How can we detect UHEs?

Detection of UHEs:

Can detect UHEs in space with satellite detectors, but UHEs are very rare, and so the detectors would have to be too big and expensive.

Instead, use detectors on the ground.

Detecting UHEs on the earth:

When UHEs get to the earth, they collide with the particles that make up the atmosphere. These collisions create big showers of secondary particles (mostly muons.)

Because there are so many secondary particles, and they can make it all the way down to the earth’s surface easily, they are much easier to detect.

Detectors:

The cloud chamber. A cloud chamber is just a container full of mist, or vapour. When a subatomic particle enters the chamber, it collides with air or alcohol vapour molecules, producing free ions. Vapour in the chamber condenses around these free ions, forming droplets. The droplets are what form the visible trail.

An example of trails formed in a cloud chamber. Different particles, such as muons and beta particles, curl around different amounts, and so can be distinguished from each other. (The trails curl because the different charges on the particles interact with the Earth's magnetic field.)

From http://freeweb.pdq.net/headstrong/cloud.htm

The scintillating photodetector.

The scintillating photodetector consists of three main parts:

The scintillator: is made of a special material that fluoresces when a particle passes through it. This means that when a secondary particle passes through the scintillator, the scintillator emits some photons (particles of light.) The scintillator is shaped so that the photons travel over to the photomultiplier. The scintillator is wrapped in light-proof tape and fabric to prevent photons of light from the lab from giving a false signal, or "noise."
The photomultiplier tube: amplifies the signal from the scintillator. Each photon (except those which are lost to absorption, reflection or something else) from the scintillator enters the photomultiplier tube and hits a cathode, which then emits an electron. The electron then hits a dynode, which emits two to five more electrons. Up to 14 dynode stages are used, so that eventually there can be 10⁷ electrons created for each photon entering from the scintillator.
The computer: takes the signal from the photomultiplier tube and analyses it. The energy of the original secondary particle can be determined, since it is proportional to the strength of the signal from the photomultiplier tube.

The scintillating photodetector becomes even more useful when there are several of them. The computer can compare the signals from several photomultipliers with a very accurate Global Positioning System (GPS--a satellite mapping system) timing module to determine if signals are simultaneous. If they are simultaneous:

it is likely that each particle that was detected came is part of the same secondary shower, and so was created by the same UHE.

the angle that the shower hit the earth at can be determined, so the direction the UHE came from can be found, which could help pinpoint the source of UHEs.

This diagram shows three detectors being hit by a shower that is coming in at an angle theta . Notice that the detector on the right will detect a particle first, followed by the detector in the middle, followed by the detector on the left. Thus the time between the signals detected will indicate the angle at which the shower is arriving.

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