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DIY Geiger Counter - Learn to build a Geiger Counter

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radhoo

Sat Dec 01 2012, 02:41PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

A short story of building a Geiger counter, from my first version 1.0 with its numerous errors and mistakes, to a final 3.2 version that follows several build indications to offer a reliable functionality.

Also see:
v3.3 - online Radiation monitor station (Geiger Counter with Ethernet connection)
v3.4 - portable Geiger Dosimeter (to be added soon)

______Version 3.2 May 25, 2012__________________________________________________ _________________________
1. The High voltage supply
The goal is to have a counter working at 3V (2xAA batteries) with very little power consumption. This excludes using 555 timers or other fancy ICs. My choice is a feedback based one transistor oscillator. It provides 400V output, for 100mA at 3V. I am using a ferrite core transformer, 17 turns for primary and 17 for the feedback coil, with 0.2mm CuEm wire. The secondary has 550turns with 0.1mm. The oscillator is operating at a frequency of aprox. 23KHz.
The transformer's output is rectified and multiplied by the 3 stages multiplier (10nF 1KV capacitors / BYV26E fast diode).
Note:A filter cpaacitor must be added parallel on the output, as Steve indicated in this thread.
2. The voltage regulator
The geiger tube requires very precise voltages to operate. For this setup, I need to get 400V from this supply, regardless of variations in the input voltage (weak batteries, etc).
This task is achieved by the string of Zeners and the Q2 transistor. When the voltage produced by the multiplier exceeds the Zener threshold , the Q2 transistor opens, and pull down the base of Q1, stopping the oscillation. By doing so, the output voltage remains at a constant level, that equals the Zener threshold. The multiplier must produce a little more than the voltage that triggers the zener string.
3. Connecting the Geiger tube
An essential component is the Anode resistor (10M in my circuit). The resistor's minimum value, for any given tube, is specified in the datasheet, but it is a good practice to use a higher value where possible.
The anode resistor is responsible of the following:
- reduces the anode voltage when the tube is conducting. This allows the tube to recover after the discharge. Without this, the tube would conduct continuously and would be destroyed quickly.
- it limits the current flowing through the tube, leading to longer tube life
- it has an effect on the plateau length, a lower value of R1 gives a shorter plateau length
4. Measuring circuits
With the cathode connection, the measuring circuit's task will be to count and evaluate the positive pulses connected at the cathode resistor R2.
For the circuit v3.2, we will collect the signal, in point A, at the emitter of transistor Q4. Here is how the pulses look like:
300x146

I was able to successfully connect it to my CD4026 digital counter , as seen in the video. It works great as a counter showing the individual pulses.

Circuit Diagram:

Here is a movie showing the counter in action, featuring several Geiger tubes I was able to test it with:

Nice and clean tube (SBM19) pulses as recorded by my oscilloscope:

Construction details, photos available here:

Thanks go to Centronics, for the excellent Introduction to Geiger Muller tubes:

______Version 2.0 January 8th, 2012__________________________________________________ _____________________
Note: obsolete , please do not build. See 3.2 or 3.3 instead
This geiger counter circuit features a regulated high voltage supply, with adjustable output. It can run on a single AA battery (1.5V). It is a clicker meaning that will indicate using an audio signal each detection event: a radiation particle passing through the detector tube will ionize the gases inside and trigger a very small discharge, that will be signaled by the electronic circuit composing the clicker. The video shows several geiger tubes tested with this device:

Circuit diagram:

______Version 1.0 March 21st, 2011__________________________________________________ _____________________
Note: obsolete , please do not build. See 3.2 or 3.3 instead
Probably the simplest geiger counter possible:

A feedback oscillator using a single NPN transistor, a small ferrite transformer, a small 3 stages multiplier, a few zeners to regulate the output to 400V. A small indicator module using a led and a speaker. A SI-22G geiger muller tube, with an excellent sensitivity.

Circuit diagram:

radhoo

Sat Dec 01 2012, 02:41PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

Circuit diagram [just a low power inverter (with MJE3055) and a small signal amplifier (with 2x2n5551) ]

radhoo

Sat Dec 01 2012, 02:42PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

A GM tube tester, or another simple GM counter:

Tubes presented in the video:
BG-08, G21H
SI22G, SBM19, SI14G

The setup can be connected to a counter, such as this one:

Will post a video on that later.

radhoo

Sat Dec 01 2012, 02:43PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

Proud Mary wrote ...

I see what you mean about the pancake detector. Generally, I would only expect the tube to light up like that with excessive anode voltage.

If this is the device with three elements, I suggest you connect the grid to a variable negative supply - say 0-100V - and increase the negative voltage until the continuous conduction is just extinguished, so light is only emitted at particle strikes.

This kind of continuous conduction is very damaging to GM tubes, so it would be best not to use it until you have the problem sorted out, or the tube may be ruined.

Sorry for the confusion, please take only the video part showing the SI14G as a reference (recorded with lights on, 3:23 - 3:47).
The last frames (beyond 3:47) are pictures added to the video, and they where recorded using very loooong exposures, to join all those tiny little neon discharges into the strong glow we see.
So it's not as bad as it looks, but
- at first the SI14G works as expected, and is very sensitive to any of my test source (including weak uranium glass)
- then gradually, even with the test source removed, the number of counts rises, until it reaches an impossible level, a point where I shut down the HV, to prevent any further damaging
- it appears this uncontrolled avalanche happens in a given region of the SI14g, where the electrodes also seem corroded by looking with the naked eye (probably over-voltage stress, or old tube)

Thanks for the advise with using the third electrode to temper down the uncontrolled discharges. It might help improving the tube's stability. I also tried lower voltages, bigger resistors, but didn't help.

radhoo

Sat Dec 01 2012, 02:43PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

Version 3.2 has been added to this thread - see first post update, including the video.

Here are some pics with it:

1346582324 1938 FT111231 Geiger Counter Homemade 1

1346582324 1938 FT111231 Geiger Counter Homemade 2

1346582324 1938 FT111231 Geiger Counter Homemade 3

It puts out clean , nice pulses, a good starting point for moving to a digital counter, using a microcontroller, that being what I am currently working on:
Geiger Pulse 1

Make sure you pay attention to the Mica window tubes, presented in the videos. They show the very nice counting rate capabilities of this circuit.

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