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.

Cosmic Pixel


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.


The video above is the first operational test of the detector and the the audio track is achieved by a passive parallel voltage summing circuit below. The clicking sounds are the result of 1 millisecond pules coming from all 18 Geiger–Müller tubes as they detect an ionising particle. When an ionising particle passes through the brass plate and into a tube in the top layer, then through another brass plate into a tube in the bottom layer the LED flashes.  The LEDs are arranged to corresponds with the X and Y coordinates of tubes in the top and bottom layers see figure 1.

Parallel Voltage Summing Circuit
Figure 3. Passive parallel voltage summing circuit

To capture audio and provide a rough measure of signal in each of the two layers a passive parallel voltage summing circuit is connected to the output of each Geiger–Müller tube see figure 5.


This the signals coming from passive parallel voltage summing circuits see figure 3.
Each channel on the DSO represents the sum of detections coming from 9 GMT in each layer see figure 5.

schematic of array
Figure 4. Schematic of 400V and 5 V power supply
9 Channel Detector Circuit

Figure 5. 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

The audio output was a little hard on the ears so to make this more pleasant, I've 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 keybord.

Midi Convertor Circuit

electronics inside midiconvertor


Korg NanoKey2 USB Midi Keyboard

Inside Midiconvertor



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 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
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