Given all my previous experience with Geiger Counters (see for example my uRadMonitor project), I have decided to conceive a portable, easy to build, radiation dosimeter . I didn't have much need for yet another dosimeter (with my Radex 1706 or Terra-P), but I know there are several people not affording a commercial unit, but wanting to have their own radiation measurement tool. So here is this article, giving you just that: an efficient, stable, easy to build construction for all your radiation dosimetry needs.
I started from scratch, designing a complete dosimeter unit around the Atmel Atmega8 microcontroller and a Russian Geiger Muller tube. Here you'll see the CTC-1 tube, for high gamma doses, but the dosimeter can be used with any other tubes such as SBM-20, LND-712 or more sensitive ones such as the SBM-19 or the pancake tube SI-14B. Changing the tube requires changing the software, to adjust the dose conversion calculation. This circuit can be used with almost any geiger tube, as even the inverter's output voltage driving the tube is adjustable in the software.
Here is a video showing a high count rate when using a Si12b tube and Am241 alpha emitter from a smoke detector:
Similar to my uRADMonitor, the micro-controller takes care of everything:
1. Generates a variable duty PWM signal using Timer1, to drive the 400V inverter needed to operate the Geiger tube; The inverter doesn't need a multiplier, as the ferrite's transformer secondary puts out exactly the amount required. The transformer is made on a A22 ferrite core, with 16 turns in the primary and 600 in the secondary.
2. Uses one ADC port to measure the inverter's voltage and adjust the PWM duty cycle for constant output (exactly 400V for stable operation)
3. Counts the time, using Timer0 so we can compute the dose
4. Uses interrupt INT0 to count the pulses produced by the Geiger tube
5. Drives a 2x16 LCD to output the results.
Some other circuits on the Internet come with improper 400V inverters (some people seem not to be able to design a proper inverter), they are redundant (using 555's and additional components, when the microcontroller can take care of EVERYTHING), use the wrong signal detection/counter circuit, or other small defects that result in wrong measurements. Not to mention the complicated aspect of computing dose in sieverts out of counts per minute.
Given all these wrong designs, my detector tries to fill in some of the gaps. So here is exactly what you need: a stable design, with several improvements made over time, all packed in this nice construction that you can easily replicate.
Before getting into the construction details, and the theory involved in my geiger counter, here are some pictures of my construction. I might use them later to point out critical aspects of the construction.
Building the Ferrite transformer
Building the case
And the result
Besides using an external power adapter, I added rechargeable batteries to this device so it can operate on independently. 4 AAA NiMH batteries have been packed together using my micro-spot capacitor welder, and inserted in the bottom part of the case.
Bluetooth Module and remote radiation monitor capabilities - optional
You might be familiar with the The CD V-717 that was designed for use in fallout monitoring stations. This detector has a removable bottom with a 25 foot extension cable. The detector element (ion-chamber) is mounted inside the removable bottom. This allows for the placement of the detector element outside of the shelter area while the metering section of the metering unit would remain inside of the shelter area connected to the detector with the 25 foot cable.
In a similar fashion remote operation is possible on my dosimeter, by using... Bluetooth. Hooked to the main microcontroller, there is an UART Bluetooth module packed with its 3.3V regulator. The software running on the atmega8, sends measured dose rate values, via UART (Rx/Tx) to the Bluetooth module.
By doing so, a mobile phone can be used to read the radiation data, from a remote location, without being exposed to what the dosimeter actually measures. Currently I wrote an application for Android phones, that can search nearby bluetooth modules, find the dosimeter, connect to it and display the dose rate, as received via the radio connection. Here is a demo video:
For the Android software, see the bottom of this article.
Showing the Dosimeter on TVR Timisoara
Thanks to Andrei Borosovici, I had the chance to discuss this topic on "Vezi ce-ti doresti" TV Show running on TVR Timisoara. Here is the 14.01.2013 recording, in Romanian:
More on Geiger Counters
Here is a list with must-read resources:
uRadMonitor - a Geiger counter radiation monitoring station (v3.3)
Centronic Geiger Muller tubes
A simple Geiger counter (v3.2)
This work is free software, licensed under GPL v2; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. For other licensing options, feel free to contact me.
License for using my work
HEX Firmware for the Atmega-8
Source code and other resources
Eagle SCH and PCB: download
Dosimeter source code: The code is available on Github, under GPL. Use it only if you understand the terms of Open Source software distributed under GPL.
You can also download it here.
Optional, if UART Bluetooth module has been added to the dosimeter: Bluetooth Dosimeter Android Application
Here are some variants created by my readers
2013-11-22: black case and software improvements
This device has received an upgrade, improving its functionality by rewriting the code to embed the latest results form the uRADMonitor project. See it here.