Cosmic rays detection theory
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This document discusses cosmic rays and methods for detecting them. It begins by explaining that cosmic rays are high-energy particles originating from space that bombard Earth's atmosphere. When they interact with air molecules, they produce showers of secondary particles. Two detectors placed above one another can potentially detect these particles simultaneously, filtering out background terrestrial radiation through coincidence detection. The document then discusses various detector types like Geiger counters and scintillators, and factors to consider in the detector design like shielding against penetrating radiation and accounting for detector pulse widths.
Cosmic rays detection theory
- 1. Cosmic Ray Detector Equinox- Astronomy Club of IIT Guwahati 1 Equinox Club
- 2. 2 What are Cosmic Rays Equinox Club
- 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. Equinox Club
- 6. 6 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. Equinox Club
- 7. 7 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). Equinox Club
- 8. 8 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. Equinox Club
- 9. 9 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. Equinox Club
- 10. 10 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. Equinox Club
- 11. 11 Theory of Detecting Cosmic Rays Equinox Club
- 12. 12 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. Equinox Club
- 13. 13 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. Equinox Club
- 14. 14 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. Equinox Club
- 16. 16 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. Equinox Club
- 17. 17 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. Equinox Club
- 18. 18 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. Equinox Club
- 19. 19 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. Equinox Club
- 20. 20 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. Equinox Club
- 21. 21 Issues to consider in the design of Muon (cosmic ray) Detectors Equinox Club
- 22. 22 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. Equinox Club
- 23. 23 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. Equinox Club
- 24. 24 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. Equinox Club
- 25. 25 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. Equinox Club
- 26. 26 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. Equinox Club
- 27. 27 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. Equinox Club
- 28. 28 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. Equinox Club
- 29. 29 Scintillators-Photomultiplier Advantage- a photomultiplier has a very fast response time and so more accurate than Geiger–Müller Detector in coincidence mode. larger surface areas Disadvantage- cost and complexity. Equinox Club
- 30. 30 Assignment for Next meeting Read up on the following detectors. Google Doc. Equinox Club