Solid State Cosmic Ray Detector

Photodiode based, coincidence counting cosmic ray detector designed for HAB experiments

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Cosmic ray detectors are a common physics experiment and frequently flown on balloons to measure the change in cosmic ray flux with altitude. These experiments are typically performed with off the shelf Geiger–Müller tube based detectors. G-M tubes are typically quite expensive and involve high voltages which can become difficult to manage in the near vacuum experienced with high altitude balloons. A reverse biased photodiode can be used in much the same way as a G-M tube but with lower cost, lower voltage, almost no dead time, and the additional benefit of an output voltage proportional to the radiation energy.
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Cosmic rays are extremely high energy radiation that come from outside our solar system. Cosmic rays with energies up to 300 EeV have been detected[1], however the peak energy density is about 0.3 GeV[2]. Even these lower energy particles have vastly more energy than even the LHC can produce at 4 TeV[3]. These cosmic particles are primarily (99%) atomic nuclei stripped of their electron shells, with the vast majority (90%) being simple protons[4].

These energetic particles collide with other molecules when they enter the Earth’s atmosphere. This collision produces a cascade of secondary particles within about a degree of the original trajectory. These secondary particles include muons, protons, alphas, pions, electrons and neutrons[5]. The primary component (over half) of these air showers at sea level is muons resulting from the decay of pions and kaons (typically within meters).

Cosmic ray flux varies with altitude, latitude/longitude and azimuth angle. Early experiments with cosmic ray detectors confirmed the change in flux with altitude, indicating their extraterrestrial origin[7]. The flux increases with altitude with a peak around 15km but then falls off rapidly. Because of their charged nature, cosmic rays are deflected toward the poles by Earth’s geomagnetic field resulting in the change in flux vs latitude[6]. The charge of the particles also causes a change in flux vs azimuth, with the positive charge leading to increased flux from the west[8].


  1. Nerlich, Steve (12 June 2011). "Astronomy Without A Telescope – Oh-My-God Particles". Universe Today. Universe Today.
  2. Nave, Carl R. "Cosmic rays". HyperPhysics Concepts. Georgia State University.
  3. "Facts and figures". The LHC. European Organization for Nuclear Research. 2008.
  4. "What are cosmic rays?". NASA, Goddard Space Flight Center.
  5. Morison, Ian (2008). Introduction to Astronomy and Cosmology. John Wiley & Sons. p. 198. ISBN 978-0-470-03333-3.
  6. Clay, J. (1927). "Title unknown". Proceedings of the Section of Sciences, Koninklijke Akademie van Wetenschappen te Amsterdam 30: 633.
  7. "Nobel Prize in Physics 1936 – Presentation Speech". 1936-12-10.
  8. Rossi, Bruno (August 1930). "On the Magnetic Deflection of Cosmic Rays". Physical Review 36 (3): 606.

  • KiCad updates

    Ethan Harstad01/20/2015 at 20:42 0 comments

    Now that KiCad has updated binaries for Windows I have decided to update my library to the new library format. This will involve rebuilding the schematic and the board but as soon as that is done it will be sent out to the fab and hopefully on a balloon in about a month.

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bill wrote 03/10/2018 at 16:50 point

I have three old GeLi detectors I am going to re-drift and put in series as a telescope. 

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Robert Hart wrote 01/23/2015 at 22:30 point

Hi, I have been working on something similar, I have been building cosmic ray detectors for some years now using Photo-multiplier/Sintillator, Spark Chambers and Geiger Muller tubes. At present I've been exploring a number of Pin Diode and amplified designs.

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Ethan Harstad wrote 01/23/2015 at 22:40 point

Hi, great to hear from you. I might suggest you take a look at SensL's so called silicon photomultipliers (SiPM) as well. They have some new detectors that are optimized for the blue output of cheap scintillators. I am also working on a much higher performance version of this that uses embedded fiber readout from large scintillator panels to one of these SensL SiPMs. Of course that is a much more expensive route ;)

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