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 earths surface easily, they are much easier to detect.
- 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
- 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 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
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