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

I have a few Geiger counters (Ludlum and Eberline) with Analog Metering Systems 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 is a mixture of isotopes, each emitting different particles and gamma energies and a Geiger Counter cant provide an accurate estimate for the dose since it is not an energy-discriminating instrument.

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

Fortunately, all counters using an Analog Metering System / Scale are equipped with one or more trimmer-potentiometers to calibrate the needle reading to the registered count rate.

Digital counters also can benefit from this setup if their analog front-end (amplifier) / pulse detection circuit allows for adjustments.

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 overpriced monstrosity (which also seems to have been designed at least 20-30 years ago) , I concluded that using a modern Function Generator in Pulse mode will do an even better job and way more accurately, while providing more or less identical functionality. The Ludlum Pulse Generator allows for HV adjustment, besides rate calibration but this can easily be done with an inexpensive Fluke 80K-6 HV Probe and a multimeter (an even better option is Fluke 80K-40 probe since it has 1GOhm impedance and it will cause much lesser voltage drop in the HV circuit).

 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. The DC-blocking cap protects the output of the generator from being blown up by the High-voltage tube bias. 

I setup my Function Generator (Rigol Technologies DG1022Z) with almost the same parameters for the pulses as the Ludlum 500 Pulse Generator 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 aspect is fully adjustable with my Generator).

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 and different tubes have different "dead time" - for example LND7311 in the Ludlum 44-9 is twice as faster (minimum dead-time of 20 μs) than its low-voltage "sibling" LND7317 (minimum dead-time 40 μs).

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 500mVpp (peak-to-peak), and the Generator output is inverted - GM tubes produce negative polarity pulses as the gas discharge just shunts the HV DC tube bias to ground causing a brief voltage drop. The amplitude adjustment can be varied to calibrate the pulse height threshold of the instrument (pulse sensitivity).

Generator De-coupler / Pulse Injector

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 total 5nF / 6kV. Alternatively single 5.6nF/ 3kV cap will work just as well - the voltage 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.

Important! - Failure to properly decouple / block the Geiger's DC bias from the Signal Generator will destroy your Signal Generator! You can not simply connect the Geiger counter directly to the output of the Signal Generator - such mistake will cost your generator!  Use of a DC-blocking capacitor inline is an absolute must! 

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 on the metering system.

The generator's frequency is dialed as Freq = CPM divided by 60 (the actual pulse frequency corresponds to CPS) .

The desired rate should a rate located in the middle of the scale and re-checked with frequencies causing full needle deflection and little needle deflection (just past the begining of the scale) . This will expose any non-linearity of the circuit/metering system.

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 after a slight adjustment. 

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" $350 - 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 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 calibrated activity 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.