1. 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

2. 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

3. 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

4. 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

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

6. 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

7. 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

8. 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

9. 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

10. 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?

11. 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

12. 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!

13. 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

14. 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

15. 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

16. 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.

17. 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

18. ~END~
(Questions?)