Cosmic Ray (Muon) Detector
This project was conceived from an interest in Radio Astronomy and a discussion at a Dorkbot Meeting about a number of different project ideas I've had. The one idea that seemed to strike the most interest was a Cosmic Ray Detector.
There is some good information around about how to build a Cosmic Ray Detector e.g. CosmicRays.org and The Cosmic Connection but most designs seem expensive, clumsy or just difficult to build.
The main aim of this project is to develop a detector which is easy to build, low cost and has some kind of usable output to graph, visualise or sonify. Note that in many cases here I'll use components and materials not necessarily the most ideal for the purpose, making use of what is available or can scrounge including surplus or hacked equipment.
What are Cosmic Rays?
Cosmic rays are energetic particles originating from deep space that hit our atmosphere at high speed.
There are a variety of sources including our own Sun but most come from interstellar events like Supernova, Black Holes and yet unknown happenings in the outer most reaches of our universe.
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 and iron nuclei and remainder electrons or other sub-atomic particle remnants.
When these primary particles hit our atmosphere 30km above the Earth’s surface they hit with such tremendous energy they cause a nuclear reaction producing a shower of subatomic particles called pions. The charged pions decay into muons and muon neutrinos whereas the uncharged pions decay into pairs of high energy photons which 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 to 25 percent and hit the Earths surface at near the speed of light ~0.998c...
Another interesting phenom of cosmic rays is that they are an everyday demonstration of Einstein's theory of relativity. This is because a muon at rest will disintegrates in about 2 x 10-6 seconds and so should not have the time to reach the Earth's surface given their travel distance. However as they move at close to the speed of light, time dilation extends their life span as seen from Earth and so can be observed at the surface before they disintegrate.
This is apparently happening 200 times every second, on every square metre, across the entire surface of the earth. With so much energy behind them they pass through just about everything, penetrating deep into the Earth’s surface, without anyone really being aware of their existence.
Below is a segment from an episode the TV program called "Cosmos" with Carl Sagan a legend in my youth, sadly now no longer with us. Although a little out of date, I could never have put the mystery of cosmic rays better.
Cosmic ray detectors, video from the University of Utah
Video on cosmic rays and their detection. About the Fly's eye and HiRes cosmic ray detectors. From the University of Utah
Websites about the intersection of particle physics, astronomy, and cosmology
Theory of Detecting a Muon
Unfortunately a muon 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 will be equal amounts of terrestrial radiation known as background radiation 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 two detectors are needed placed one above the other, feed into electronics that can monitor coincidence quickly thus effectively filtering out terrestrial noise.
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.
Issues to consider in the design of DIY Muon Detectors
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 Cobalt-60 gammas which can be found just about everywhere have energies up to 1.3 MeV and so can penetrate 10mm of lead. In all detector arrays designs either Geiger–Müller or Scintillator-Photomultiplier configurations, as 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, but increased spacing also has the negative effect of decreasing the aperture of the detector.
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 consequent emission of gamma and Bremsstrahlung radiation.
Again why coincidence detection methods are the only real reliable way to discriminate between terrestrial radiation and cosmic sources.
Muon Detector using Geiger–Müller Tubes
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.
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.
Detector Using 2 Pancake Geiger–Müller Tubes
I'm also making a two tube variation of the 3 tube design using Pancake Geiger–Müller tubes as they have a larger surface area.
Issues with detectors using Geiger–Müller Tubes
What should the count rate be?
The theoretical rate of cosmic rays is of the order of 1 count per minute per cm2 of active area, but it does depend on solid angle, so this number is only approximate. Assuming each GM tube has a broadside active area about 10 cm2, the number of counts maybe 10 per minute.
If the rate of each tube; S1, S2, and S3 counts per second and the coincidence gate width is τ seconds, then if counter #1 is ON S1τ , and the random coincidences in tube #2 at a rate R12 = S1S2τ random coincidences per second. So the random coincidence rate should be less than 1% of what is expected for the real coincidence rate, or about 0.1 counts per minute for a 10 cm2 Detector. (Many thanks to Bob S for this information)
Compton Scattering
One of the reasons for false counts in a Geiger–Müller array detector maybe due to Compton Scattering, 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 a high energy photon into the adjacent detector causing a false coincidence detection. In other words this causes cross-talk interference between GM Tubes
Consequently radiation shielding is required between each GM Tube of either 6mm of lead, 12mm of copper or 25mm of aluminium (note Iron is unsuitable). (Many thanks to Bob S for this information)
August 13th 2009 - To date tests carried out using radiation shielding between the GM tubes don't indicate that this is a real problem, however any cross-talk between each the tubes or the electronic is a real concern and should be design into any detector array.
Detector pulse width
In theory the Geiger–Müller tube is a very good detector of Muons (Cosmic Rays) however it would seem that filtering out background radiation using a simple coincidence detector is problematic due to the Geiger–Müller tube response and decay time (Pulse Width) when an ionising particle has been 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.
Coincidence
Although coincidence implies simultaneously, in reality we are talking about almost simultaneous, this is because a muon created by a cosmic event is travelling at near the speed of light 0.998c, so if the detectors are only spaced 2.5cm apart the actual flight time of a muon would only be 0.08ns. However as the detector and electronics response and delay times are slower than this, we can say in "real-life terms" it is simultaneous.
Sonification of Muons via Arduino and Max/MSP
I loaned my 3 Tube Muon Detector to Sebastian Tomczak who has rigged it up for some experimental sonification.
More information available on his website Little-scale
Muon Detector using a Geiger–Müller Array within Lead Shielding
I am currently building a Geiger–Müller Array with the aim of exploiting an effect called Electromagnetic Cascade or Particle shower as a means of significantly increasing the effective aperture and reducing other issues identified in my DIY experiments.
Prototype 1. Muon Detector using a Geiger–Müller Array within Lead Shielding
The circuit for this design is not that dissimilar from my three tube Geiger–Müller array however this design has nine coincidence outputs which will be feed into a Arduino embedded CPU for further processing.
Prototype 1. Schematic Draft
It should be mentioned at this stage that this idea is very much an experiment and I really have no idea if it will really work. However if muons do induce an Electromagnetic Cascade in Lead as demonstrated in other experiments there should be an improvement compared to using Geiger–Müller tubes only on their own.
Prototype 1. Expected aperture and number of tubes using lead shielding compared to no lead shielding.
The other expected benefit using this method is that coincidence detection is also moved into the vertical where in a conventional detectors coincidence detection requires layers of detectors at the top and bottom.
Muon Detector using Scintillators and Photomultipliers
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 to 5 MEV and emits light between 420nM and 450nM (i.e. blue light)
The scintillator block will be coupled using Dowel Corning DC4 to a 10 stage photomultiplier BURLE S83020F which is very sensitive to light between 350nM to 500nM.
Dowel 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...
Muon Detector using Fluorescent Tubes
This project is deliberately aimed at developing a very low cost cosmic ray detector using common Fluorescent Tubes. It is 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. I found information about this on the CosmicRays.org website called a Spark Chamber.
There is another link here at the Teachers CERN Website at the bottom of their page, unfortunately there is little/no information about how this actually works, but it should be fun finding out.
(C) www.cosmicrays.org
I am also building a detector using Geiger–Müller Tubes as a test unit to use as a standard to measure the performance of the Fluorescent Tube Detector against. It will also help resolve some issues identified when using Fluorescent Tubes to fine tune the final design...
Below is what I have done so far:
Prototype 1 - Detector using fluorescent tubes (Very Unstable)
Note: This prototype is too unstable and not recommended.
First Prototype demonstrated at Dorkbot Meeting
My first detector prototype was not that dissimilar to the CERN example, except the fluorescent tubes are placed between three metal plates. The outer plates are connected together by bolts and connected to the Negative rail of the supply and the centre plate is insulated by the fluorescent tubes and connected to the Positive rail of the supply. So far I have found the best result with small 6W fluorescent tubes is around 650V DC
First Prototype Built
Like the CERN example, when a muon flys through the fluorescent tube, the gas inside ionizes due to the high voltage field across the plates. As a result of the ionization the resistance across the plates will fall slightly and so it should be possible to measure this as a change in current flow in the high voltage source.
Schematic of first tests.
The reason for two rows of fluorescent tubes is to sense the crude presents of coincidence occurring in the top and bottom rows of fluorescent tubes due to a muon flying through both. I'm speculating that the resistance in the detector should be half compared with only one row detecting something, due to terrestrial noise. If the output is feed into a data logger and also speculate that over time the difference between cosmic and terrestrial detections could be filtered.
Off on a tangent again.
I couldn't help noticing the similarity with flash tubes and other types of gas filled trigger electronics like a Thyratron thermionic valve. Basically these tubes are biased at a voltage below ionization and when a high voltage trigger is applied briefly in the gas path between the Cathode and Anode, the gas to within the tube ionizes, the resistance to falls rapidly between the Cathode and Anode and like SCR current flows until power is removed.
Consequently I tried biasing the individual fluorescent tubes using their standard electrodes with a DC voltage somewhere below their point of ionization ~70V through a high impedance RC network. The RC network preventing sustained ionization, so producing just a pulse.
Schematic of trigger experiments various component values where tried
However, to my surprise I got quite the opposite, as I measured a voltage spike across the electrodes rather than a dip and so it would seem biasing may not be required as a strong positive spike can be clearly observed on a CRO without any biasing.
Schematic of experiment first tried and demonstrated at a Dorkbot meeting.
Summary
Nevertheless, even though "something" is causing clear observable pulses on a CRO in all variations tested above, it is difficult to confirm they are actually due to Cosmic Rays or Terrestrial Radiation over something like coronal discharges within the tube itself.
All attempts to find RFI sources have drawn a blank as pulses disappeared when the high voltage supply was switched off, other Electrical Interference has also been ruled out shielding inside a metal box. I also ruled out the supply itself without the detector and could not find any other interference sources.
I should also note that early in my building and testing of these ideas, I found that most HV supplies I built had quite allot of noise or ripple present, specially the type often recommended for Geiger Counters, so I spent quite a bit of time trying to eliminate this, with improved voltage regulation and a good bank of capacitors.
Results
Tests with an xray source have confirmed the system dose detect radiation, however once the gas inside the tube ionizes, spurious pulses re-occur randomly after, which I suspect is caused by photons being emitted inside the tube causing new avalanches occur. Increasing the impedance of the high voltage supply and placing a discharge resistor in circuit does reduce some of the problem, but this also decreases the output signal. Also I have moved away from using the filament electrodes of the lamp, although this also detects radiation successfully with a high output voltage it also significantly increases the problem of oscillation and other spurious pulses.
So I have moved to a new improvement prototype with better coupling and RFI controls see: Prototype 2 for details.
Radioactive Source for testing a detector
In the process of developing my low-cost Cosmic Ray Detector (Muon Detector) it has become increasingly clear that I first should confirm that a common fluorescent lamp can be used to detect radioactive particles when placed between a high voltage electric field.
Although the idea has been demonstrated in a simple experiment describe at the High School Teachers CERN lab and the CosmicRays.org website, the detector described relies solely on visual confirmation of faint flashes over multiple tubes.
Where my muon detector relies on the assumption that a current draw can be measured in the high voltage supply, when the gas inside the fluorescent tube ionises. Consequently, it will be very important to see if under controlled conditions a radioactive particle will trigger such an effect.
Basic Test Schematic.
Unfortunately, I haven’t yet been able to source a radioactive sample to use in a test, as there seems legal restrictions in Australia that prevent amateur experimenter owning such samples, nor putting them through the post even with a weak level of radiation.
I have even visited numerous antique stores with my Geiger Counter looking for uranium based ceramics or glass without success, well nothing that is good enough for testing. But I have had many strange looks and questions when I pull out my pocket Geiger Counter.
So I build a low-level x-ray source using a Vacuum Tube and high voltage ~50KV. Although slightly more dangerous an xray source can be switched on and off at the flick of a switch which has een bvery useful for testing the detector.
Prototype 2 - Detector using fluorescent tubes (improved still unstable)
Note: This project is still very much a work in progress and there are several issues to iron-out, if you have any questions or comments to contribute please feel free to contact me.
I found an easier and more effective way to capacitively couple the High Voltage supply to the gas inside the fluorescent tube using self-adhesive copper tape either side of the tube.
Prototype 2 initial test rig connections
This allows easier wiring and mounting multiple tubes within an enclosure and may also allow for the building of a larger array detector. First I have built a simple two tube detector and after some experimentation I have found a few more refinements to implement into the next prototype coming soon.
Simple two tube prototype in testing
I am currently building a unit Detector Using Geiger–Müller Tubes as a test unit to use as a standard to measure the performance of the Fluorescent Tube Detector against. This will greatly help resolve issues and fine tune the design...
Electrical characteristics of the fluorescent tube detector.
The resistance of the gas within a fluorescent lamp is virtually an open circuit at room temperature until a sufficient electric field is applied where the gas atoms absorb enough energy to emit a free electrons resulting in ionization.
Once the gas is ionised the electrical characteristics of the fluorescent lamp changes and begins to conduct electricity with a negative differential resistance and so more current flows. Consequently, the electrical resistance of the fluorescent lamp drops dramatically from many mega ohms to hundreds of ohms.
Ionisation of the gas within fluorescent lamp occurs at around 300 to 600V depending on the size of tube, environmental factors and the type of gas mixture used by the manufacturer this is usually a low pressure mercury vapour and a mixture of either argon, xenon, neon, or krypton.
In the cosmic ray (muon) detector the electrodes are not in direct contact with the gas inside the tube but capacitive coupled through the glass wall as identified in Diagram 1.
Where C1 represents the capacitive effect of these electrodes and R1 in series with VD represent the gas inside the tube. CP represents the parallel capacitance of the overall coupling effect from the two electrodes across the entire tube.
When a high voltage is applied E (~600 to 1200V) across the circuit it creates an electrostatic field within the gas and when this charge reaches sufficient energy the gas ionises (emitting a flash of light) causing the field to collapses and discharging C1 until the charge can build once again. If not quenched rapidly or the voltage applied is too high the circuit forms a basic relaxation oscillator and the process repeats itself rapidly proportional to the time constant of C1 and R1.
In the detector configuration in below the voltage E is set at a point just below ionisation.
!he 1M Resistor forms a series LC network with the tube capacitance C1 ensuring that only a small amount of energy is stored which can be easily quenched shortly after being triggered. The 4 10Meg Resistors also play an important role by reducing parallel capacitance, preventing the voltage across the tube C1 to run away due to the high impedance of the supply causing oscillation and also ensures a zero bias to the filaments of the fluorescent tube reducing the production of spurious pulses and discharges within the tube.
Diagram 2
Consequently, when a Muon passes through the tube, some of the gas molecules are ionised, creating positively charged ions, and electrons. The strong electric field created by the electrodes accelerate the ions towards the negative side of the tube wall and the electrons towards the positive side of the tube walls. The ion pairs should gain sufficient energy to ionise the gas further through collisions along the way, creating an avalanche of charged particles and discharging the energy in C1 resulting in a short pulse of current which can be measured across the 1 Meg resistor as a negative travelling pulse of voltage, not dissimilar to a Geiger-Müller tube.
Issues using Fluorescent Tubes
I have been testing a number of different design variations and have identified the following issues.
1) Power supply requires good filtering and regulation - Completed High Voltage Regulated Power Supply
2) Tubes vary in voltage requirements from one tube to another even between the same make, model and age
3) Oscillation is a problem as the supply voltage and/or coupling plate surface area increase
4) Internal filament electrodes must be insulated, even loose coupling increases oscillation and spurious pulses
5) Coupling plates should be positioned back 1cm from the tip of the internal filament electrodes
6) Oscillation occurs as the circuit forms a basic relaxation oscillator
Although oscillation is an unwanted artifact, it would also seem there is a point before oscillation begins where the tube increases sensitivity to radiation as the voltage increase and approaches a point where oscillation begins. However, radiation (cosmic or terrestrial) is also what first triggers the tube to jump into an unstable state before free oscillation begins.
Nevertheless, I will investigate this further to see how oscillation could be regulated through some form of negative feedback or quenching circuit as this may yield useful results.
Compton Scattering
Tests using a Geiger–Müller array detector have revealed a problem which will equally effect fluorescent tubes detector called Compton Scattering, this is 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 a high energy photon into the adjacent detector causing a false coincidence detection. In other words this causes cross-talk interference between Detector Tubes
Fluorescent Tube Detector Electronics
The electronics component of the detector is an important part of a Cosmic Ray Telescope and comes in two main parts.
Detection CircuitOne of the problems with building a detector using fluorescent tube is that although it functions very much like a Geiger-Müller tube the voltage is higher; the signal has a very high output impedance and is capacitive coupled. So the following considerations needed to be included in the circuit design:
- The amplifier section must tolerate high voltages and have a high input impedance.
- Capacitive coupling introduces an AC signal characteristics and amplitude variations, so the output needs shaping through a Schmitt trigger for better stability and noise immunity.
- The high voltage power supply must have good filtering and be regulated to decrease amplitude variations and introduction of noise.
The main problem with the physical muon detector is that in essence it is a radiation detector and along with muons showering down from the skies there also equal amounts of terrestrial radiation present in the environment including the detector itself. Although this is in very small quantities it is sufficient to make it difficult to discriminate between a terrestrial or cosmic events.
However, a muon does have sufficient energy to pass through the physical detector easily, whereas terrestrial radiation will not. Therefore anything detected in two or more detectors placed one above the other, simultaneously (coincidence) is almost certainly a cosmic event. Consequently, having electronics that can measure coincidence across two or more detectors is essential.
To do this task the output of the Schmitt Trigger is feed into a logic gate eg: AND Gate where the output could be captured by a data logger recording counts over time or other devices.
Pulse Shortening
Tests using other more reliable detector systems such as Geiger–Müller Tubes have shown that a simple coincidence detector using a AND Gate by itself is problematic due to the response and decay time (Pulse Width) of the detector when an ionising particle has been 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.
Electromagnetic Cascade Induction
I am currently investigating an effect called a Electromagnetic Cascade or Particle shower as a means of increasing the aperture of a Cosmic Ray (Muon) Detector.
In a 1964 publication on the historical understanding of the nature of the cosmic radiation, the scientist Bruno Rossi describes 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 many as 35 per hour as the thickness of lead increased peaking at 1.5cm and then falling slowly.
Shower curve obtained by Bruno Rossi’s experiment
When the upper part of the lead shielding was removed the rate fell to about 2 detections per hour. This effect has now been confirmed to be a direct result a Particle showers induced within the lead as the result of a high energy muon passing through triggering an avalanche of electrons, positrons and gamma rays.
Figure 2. Muon induced electromagnetic cascades in lead plate.
A demonstration of cosmic ray induced electromagnetic cascade was also shown to work by Peter Dunne, Preston College, Preston, Lancashire, UK
Cosmic Ray effects on Earths' climate
From the available research their is no convincing evidence that confirm cosmic rays are 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 physicist 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 the amount of 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.
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For the record, the Administrator of this website agrees with the findings of the IPCC assessment of the scientific, technical and socio-economic information relevant for the understanding of the risk of human-induced climate change.