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 wholes, but most are the accelerated remnants of supernova explosions.

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 particels.

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 eV (electron volts), far more than any particle accelerator built here on earth, like the Large Hadron Collider (LHC).

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). Where 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.

Common interstellar events on earth of which most people are unaware
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, passing through just about everything. They can penetrate mountains, buildings, our bodies, and deep into the Earth’s surface, without anyone really being aware of their existence other than scientists and obsessive geeks

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.

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. The oldest and most famous example of this is the Cloud chamber. There is an operational cloud chamber installed and running at the South Australian Museum and is well worth a look (I think its fascinating).

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.

The problem using a radiation detector for a cosmic ray observation is that there is larger amounts of 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.

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.

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.

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.

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 = 0.01. Thus the chance of a false detection is reduced by the use of coincidence detection.

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.

Penetrative Terrestrial Radiation
I've been very surprised how penetrative local terrestrial radioactive sources can be. 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.

Compton Scattering
Compton Scattering is an effect where 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.

In other words placing detectors too close to each other may cause cross-talk interference in coincidence mode, and so radiation shielding should be added or the detectors spaced further apart. However increased spacing also has the negative effect of decreasing the aperture of the detector and so the expected count.

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.

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.

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.

The major advantage of Scintillator-Photomultiplier detectors over a Geiger–Müller Detector is that a photomultiplier has a very fast response time and so more accurate than Geiger–Müller Detector in coincidence mode. Also as Scintillator panels can be made to have a much larger surface areas means a greater number of muons can be detected compared to other radiation caused by terrestrial sources, further increasing accuracy.

The major disadvantage of Scintillator-Photomultiplier detectors is cost and complexity.

Lead Shielding
Lead plays an important role as a material to shield 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 Pb-210, which undergoes beta decay, with the consequent emission of gamma radiation.

Again why coincidence detection methods are the only real reliable way to discriminate between terrestrial radiation and cosmic sources.

This project was an experiment to see if a multilayered array of Geiger–Müller Tubes (GMT) could track ionizing particles as they pass through.  The result is an interesting display demonstrating how cosmic rays travel down through the atmosphere at different angles.

In the video random flashes are the result of terrestrial background radiation in and around the 18 GMTs but when you see a line of 3 or more simultaneous flashes these are the result of a muon (cosmic ray) passing though. The red LED flashes when more than three blue LED flashes and the level control sets the sensitivity.  

This is a very rough animation of how I first imagined the tubes would be triggered when a Cosmic Ray (Muon) flies through the detector array.

 

Second Prototype still more work to do 24th March 2012, but getting there.
 

First prototype above (June 2011) used 20GMT

In my first prototype I soon realised that doing coincidence in software was too difficult as the processing speed was fast then expected and so a gate array would be needed to pre-process the coincidence. Needless to say this would have made the detector quite large.

 

Consequently I redesigned a more compact designed using a smaller 18 GMT array and double sided PCBs rather than the stripboard I used above.

 

Schematic of 400V and 5 V power supply

 

Circuit design for the 9 Channel  Geiger–Müller Tube Detector to 5V TTL this detector uses two of these giving a total of 18 outputs.

Note: The IC used in this desing a 74HC14 and not 74LS14.  The 74HC14 is a high-speed Si-gate CMOS device Low-power Schottky TTL. It provides six inverting buffers with Schmitt-trigger action. It transforms slowly changing input signals into sharply defined, jitter-free output signals.

 

The above circuit is design to discriminate between terrestrial background radiation and strikes that are the result of a muon passing through. This is achieved by adding a resistor in series with the LED array and measuring the voltage drop across it.  The greater the number of LEDs are lit simultaneously the higher the voltage across it.  A darlington transistor amplifier increases the voltage to set a level using a schmitt trigger which drives an LED indicating a muon was detected.  

Final PCB design of the 9 Channel  Geiger–Müller Tube Detector to 5V TTL

20 wired GMTs installed between two plastic brackets which was changed to 18 for practical reasons.

Drilled completed bracket for holding GMTs

Drawing dimensions of the bracket for holding GMTs

Geiger-Müller Tube (GMT) CI-1G
I'm also using Russian Geiger-Müller tubes in this experiment which are described as being Gamma sensitive and available on ebay at very low cost less than the common SBM-20 Tubes that I have use before.

Specifications
Gamma Sensitive: unknown rate
Working Voltage: 360 - 440V
Plateau: Length/ Inclination: 80V/0,125%/V
Own Background: 0,4 Pulses/s
Load Resistance: 5 - 10 MOhms
Working Temperature Range: -400 +500 С
Length: 90mm
Diameter: 12mm

This  cosmic ray  detector works by detecting  muons which are a by-product of cosmic rays hitting our atmosphere. It detects these muons using Geiger Muller tubes - the very same type of detector used in a Geiger counter to measure radiation. However, this detector uses 18 Geiger Muller tubes that are arranged in an XY array of 9 tubes oriented on an X-axis and 9 tubes on a Y-axis.

Called a Hodoscope (from the Greek "hodos" for way or path, and "skopos:" an observer) it is a type of detector commonly used in particle physics that make use of an array of detectors to determine the trajectory of an energetic particle.

When a muon flies through the detector, it will trigger two tubes simultaneously.  By graphing which of the two tubes are triggered on an array of 81 LEDs, it gives an indication that a muon was detected as well as where it struck. 

The detector minimises background radiation using some shielding (brass plates) between the layers of tubes and also method of called coincidence detection.  Muons travel through matter very easily passing through the brass plates and both axes of the detector without effort, whereas the terrestrial radiation will not.  Consequently anything detected in both axes of the detector simultaneously is more likely to be a muon than local background radiation in, around and near the detector.

Figure 1. Basic overview operation of the 81 (9x9) Pixel hodoscope

Figure 2 Primary overall circuit  using a simple LED Matrix for coincidence detection.

Final PCB design of the 9 Channel  Geiger–Müller Tube Detector to 5V TTL

Geiger-Müller Tube (GMT) SI-22G

I'm using those good old Russian tubes again for this project. These are quite large 220mm with a diameter of 19mm. 

SI-22G Specs.

Working Voltage 360 - 440V
Initial Voltage 285 - 335V
Recommended Operating Voltage 400V
Plateau Length 100V
Plateau Slope 0.125% / 1V
Inherent counter background (cps) 1.16 Pulses/s
Cobalt-60 Pulse Gamma Sensitivity 540 pulse/mkR
Interelectrode Capacitance 10pF
Load Resistance 9 - 13 MOhms
Working Temperature Range -500 +700 ?
Length 220mm
Diameter 19mm

Bottom Layer (foam layer is to prevent tube damage and prevent slipage)

Assembled with shielding

This detector is to be used as test unit, to measure the performance of my other project using Fluorescent Tubes against in order to clarify and identify any issues, also to better understand and to also demonstrate the principles of a cosmic ray telescope.

Audio Recording of the Detector Outputs (1.35 MB Mp3)

Above is one of the first visual test of the circuit, which has been updated a number of times since this video. Although the new circuit produces less false positives and hence less flashes, the video here offers a good demonstration of what the output data looks like. The LED flash times are slowed by a one-shot timer, as the pulses are so short they would not be visible if the LEDs where driven directly.

 

Version 7. 1st August 2009

 

After a number of different configurations and tests, I have distilled the design down to a simple circuit seen here. The outputs 4,5,6 give a positive 5V logic pulse when a coincidence is detected in two or more of the tubes.

 

 

Although Geiger–Müller tubes are sensitive to Muons, the response time to decay (Pulse Width) when a muon is detected is relatively long for measuring the probability of coincidence in two or more the tubes. This means the wider the Pulse Width the greater the number of false positives. Consequently some means of pulse shortening is required to shorten the Pulse Width to a shorter period to decrease the probability that it is terrestrial radiation hitting the tubes in close succession.

 

 

The CRO trace above demonstrates what happens in this circuit when a wider pulse in feed in. The bottom trace is the output and only responds to a negative travelling pulse regardless of pulse shape. This is also important as the Geiger–Müller tube is positively biased and when a particle is detected the output swings negative, so the circuit ensures that only the first micro second of the detector pulse is processed in the coincidence circuit.

 

 

Why 3 tubes? I hear you say, well it is all about increasing the detectors aperture size to muons as most Geiger–Müller tubes available are longer than they are wide. So as an attempt to achieve a larger aperture as well as coincidence detection without using rows of tubes in each layer I am combining the coincidence outputs from top-middle and middle-bottom detectors.

 

 

The prototype provides six positive 5V logic outputs to a din socket. 1) Top tube - all detections 2) Middle tube - all detections 3) Bottom tube - all detections (all detections) meaning both cosmic and background radiation 4) Top and Middle - coincidence detection 5) Bottom and Middle - coincidence detection 6) "Top & Middle" or "Middle & Bottom" - coincidence detection (coincidence detection) meaning a stronger likelihood of a cosmic source than terrestrial source being detected. Outputs 4 and 5 are the main outputs, but to give some visual information that the detector is working I've added some LEDs driven by one-shot timers to give a 1/8 second flash, as the output pulses are only a micro second and wouldn't be seen if used to drive the LEDs directly. Output 6 drives the blue LED indicating a confirmed coincidence in two or more tubes and where outputs 1,2 & 3 drive the red LEDs indicating any detection in the adjacent tube.

 

 

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 = 0.01. Thus the chance of a false detection is reduced by the use of coincidence detection.

 

This Geiger–Müller Array (or Geiger tube telescope) exploits an effect called Electromagnetic Cascade as a means of significantly increasing the effective aperture of the detector while reducing other issues I've identified in experiments with cosmic ray detection.

Prototype v1. Construction of Lead Block Array

In a 1964 publication Bruno Rossi first described an experiment where cosmic rays could penetrate dense materials. Finding that cosmic radiation at sea level could penetrate over 1m of lead. In these same experiments he was also surprised to record a higher rate of detection as the thickness of lead increased peaking at 1.5cm and then falling slowly.

Consequently using a similar approach it is possible to improve a standard Geiger–Müller Array using a lead block in which the tubes are placed in an area which optimise these effects.

A Muon passing through lead will create electromagnetic cascades where they are detected by Geiger–Müller tubes.

The benefit of using this method not only enhances the count rate, but also allows the tubes to be moved into a parallel arrangement doubling the aperture of the detector over a conventional Geiger–Müller Array where the tubes are placed one above the other. While at the same time significantly reducing terrestrial interference with 15mm of lead shielding between each tube.

Prototype v1. Cut Lead Block block to size, drill holes, insert tubes and terminate.

Worse case terrestrial radiation may have energies up to 1.3 MeV but do not have enough energy to penetrate more than 10mm of lead and also cannot produce Electromagnetic Cascades, where muons created as a result of cosmic rays at sea level still have a mean energy of 4 GeVh or more and so can penetrate a metre of lead easily creating numerous Electromagnetic Cascades as they pass through.

Prototype 1. Schematic of a two tube Geiger-Müller Coincidence Detector.

Using the circuit above the negative sawtooth pulses from each Geiger–Müller Tube (GMT) is converted into brief 5V square wave signals through a 74HC14 Schmitt Inverter and then further shorted to around 5uS. Unfortunately a Geiger–Müller tube response time to decay (Pulse Width) is relatively long for measuring the probability of coincidence in two or more the tubes. This means the wider the Pulse Width the greater the number of false positives. Consequently some means of pulse shortening is required to shorten the Pulse Width to a shorter period to decrease the probability that it is terrestrial radiation hitting the tubes in close succession.

The CRO trace above demonstrates what happens in this circuit when a wider pulse in feed in. The bottom trace is the output and only responds to a negative travelling pulse regardless of pulse shape. This is also important as the Geiger–Müller tube is positively biased and when a particle is detected the output swings negative, so the circuit ensures that only the first 5 micro seconds before entering a 74HC02 NOR gate which acts as a coincidence detector. Only when pulses are detected from both GMTs simultaneously within a 5uS window will there be an output, confirming the presents of an Electromagnetic Cascade within the Lead Block as a result of a high energy particle passing through.

Note that the output of the above circuit uses a 5V TTL to USB PCB this is because they are so cheap and I see no reason to reinvent the wheel, thank you to all Arduino enthusiasts everywhere. This PCB was purchased from Little Bird Electronics.

This allows me to link the output to the computer and log the count and even upload a live graph to the website. Unfortunately there aren't any win32 freeware or linux packages around and as I'm not a coder, I purchased a software package called Rad 2.4.5 which allows this and runs on both Win32 and MacOS.

Prototype 1. First rough prototype built - still more refinements but basically it works!.

Data-sheets of components used in the design
74HC02.pdf (451.58 KB)
74HC14.pdf (318.64 KB)
LM2575.pdf (725.34 KB)

Geiger-Müller Tube - SBM-20 / SBM-20U

I'm using Russian Hard Beta/Gamma sensitive Geiger-Müller tubes which are commonly available on ebay at low cost. I've purchased a few of these and they work quite well for gamma and xray sources. Which is ideal for this experiment as they are insensitive to low energy radiation and detect radiation along the wall of the metal tube.

Specifications
Minimum Anode Resistor (meg ohm) 1.0
Recommended Anode Resistor (meg ohm) circuit diagram 5.1
Recommended Operating Voltage (volts) 400
Operating Voltage Range (volts) 350 - 475
Initial voltage (volts) 260 - 320
Plateau length (volts) at least 100
Maximum Plateau Slope (%/100 volts) 10
Minimum Dead Time (at U=400V, micro sec) 190
Working range (mkR/s) 0.004 - 40
Working range (mR/h) 0.014 - 144
Gamma Sensitivity Ra226 (cps/mR/hr) 29
Gamma Sensitivity Co60 (cps/mR/hr) 22
Inherent counter background (cps) 1
Tube Capacitance (pf) 4.2
Life (pulses) at least 2*1010
Weight (grams) 10 / 9

Just a note to say this project is currently having a rethink, due to some problems with noise induced by the PCB, even with shielding. So looks like I'm going to do a complete redesign. I still think it will work but each diode may will need to be FET buffered before amplification.

A Silicon Pin Photo-diode like the VBPW34FAS has been successfully used for the detection of Gamma Radiation and so in theory should also detect Cosmic Rays (Muons).  However, one very big drawback is their very small surface area. This is further complicated as they cannot easily be wired in parallel without decreasing sensitivity due to their inherent capacitance.

Experimentation has shown that 8 x photo-diodes can be paralleled before the problem becomes too over whelming.  This experiment will also use lead shielding to take advantage of electromagnetic cascade which can increase the aperture of a detector by allowing coincidence to be measured between individual detectors laying side by side and so providing a surface area of 16 x VBPW34FAS spaced apart evenly.

The below schematic is the first photo-diode amplifier stage which is based around the MAX4477 low-noise dual opamp and will be latter amplified using a high speed comparator and then coincidence circuit.  I will be develop this latter once the photo-diode amplifier stages are enclosed in shielding and optimum component values are determined.

Draft PCB of Pin Diode amplifier section

Silicon Pin Photo-diodes are attached with a modified polyester varnish coating to reduce moisture

Silicon Pin Photo-diodes covered in black resin and foil to block light

Using a neutron detector to measure cosmic rays may sound odd, but this has been a common way to measure the level of cosmic ray levels since 1948. This is because if the primary cosmic ray that starts a cascade has an energy well over 500 MeV, and so many of its secondary by-products will be neutrons that will reach ground where they can be detected.  These systems are commonly called Neutron Monitors

Recently some Russian Boron Coated Cathode Corona Pulse Neutron Tubes (SI-19N) became available on ebay at low cost and so I thought I'd see if I could also make one these guys.

List of materials required to build a Neutron Monitor:

1. Reflector - 75mm of UHMWPE (Ultra-high-molecular-weight polyethylene)  or 280mm of Paraffin Wax.  Low energy neutrons cannot penetrate this material, but are not absorbed by it. consequently, non-cosmic ray induced neutrons are kept out of the monitor and low energy neutrons generated in the lead are kept in which is largely transparent to the cosmic ray induced cascade of neutrons.
2. Producer - 40mm of Lead . Fast neutrons that get through the reflector interact with the lead to produce, on average about 10 much lower energy neutrons. This both amplifies the cosmic signal and produces neutrons that cannot easily escape the reflector.
3. Moderator - 20mm of UHMWPE (Ultra-high-molecular-weight polyethylene) or 32mm of Paraffin Wax.  This slows down the neutrons now confined within the reflector, and makes them more likely to be detected.
4. Nuetron Detector Tube - Boron Coated Cathode Corona Pulse Neutron Tube SI-19N. After very slow neutrons are generated by the reflector, producer, moderator, and so forth, they encounter a nucleus within the proportional counter which causes it to disintegrate. This nuclear reaction produces energetic charged particles that ionizes the gas in the proportional counter, producing an electrical signal. 
5. Electronics - Very similar to a typical Geiger Counter circuit but with an adjustable High Voltage Regulated DC Supply 2-3kv

Boron Coated Cathode Corona Pulse Neutron Tube SI-19N

Specifications:
Operating  Voltage - 2000-2800 V
Working Voltage - 2400 V
Slop of plateau, % per 100 V - ≤ 3
Neutron sencitivity, pulses per neutron/ cm2 - 100
Plateau Length - 800 V
Own background, pulses/min - 5
Temperature range , 0 C  -  -50... +150
Diameter - 33.3 mm
Length  - 218 mm

This Cosmic Ray detector uses three Geiger–Müller Tubes (GMT) where the detection of coincidence between any 2 or 3 GMT is displayed as a colour.

Recently I obtained some blocks of Plastic Scintillator BC412 which measure 89mm x 89mm x 38mm and is ideally suited for detecting muons.

Scintillation occurs in the BC412 when exposed to ionising radiation with an energy between 100 KEV and higher and emits light between 420nM and 450nM (i.e. blue light)

The scintillator block will be coupled using Dow Corning DC4 to a 10 stage photomultiplier BURLE S83020F which is very sensitive to light between 350nM to 500nM.

Dow Corning DC4 is mainly used in Aviation as an electrical insulator but because it is clear with a refractive index range of 1.4 to 1.5 it is similar to glass making it an ideal low cost choice Optical Coupling Grease between the scintillator plastic and photomultiplier glass envelope.

Proposed layout of Prototype Version 2

More information soon...

This project was deliberately aimed at developing a very low cost cosmic ray detector using common Fluorescent Tubes. It was based on variation of an experiment performed in 2000 by the CERN (European Organization for Nuclear Research) laboratories by Dr. Schmeling which found a simple method for detecting and visualizing cosmic rays using everyday fluorescent tubes inside a wire mesh of feed with a high voltage.

There is a link here at the Teachers CERN Website at the bottom of their page, unfortunately there is little/no information about how this actually works.

However one of the very best websites I have found on working Fluorescent Tube cosmic ray detectors is www.astroparticelle.it if you are interested in this type of design you could do no better.

I have now begun building a number of other types of detectors due to a number of issues I've identified in experiments using Fluorescent Tubes.

The first experiment in which radio emission was detected from high energy particles was an array of dipoles was operated by a team of British and Irish physicists in 1964-5 at the Jodrell Bank Radio Observatory in conjunction with a simple air shower trigger. The array operated at 44 MHz with 2.75 MHz bandwidth. Out of 4,500 triggers a clear bandwidth-limited radio pulse was seen in 11 events. This corresponded to a cosmic ray trigger threshold of 5x10^16 eV and was of intensity close to that predicted. This has been further confirmed by many otherexperiments over the years and is now currently used in many well known projects such as LOFAR and at the Pierre Auger Observatory.

Cosmic rays are known to initiate a cascade of particle collisions in which large multiplicities of secondary particles of all kinds are produced. In the creation and annihilation of these secondary particles air-showers of charged particle are produced and travel down through the atmosphere near the speed of light to the ground. A large part of these charged particles consist of electron-positron pairs which emit a radio signal as they are deflected by the Earth's magnetic field. This process is known as a geo-synchrotron emission.

Radio signals produced are in the region of 10-100 MHz where most power received is at the lower frequencies due to coherence effects and so the spectrum begins to fall off at around 50 MHz. However due to short duration of the radio pulse typically less than 10nS it becomes increasingly difficult to measure at low frequencies and so most observations of cosmic ray induced radio emissions are made between 40 and 60Mhz.

From the available research, there is no convincing evidence confirming  cosmic rays have a “major factor” in determining cloud cover. The ionising of air by cosmic rays will impart an electric charge to aerosols, which in theory could encourage them to clump together to form particles large enough to form cloud droplets, called "cloud condensation nuclei".

However, the majority of physicists who research this area say such clumping has yet shown to occur. Even if it does, it seems far-fetched to expect any great effect on clouds in the atmosphere. Most of the atmosphere, even relatively clean marine air, has plenty of cloud condensation nuclei already.

It is also not even clear whether the satellite measurements of changes in cloudiness are correct or how these changes have affected temperature, as it is unknown if clouds cover may mitigate global warming or amplify it.

Visit the CERN CLOUD experiment.

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