Friday, April 30, 2021

My XRF Setup - Part 2 / The Hardware

 Amptek (Amptek) is one of the leading companies for Space instrumentation, experimental and research XRF equipment.

They have a fantastic line of products called X-123 Spectrometer - (1) detector element and preamplifier, (2) Digital Pulse Processor and MCA and (3) Power supply.  It is all-in-one device which only requires an external power and connection to a computer. The PC software by Amptek called DppMCA is used to configure, control the X-123 operation and receive & visualize the accumulated spectrum.

Once the acquisition process is started, X-123 doesn't even need the computer connection until it is time to receive, save and display the data, integrated by the internal to X-123 multi-channel analyzer (MCA) located on the DP5 module. 

X-123 Spectrometers are offered with a variety of detectors -  Si-PIN, SDD, Fast SDD or CdTe and can employ different length extenders between the case and the detector element. Fast SDD is their top-of-the-line model, while Si-PIN is more of a general use detector.

I got my detector from George Dowel (GEO Electronics) as Geo-123. Internally, the unit is identical to the commercial Amptek Si-PIN X-123 unit - George uses the OEM modules and installs them in a custom-machined enclosure. The enclosure is a bit larger than the commercial Amptek version but this is an advantage - the aluminum alloy enclosure actually acts as a giant heatsink for the heat pumped by the TEC module, located inside the detector element and larger surface area results in better heat dissipation.
If there is one thing I wish for, is to have at least 1" or more extension between the detector and the main enclosure - this could help a lot with detector placement in relation to the sample and the exciter.

The "business end" of the unit - the 25 mm2 / 500 μm Si-PIN X-Ray detector element (model FSJ32MD-G3SP) with a thin, very fragile 1 mil Beryllium window.
(!) This window must never be touched by hand or come in contact with any object - such thing could turn into a very costly mistake!

Out-of-the-box there is a red polyethylene protective cap installed. There is actually very little reason for the red protective cap to be removed and the detector works with the cap on. I would expect to see some attenuation in the very low end of the range (0 to 2 keV) when the cap is on but even with this cap, Calcium K-lines are actually detectable.
(George supplies a spare modified cap with a built-in thin Kapton window )

Energy resolution is 190 - 225 eV FWHM @ 5.9 keV, peaking time 25.6 μs and Peak-to-Background ratio: 2000/1 (typical).

Plot showing the efficiency as a function of energy for Si-PIN detector. 
The optimal energy range is 1 to 25 keV with efficiency >25%. Below 1 keV the loses from X-Rays traveling thru the air are significant - only 1cm of air will stop 90% of the X-Rays.
Above 25 keV the detector is still useable up to around 60-100 keV with a decreasing efficiency.

The X-123 Spectrometer supports USB 2.0 (mini-USB Connector), RS-232 (2.5mm jack) and Ethernet (RJ45) computer connections. 

USB works just fine and it is very fast so I never had the motivation to try any of the other interfaces. The Ethernet connectivity might require a future software release for full implementation, according to one Amptek document, but the orange data light on the port is a useful indicator - it is lit solid if the data acquisition is stopped and it is blinking when the MCA is running and storing data.
Other connectors on the back are the proprietary jack for the External Power supply and there is also a well documented auxiliary connector for gated counts and other functions.

The "sandwich" of DP5 Digital Pulse Processor (top board) and PC5 power supply module on the bottom. A ribbon cable connects DP5 to the PA230 Pre-amplifier board.

External power is supplied with a very small, proprietary connector (George provides a spare connector in the kit). 
The power adapter is regulated and rated for 5V / 2.5A. 
The current rating is very important - while the unit only needs 500-700 mA during normal operation, there is a short, high-current transient of around 2A during the boot up sequence and any current limiting bellow 2A could damage the internal power supply PC5 module.
All of the power conditioning and the generation of various voltages is done internally by the PC5 power supply module.

The Amptek software - DppMCA is quite good and I really like it! It is available on the AmpTek web site for free. The software is fairly easy to use and provides extensive toolset for data acquisition and analysis. The peak identification feature using energy reference libraries is very useful. The UI is logical and easy to use and ability to customize the color schemes.

There are a few features I wish it had but overall it does its job very well and it is well integrated with the hardware DP5 Pulse Processor.
Speaking of the DP5 module, the built-in hardware MCA in X-123 is quite impressive - 256 to 8192 channels (I normally use it in 4096 channels configuration) and 24 bits per channel (16.7 million counts). Acquisition time is selectable from 10 ms to 466 days. 
The MCA can be set to work in two modes - NORMAL and DELTA. In Delta mode it shows the spectrum, refreshed every second with pulses integrated over the past 1 second.

Combination of coarse and fine amplifier gain yields an overall Gain, continuously adjustable from x0.84 to x127.5 - the amount of preamp Gain determines the spread of the spectrum over a specified number of channels in the MCA.
For example, when using 4096 Channels, a Gain of x18.5 allows coverage of 0 to 62 keV range.

The Si-PIN detector response is quite linear and 2 point calibration is all that is needed for most applications.
I use pure, 99.9% Copper (Cu) foil - the Kα1 line at 8.05 keV and the Am-241 X-rays at 59.54 keV at the high end of the spectrum are sufficient for channel/energy calibration but more intermediate points can easily be added if necessary using different pure metals.

Gadolinium (Gd) is a Rare-Earth Element which is very interesting to XRF with its many peaks and also can be used as a calibration aid since both L and K-lines show up nicely at the low and high energy range of the detector.

XRF of a 99.9% pure 1" disk of Gadolinium (Gd).

Most of the Gd peaks can be easily identified - Kα1, Kα2, Kβ1, Kβ2, Lα1/Lα2, Lβ1, Lβ2 and even Lγ1, Lγ2 and Ll are visible in this plot. Obviously, Lα1 and Lα2 can not be separated - they are only 30 eV apart - way too close for the 190-225 eV resolution of the Si-PIN detector.

The very low count "hash" above the group of Gd L-lines is caused by Np-237 L-lines coming from the native spectrum of the exciter - 25 μCi of Am-241. Am-241 decays to Np-237 and the L-lines of the Neptunium are Rayleigh scattering and somewhat visible in the spectrum. 
The Exciter's X-Ray beam is collimated and reduced down to about 3mm spot for a precise sampling so the overall count rate is low as expected but the spectrum is nice and fairly clean. Longer integration times are to be expected with this type of exciter and I use a different exciter with a broader beam for general purpose.

to be continued....

Thursday, April 29, 2021

My XRF Setup - Part 1 / How it all works?

 What is XRF? 

XRF stands for "X-Ray Fluorescence" and there are two main types: Energy Dispersive XRF and Wavelength Dispersive XRF. 

I'll focus on the Energy Dispersive, Direct Excitation (2D) Method as this is what I use. 

EDXRF is a Non-Destructive method for material analysis, used to determine the Elemental composition of a material or a chemical compound. It is an extremely useful tool to analyze raw materials, minerals, alloys, etc.

The Physics behind XRF is absolutely fascinating and at the same time relatively  simple to understand.

The description of the whole process can be boiled down to this: exposing a test sample to a beam of X-Ray radiation and detecting the energy of the secondary / characteristic X-Rays emitted by the atoms in the sample and then building a histogram of the energy spectrum in order to identify specific secondary X-Ray peaks.

When a material is irradiated by short-wavelength ionizing radiation like X-Rays or low-energy Gamma Rays, the electrons from the innermost electron shells, the ones closest to the nucleus (K, L, M shells) will become excited and are expelled from the atom. This causes a vacancy in that lower electron shell and it is immediately filled with an electron from a higher-energy shell.

For example, if the electron is ejected from the K-shell, this vacancy will be filled by an electron from the L or M shells. When the electron makes the jump from a higher-energy shell to a lower-energy shell in order to fill such vacancy, it must give off the excess energy and does it so by emitting a photon with energy equivalent to the difference. This secondary photon is again, an X-Ray photon but with a very specific energy to the particular element due to the unique binding energy between the nucleus of each element (with its protons) and the surrounding electron shells. 

This, secondary emitted photon is called a Characteristic X-Ray. By detecting the energy of these characteristic X-Rays we can determine which Element from the Periodic Table the examined atom belongs to. 

There is a number of such characteristic X-Rays emitted, based on which shell, the electron comes from to fill the vacancy of the electron expelled by the primary X-Rays - if the vacancy is in the innermost shell (K-shell) and it is filled from the L-shell it is called Kα energy, if it is filled by the M shell is Kβ energy and so on. Vacancies in the L-shell are filled from the M-shell and are called Lα and when filled from N-shell - Lβ

These characteristic X-rays energy are published in lookup XRF tables.

Overlaps between Kα/Kβ and Lα/Lβ energies for some elements exist so identifying an element often relies on identifying multiple energy peaks in the spectrum, coming from different transition lines.

This is a typical XRF histogram as produced by the MCA software. I used a small sample of pure (99.98%) Cobalt metal for this test. The two blue peaks on the very left are the Kα and  lines of Cobalt. 

The other peaks in the spectrum above are just parasitic peaks coming from the X-ray exciter or the environment - for example the tall green peak on the right of the Cobalt peaks is the Bromine 1-line @ 11.92 keV immediately followed by the Br 1-line @ 13.29 keV, all coming from the plastic clamp I used to hold the small Cobalt metal sample in front of the detector - the Bromine was likely used in manufacturing of the dye or filler of the plastic and even though the clamp was just partially exposed to the detector the Bromine (Br) lines were still detected.

What do I need for XRF?

At a very basic level - two things : X-Ray Source, X-Ray detector and a computer.

The Amptek XRF Kit - Mini-X X-Ray tube and the all-in-one X-123 detector.

X-Ray source: obviously the best source is an X-Ray tube in the 40-60 kV range but these require forced cooling, HV power supply, lots of shielding and a collimator. Such setups are large and very expensive. There are small and portable tubes but they usually have an even higher price tag.

Alternatively a radioactive isotope emitting low-energy gammas / X-Rays can be used as an exciter - Cd-109, Fe-55 or Am-241 just to name a few. The intensity is usually lower than the beam from an X-Ray tube, even when mCi amounts of activity are used so counting times are longer, but it is a very portable and uncomplicated method to produce the primary X-Rays. It is important that the energy of the exciting X-Ray beam is higher than the characteristic energy to be detected.

Needless the say, regardless whether the exciter is an X-Ray tube or a Radioactive Isotope, caution must be exercised at all times dealing with ionizing radiation. 

X-Ray Detector: the X-Ray detector must have very high-resolution (typically 122-200 eV or less) as some peaks are really close to each other and high efficiency in the low-end of the X-ray energy spectrum - typically, efficiency is >25% in the 1 to 25 keV range but my detector covers energies all the way up to 60+ keV at a reduced efficiency.

The required resolution and energy range are normally outside of the capabilities of most Gamma Spectroscopy detectors. Even the thin-crystal GS probes designed for the X-ray region will not have the resolution needed - some limited XRF might still be possible though.

 A specialized semiconductor X-Ray detector device is needed - Si-PIN, SDD or CdTe detector.

These detectors are very expensive, complex and very delicate devices employing a thin and very fragile (0.5 mil or less) Beryllium window and are usually evacuated or filled with low-pressure Helium gas. Inside the detector device are housed many components: the Si-PIN or SDD detector semiconductor chip, an input FET transistor for the preamp, a temperature sensor, a built-in multi-stage thermo-electric cooler (TEC) with a delta of ~85°C which reduces thermal noise in the detector chip and a temperature sensor. The heat pumped out from the chip must be constantly dissipated in the environment thru the mounting stud and back surface thermal interface.

The detector is connected to a charge-sensitive pre-amplifier and the output of the pre-amp is fed into a Digital Pulse Processor (Dpp) which does the pulse detection, pulse shaping, ADC and pulse-sorting as it has a built-in Multi-Channel Analyzer (MCA) (8k channels). 

A Power Supply module generates the bias for the detector, the power to the TEC module and controls the temperature of the detector chip, besides powering the pre-amp and Dpp.

Because of the very low Characteristic X-Ray energies of light elements it can be extremely difficult to detect these elements as their secondary X-rays are easily absorbed even by even air - lightest elements emit energies <1keV.

 High-Intensity primary x-ray beam, very thin Be-window, Silicon-Drift Diode (SDD) detector vacuum chambers and even Helium-filled test chambers are often needed for elements lighter than Potassium to be detected.

Typical Si-PIN detectors work well for elements heavier than Scandium (Z>21) but I am actually able to observe even the Calcium lines - not in great detail but visible in the spectrum.

Tuesday, April 20, 2021

Gamma Spectroscopy - Lead Shielding / Castle v2

I decided to rebuild my Lead Castle / Test chamber for Gamma Spectroscopy. The old vertical design was working great but after awhile it gets annoying to add or remove samples - every time I have to partially disassemble and remove the lead modules (which are quite heavy) in order to access the sample chamber, add or replace the sample and then put everything back together

I redesigned the castle so the detector is in a horizontal position and can stay in while I am only manipulating the sample and the end shielding.

For the new design I reused the main shield sleeve from the old chamber - 6" long Schedule 40 PVC pipe with 4" diameter. There is about 0.5mm of copper foil wound directly around the pipe and a little over 1/2" of Lead sheet (1/8" thickness). The whole package is tightly wrapped with duct tape. The outer diameter is 6". 
The outer shield doubles the amount of lead around the test chamber and the detector's NaI(Tl) crystal.

For the inner shield I used 12" long section of 2" copper pipe and wound 5 turns of 1/8" thickness Lead sheet. Again, everything is tightly wrapped and covered with duct tape. 

The inner shielding is twice as long as the outer sleeve (12" vs 6") and shields the length of the entire detector assembly.

I built a special, custom wooden crate to hold the lead shielding. The base is made of 3/4" pine 6" x 16". The two sides around the large lead sleeve are glued and screwed with 2" 1/2 long decking screws to the base. 
Two threaded rods, inserted in transparent vinyl tubing, together with washers and nuts re-enforce the top part, maintain the spacing of the sides while allowing the outer sleeve to be removed from the top after the inner sleeve is removed thru the front or the back. The front threaded rod (which also has a larger diameter) doubles as the front carry handle.

The rear portion is a boxed off extension and it has 3/4 pine raiser on which the inner sleeve lays on so it aligns with the outer sleeve. A second carry handle is attached to the top of this box.
Assembly takes place in reverse order. Outer sleeve is dropped from the top into place and then the inner sleeve is inserted.

The 6" long second box section sits on top of the base and the inner sleeve rests on it. It is attached to the base from the bottom side with the same long decking screws and wood glue. A 1" "lip" is left on the back to support a lead shield cap. 

This is the end through which the GS detector is inserted and the detector position is adjusted inside the copper tube lining in order to place the front face of the crystal right against the sample.

The front end of the chamber with the inner sleeve nested inside the outer sleeve. The white front face of the detector is visible inside the copper lining. 
The bottom of both compartments is lined with 1/8" rubber lining. The threaded rod on top is a structural components as the whole crate becomes very heavy (~65 Lbs.) and it acts as a handle as well.

After the test sample is placed in the chamber, a front shield/cap can be quickly assembled of lead bricks.

The complete lead castle (v2). 4 rubber feet are installed on the bottom of the base.
While this design puts a slightly less lead (5-7mm) around the crystal than the vertical design with cast inner shielding, it is much more convenient to use and operate.

Two coats of polyurethane varnish as a finishing touch.

Wednesday, March 3, 2021

Eberline ASP-1 LED modification - "visual pulse indicator"

Eberline ASP-1 is one my favorite "80s era" Geiger Counters. 

The electrical design is elegant, downright beautiful. ASP stands for "Analog Smart Portable" but "Analog" refers to the metering system only - the meter is actually digital with an 8-bit microcontroller (Intel 80C31 @ 6MHz), firmware with a very robust algorithm, stored on 27C32 EPROM and an AD7524 DAC to drive the metering system. Functionality-wise it is way more advanced and more flexible instrument than Ludlum Model 3, with a lot more features, measuring units and ranges and more sophisticated digital circuitry.

The speaker of the unit is not terribly loud - it is more of an acoustic air-tube type headphone transducer than a proper speaker.

I decided to add a "visual click" with a LED, just like on the more modern counters. 

The good news is that it is super-easy to mount the LED - since the metering system has a backlight feature there is no need to modify the case at all.

The LED is mounted behind the metering system and projects the light on the white, semi-translucent meter backing, using it as a "projection screen".

When the LED is off, the metering system looks just like before.

Behind the scenes. The Anode of the LED is connected to a wire, heat-shrink insulated and then mechanically attached with hot glue to the PCB, just behind the metering system. The LED Cathode is connected thru a 220 Ohm resistor to the GND lead (the left lead on the picture) of the Speaker switch and this is also a second anchor point.

The Anode lead wire of the LED is connected to junction point between R138, R139 and pins 1,2,4 of A104 (CD4001B) on the bottom of the PCB.

The wire is secured with hot glue to the PCB and uses the notch of the edge connector to change sides.

This schematics shows how the LED is integrated in the circuit.

The mod is using the speaker circuit to pulse the LED as this circuit is designed to provide long enough pulses (~2 ms) for the speaker, using a free-running 2kHz generator and a mono-stable trigger. 
The pulses behind the pre-amp which are counted by the MCU are extremely short (just a few uSec) and they will not light up the LED for a long enough time to be visible - using the audio circuit solves this.

The LED will light up permanently if there is an Alarm condition (Overload).

Turning the Audio feature off will disable the visual pulse indicator as well but the Alarm will still work as designed and the LED will still respond to an alarm condition.

Friday, February 19, 2021

SE International Radiation Alert Ranger - making "protective cap accessories".

One thing I really like about the design of the SE International Radiation Alert Ranger is the protective cap on the back for the "pancake" detector.

This cap is great to protect the delicate mica window from contamination and mechanical events that can destroy the detector.

I realized that this cap can easily be turned into a useful  "charged particle filter" or even a check-source.

I went ahead and ordered a few spare caps from SE International at $1.50 a piece.

The  assortment of "cap accessories I made - a check-source, an Alpha filter and an Alpha + Beta filter. The Alpha filter is used for Beta + Gamma measurements and the Alpha + Beta filter is used to measure Gamma only.
The "Alpha Filter" (the cap with the black insert) is made by cutting a disk of self-adhesive aluminum foil (thickness 0.1mm) and applying it on the inside of the cap. The foil in combination with the plastic of the cap will stop all alphas, while letting most betas come trough. 

For the "Alpha filter", thicker cooking foil can be used as well and attached with double-sided adhesive tape or glue.

The "Alpha + Beta" filter (gray) is made by cutting a disk out of aluminum. The disk is a tad small than 1" 3/4 and thickness is 3mm. This disk is secured inside the cap with a piece of double-sided adhesive tape.

The "Alpha" filter (top) is placed for measuring Beta + Gamma activity only. The "Alpha + Beta" filter (bottom) in addition shields all Betas up to 2 MeV and attenuates Gamma <7 % @662 keV (Cs-137)

For the "Alpha + Beta" filter, I cut the disk out of equipment rack blank panel. The aluminum of the blank is 3 mm thick which stops all beta particles with energy up to 2 MeV and has very low Gamma attenuation (less than 7%) for the Cs-137 isotope (662 keV).

Cutting a perfect, large (1" 3/4) disk out of 3mm aluminum is a bit of a chore but nothing that can't be solved by a drill press, a grinding wheel and a file.

As an alternative, pure 1 mm Pb sheet will stop betas up to 2.3 MeV and will attenuate Cs-137 gamma by 10%, Co-60 Gamma by 5% and not only the math is easier but it is much easier to cut than 3mm aluminum - one can use regular scissors for such thin lead sheet.
The plastic of the cap should slow down beta particles a bit perhaps reducing the generation of secondary X-Rays (from the Bremsstrahlung effect)
Inserts made out of other materials can be made to fit these protective caps - I'll be making another cap with a 3 mm thick lead sheet insert for measuring extremely high activity gamma sources (normally the Ranger overflows at 350K CPM). 
By calculating the attenuation factor of the lead in the cap, I can correct the measurements which will extend the maximum range of the instrument. For example 1/8" Lead (3.1mm) will attenuate Cs-137 Gamma rays by 28% and Co-60 by 14%

The Check-Source Cap using an epoxy sealed tiny piece of Autunite crystal.
The Autnite crystal is placed on a piece of aluminum foil in a drop of epoxy. Self-adhesive tape on top sandwiches the crystal between the foil and after trimming the foil it is attached on the inside of the cap with Kapton tape. The source produces around 600 CPM.

It is very important for the autunite crystal to be completely sealed in epoxy so no radon can escape and contaminate the mica window of the pancake detector.
I used NORM for the source for 2 reasons - I don't have to worry about half-life and changes of the activity over time and secondly, I wanted to have a very low activity check-source. The lowest activity Cs-137 I have at hand is 1 uCi so I went the NORM route instead.
Alternatively (and it would be even better), one can attach a Spectrum Technique sealed Cs-137 check-source disk on the inside of the cap - like 0.05 uCi or 0.1 uCi.

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.