Cosmic Ray Detectors

Cosmic Rays

These projects where conceived from an interest in Radio Astronomy and Particle physics. I’ve now come to see cosmic ray detection as kind of poor man’s version of particle physics experiments.  As the journey of construction, experimentation and interpretation of the results has meant learning a little bit more about the fascinating world of particles and forces that make up the universe. 

If you would like to discuss ideas and your interest in Cosmic Rays with me and others here are some social media sites I also admin:

What are Cosmic Rays?

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 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.

Theory of Detecting Cosmic Rays

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.

directional coincidence

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 Muon (cosmic ray) 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 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.

Cosmic Ray (Muon) 18 Tube Drift Hodoscope

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.

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

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.
 
New version Cosmic Ray Hodoscope
Second Prototype still more work to do 24th March 2012, but getting there.
 
Rough Layout
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 array
Schematic of 400V and 5 V power supply
 
9 Channel Detector Circuit

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.  

9 Channel Detector PCB

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

 
Geiger–Müller Array
20 wired GMTs installed between two plastic brackets which was changed to 18 for practical reasons.
 
Bracket for array
Drilled completed bracket for holding GMTs

 

Bracket for array
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

Cosmic Ray (Muon) 81 (9x9) Pixel Hodoscope

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.

Matrix of GM Tubes

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

Overview circuit

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

9 Channel Detector Circuit

Figure 2. 9 Channel Geiger–Müller converter to 5V TTL

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.

I made this PCB very generic so the design could be used in other projects like the 18 tube Drift Hodoscope.

9 Channel Detector PCB

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

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

Case Wired
Complete Assembly with labling
 
Inside electronics
Complete wiring inside Box Arduino Mega PCP (not yet used) mounted on top of the passive parallel voltage summing circuit see figure 3.
 
Green LED Matrix
New Green 81 Pixel (9x9) LED Matrix Display Enclosure Inside
 
Green LED Matrix Outside
New Green 81 Pixel (9x9) LED Matrix Display Enclosure Outside
 
LED Matrix Display
Old Blue 81 Pixel (9x9) LED Matrix Display Enclosure Outside
 
Installing LED Matrix
Old Blue Inside the 81 Pixel (9x9) LED Matrix Display Enclosure
 
LED Matrix Display
81 Pixel (9x9) LED Matrix Display
 
Wiring Up
Wiring of each Geiger–Müller Tube
 
Connectors
Geiger–Müller Tube Anode and Cathode Clips each tube has been wired with LDR316 to reduce RFI
 
9 GMT Row
Basic arrangement of the tubes
 

Bottom Layer

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

Radiation Sheild

Assembled with shielding

Cosmic Ray (Muon) 81 Pixel Hodoscope with Midi Sound

The raw audio output from the Cosmic Ray (Muon) 81 (9x9) Pixel Hodoscope was a little hard on the ears so in an attempt to make this more pleasant, I modified the 9 x 9 matrix output by dividing into a 3 x 3 output using triple input NAND gates (74LS10) then monitoring coincidence between the resulting 3 x 3 matrix using AND gates (74LS08) to convert it to 9 channels, in order to drive a hacked MIDI Korg Nanokey 2 MIDI controller.

electronics inside midiconvertor

To do this, I modified the 9 x 9 matrix output by dividing into a 3 x 3 output using triple input NAND gates (74LS10) then monitoring coincidence between the resulting 3 x 3 matrix using AND gates (74LS08) to convert it to 9 channels in order to drive a MIDI keyboard.

Midi Convertor Circuit

Korg NanoKey2 USB Midi Keyboard
Hacked MIDI Korg Nanokey 2 MIDI controller

Midiconvertor

Cosmic Ray Pixel Hodoscope with generative graphics and music

My Cosmic Ray (Muon) Hodoscope now produces live generative graphics and music using an Arduino Mega and Ethernet Sheild over a network to a computer running software called Processing (PDE) and MaxMSP.

Arduino-Mega

Arduino-Mega Pin outs

Inside Hodoscope

Many thanks go to Sacha Panic for his enthusiasm for cosmic 
ray detection and talented imagination, time and skill in 
coding and graphics to make this work on my detector. 

I would also like to thank Luke Stark who has developed 
some wonderful music using MAX/MSP.

Code & Graphics by Sacha Panic:
http://omnime.blogspot.com

Music by Luke Stark
http://lukestark.com

Software 
arduino
Processing
MAX/MSP

Detector Hardware
Robert Hart 
Cocmic Ray Hodoscope
http://www.hardhack.com.au

Cosmic Ray (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.

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.

 

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

Cosmic Ray (Muon) Detector using Geiger–Müller tubes within lead shielding

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.

 

Muon Detector using Geiger–Müller Array and Lead Sheilding
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.

 

Muon Detector using a Geiger–Müller Array in  Lead Shielding
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.

 

Muon Detector using a Geiger–Müller Array in  Lead Shielding
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.

 

Geiger–Müller Array Schematic
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.

5V TTL to USB PCB

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.

 

 

Geiger–Müller Array prototype
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.

Geiger–Müller Tube SBM 20

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

Cosmic Ray (Muon) Detector using Si Pin Photodiode

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.

Cosmic Ray Draft Mechanical Layout

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.

 

Pin Diode Array Amplifier Stage PCB

Draft PCB of Pin Diode amplifier section

Si Pin Photo_Doide Array

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

Array covered with light-proof shielding

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

Cosmic Ray Neutron Monitor

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

Russian Neutron Pulse Corona boron Counter SI-19N

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. Moderator20mm 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

Neutron Monitor Layout

Draft Neutron Detector Circuit

Boron Coated Cathode Corona Pulse Neutron Tube SI-19N

Russian Neutron Pulse Corona boron Counter 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

 

Cosmic Ray (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 and higher and emits light between 420nM and 450nM (i.e. blue light)

Plastic scintillator Block BC412

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.

10 stage photomultiplier BURLE S83020F

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.

Layout sketch of muon telescope
Proposed layout of Prototype Version 2

More information soon...

Cosmic Ray (Muon) Detector using Fluorescent Tubes

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 identified in my design using Fluorescent Tubes below is what I had done so far.

Prototype 1 - Detector using fluorescent tubes (Very Unstable)

Note: This prototype is too unstable and not recommended.

Cosmic Rays with fluorescent tubes
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

Cosmic Rays with fluorescent tubes 0
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.

Cosmic Rays with fluorescent tubes 1
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.

Cosmic Ray Detector 1
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.

Cosmic Ray Detector 1

Cosmic Ray Detector 1
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

Radioactivity for testing

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.

Cosmic Rays with fluorescent 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.

Cosmic Rays with fluorescent tubes 1
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.

Basic detector circuit

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.

Coupling HV with fluorescent tube gas
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.

Coupling HV with fluorescent tube gas
Simple two tube prototype in testing

Coupling HV with fluorescent tube gas

Coupling HV with fluorescent tube gas

Coupling HV with fluorescent tube gas

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.

ionization

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.

Basic Circuit

When a high voltage is applied E (~750 to 1800V) 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. The 1M Resistor  (Note experiment with R values) forms a series RC 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 (Note experiment with values) 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.

Basic detector circuit
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.

Inside the 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 Circuit

One 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:

  1. The amplifier section must tolerate high voltages and have a high input impedance.
  2. 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.
  3. The high voltage power supply must have good filtering and be regulated to decrease amplitude variations and introduction of noise.
Coincidence Circuit

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.

directional coincidence

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.

Cosmic Ray induced radio emissions

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.

Further due to the relativistic motion of these radio emissions they are concentrated into a beam in a forward direction so the footprint of the radio signal is concentrated around the particle shower by only several hundred meters. This makes the method of particle showers detection an ideal companion for other traditional particle shower observations by providing reinforcing information about the trajectory and energy of the incoming shower. 
 
As the number of particles in the air shower and peak radio signal correlate with the energy of the original primary particle and so the radio emission increases with the angle between the particle showers trajectory and the Earth’s magnetic field.

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.