Thursday, February 18, 2021

Ludlum GM Counter Calibration for accurate CPM rate using a Function Generator

I have a few Geiger counters (Ludlum and Eberline) with Analog Scales and wanted to make sure they are properly calibrated to display the CPM rate.

I am not interested in dose rates as they are more or less meaningless when working with NORM (Natural Occurring Radioactive Materials). These are a mixture of isotopes, each emitting different particles and gamma energies and a Geiger Counter cant provide an accurate estimate for the dose.

Geiger Counters are usually calibrated to display doses from a specific isotope - Cs-137 or Co-60, while I am interested in the activity of the samples so CPM rates are more important to me.

Ludlum makes their Model 500 Pulse Generator for this type of calibration but it is ridiculously priced (a used one sold on eBay for $2100) and after studying the schematics of this over-priced monstrosity (which seemed designed at least 20-30 years ago) , I concluded that using a modern Function Generator in Pulse mode will do the job even better - more accurately, while providing identical functionality.

 This is my setup for the CPM calibration of Geiger Counters with an Analog Scale. The Function Generator simulates the GM tube inside the detector probe.

I setup my Function Generator (Rigol Technologies DG1022Z) with almost the same parameter pulses the Ludlum 500 produces.

Leading edge is set to 300 ns, Pulse width is 4 μs and trailing edge 2.25 μs. (Ludlum's pulser actually has a trailing edge of the pulses at 5 μs due to their circuit - I made my pulses wider and more defined with a steeper trailing edge but this is fully adjustable).
GM tubes in general produce very short pulses as they are quenched by the halogen gas in the tube so they can reset for the next count.

The shape of an individual pulse

Pulses with period 2ms (500.0 Hz) or 30K CPM. The scope is set to 1ms/div.

Amplitude is set for 500 mVpp and the Generator output is inverted - GM tubes produce negative polarity pulses as the gas discharge just shunts the HV DC tube bias. The amplitude adjustment can be varied to calibrate the pulse height threshold of the instrument.

To block the DC bias and inject the pulses, simulating a GM tube with my generator, I inserted a HV blocking capacitor inline - this protects the output of the generator form the 900V DC bias generated by the Ludlum meter.
At some point I'll make a little enclosure with tap points for oscilloscope and a high-voltage voltmeter.

I had some 10nF / 3 kV caps at hand and used two in series for 5nF / 6kV. Alternatively 5.6 nF/ 3kV cap will work just as well - the rating must be at least 3kV or more.
Everything is insulated to avoid a short that can damage my generator. Two Female BNC connectors are used to connect both cables thru the capacitor.

The calibration procedure is super-simple - I select a scale on the Ludlum, dial the frequency on the generator and adjust the calibration trimmer-pot for this range until I get the correct reading.
The generator frequency, Freq = CPM (desired rate, usually in the middle of the scale) / 60.
In the picture 500.0 Hz are used to generate a pulse rate of 30 000 CPM (3K in the x10 scale).

Full scale deflection on the x10 range is checked with 1.0 kHz signal for 60 000 CPM and it is dead on.

At the x1 scale, 50.0 Hz from the Generator should result in exactly 3000 CPM reading. The x1 trim-pot is adjusted to match the needle if reading is off.

Full deflection at the x1 range is then confirmed with 100.0 Hz signal

Each range is adjusted individually in the same manner - most meters have independent trimmer-pots. 

Here is a video of the Ludlum Model 14C counter driven with 600 CPM (Scale x0.1) - 10.000 Hz from the generator.

For slower rates, like the x0.1 scale, the Ludlum should be placed on SLOW mode to average and smooth out the reading. For the higher rates FAST mode can be used just as well and the reading is stable immediately.

Basically, it cost me nothing to put together this calibration setup - if I am to count the cost of the generator - it is still a "mere" $400 - not even 1/4 of the price of Ludlum 500 pulser.
The time-base of the generator has an excellent stability (1ppm) and it is many times more accurate than the Ludlum's voltage-to-frequency converter used in their circuit. Amplitude can be finely adjusted as well so one can align the counter's pre-amp sensitivity threshold.

This calibration method yields a for very accurate CPM rates and the reading can be adjusted with a pin-point precision on the scale.

Disclaimer: This method will not be applicable for Dose rate calibrations (the efficiency of the detector at certain gamma energies can not be measured without an actual gamma source) but it can be used to determine the calibration conversion constant for an instrument, already calibrated for the correct dose.
The generator just simulates a GM tube and each pulse produced will be correctly counted over time by the counter as the rate is known, fixed and super-stable.
The electronics are calibrated to ensure that the count rate is accurately displayed on the analog scale of the meter but how the detector generates pulses based on exposure to gamma rays or charged particles is not factored - it is assumed that each charged particle in the volume of the detector tube will result in a pulse.


Unknown said...

Agradezco mucho tu ayuda.
Eres lo maximo.
Continuamos en contacto.

Mil gracias

nick_las123 said...

I appreciate your collaboration, I did the test as you set it and it works perfectly. I would like you to explain in more detail why you choose that capacitance and how the high-voltage measurement can be carried out. This is because the measurement I make with the multimeter is very far from the voltage indicated in the ludlum manual.

Andrey E. Stoev said...

The Ludlum calibrator uses 5.6nF capacitor for DC blocking - the exact capacitance value is not that important but larger value will cause the pulses to become "mushy" (less defined, it just widens the pulses). Smaller value will cause less energy transfer (peak height). 4.7nF should work just as well. The important part is that the capacitor is rated for at least 2-3kV or more. In my case I used 2x 10nF in series - this drops the capacitance in half while doubling the rated voltage. For the High-Voltage measurements you'll need a very high-impedance probe to be accurate as the Ludlum's power supply is not designed for high current and voltage will drop if the input impedance of your voltmeter is low, skewing the reading. I use Fluke 80K-6 probe for my voltmeter (just like this one - ) and it works perfectly!

nick_las123 said...
This comment has been removed by the author.
nick_las123 said...

I really appreciate your collaboration, now I will try to do the test of measuring the high voltage as you indicate, because I measure it with a conventional multimeter and it gives a very low value compared to that of the Lumlum manual.

nick_las123 said...

I would like you to tell me what is the multimeter reference you use to measure that high voltage