The development of a Cosmic Ray (Muon) Detector Kit has come about as a result of tinkering with Cosmic Ray detectors over the past 10 years. I’m regularly approached by people wanting me to construct them or needing assistance. I have also collaborated with musicians, artists, educators, and enthusiasts alike. Through this process gaining some valuable experience building detectors. However, juggling new projects with job and family life pose constraints.
Consequently, I’ve recognised the need to develop a straightforward detector kit that empowers people to construct and customise their own detectors for whatever thier intentions. Interest, Education, Art, or just wacky nonsense. My reasons: I just love tinkering, I have an interest in science, and as described on the about page of this website.
The design of this kit is based around thin-walled Gamma Sensitive Geiger–Müller tubes. A technology that has been embraced by many astrophysicists and educators since the early 20th century when Cosmic Rays were initially identified. Despite the evolution of alternative detection methods involving photomultipliers coupled with scintillators, the Geiger–Müller tube in my opion currently persists as the most economical and accessible technology for Do-It-Yourself (DiY) projects.
A Geiger–Müller tube (GMT) typically functions at a high voltage around 400V DC or even higher, depending on the manufacturer. There are many low-cost versions available, such as surplus ex-Soviet models (as below) and other more modern Chinese manufacturers, most of which operate within the range of 350 to 450V. Despite this elevated voltage, these tubes consume very little power, operating in the microamp region since they operate in series with a high-value resistor typically around 10 Megaohms.
The bare minimum requirement for any cosmic ray detector involves employing at least two GMT tubes enclosed inside a non-ferrous metal shielding and a coincidence circuit. This will effectively filter out any low-energy terrestrial background radiation from Cosmic Ray initiated Muons.
The output impedance of a basic GMT circuit is very high and so is also susceptible to noise from RFI and static. Therfore it is crucial to use suitable shielding and a regulated low-noise power supply to ensure the reliability. To meet this requirement, I have designed a power supply using the Analog Devices LT8365 Low IQ Boost Converter and voltage multiplier with an adjustable feedback circuit. This allows the GMT voltage to be set at a level that multiple GMT can be trigger consistently and also provides compatibility with various GMT manufacturer voltages, enhancing the flexibility and adaptability of the design.
Setting the correct voltage is very important as when an ionizing particle traverses the GMT, the gas inside ionises and so the resistance across the Anode and Cathode rapidly decreases and then snaps back to its initial state. The happens within a specific voltage range, too low voltage and no ionising occurs, and too high and ionisation will not quench, possibly damaging the tube.
Another critical aspect to a detector circuit involves the conversion of the pulse generated by a Geiger–Müller tube (GMT) down to a usable level to use within low voltage logic circuits. At the juncture where the high-value resistor ~10M and GMT connect in series, a voltage drop will occur with regards to ground. At this point a low-value capacitor ~20pF is coupled to the input of an Inverting Schmitt Trigger which has been bias high (1) with another high value resistor ~100K at the Logic circuits supply voltage +VCC meaning the output of the Schmitt Trigger will be Low (0). When the negative pulse of the GMT is seen by the Schmitt Trigger it will register as Low (0) and and the output will be 1, for the duration negative traveling transition.
Another aspect of the a cosmic ray detector measuring a coincidence of pulses from the two GMT. Now that we have logic level pulses it is a simple matter of using an AND Gate.
Named a “coincidence circuit” this invention emerged in 1930, courtesy of Bruno Rossi, an Italian experimental physicist captivated by the mysteries of Cosmic Rays. Motivated by a desire for a more practical and dependable method of measuring these phenomena, Although this idea may seem simple Rossi’s invention marked a significant advancement and not just in cosmic ray detection.
The main concept behind a ‘coincidence circuit‘ in signal processing is that if a detector identifies a signal pulse amidst the random noise pulses inherent in the detector and environment, there is a certain probability that the detected pulse is actually a noise pulse. However, if two detectors simultaneously detect the same signal pulse, the probability that it is a noise pulse in both detectors is statistically squared. The rate of background radiation in most locations is very low, only a few counts per minute, but with the addition of shielding, this rate is further reduced. Consequently, the likelihood of coincidence detection being cosmic ray muons rather than background radiation is significantly increased.
Shielding in a Cosmic Ray Muon Detector
Use non-ferrous, non-magnetic metals to avoid the potential of deflecting charged particals. Lead, Aluminum, or Copper being viable options. Copper, in particular, is a favorable choice due to its ease of workability, readily available in many tube diameters, electrical conductivity reducing radio frequency interference (RFI), and lower toxicity than lead.
Shielding is mainly used to increase the statistical difference of noise in the coincidence circuit. With 2 main aims: 1) To differentiate the noise within each Geiger–Müller tube due to natural radioactive decay in matter e.g. gasses, glass and metals surrounding the detector and materials used to manufacture Geiger–Müller tube. 2) Cosmic Ray muons start with very high energy and speed, having the capacity to ionise many atoms before their energy is exhausted. Shielding in combination with the coincidence circuit helps to differentiate between slow moving particles from background radiation and muons which will pass through any shielding applied.
It is very easy to shield detectors from Alfa radiation which is most common in our environment. Beta radiation can be stopped with conductive metals like aluminum or copper. Gamma radiation being very penetrative is difficult to filter, however it occurs at very low count rates, random times and directions. Consequently this noise is very low compared to Cosmic Ray muons and so can be relativelly ignored.
Help me make this happen sooner
This project is still a work in progress, depentant time and funds. I’m hopping to offer 4 basic options:
1. Open Source instructions, parts lists, cad files etc.
2. PCB and Component Kits
3. A populated assembled PCB Kit
4. Custom built variations for people with interesting projects, where I have the time
Up to now, I’ve personally financed all the prototyping expenses from my own resources. Should you wish to contribute financially, regardless of the amount, your support would be greatly appreciated. I want to let you know that this endeavour is not driven by commercial interests, as outlined earlier. For more extensive endorsements of this project, you will receive exclusive access to CAD files and Part Lists, securing your priority status when ordering a kit. Regardless of how modest, I want you to know that your help will bring this initiative to fruition.
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