a medium HAWC observes the Cherenkov radiation created when a particle shower passes through water inside on of the tanks This is what HAWC monitors via photo-multiplier tubes Geometry of Cherenkov light shockwave propagation Cherenkov radiation glowing in the core of Idaho National Lab’s Advanced Test Reactor

Introduction: Water Cherenkov Detection Principal The brightness of the detected Cherenkov Radiation provides information about the spread and intensity of particle shower, which provides information about the originating gamma-ray burst. Water Cherenkov Detection Principal

Investigation: Water Properties Inside Tanks Water attenuates light, thus affecting brightness of collected light Problem Description: Need to know properties of water to better understand the nature of the particle shower and originating gamma-ray burst Absorption of light Scattering of light: incoherent Mie scattering assumed I: measured intensity I0: initial intensity λs: scattering length λs: absorption length d: distance traveled by light Intensity of collected light as a function of initial intensity, scattering length, absorption length, and distance traversed by the light

Investigation: Water Properties Inside Tanks Absorption of light: reduction of measured light intensity Scattering of light: may reduce or increase intensity of collected light Two unknowns: need way to decouple measurement of scattering length and absorption length. Both λs and λa are unknown

Investigation: Design a System for Measuring Water Quality Research Goal: Design a system for measuring water quality. Measure scattering and absorption length by using two detectors and a laser of known intensity sending a light pulse Considerations: Geometry of water container affects measurements, therefore in situ measurement is best Must decouple measurements from each detector to accurately determine water properties Different configurations are possible for the detector and laser layout: What is the best layout?

Investigation: Optimizing the System Design Detector design algorithm Use simulation environment to build a detector setup inside a water tank (GEANT4 Toolkit) Set water scattering and absorption lengths Run simulation and obtain detector response Solve inverse problem, i.e. estimate water parameters from detector response Repeat for varied water parameters and detector layout Possible detector layout

Investigation: Simulated “Detector Response Surface” Primary detector response under varying water conditions Secondary detector response under varying water conditions. Hits are mostly function of scattering length!

Investigation: Simulated Detector Response Rerun simulation with fixed water properties, for example, a scattering and absorption length each of 10 meters Each detector will count a certain number of hits Using the hit count in each detector, “slice” the detector response surfaces to generate a “detector response curve” corresponding to the fixed water parameters

Investigation: Solving the Inverse Problem Intersection of primary and secondary detector response curves is at 12 x 8.5 meters; not quite 10 x 10 Histogram of curve intersections for 1500 runs at 10 x 10 meters

Investigation: Statistical Analysis of Results End result: Scatter plot showing likely solution for water properties Should produce a “solution ellipse” Characterizing the solution ellipse Spread Length/Shape Smoothed histogram of curve intersections

Investigation: Future Work Port simulation environment to large cluster to run more massive simulations and produce more accurate statistical results Vary other experimental parameters: Detector spacing Detector angle with respect to laser light Other optical scattering models Implement a hardware version inside a test tank.

Summary: HAWC monitors for gamma-ray bursts Gamma-ray bursts induce particle shower which can be detected as energetic particles pass through water and generate Cherenkov light To understand intensity of gamma-ray burst, you need to know your losses from water impurities A system must be designed that can individually measure light absorption and light scattering Optimal configuration exists for maximal decoupling of absorption and scattering measurements

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