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Homemade digital Radiation Dosimeter

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radhoo

Tue Dec 25 2012, 09:31PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

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.

Short description
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.

Similar to my uRADMonitor, the microcontroller 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 invertor 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.

Some pictures

Circuit diagram:

Eagle SCH and PCB files:

I plan to add an UART Bluetooth module to enable remote operation (readings can be received on a phone running Android OS - I'll write a simple app for that), and a demo video - just didn't have the time to make it yet.

The software:
Here is the hex file needed to program the atmega8 :

. More versions will follow as the project evolves.

Proud Mary

Mon Dec 31 2012, 03:15PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

You are a good craftsman, Radhu!

One thing missing from your design, which we find on all modern pocket dosimeters, is an alarm function.

A good alarm default setting would be 10 Î¼Sv/hr, which is higher than any natural background radiation, except perhaps around a uranium mine, or geological exposure.

Ideally, the user should be able to set the alarm level themself, if the 10 Î¼Sv/hr is too high or too low for a particular application or environmental situation.

I found a dose rate of ~ 100 Î¼Sv/hr gamma inside an old uranium mine where it cut into a pitchblende (uraninite) seam. Of course, the actual dose rate must have been much higher, due to the concentration of alpha emitting radon which is always found in uranium mines, but could not be detected by my dosimeter.

Sulaiman

Mon Dec 31 2012, 06:49PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Excellent !
Almost got me sucked into yet more projects....
As above, nice craftsmanship .. I'd be very pleased with myself.

Just out of curiosity;
does the processing required for the hv psu add much/any error to the count accuracy?

radhoo

Thu Jan 17 2013, 03:16PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

Thank you, guys.

@Proud Mary: the alarm feature is possible: this dosimeter produces digital sound pulses: when the microcontroller detects a radiation event from the tube, it uses a digital pin to control the speaker, to re-create a very short pulse to signal the detection event. This principle can be used to sound an alarm, so all it takes is a modification in the software that I should be able to do somewhere soon.

Where do you find all those interesting mines to visit? :)

Having the user select the alarm threshold is also possible, via two push buttons (that need to be added somewhere in the case's surface), or via a pot , the latter would make a selection over a predefined interval.

@Sulaiman: the hv supply uses one of the uC's timers. All it is needed is a PWM signal with variable duty cycle. The duty cycle is used to control the voltage, which is also measured via another pin working as a digital voltmeter. If the voltage exceeds the threshold of 400V, the duty cycle gets decreased. The opposite for less than 400V.
Counting the pulses uses one of uC's interrupts. I didn't notice any issues such as interferences between the two, so far.

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:

Here is a video showing a high count rate when using a Si12b tube and Am241 alpha emitter from a smoke detector. Please disregard the uSv/h indication which is most likely incorrect for this particular case:

The dosimeter allows changing the tube, via the small white jumper. The built in STS-1 gets disconnected, and a connector can be used to attach another tube, like I did with the SI12B . If case surface space allows it, I'll try to put a nice BNC connector of some kind.

Proud Mary

Fri Jan 18 2013, 07:29PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Is this a Class 1 bluetooth device, Radu? What is its working range in real world conditions?

radhoo

Fri Jan 18 2013, 11:23PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

Unfortunately this is only a class 2 bluetooth module. The tested range is as expected, somewhere close to 10m without obstacles.

radhoo

Sun May 26 2013, 10:44AM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

The entire project (both source code, and circuit design files) has been published on google code, as OPEN SOURCE, under GPL v2. Here is the link:

Linas

Sun May 26 2013, 12:33PM

Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721

I am so envy on your android programming skills...

radhoo

Mon May 27 2013, 09:59AM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

Thanks Linas, but programming is just an ordinary job. Given your other skills, I'm sure it would be piece of cake to learn it, if this is what you want.

radhoo

Fri Nov 22 2013, 02:08PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

Now the device got an upgrade: case painted black and a new, improved software:

A demo video showing a comparison between SBM-20 and SI-29BG, when connected to the external port:

This is an open source project, code and schematics freely available. See:

for more details.

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