Geiger counter / radiation meter modification

[Solidus]

Hello all,

I had a project which I thought would be an interesting one and one that I can use to gain some actual research-type information from.

To provide background, Geiger counters and radiation survey meters are devices use to measure intensity of radiation / rate of release. Geiger counters, by design limitation, can only measure counts, as they cannot discriminate between different types / energy levels of radiation. However, radiation survey meters, which use an ionization chamber which is energy-compensated, can distinguish this amount and display an instantaneous rate typically in rad/h or microSieverts/h. This is absorbed dose - that also factors in the energy of emitted radiation to calculate the amount of energy that is being imparted to the instrument.

Now, as all of us probably remember from calculus, instantaneous rate, when precision is needed, is useless for all but the ballpark estimates. So, what I wanted to do was to modify one of these counters to not only display instantaneous dose, but to integrate that as to provide total dose received. However, from looking at schematics of the meters, it is just not that simple.

I was originally thinking about configuring an op-amp into an integrator circuit and running that through a 7-segment display driver to provide a total, but the physics of the circuit doesn't agree with me. First of all, even under intense amplification (multiple triode stages), the current is only in the range of 5 microamps (the meter itself is an ammeter), and also even though y'all have known me to be a fool in the ways of electronics, even I know that "tapping" a line off to run an integration circuit would sap the current to a degree that can't be determined without modeling the circuit, thus making both integrated and rate meters completely unusable.

So, how would I start to work to accomplish something like this?

To satisfy curiosity, what I would like to do is to place the instrument chamber outside for various periods of time to measure background radiation, as well as place it under certain levels of radiation to make educated guesses as to the distribution of radiation present.

 

[BobK]

I would think an opamp voltage follower would allow you to tap the signal. Look for an opamp with very low input bias and input offset current. A random microchip one I looked at was 1pA.

Bob

 

[Solidus]

Okay, now how would I configure a circuit that would manage either of the two options:

1) integrate the result and output to a 7-segment display driver
2) Have a certain frequency of "sweeping" or "scanning" in which, in sync to that frequency, the logic circuit records the instantaneous value of the counter and sums all values to date to output to display driver.

Obviously, the second would be much harder to configure correctly, although I would construct the sweep generator using a 555 timer. That option, however, by modifying the voltage gain of the summation sequence, could have a much higher range than the first option, as without modification of the gain, the integrator circuit would have a finite range.

Tell me if I'm on the right track with this or if this is just idiotic ramblings of a madman.

 

[duke37]

All radiation counters produce pulses. The Geiger counter produces pulses of fixed amplitude whereas a proportional counter will give puses depending on the radiation.

Since pulses are detected you cannot get an instantaneous reading, you will need to integrate over a certain time.

The way to do what you want is to use a micro (not my scene) to count the pulses over a time period that you define. If you have a proportional counter, then the counts can be sorted into different sizes before counting.

 

[Solidus]

So such an operation isn't practical with standard components?

I'm just teaching myself more electronics by the project and slowly learning the art, so the idea of programming a microcontroller to do the nasty work for me seems to be cheating to me.

 

[BobK]

If the pulses are of fixed amplitude, a leaky integrator should work to average over time.

Bob

 

[jimmyhackers]

someone thinking about exploring the radiation ravaged post apocalyptic washington dc???????

 

[john monks]

What duke37 is describing is a multi-channel analyzer(MCA).
The way we normally check for radiation energy levels is with a scintillation chamber of some sort and a photomultiplier. This is normally done with a clear crystal attached to a photomultiplier tube. The crystal gets hit with particles or photons and emit a faint pulse of light that is picked up by the photomultiplier tube. This tube looks at the height of the pulse coming out of the tube. typically the pulse looks like a tail pulse. And the area under the pulse is proportional to the energy of the particle hitting the crystal.

The attached explains in greater detail.

http://www.berkeleynucleonics.com/resources/Nuke-catalog.pdf