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.

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

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

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 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

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

Assembled with shielding