9: COSMIC RAYS Flashcards
How were cosmic rays discovered? (3)
- In 1910-13, Pacini measured ionisation above and below water. The ionisation level was lower below water than above the surface. It was expected to increase with depth as you move closer to the Earth’s crust.
- In 1911-13, Hess flew ~ 5km and found that up to above 1km the ionisation level decreased with altitude. Above this, the ionisation level increased reaching ~ 4x that at ground level. Hess concluded that this was caused by a source of ionising radiation from outside the atmosphere.
- In 1913-19, Kolhorster confirmed the increasing ionisation continued up to 9km.
What were cosmic rays initially believed to be?
It was initially believed that they were ultra-high-energy protons.
What is the latitude effect and what did this prove about cosmic rays?
The latitude effect says the number of cosmic rays was larger at the Earth’s poles than at the equator.
The effect can be understood if CRs are charged particles. Charged particles spiral along magnetic field lines. Since the Earth’s magnetic field is a dipole, the field lines at the poles should channel CRs most effectively at the surface of the Earth.
CONCLUSION: CRs are charged particles.
What charge do most CRs have and how was this found?
CRs have positive charge. This was found by the direction the particles bent in the Earth’s magnetic field.
What is the East-West effect?
If CRs were positive, low-energy and hence low-rigidity particles arriving from the East, they would be blocked by the Earth. Higher energy particles with a larger radius of curvature would not be blocked. From the West, there is no constraint and so lower energy particles arrive too. 14% more particles arrived from W than E.
How are low energy CRs measured?
They are stopped and measured in small detectors on satellites or balloons in the upper atmosphere.
What is the advantage of measuring low energy CRs in this way?
Primary CRs are detected rather than secondary particles generated by CR interactions in the atmosphere.
How do detectors measure the CRs?
They measure the ionisation caused by a CR along its path (or track) through the detector, and the range of track in the detector material.
When does a particle emit Cherenkov radiation?
If it moves into a medium in which its speed is faster than the speed of light in that medium, c/n, where n is the index of refraction.
When detectors combine ionisation and Cherenkov measurements, what do they determine?
The charge and the number of particles.
Why do high energy CRs (> 10^12 eV) need a large detector and what detector do they use?
They are hard to stop. They use Earth’s atmosphere.
A high-energy CR entering the atmosphere will produce what and how?
Nucleonic cascades and electromagnetic showers. A CR proton collides with a nucleus in the atmosphere, ejecting a nucleon and producing pions. The primary CR and the secondary particles go on to interact with further nucleons in the first nucleus and in additional nuclei. Pions decay through various paths to give gamma rays, e+-, neutrinos etc.
What do we detect at intermediate energies and how?
We detect Cherenkov light produced by the Earth’s atmosphere from primaries and secondaries, using photo-multiplier tubes in combination with mirrors or lenses.
How are the highest-energy CRs detected and give a real example of one such detector?
Extensive Air-Shower Arrays.
Pierre Auger observatory in Argentina uses detectors which contain 12 tons of purified water and photomultiplier tubes to detect the Cherenkov radiation of incoming secondary particles from > 10^18 eV primaries as they pass through the water. It is then possible to reconstruct the energy and arrival direction of the primary CR from the shower of secondaries that was detected.
What is the range of energies CRs can have?
Non-relativistic particles to energies > 10^20 eV.
