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. Furthermore, the dose calibration is taking place right on the scale where specific CPM rate equates to a dose based on the efficiency of the probe.

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. 

The metering system is nothing more than a mA-meter or uA-meter and the counter's circuitry converts count rate to a specific current which will deflect the needle to a specific rate marked on the scale.

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 recently 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 would be Fluke 80K-40 probe since it has 1GOhm impedance and it will cause much lower voltage drop in the HV PS circuit thus less measurement error).

 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 by sending out pulses with the correct shape and timing and polarity. 

The DC-blocking cap protects the output of the generator from being blown up by the High-Voltage tube bias coming from the counter's HV PS. 

I setup my Function Generator (Rigol Technologies DG1022Z in my case) with almost the same parameters for the pulses as the Ludlum 500 Pulse Generator produces. Any function generator with a dedicated "Pulse Mode" will work. 

A square wave signal is not useable as the pulse width must remain constant while varying the frequency!

• Leading edge is set to 300 ns
• Pulse width is 4 μs
• 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 instead, but this aspect is fully adjustable with the generator).
• Amplitude is set for 500mVpp
• Pulse output is set to Inverted

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 across the tube's anode resistor. The amplitude adjustment can also 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 GM tube bias generated by the Ludlum meter.

I had some 10nF / 3 kV capacitors 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 for the safety of the equipment.

Turns out Pomona makes the perfect enclosure for the DC blocking capacitor. The male BNC is attached directly to the front panel of the Function Generator, and it provides a Female BNC for the output after the capacitor.

The enclosure is very compact and fits perfectly both capacitors I used.

Important! - Failure to properly decouple / block the Geiger's DC bias from the Signal Generator's output will destroy the front-end of your Signal Generator!

You cannot 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 analog metering system.

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

The desired rate should be a rate located in the middle of each scale and re-checked with frequencies causing full needle deflection (end of scale) and little needle deflection (just around 5-10% past the beginning of the scale). This will expose any non-linearity of the circuit/metering system.

In the picture 500.0 Hz signal is 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 move the needle and match the markings on the scale if reading is initially 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 lower rates, like the ones on the x0.1 scale, the Ludlum should be placed on SLOW response mode to average and smooth out the reading - this will smooth out the pulse jitter on the meter's needle. For the higher rates, FAST mode can be used just as well, and the reading will be stable immediately.

Basically, it cost me nothing to put together this calibration setup - if I am to count the cost of the generator (which I already had) - it is still a "mere" $350 - not even 1/4 of the price of Ludlum 500 pulser. A Fluke 80K-40 probe for the High-Voltage measurements will add another $100.

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. In addition, pulse 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 cannot 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 be registered and will result in a pulse.