3. 3
Cosmic rays
Cosmic
rays are energetic particles
originating from deep space that hit our
atmosphere 30km above the Earth’s
surface. They come from a variety of
sources including our own Sun, other stars
and distant interstellar objects such as
black holes, but most are the accelerated
remnants of supernova explosions.
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Cosmic Rays
Although
commonly called cosmic rays
the term "ray" is a misnomer, as cosmic
particles arrive individually as a primary
particle, not as a ray or beams of
particles. 90% are Protons, 9% helium
nuclei, and the remainder electrons or
other particles.
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Matter smashing energy
When
these primary particles hit, they do
so with such tremendous energy they rip
their way into our atmosphere with atom
smashing power. Cosmic rays are
commonly known to have energies well
over 1020 electron volts, far more than any
particle accelerator built here on earth,
like the Large Hadron Collider (LHC).
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Matter smashing energy
These interactions produce an exotic zoo of high energy
particles and anti-particles high in the earth's
atmosphere such as positive and negative
pions and kaons that subsequently decay into
muons and muon neutrinos (including cascades
of protons and neutrons as a result of nucleonic decay).
uncharged pions decay into pairs of high
energy photons they become the starting points of
large cascades of electrons, positrons and gamma rays.
The resulting flux of particles at ground level consists
mainly of muons and electrons/positrons in the ratio of
roughly 75% : 25% still with energies greater than 4GeV
travelling at near the speed of light ~0.998c.
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Common interstellar events
Muons created by the interaction of cosmic
rays and our atmosphere lose their energy
gradually. Muons start with high energies and
therefore have the capacity to ionise many
atoms before their energy is exhausted.
Further, as muons have little mass and travel
at nearly the speed of light, they do not
interact efficiently with other matter. This
means they can travel through substantial
lengths of matter before being stopped.
Consequently, muons are all around us.
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Time travellers
Muons created by the interaction of cosmic rays are
an everyday demonstration of Einstein's theory of
relativity. A muon has a measured mean lifetime of
2.2 microseconds. Consequently, they should only be
able to travel a distance of 660 metres even at near
the speed of light and should not be capable of
reaching the ground.
However Einstein's theory showed that time ticks
slowly for particles moving at speeds close to that of
light. Whilst the mean lifetime of the muon at rest is
only a few microseconds, when it moves at near the
speed of light its lifetime is increased by a factor of
ten or more giving these muons plenty of time to
reach the ground.
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Detection
Unfortunately
a muon created as a result
of Cosmic Rays is not easily seen, but their
after-effects when passing through is a
little more easier, typically most forms of
radiation detectors will do the job.
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Detectors
The
oldest and most famous example of
this is the Cloud chamber.
Other radiation detectors can be used
like Geiger Counters, Spark Chambers,
Resistive Plate Chambers and materials
called Scintillators which give off light
when an ionizing particle passes through
them.
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Issues
Terrestrial
radiation- as much 73% of
background radiation is due to the
natural decay of matter. Although in small
quantities it is sufficient to make it difficult
to discriminate between a terrestrial or
cosmic source.
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Solution
Cosmic
particles travel at nearly the
speed of light and so do not ionise very
efficiently and hence can travel through
matter very easily passing through both
detectors without effort, whereas the
terrestrial radiation may not.
Consequently anything detected in both
detectors simultaneously is more likely to
be a cosmic event than terrestrial.
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Simultaneity
Well
almost simultaneously, if a muon is
travelling at 0.998c and the detectors
where spaced 5cm apart the actual flight
time of a muon would be just 0.16ns.
However as the detector and electronics
response and delay times would be much
slower than this, we can say in "real-life"
terms it is simultaneous.
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Solution
Consequently
at least two detectors are
needed placed one above the other,
feed into electronics that can monitor
coincidence between the two detectors
quickly thus potentially filtering out most
terrestrial radiation.
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coincidence circuit
In
physics, a coincidence circuit is
an electronic device with one output and
two (or more) inputs. The output is
activated only when signals are received
within a time window accepted as at the
same time and in parallel at both inputs.
Coincidence circuits are widely used
in particle physics experiments and in
other areas of science and technology.
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coincidence detection
The main idea of 'coincidence detection' in
signal processing is that if a detector detects
a signal pulse in the midst of random noise
pulses inherent in the detector, there is a
certain probability , P, that the detected
pulse is actually a noise pulse. But if two
detectors detect the signal pulse
simultaneously, the probability that it is a noise
pulse in the detectors is P² . Suppose P=0.1
Then P²=0.001. Thus the chance of a false
detection is reduced by the use of
coincidence detection.
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Muon Energy
Muons created by the interaction of cosmic rays
and our atmosphere lose their energy gradually by
ionisation of the material through which they pass.
As they start with high energies they have the
capacity to ionise many atoms before their energy
is exhausted.
Also, as they travel at nearly the speed of light,
they tend not to ionise very efficiently and hence
can travel through substantial lengths of matter,
some metres of lead, before being stopped.
Consequently, coincidence detection methods
are the only real reliable way to discriminate
between terrestrial radiation and cosmic sources.
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Penetrative Terrestrial
Radiation
For example natural Cobalt-60 gammas can
have energies up to 1.3 MeV and so could
penetrate upto 10mm of lead. In all detector
array designs either Geiger–Müller or
Scintillator-Photomultiplier configurations, this
can cause a substantial number of false
detections. This particularly becomes a
problem of detectors with small surface areas
(aperture). Consequently, it is recommended
that radiation shielding be included in your
design to reduce the problem and increase
reliability.
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Compton Scattering
An
interaction between charged
electrons within the detector and high
energy photons result in the electron
being given part of the energy, causing a
recoil effect of another high energy
photon, which may enter into the
adjacent detector causing a false
coincidence detection.
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Lead Shielding
Lead
shields against environmental
radioactivity due to its high density and
atomic number together with reasonable
mechanical properties and acceptable
cost.
This role is however hindered by the
unavoidable natural presence of Pb210, which undergoes beta decay, with
the consequent emission of gamma
radiation.
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Geiger–Müller Tube Detector
Pulse Width
The
Geiger–Müller tube is a very good
detector of Muons however it would seem
that filtering out background radiation
using a simple coincidence detector
systems alone is problematic due to the
Geiger–Müller tube response and decay
time (Pulse Width) when a muon has
passed through and is detected.
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Geiger–Müller Tube Detector
Pulse Width
Consequently, the wider the Pulse Width the
greater the number of false positives. The
means a pulse shorting or quenching circuit is
also needed to shorten the Pulse Width to a
period closer to the expected flight time of
the Muon between tubes, but not too narrow
that the electronics cannot measure relative
coincidence. Some improvement might also
be achieved by spacing the tubes further
apart, but this also has the negative effect of
decreasing the aperture of the detector.
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Detector using Scintillators
As muons travel at nearly the speed of
light, they tend not to ionise very efficiently
and hence can travel through substantial
lengths of matter, some metres of
lead, before being stopped. This means that
although a Scintillator-Photomultiplier
detector has the potential to measure the
energy of an ionising particle they can not
discern between a muon and any other
radiation caused by terrestrial sources and so
must be used in a coincidence detection
mode.
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