The Hidden Landscape of Radioactivity
Test for Radioactive Contamination
If you’re testing for contamination, your GM tube should be sensitive to alpha radiation as well as beta and gamma (like the GMT-01).
Geiger counters can only test for gross levels of contamination that show up clearly above background radiation; they are not the proper instruments for detecting low-level contamination. That said, here is how to test for radioactive levels above background:
- Establish the background radiation level by measuring CPM for at least 20 minutes, and longer is better. Note the lowest and highest levels and then average them all to establish the baseline minimum, maximum, and average. With my data from Step 4, a 298-minute sample, my average CPM was 16, and min to max was 6 to 28.
- Position the GM tube very close to the top surface of the material you’re testing, and run the counter, recording the CPM output. The longer the run, the more accurate the results.
- Compare the radiation output of your sample against your baseline.
Alternative Sound Output
If you want louder clicks from your Geiger counter, you can use the alternative sound output circuit; see the schematic at http://makeprojects.com/v/29. In this circuit, the pulse signal from the comparator triggers a 555 timer chip, which is set up in monostable mode to stretch out the pulse it receives on its trigger. The output pulse from the timer flashes the LED and outputs an audible click to the speaker via pin 3.
The alternative circuit’s components (555 chip, caps, resistors) fit on a small breadboard. You can connect it to one of the main board’s digital outputs, or to where the standard light and sound output connects (LM339 pin 14), either replacing the original output or making it switchable.
Digital Meter Adapter
You can do more with your Geiger counter by wiring its digital output to a standard 3.5mm mono earphone jack, connecting ground to the sleeve contact and signal to the tip.
This output jack lets you connect the meter to my company’s Digital Meter Adapter (DMAD) an add-on that shows CPM or counts per second (CPS) along with milliroentgens per hour (mR/hr). Onboard switches also configure the adapter to work as a true random number generator, with multiple ranges. The DMAD (see inset, opposite) has a second 3.5mm plug for output, which lets you connect any of its output functions to a computer via 3.5mm to RS-232 serial cable.
Want to help keep track of radiation levels nationwide? A 3.5mm output jack, as connected to the Digital Meter Adapter above, will also let your Geiger counter become a monitoring station for the Radiation Network (http://radiationnetwork.com), home of the National Radiation Map. The network software is sold with an adapter cable ($79 together).
For schematics and a template for an acrylic enclosure, see http://makeprojects.com/v/29.
Caution: The Author and Publisher do not make any warranties (express or implied) about the radiation information pro-vided here for your use. All information provided should be considered experimental. Safety and health issues and concerns involving radioactive contamination should be addressed, confirmed, and verified with local and national government organizations or recognized experts in this field.
This project first appeared in MAKE Volume 29, pages 100-111.