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  • Published on: 15th October 2019
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What are cosmic rays?

Cosmic rays are high energy, subatomic particles, such as protons or Alpha particles  (Helium nuclei), that are traveling through space at almost the speed of light, roughly 0.999c (where c is the speed of light, 300, 000, 000ms-1). They collide with our atmosphere and create a cascade of secondary particles that ‘shower’ us on Earth. They make up roughly an eighth of all background radiation we experience (including artificial background radiation).

Cosmic rays were discovered by an Austrian physicist called Victor Hess on the 7th of April 1912. Hess sent a balloon up 5300m during an eclipse and saw that the radiation count of the atmosphere did not change which led him to conclude that the source of the radiation had to be coming from outer space, not the just the sun.

Where and how are they produced?

We do not know where a specific cosmic ray originated as they travel in nonlinear paths due to being pulled about by magnetic fields. We do however have an idea where cosmic rays in general come from. They are thought to originate from places such as supernovae, supermassive black holes, colliding galaxies, pulsars and the sun.

A supernova is the death of an average mass star. Supernovas occur when the star has used up all its fuel and gravity begins to overcome the outward acting pressure of the hot gases, the core collapses in on itself and sends out shock waves which create a huge explosion. This occurs because of a reaction, e- + p → n + ve , that happens in the star which uses up electrons, an essential product in maintain gas pressures which support the star. In these explosions atoms are smashed together and fused to create heavier atoms but also causing radiation to be created and shot off into space at very high speeds, eventually hitting our atmosphere as a cosmic ray.

 A black hole is the death of a very high mass star. Again, the fuel is used up and gravity begins pulling the star’s mass in, causing the core to collapse. The star explodes due to shock waves but the core is not destroyed. The core continues to pull matter in, becoming smaller and denser until its gravitational pull is so strong that it becomes a black hole (the gravity of a black holes is so strong that not even light can escape it). The biggest black holes are called supermassive black holes and are over a million times more massive than our sun! They are theorised to be at the centre of every galaxy.

The data collected from the High Energy Spectroscopic System (HESS) telescope in Namibia allowed astronomers to estimate the general direction that the cosmic rays were coming from. A lot of the cosmic rays seemed to be coming from the centre of our galaxy, where the supermassive black hole is. This led to the theory that cosmic rays may come from black holes. Matter orbits the supermassive black hole at very, very high speeds which also creates a lot of energy. The energy is given off by way of radiation, or cosmic rays. This theory would explain how some of the cosmic rays that we see hit the earth have such high accelerations and such high energies.

A galaxy is a group of a huge number of stars along with gas and dust, which are held together by mutual gravitational attraction. When galaxies collide, or merge they stick together. As they move closer the strong gravitational pull between them causes the galaxies to change structure. Spiral arms are pushed and pulled and the galaxies end up looking blob like in the sky. Although the process of colliding two galaxies may look gentle it sends huge shockwaves out into space, carrying with it high energy radiation

Pulsars are neutron stars (the remaining core of a once supergiant stars that died through a supernova) that are oscillating at very high speeds. The fastest pulsars spin 1000s of times a second. Pulsars spin because they were once spinning stars, as they got smaller (about the size of Australia!) the spin rate increased due to the conservation of angular momentum (just like how a spinning figure skater becomes faster as he pulls his arms in). Pulsars also have a very strong magnetic field. All stars have a magnetic field but it is spread over millions of km2 so it is fairly weak but when the star shrinks the magnetic field is much more concentrated so becomes stronger by the same factor, i.e. If a star shrinks to become a neutron star by a factor of 108 then the magnetic field strength will increase by a factor of 108. The magnetic field is useful to us in determining whether something is a pulsar or not because as the pulsar rotates the magnetic field throws beams of radiation out which we can detect in pulsating radio signals. Unlike normal stars, neutron stars have a solid surface so sometimes have ‘starquakes’ (due to shifts and fractures in the surface). These quakes cause an extraordinary amount of radiation to be released. For a fraction of a second this radiation burst causes the pulsar to shine brighter than the whole galaxy! Both sources of radiation can travel to our atmosphere as cosmic rays.

Pulsars often have charged particles orbiting around them. These particles are accelerated to very high speeds when they are moving around the neutron stars magnetic poles and produce two beams of radiation shooting out at opposite ends. This is also a source of cosmic radiation. All the energy given off by radiation causes the pulsars to slow down over time.

The sun is the source of the lowest energy cosmic rays that hit our atmosphere. The solar cosmic rays are created during solar flares. Solar flares are large explosions that occur on the Sun’s surface, caused when energy from the sun which is stored in 'twisted' magnetic fields is suddenly released. Material is heated to millions of degrees in a few minutes causing a burst of radiation which hits our atmosphere.

How are they detected?

Four cosmic ray detectors scientists use are the Muon Observatory system, HiSPARC system, cloud chambers and the MX10 hand held detectors.

The Timstar Muon Observatory system consists of four types of equipment, Geiger-Müller tubes, a muon observatory, a coincidence box and a counter. The Geiger-Müller tubes are only suitable for detecting low levels of radiation and are sensitive to alpha, beta and gamma radiation. They each have a large 28.6mm diameter ‘window’ which means they will typically give 3 times more counts per minutes that an average Geiger-Müller tube. Geiger- Müller tubes are mounted onto the muon observatory which can be configured in one of two ways, shower mode or telescope mode.

When in shower mode the Geiger-Müller tubes are arranged in a triangle at the end of the observatory. Using the coincidence box (a box which each Geiger-Müller tube is linked to. When they detect radiation a specific light on the box flashes to tell you which tube detected radiation) we can tell if there has been a particle shower caused by cosmic rays because all 3 light will come on at once, in a coincidence hit. If you are only interested in high energy radiation then you can block the lower energy radiation by using a number of steel plates which will absorb radiation. Some experiments you could do using this set up could include investigating the correlation between amount of coincidence hits and the barometric pressure (or atmospheric pressure, which is the force exerted by the atmosphere at a given point. Or the weight of air). It is thought that the higher the barometric pressure is the more coincidence hit there should be as there is more particles in the atmosphere which can be hit by a cosmic rays and cause and shower. If you had lots of Muon Observatory systems you could set them up in a wide circle and look to see if when there is one coincidence hit, if all of them have had a coincidence hit too. Showers can be up to 7 km2.

When in telescope mode two or three (if suppression of random coincidences is wanted) tubes are arranged in a line, perpendicular to the observatory (which will usually be in a near vertical position as almost no muons are detected when it is in a horizontal position). If a muon passes through all tubes then all three lights on the coincidence box light up. This would send a pulse to the counter. Again you can control what radiation you are receiving by using the steel plates. An experiment you can do using this set up such as looking at correlation between your angle to the vertical and how many cosmic rays pass through all the Geiger-Müller tubes at once (muon flux is directly proportional to sin2θ, where θ is the angle to the vertical). You could also look at how the coincidence hit rate changes as you move down from the upmost level of a building down to the basement. It is good practice in any experiment to note the outside barometric pressure and the temperature.

The HiSPARC system is a project for high schools and other academic institutions where they can join together to measure cosmic rays with extremely high energy. They set up detectors on the roofs of the building in ski cases to protect it from varying weather conditions. This is useful in estimating how large the showers are and means data is more accurate as its spread over a wider area. There is less chance of a random hit.

The HiSPARC consists of 2 plastic scintillators, 2 photomultiplier tubes, 2 ski-boxes, GPS device and antenna which included a timestamps which ~5 ns accurate, HiSPARC II control box and USB 2.0.

Cloud chambers have been used since 1912, when the Scottish physicist, C. T. R. Wilson invented it. Cloud chambers are very helpful in particle physics as they allows us to see the paths of individual particles (each type of particle has a different path through the cloud chamber). There is a supersaturated vapour used as a detector. This vapour condenses into small droplets around ions that are produced by charged particles, such as alpha particles, beta particles or protons.

The MX10 hand held detector is a product of the MediPix2 collaboration between CERN, Jablotron and the Czech Technical University (CTU). Its Timepix chip detects radiation with incredible accuracy. It has 256x256 pixels each of which individually detect radiation making a really clear picture of where exactly and what is hitting the chip.

To use the MX-10 you need to plug it into your laptop or computer and download the pixelman software which allows you to see what is happening on the chip, analyse the data and also log the data.

How do they reach us?

A cosmic ray travels through space, being pushed and pulled by various magnetic fields. Eventually it enters our atmosphere as a primary cosmic rays and hits atoms creating a ‘shower’ of secondary particles. This causes a chain reaction of particle collisions and ‘showers’. The original energy of the cosmic ray splits at each collisions so the secondary particles we observe on the ground have much lower energy than the primary cosmic ray they came from. The most common particle observed from cosmic rays is the muon.

What is a Muon?

Muons were discovered in 1937 by Carl Anderson when he was studying cosmic rays. They were originally thought to be mesons but are in fact part of the lepton group as they do not interact with strong forces. Muons have charges of both 1e and -1e (where e is the charge on an electron, 1.6022×10-19C) and are fundamental (or elementary) particles, which means they cannot be broken down further. With a mass of 1.883 531 594 x 10-28 kg the muon is 200 times more massive than an electron. They also have a spin of ½. I don’t fully understand spin but I know that ‘Spin is a property that particles have that cause them to act a little like magnets. Having spin of 1/2 means that muons can exist in one of two states spin up or spin down.’  Muons are not found in everyday matter and so still baffle scientists as to how they fit into the scheme of things.

Why do we care?

Cosmic rays have much more energy the higher up you go, this is because energy of the cosmic rays slits at each collision (due to energy conservation). Therefore airline pilots are frequently hit by ionizing particles of much higher energy. High doses of this radiation are known to increase the risk of cataract problems. Astronauts are very much affected by cosmic rays too as they are not very shielded from the high energy particles that hit them when in the ISS (International Space Station). This becomes more of a problem as we start travelling further afield to Mars or asteroids, where astronauts will have to venture outside of our protective magnetic field. They will be hit by even stronger and more dangerous particles and will be at very high risk for cancer .

Cosmic rays can also cause damage electrics. We rely on electronics a lot in modern day life, from things like using our phones, to life support at a hospitals. High energy cosmic rays can damage power lines causing electrical blackouts for varying periods of time. This can have a catastrophic outcome for hospitals and factories that rely on a constant power input. Cosmic rays can also interrupt specific radio frequencies that are used by aircraft to communicate which is very dangerous. Flights are often delayed or cancelled so as to avoid this. Computer systems on-board space craft are also susceptible. A hit from a high energy cosmic ray could jeopardise the whole mission and could lead to the spaceship becoming marooned. Cosmic radiation from the sun can interfere with signals to and from satellites meaning connections are lost. This can have an effect on things like the navigation systems on cars, trains and ships, personal and government communications and also television signals.

Finding out more about cosmic rays will help us to figure out how to shield ourselves from them. Enabling us to travel further into space to learn more about our universe, protect aircraft pilots and also ensure that we are getting power to all the places that need it.

Fascinating Facts

• Lots of cosmic rays hit our atmosphere a second but about 30 cosmic rays fly through our bodies every second.

• Cherenkov radiation occurs when a particle exceeds the speed of light in air (or water if the detector is under water). It is likened to a sonic boom but it’s the light equivalent. The radiation manifests itself in a flash of faint bluish white light in the atmosphere. It was named in 1934 after the man who first figured out what Cherenkov radiation was, a Russian physicist named Pavel A. Cherenkov.

• Every second each m2 of the Earth is hit by a hundred or so particles that have originated from cosmic rays.

What is angular momentum?

Angular momentum is the measure of momentum for objects moving in a circular motion. It relates the velocity of the body and how close the body is to the point its moving round (rotation axis). It is always conserved, even if the object begins moving in a straight line. The new radius length is cancelled out due to the decrease in velocity. The angular momentum of a system also stays constant unless an external force acts upon it.

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