Saturday, January 30, 2010

Geiger Counter kit for Near Space

I've started my experiments with the Electronics Goldmine geiger counter kit (C6979). The kit was on special for $70 (down by $10). It operates from a nine volt battery and uses a 555 timer and transformer to boost the voltage to 600 volts. The 555 timer operates at 128 Hz. So when there's a detection of a cosmic ray, there are six (some times five) pulses during the GM tube's dead time.

Gas molecules inside a GM tube become ionized at the passage of a subatomic particle. The ionized gas lets electrons, pushed by the high voltage on the tube, pass from the wall of the GM tube to the center conductor. This makes the GM tube act like a switch at the passage of a cosmic ray. While the tube remains ionized, it's unable to detect other radiation events. How quickly the GM tube clears out this ionization is called the tube's dead time. The shorter the dead time, the more frequently the tube can detect radiation. In near space, I have detected up to 800 counts per minute. On average then, there is 75 milliseconds between detections. As long as the GM tube's dead time is less than that, it should accurately detect radiation levels in near space.

I'd like to try placing a capacitor across the GM tube to smooth out the voltage spikes. If that works, then the flight computer doesn't have to divide the number of counts by six to get the real radiation levels. Perhaps it will also let the tube clear out faster (reducing its dead time).

It's a short video about my experiments to date. Look for an article in Nuts and Volts this year.

Onwards and Upwards,
Your Near Space Guide

video

2 comments:

  1. Did you have any trouble with the kit? I just put mine together and the led stays on and the speaker just hums. And I'm not living at Hanford! I measured the voltage across the tube and it's only 25 v. I suspect something is wrong with the boost circuit? The primary of the transformer is 9 v and the secondary is -5 v. I thought it might be the high frequency is causing my meter not to measure right? Any ideas or suggestions would be appreciated.

    bRANDON

    ReplyDelete
  2. Do you have access to an oscilloscope? The frequency of the 555 timer is around 150 Hz and a DMM may have trouble measuring the voltage properly.

    You checked that there were no solder shorts and backwards components?

    Another thing to check is that you have good contact between the wires and the GM tube body. Since you don't solder this part, the wires need to press firmly against the tube.

    ReplyDelete