The Sericho Pallasite Meteorite - XRF Analysis

I obtained 2 fragments of the Sericho Pallasite Meteorite, "discovered" in Habaswein, Eastern Kenya in 2016. 

The meteorite has been known to local populations for many years but it wasn't until 2016 when the meteorite was officially classified as such. This was a huge meteorite - so far 2.8t were recovered.

A highly sculpted complete fragment of the Sericho meteorite. This specimen exhibits the "classic" meteorite look and the fragment is complete, not cut from a larger piece.

The second specimen is an end-cut piece from a larger fragment. It exhibits the typical "fusion crust" from the entry in Earth's atmosphere. 

The back side of the second piece with straight polished cut reveals the pallasite nature of the meteorite and a structure of olivine crystals.

The XRF Analysis setup - the polished cut of the meteorite is exposed to the 59.54 keV X-Ray source and the X-Ray detector. I placed the source a bit further away decreasing the intensity in order to eliminate parasitic peaks coming from Np, Au and Ag in the source itself. The weaker beam resulted in a long acquisition time - nearly 6 hours but produced a fairly clean spectrum.

XRF analysis plot.

As it turns out from an XRF point of view, much like most other meteorites, Sericho is typical and quite boring - no exotic metals are present - just Iron, Nickel and traces of Cobalt and Chromium.

The plot prominently features the Kα1 and Kβ1 peaks of Iron and smaller peaks of Nickel. Cobalt is in very low concentration (0.8%) and masked but if one looks for it, it can be seen in the irregular shape of the base (on the right side) of the Ni Kα1 photopeak. The Ni Kα1 at 7.48 keV is too close to the 7.65 keV of the Kβ1 of Cobalt just at the edge of the detector resolution. The 6.93 keV Kα1 line of the Co is dominated by the Kβ1 Fe at 7.06 keV and can not be differentiated. Chromium can not be detected at all with my setup due to the trace amounts (0.03%)

My XRF Setup - Part 3 - Exciter

The exciter is the second main component of an XRF setup - this is the source of the primary X-Rays.

Two type of Exciters are generally used - X-Ray tube or Radioactive Isotope.

X-Ray tubes 

Pros:

- provide high-intensity beam

- low limit of element detection

- easy on/off capabilities 

- fast integration times

- fairly clean and uniform spectrum

- very small spot of irradiation / sampling 

Cons:

 - big, heavy, very delicate

- require additional cooling

- large, hazardous HV power supplies

- need for safety interlock system

- heavy beam collimators

- substantial shielding is required

- consideration must be made about beam scattering and reflection

- Not as portable

Radioactive Isotope source 

Pros: 

- smaller, lighter and simple to use

- 100% reliable

- very portable for field use

Cons:

- low intensity beam requires long acquisition times

- shielding is required as well a shutter-type on/off system

- highly regulated

- danger of contamination if source is damaged

- spectrum is not as clean and can contain various peaks

 The holder of the exciter was designed with TinkerCAD and 3D printed

While this method works and it is a convenient way to use a number of small individual sources, the main problem is that they need to be placed at some distance from the Object Under Test which decreases the flux and irradiates a larger area of the specimen. One can not easily select the area being sampled.

I made a small, single, directed source with a  Lead collimator / shield which works very well and I can place it much closer to the specimen without the detector picking up the primary X-ray.

Update: X-Ray tube is added as yet another option to do XRF excitation and I built a custom controller for it. See THIS post.

My XRF Setup - Part 2 / X-Ray Spectrometer

 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. CdTe detectors are great for the higher energy region - up to 150 keV at the expense of resolution and internal noise.

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 for a SiPIN detector is 1 to 10 keV. 

The range of 10 keV to 25 keV exhibits a drop in efficiency to ~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 keV with a rapidly 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.

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 Kβ 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 Kα1-line @ 11.92 keV immediately followed by the Br Kβ1-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 - three 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 the component's 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 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.

Gamma Spectroscopy - Lead Shielding / Castle v2

I decided to rebuild my Lead Castle / Test chamber for Gamma Spectroscopy. 

The old vertical Lead Castle was working great but after awhile it became evident  that the design is not the most convenient one, especially if it is used often - every time I needed to access the test chamber, I had to partially disassemble and remove most of the lead modules (which are also quite heavy) and then put everything back together. 

Horizontal design is a more convenient option and creates a more accessible test chamber on the expense of the desktop footprint.

I redesigned the castle so I can use some components from the old castle - the detector is also in a horizontal position and can stay in place while I am only manipulating the sample and the end-cap shielding.

There are 3 main shielding components - outer sleeve, inner sleeve and end-cap.

For the new design I reused the main shielding 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 plastic form and almost 1" of Lead (rolled sheet with 1/8" thickness). 

The whole package is tightly wrapped with duct tape. The overall outer diameter is 6". 

The outer shield doubles the amount of lead around the test chamber and the detector's NaI(Tl) crystal in this configuration.

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 cover made of 6"x 6 x 1/4" Lead plate and 6" x 4" x 1/8" copper plate (towards the chamber) is used to close the opening of the chamber and another 1.5" inches of lead bricks are stacked. The front lead cap on the detector side has an opening for the coaxial cable.

The complete lead castle (v2). 4 rubber feet are installed on the bottom of the base.

While this design puts slightly less lead (5-7mm less) 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.

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 only to the metering system - the counter is actually digital with an 8-bit microcontroller (Intel 80C31 @ 6MHz), firmware with a very robust algorithm, stored on 27C32 EPROM, an AD7524 8-bit DAC to drive the metering system and multiplexed read of the ranges and configuration dip-switches. Functionality-wise it is way more advanced and more flexible instrument than Ludlum Model 3, with a lot more features, measurement units and ranges and far more sophisticated circuitry.

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

I decided to add a "visual click" with a LED, just like the one found on the more modern counters. As I mentioned the electrical design is beautiful and it wasn't difficult to figure out how to implement this mod.

The good news is that it is super-easy to mount the LED from a mechanical stand point as well - since the metering system has a backlight feature there is absolutely no need to modify the case - this is a slick "no drilling", very unobtrusive mod.

The LED is mounted behind the metering system and projects the light on the white, semi-translucent meter backing, using it basically 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, junction is heat-shrink insulated and then mechanically attached with hot glue to the PCB, just behind the metering system. The LED Cathode is connected with 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, providing mechanical rigidity.

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 dabs to the PCB and uses the notch of the edge connector to transition PCB sides.

This schematics shows how the LED is integrated in the electrical 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 not only solves this problem but takes advantage of the AES-1 ability to divide high click-rates with user-selectable division factor (when a scintillating detector is used for example).

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

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

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. Adding the thin aluminum foil is not "a must" as the plastic of the cap will be completely sufficient to block Alphas but it can double as a "soft beta" filter as well.

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 Lead (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% and this can be used as a crude dose rate correction.

The Check-Source Cap using an epoxy sealed tiny Autunite crystal.

The Autunite crystal is placed on a piece of aluminum foil substrate and suspended inside a drop of epoxy. I made a shallow cavity in the plastic to accommodate the epoxy drop and after trimming the foil it is attached on the inside of the cap with Kapton tape. The source produces around 600 CPM when the cap is in place.

It is very important for the Autunite crystal to be completely sealed inside the epoxy drop so no radon can quickly escape and contaminate the mica window of the pancake detector. In addition, this cap is normally stored in the carry pouch and only used as needed.

I used N.O.R.M. 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 that I have no problem carrying around and traveling with. Unfortunately, the lowest activity Cs-137 disk source I have at hand is 1 uCi so I just went the NORM route instead.

Alternatively (and it actually would be even better), one can attach a Spectrum Technique sealed Cs-137 disk source on the inside of the cap - something like 0.05 uCi or 0.1 uCi or even a tiny slab of uranium glass - the larger cap is just more difficult to lose and easier to work with acting as a "holder' for the source.

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.