Cosmic Radiation Beechen Cliff Physics

1. Cosmic Radiation Beechen Cliff Physics

2. Cosmic radiation: introduction • The year is 1900 and so far scientists know: • There is high energy radiation coming from radioactive sources • This radiation passes through thin gold leaf foil, or thin metal or several cm of lead • They don’t currently know how harmful it can be • One cm of lead approximately halves the intensity of gamma radiation • But there was more that still couldn’t be explained

3. Cosmic radiation: spontaneous discharge After 1900 it was noticed that a charged electroscope discharged itself spontaneously To check as to why this happened electroscopes were made more and more isolated from electric fields and the surrounding air The only thing left that could be causing the discharge was ionising radiation

4. Cosmic radiation: spontaneous discharge • The ionising radiation creates positive and negative charge carriers inside the electroscope which travel towards their opposite charge • This reduces the charge on the electroscope • Even when 5cm of lead was used to surround the electroscope to reduce the amount of ionising radiation the loss of charge was only reduced by 30%! • There must be a type of radiation that can pass through over 5cm of lead.

5. Cosmic radiation: spontaneous discharge In 1900 an Englishman called Wilson took his electroscope into a deep railway tunnel to see if the radiation came from the sky There was hardly any difference so it seemed unlikely that the radiation came from outer space - so surely the source of this radiation was underground If this was the case then surely the higher you go the amount of discharge should decrease

6. Höhenstrahlung – Height radiation So scientists wen up mountains with their electroscopes! The results were unclear and some scientists suggested the radiation may well be coming form the mountains themselves A new method must be used to get to high altitude to measure the effect on the electroscope

7. Wulf and Hess With an increase in height comes a decrease in pressure which may influence the rate at which charge carriers are created – new electroscopes had to be designed Theodor Wulf made one that was airtight and had a magnifying window in 1909 In 1910 Wulf took his device up the Eiffel tower – at 80m the gamma radiation intenity halves and he was up at 300m He measured a small decrease in the charge lost, but not as low as he was expecting – further investigation was required

8. Wulf and Hess • Victor and Hess in 1911/12 showed with balloon flights that the discharge speed actually increased with altitude • In 1936 he received a Nobel prize for this work • Hess performed his measurements up to 5km • Up to 2km the intensity of the radiation did decrease but from 2km upwards the intensity increased – he called this increase Höhenstrahlung – Height radiation • Kolhörster was a contemporary of Hess and confirmed his results up to 9km in his own measurements

9. Cosmic Radiation After the first world war the German Reneger found a maximum height for radiation before it decreased again up to 30km Robert Millikan (US) carried out a extensive project into cosmic radiation. He obtained lower radiation intensities at low altitudes up to 1.5km that Hess and Kolhörster This put fresh doubt in the origins of this radiation and was not resolved after a second set of results were found

10. Robert A. Millikan He took his electroscopes to two different lakes on two different mountains measuring the radiation intensity in the air and lake water The deeper in the water the slower the rate of discharge At the higher altitude lake the electroscope had to go deeper (2m) into the water to get the same discharge speed Millikan realised that the radiation passes through 2m of water the same as 2000ft of air i.e. the same amount of matter

11. Cosmic Radiation Millikan’s conclusion was there could only be one explanation for the source of the radiation It comes from outer space: cosmic radiation Initially Millikan thought it was a higher energy gamma radiation which is why they are still sometimes called cosmic rays. Jacob Clay however showed that the radiation should be mostly positively charged particles Further research found these to be stripped atomic nuclei such as Hydrogen (proton) These travel at near light speed velocities

12. Latitude Effect The Dutchman Jacob Clay measured the discharge speed whilst on route from Genoa to Java He determined that there was less radiation nearer the equation than at higher latitudes This was confirmed in the US by Arthur Compton and the reason was determined to be the Earth’s magnetic field The cosmic radiation must consist of charged particles that are subjected to a Lorentz force Careful analysis showed these to be positive

13. Radiation from Space The Sun is the first source of cosmic radiation. We consider the Sun to have low energy particles sent out as cosmic radiation but compared to Earth sources they are still high energy On Earth radioactive decay provides particles with energies up to several MeV Our Sun produces particles with energies up to 100GeV At the LHC at CERN create particles up to 3.5TeV

14. Radiation from Space The radiation from the Sun is very sensitive to the Earth’s magnetic field Why and how so? The field is sufficiently strong to deflect most of the particles to the poles due to the Lorentz force At the equator the amount of radiation detected from the Sun is lower than at the poles High energy particles are less likely to be deflected by the Earth’s magnetic field and as such are just as likely to be found at the poles or equator

15. Sunspots Light intensity is fairly constant during a year Magnetic processes in the Sun and on the surface, however affect the amount of cosmic radiation given off During large sunspots huge amounts of particles are given off Sunspots are regions on the Sun’s surface where the temperatures are roughly 1000oC below average, around 5000oC This is why they appear darker on images of the Sun

16. Sunspots

17. Sunspots Every 11 years there is a maximum and minimum number of sunspots – reason unknown They linked to the Sun’s complex magnetic field During a sunspot the magnetic field is at its strongest This means we are most protected from radiation coming from outer space But on the other hand the Sun is most turbulent

18. Sunspots The surface solar flares emit large numbers of charged particles with great speed into space Particles headed for Earth are deflected by our own magnetic field to the poles Here they collide with nitrogen and oxygen molecules in the upper atmosphere This creates the polar or northern lights http://www.youtube.com/watch?v=FcfWsj9OnsI

19. Sunspots These solar eruptions also disturb radio communications and can pose a real danger to satellites and astronauts Because of this repeated 11 year cycle on the Sun – there is a similar cycle on Earth for the amount of cosmic radiation that reaches us.

20. Solar Wind Our Sun emits a constant stream of charged particles, predominantly protons and electrons These particles move at speeds of up to 700km/s The Earth’s magnetic field protects us against most of these – however the solar winds increase the sphere of influence of the Sun’s magnetic field The area dominated by the Sun’s magnetic field is called the heliosphere – this is larger than the total size of the solar system

21. Solar Wind

22. Further away Our Sun is not the only star emitting radiation – all stars do so Some of them are far brighter than the Sun (which is actually very average) Bright stars emit particles with much higher energies- they have much bigger heliospheres Novas or supernovas (exploding stars) emit radiation and particles with energies up to 1015eV

23. Further away Radiation up to this energy stays within our galaxy Our magnetic field has little effect on these particles – but the Milky Way has its own magnetic field! It is weak but has massive distances to act over so the force is strong enough to deflect particles so they start to spiral around in the galaxy

24. Further away The radiation cannot leave – but we cannot exactly determine its origins Some radiation gets past the Sun’s and Earth’s magnetic field resulting in collisions in the atmosphere creating C-14 It is thought that cloud formation is also connected with cosmic radiation

25. Higher energies Particles with energies greater than 1015eV regularly reach Earth Where from? Possibly black holes at the centre of a galaxy or colliding galaxies! When particles have energies of over 1018eV then they can leave out galaxy They move too fast to be effected by the Milky Way’s magnetic field Most obvious miss Earth so we cannot measure them

26. Higher energies However if higher energy particles can leave our galaxy what can we infer from that? High energy particles hitting Earth may come from distant galaxies As they have such high energies they are only slightly deflected meaning we can determine the origins of the high energy particles With this we can track down the processes that create these particles

27. High energy particles This is one of the goals of the HiSPARC project: Detect ultra high energy cosmic radiation Determine where it cam from and how it was created

28. High energy particles When energies are over 1020eV then they interact with the cosmic background radiation left over from the ‘big bang’ After travelling over 100 light years the particles have lost significant amounts of energy Meaning that any particles detected in this energy bracket come from our own cluster of galaxies These particles have 10million times more energy than anything created on Earth