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 a method 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

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 raw audio output from the detector 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.

Midi Convertor Circuit

electronics inside midiconvertor

Midiconvertor

Korg NanoKey2 USB Midi Keyboardhttp://www.korg.com/us/products/controllers/nanokey2/

Inside Midiconvertor

Why monitor coincidence?

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.

What are 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...

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