The High-Altitude Water Cherenkov Gamma-Ray Observatory

Detecting Cosmic Rays

When high-energy cosmic rays and gamma rays enter the atmosphere, they set off a chain-reaction particle cascade known as an extensive air shower. The properties of these showers can be exploited to determine the type, energy, and direction of the primary cosmic ray particle. We describe how below.

Cherenkov Radiation

As the extensive air shower traverses the earth's atmosphere the relativistic charged particles in the shower emit Cherenkov light. Cherenkov light is the electromagnetic equivalent of a sonic boom. It occurs when a charged particle travels through a medium faster than light can travel through that medium. This speed is determined by the index of refraction of the medium. For example, the index of refraction of air at standard pressure and temperature is about 1.0003, so the speed of light in air is roughly

cair = c / nair = c / 1.0003 = 0.9997·c

Cherenkov light is emitted primarily into the ultraviolet spectrum in a small cone around the direction of motion of the particle. A typical charged particle in an air shower will produce 10 to 20 Cherenkov photons per meter as it moves through the air. Since there are roughly 108 to 109 charged particles in an air shower near its maximum extent, these showers produce large amounts of ultraviolet Cherenkov radiation in the forward direction.

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Detecting an Extensive Air Shower

Extensive air showers
The observation of an extensive air shower particle cascade using by collecting Cherenkov radiation (left) and observing the shower particles at ground level (right). From the Milagro collaboration (copyright © 2002, University of California).

Because extensive air showers produce Cherenkov light, they can be detected by collecting Cherenkov photons with sensitive ultraviolet telescopes. Such instruments are called imaging air Cherenkov telescopes, or IACTs. An IACT consists of a large upward-facing mirror to focus the Cherenkov light generated by the air shower onto an array of UV-sensitive photomultipliers. The properties of the shower image in the focal plane can be used to determine the energy, arrival direction, and the type of particle which initiated the shower (i.e., a gamma-ray or a cosmic-ray nucleus).

Another way to measure air showers is to deploy an array of particle counters on the ground and observe the particles in the shower once they reach ground level. This technique dates back to the 1930s. Traditionally an air shower array is composed of a sparse array of plastic scintillators, which emit a short burst of UV light when they are penetrated by a charged particle. Alternatively, the particle detectors can be tanks full of water. When particles from the shower pass through the water, they emit Cherenkov light because they travel faster than the speed of light in water (about 0.77·c). Such arrays are called water Cherenkov detectors.

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Cherenkov Telescopes vs. Air Shower Arrays

Why bother having two different detection techniques for gamma-ray air showers? Because each technique has its own strengths and weaknesses, and so measurements using the two methods complement each other.

For example, in air showers created by cosmic rays below 1 TeV, few particles penetrate to ground level. However, the Cherenkov light will reach the ground and trigger an IACT. Therefore, the IACTs have a lower energy threshold than air shower arrays. Unfortunately, they are also optical instruments with a small field of view, and they cannot be operated during daylight hours (when the Cherenkov radiation is overwhelmed by sunlight) or during inclement weather (when the atmosphere is blocked by clouds).

Air shower arrays can operate during all weather conditions and any time of day. However, they are much less sensitive to low-energy showers than the IACTs. And for gamma-ray observations, they are also much less efficient at distinguishing cosmic-ray air showers from gamma-ray air showers.

The relative merits of the two techniques are summarized below:

Cherenkov Telescope Air Shower Array
Energy Threshold Low (<200 GeV) High (>10 TeV)
Background Rejection Excellent (>99.7%) Moderate (>50%)
Field of View Small (<2°) Large (>45°)
Duty Cycle (uptime) Low (5%-10%) High (>90%)
Complementary detection characteristics of imaging air Cherenkov telescopes (IACTs) and extensive air shower arrays.

Note that the most recent generation of air shower arrays — ARGO-YBJ, Milagro, and now HAWC — have been designed explicitly to marry the low energy threshold of IACTs with the high duty cycle of arrays.

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Removing Cosmic Rays from the Data

The HAWC observatory is sensitive to cosmic rays and gamma rays — specifically, the extensive air showers of charged particles created when TeV cosmic rays and gamma rays enter the Earth's atmosphere.

Pattern of hits from a γ-ray air shower. Most of the signal is located near the core of the shower.

Because the flux of cosmic rays is about 1000× larger than the flux of gamma rays, the gamma-ray "signal" is overwhelmed by the "background" of cosmic rays. This is a huge experimental challenge for gamma-ray observatories, because we must find clever and efficient means to remove the cosmic rays from our data in order to do gamma-ray astronomy.

In HAWC, we can discriminate air showers from gamma-rays and cosmic rays by observing the spatial pattern of the "hits" observed in the detector when the showers reach ground level. In gamma-ray showers, most of the signal at ground level is located near the shower axis, i.e., along the direction of the initial gamma-ray. In contrast, charged cosmic rays tend to "break apart" when they shower in the atmosphere, creating much messier signals at ground level.

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