Ordering nuclear surplus from Russia and custom crystals from China to perform gamma-ray spectroscopy at home
In the fall of 1998 I played Fallout 2:
It permanently altered the course of my life - I have been obsessed with radioactivity ever since.
The ability to detect/see radioactive particles feels like a superpower. I’ve already built a geiger counter and a cloud chamber, what’s left?
A lot.
Whenever I start a new hobby or interest, I take the same approach:
Radiation detection will be no different. My homemade geiger counter was built around a Soviet SBM-20 Geiger–Müller tube - a fun starting point, but limited in its detection capabilities. My goal is to have the ability to measure (in useful ways) every major type of ionizing radiation (alpha α, beta β, gamma γ, and X-rays). (Neutron radiation will be out-of-scope until I get around to building a fusor)
The process will be:
And with all of that completed, we will unlock a new ability:
Gamma rays are the highest-energy form of electromagnetic radiation. Each isotope produces gamma rays with energy focused at particular levels. By measuring this spectrum of energies it is possible to distinguish radioactive isotopes.
Let’s get started.
Radiation-measuring instruments come in various forms:
It would be nice to have at least one of each of these instruments.
Survey meters are the classic handheld radiation measuring devices. Models differ in how the detector attaches (or the detector may be internal), how much control you have over the pulse detection (threshold, high voltage), the quantity displayed (raw counts vs rates), and how rugged they are.
We’ll start by getting a consumer geiger counter, aiming for a portable device which offers data logging capabilities and easy modification. The GQ GMC-300E Plus is a popular unit with an active forum documenting successful modifications.
With modification (to follow) the GMC-300E Plus can work with a variety of Geiger-Müller tubes, but we plan on using many other detector types. For that we need a more serious instrument.
Modular analog survey meters fit the bill - they are rugged, reliable, and highly customizable. Ludlum is the gold-standard for this market. Their Model 3 is the classic, we will go with the Model 3-97 variant (which includes an internal 1"x1" NaI(Tl) detector):
For external probes, the high-voltage (HV) may be adjusted from 400V to 1500V - this gives us some flexibility, but even more range would be useful. The Ludlum Model 12 has a wider HV range (400V to 2500V) and adds an adjustable discriminator (to set a threshold for which pulses should be counted):
We have our lineup of pulse-counting instruments, let’s move on to the next category.
The purpose of a dosimeter is to measure the cumulative absorbed dose of ionizing radiation. They are pocket-sized and are affixed to the clothing (usually the chest or torso area) in situations where occupational exposure may occur. By measuring the dose in a standardized location, the dosimeter can estimate doses for major organs and different depths in the human body.
In the classic pen form factor, we start with an Arrow-Tech Model 725:
It has a huge range of 0-5 Roentgen(R), which is really only useful in the case of an accident or if it were placed directly in the beam of an x-ray. For practical purposes, we should have a dosimeter with a finer resolution. The Arrow-Tech Model 138 has a range of 0–200 mR, much more practical:
Speaking of being placed directly in the beam of an x-ray, here is what the Model 138 looks when imaged with x-rays:
They are surprisingly simple: a filament is placed in a tiny air-filled ionization chamber and when X- or gamma-rays ionize the air in the chamber, the filament deflects electrostatically. This deflection is visible on a dose-calibrated scale visible through a built-in microscope.
This process depends upon the ionization chamber starting at a known voltage. To achieve this, we need a charger. Two types exist, the first uses batteries and a DC-DC step-up to smoothly adjust the voltage applied to the center electrode:
The other type presses a plunger against a piezo crystal that generates the necessary voltage:
With the classic pen dosimeter solved, we can move onto the modern implementation: an electronic personal dosimeter (EPD). As these instruments are intended for monitoring the cumulative dosage of workers who may be exposed to ionizing radiation, they add additional functionality to the pen dosimeter: continuous real-time readings, long-term dose logging, and alarms.
We go with the Thermo EPD Mk2.3:
That covers general-purpose dose-measuring, but there’s a special-purpose instrument in this category which may come in handy if (when) we play with X-rays: an X-ray exposure-rate meter. The Nuclear Associates RAD-CHECK Plus Model 06-526 is a special-purpose instrument containing an ionization chamber (with optional external chamber), calibrated for the range of typical diagnostic X-ray beams (30kVp-150kVp).
It can measure X-ray dosages from 0.001-1.999R, and exposure rates of 0.01-19.99R/min.
By attaching the Model 6000-528 external ionization chamber we gain flexibility in where we can take our measurements:
When working with X-rays, exposure time is a primary concern (for safety reasons, but also for ensuring proper exposure of images). The Electronic Control Concepts Model 8700 measures total exposure time of an X-ray beam (for calibration of an X-ray source), and also the number of X-ray pulses produced. It does this using a silicon PIN photodiode-based sensor.
The last piece of X-ray-specific instrumentation allows us to measure the peak energy level of an X-ray generator. The Nuclear Associates kVp Meter Model 07-479 contains two silicon PIN photodiodes with two different sets of filters. By comparing the ratio of the accumulated doses between the two diodes, the instrument can derive the peak kVp of an X-ray beam.
Owning these X-ray measuring instruments is overkill, but who can resist eBay?
We return to our ultimate goal: gamma spectroscopy. Each gamma-ray interaction in a detector produces a voltage pulse. The peak amplitude of that pulse is proportional to the deposited energy from the ray. A multichannel analyzer (MCA) takes these voltage pulses and bins them by amplitude, producing a spectrum / histogram.
As an example, here is the gamma spectrum of background radiation on the earth’s surface:
We can see peaks for potassium-40 and the daughter products of thorium and uranium (lead-212, bismuth-214) marked in the image.
Thanks to the open-source Theremino project, we can make our own MCA. Binning occurs in the Theremino MCA software, but acquiring the voltage peaks requires supporting electronics. First we need an adapter which can provide high-voltage to the gamma spectroscopy probe (detailed later) and stretch out the length of the pulses, then we need an analog-to-digital converter (ADC) to sample the pulses and digitize them.
The probe adapter is a relatively simple build. We start by having the PCBs made:
Then we order all the necessary components:
For a shielded enclosure, we’ll use my go-to: an electrical junction box. After soldering everything together, we have our probe adapter:
For the ADC, China comes to the rescue: we can modify a dirt-cheap USB audio card to serve our purposes.
We will need to make the following modifications:
After making these modifications, we have our ADC:
Together with the Theremino software, we have our MCA. We can now move on to the detectors.
All radiation detectors work in the same way: an interaction occurs in a medium, causing an effect that can be measured.
Charged particles (alpha, beta) directly cause ionization or excitation along their tracks.
Photons (X-rays, gamma-rays) interact via:
A Geiger-Müller tube consists of a chamber filled with a gas mixture. A large voltage difference (hundreds of volts) is established between a central electrode and the conductive walls of the chamber. When a radioactive particle strikes the tube, ionization occurs in some of the gas molecules (either directly, or as a secondary effect of interaction with the walls). These ions migrate in the gas due to the strong electric field caused by the voltage difference and cause further ionization, starting an electron avalanche. The avalanche quickly turns the gas near the electrode into a conductive plasma, completing a circuit - current briefly flows causing a voltage sudden voltage dip which is detected as a pulse. The electric field collapses and the tube recharges, ready for the next event.
By designing the GM tube to limit (or allow) particular types of ionizing radiation to penetrate the chamber, we can produce a variety of different detectors.
We’ve already mentioned the vintage Soviet SBM-20, which can detect beta and gamma radiation (its thick metal walls prevent detection of alpha radiation), as well as X-rays above 40keV.
The M4011 is a modern GM tube with a similar construction (using glass walls instead of metal), and the same detection capabilities.
If we want to measure alpha radiation as well as low-energy X-rays (<=40keV, the range where bone and tissue absorption increases) we need tubes with thin windows to admit these forms of ionizing radiation.
The vintage Soviet SBT-9 GM tube (used in Russian spacecraft!) has a mica end-window, which enables it to detect low-energy X-rays and alpha radiation (as well as the usual beta and gamma radiation).
Another vintage Soviet tube, the SBT-11A has a larger mica window, making it even more sensitive to alpha radiation and low-energy X-rays.
And while we are at it, since we already have Ludlum meters we will get a Ludlum Model 44-9 GM probe which contains an even larger mica window:
With a collection of tubes, we can now work on modifying our digital geiger counter to accept them.
Thanks to the experimentation of other owners, we have proof that the GMC-300E Plus can handle all of our tubes. By adding a DIP switch, we can swap between our choice of GM tube or use more than one GM tube at once.
The SBT-9 needs a hole cut out of the side to expose the mica window:
And the SBT-11A is mounted externally:
With GM tube detectors taken care of, we can move on to a more exotic detection technology.
A scintillation detector measures ionizing radiation by combining two components: a scintillator (usually a crystal) that emits photons after an interaction, and a photomultiplier tube (PMT) which uses a photocathode to detect photons via the photoelectric effect (multiplying the signal with a series of dynodes).
The intensity of the light produced by the scintillator is proportional to the energy deposited by the incident radiation - this is what allows us to create the histograms used for gamma-ray spectroscopy. By identifying the energy-level of the peaks in the histogram, we can infer the source(s) of the incident radiation.
With an understanding of how they work, we can now build some scintillation detectors.
When it comes to scintillators, Thallium-doped Sodium Iodide (NaI(Tl)) is the most well-established and extensively used. It has high stopping power, very high luminescence, and the blue light they emit (~415nm) matches well with the optimal wavelength (420nm) for the bialkali photocathode found in most PMTs. The drawback is that NaI(Tl) is hygroscopic and must be hermetically sealed in a metal container (with an optical window) to prevent absorption of moisture from the environment (which degrades their performance).
We buy two NaI(Tl) crystals. The first a 1" (diameter) x 1" (height) cylinder:
The second a more impressive 2.5" x 2.5" cylinder:
Aside from the obvious (a bigger detector captures more radioactive particles), the thicker crystal increases sensitivity to high-energy gamma-rays (>= 1 MeV) by increasing the probability that Compton-scattered photons are detected before leaving the crystal.
There are some drawbacks to NaI(Tl), however. The metal encapsulation stops alpha-radiation and most beta-radiation, and low-energy X-rays are heavily attenuated. They are designed to prioritize gamma-radiation. If we want to be able to detect alpha, beta, and low-energy X-rays, we will need an alternative scintillator.
Thalium-doped Cesium Iodide (CsI(Tl)) is the solution. It is less hygroscopic which allows a thin metal film to be used in place of a thick metal enclosure - as a result it can detect alpha, beta, and low-energy X-rays. In addition, if we keep the crystal thin we can reduce the chances of a gamma-ray interaction. The drawback of CsI(Tl) is that it emits green light (~550nm) which is not a good match for typical bialkala PMTs (resulting in smaller pulses and lower resolution).
As a reference point, the commercial RAP47 scintillation probe uses a 1" diameter x 1mm thick CsI(Tl) crystal tuned for 20-40keV X-rays. We’ll use the same size for our CsI crystal, and will experiment with even thinner (which should make it even more selective for 20-40keV X-rays).
Obtaining a polished cylindrical CsI(Tl) crystal in these dimensions requires a custom order. I emailed numerous suppliers around the world and found one who could produce a small batch of 1" x 1mm and 1" x 0.5mm discs at a reasonable price: OST Photonics in China.
With our scintillation crystals obtained, we can move on to the other half of our scintillation probes.
As described earlier, a photomultiplier tube (PMT) detects photons (as little as a single photon!) with a photocathode which emits electrons when hit. These electrons are multiplied via secondary emission in a high-voltage (500-2000V) dynode chain, multiplying at each stage until collected at the anode and producing a measurable pulse.
To achieve the multiplying effect, each successive dynode requires a higher voltage than the last. This is achieved through a voltage divider attached to the end of the PMT.
When choosing PMTs for our scintillation detectors, we have a two major considerations:
We’ll start with the 2.5" NaI(Tl) crystal. The Hamamatsu R6233 is a 3" diameter PMT, with a peak sensitivity near 420nm:
To align the 2.5" crystal with the center of the 3" PMT optical window, we model and print a gasket:
For the PMT voltage divider, we will model and print a socket:
For connecting to the PMT pins, Molex 08-50-0106 are the perfect size. We can then solder the necessary components (resistors and a capacitor) onto the crimps. This allows us to easily remove/replace the PMT without having to desolder anything.
Next we tackle the 1" NaI(Tl) crystal. The Hamamatsu R6095 is a 1" diameter PMT, with a peak response near 435nm:
For the 1" CsI(Tl) crystals, the Hamamatsu R268 is a suitable PMT with 1" diameter and quite good response at 550nm:
Both the R6095 and the R268 use the same 14-pin style socket, so there is an opportunity to create a generic voltage divider PCB. The R6095 has 11 stages of dynode, while the R268 is a 10-stage tube - we could simply leave out one resistor in the divider.
The finished PCB:
We can also model and print a socket which will work for both PMTs:
By designing the socket in two pieces we can directly embed female DB25 crimps in the adapter for connecting to the PMT pins and soldering directly to the PCB.
We are now ready to assemble our scintillation detectors.
We start by coupling the scintillation crystal to the PMT. Both glass surfaces are cleaned with acetone, and an optical grease is applied to one of them. Optical interface compounds have a refractive index very close to glass, and their purpose is to eliminate glass-to-air and air-to-glass transitions. At each transition, a photon may be reflected back or refracted in another direction - by improving this coupling we can increase the amount of photons which reach the photocathode.
Our NaI(Tl) crystals are encapsulated in metal housings lined with a reflective coating (to maximize the amount of photons that leave the optical window), which makes coupling with the PMT easy. Our CsI(Tl) crystals are bare, which means after coupling with the optical grease we need to take the additional step of wrapping the exposed crystal with a reflective material. White PTFE tape works great for this purpose.
For our CsI(Tl) detectors, we want the end of the crystal to have as transparent a barrier as possible (since we are targeting low-energy X-rays). To do this, we will leave a window of 0.8 mg/cm² aluminized polyester known as Alpha-Film (aka Rad-Film). This material will block light without substantially reducing low-energy X-ray transmission.
With our crystals coupled to the PMTs, we wrap everything in black electrical tape to prevent light leakage (we do not want any external photons to reach the photocathode).
Our PMTs are ready to plug into their sockets:
PMTs are extremely sensitive to magnetic fields - even the earth’s magnetic field can cause noticeable shifts in their output. To combat this, we will surround each detector by 0.8mm of solid mu-metal:
All of this will need to be packed into an enclosure. We can model and 3d print what is needed:
For the CsI(Tl) probes, the bottom is left open to maximize transmission:
For the connectors we will go with BNC, and for the cables we will go with RG58/U coaxial cable. A chassis-mount BNC adapter works great for our design:
Putting it all together, we have our lineup of gamma spectroscopy probes:
And a complete gamma spectroscopy system:
If we want to use our gamma spectroscopy probes for their intended purpose (producing a spectrum of energy peaks to identify radioactive isotopes), we will need to shield it as best we can from background radiation. Beyond cosmic rays, our probes will count gamma rays given off by concrete and other materials in the surrounding environment. We should do our best to block as much of this noise so that our signal produces clean, distinct peaks without getting lost in the noise (especially problematic for low-activity samples and trace isotopes).
Everyone is familiar with the use of lead as a shield against radiation. Lead hits the sweet-spot: it is dense, soft/malleable, has a low melting point (so is castable), is radioactively quiet, and is reasonably priced (relative to the denser and harder tungsten, for example). It is also commonly used in roofing as a weatherproofing material (lead flashing) which makes it easy to obtain.
There is one problem, however. When lead is hit with high-energy gamma rays, it emits X-rays (~75-85keV). If we want a clean spectrum, these X-rays will need their own layer of shielding. A graded shielding method addresses this - we can add a layer of copper to address the X-rays from the lead. Of course, copper itself exhibits a similar phenomenon, so another layer could be added to address that, and so on. Unfortunately, each additional layer of shielding adds additional backscatter, which adds noise back to our spectrum. We have to strike the right balance.
Thanks to the efforts of users at the Gammaspectacular forums we can come up with target dimensions. To cut the background radiation in half, we will need 13.27mm of Lead, and to cut the lead X-rays by 75% we will need 2.04mm of copper.
We end up with 10.5mm of lead surrounding a 4" diameter copper tube (2.0mm thick), 12" in length. The entire outer surface is covered with tape to reduce lead exposure when moving the shield.
This gets us close enough to our goal:
For a cap, we use a similar arrangement: a copper disk (2.82mm thick) on top of lead flashing (3.84mm thick).
When in use, the top will be left often but the sample will be placed at the very bottom of the tube for maximum shielding:
(The ideal graded shield would consist of lead, tin, then copper. We will consider adding a tin layer if our current design is insufficient.)
With everything built, we need to calibrate the spectrum generated by our MCA. To do this we a material which produces a known spectrum - a radioactive source.
We obtain Cs-134 and Tl-204 check sources:
Cs-134 produces strong peaks at 605 keV and 796 keV. By maintaining source-to-detector distance and keeping our shield in place, we can use either of these lines to calibrate our MCA and probes. Once calibrated, we can characterize the performance of our probes.
The measure commonly used to quantify the resolution of a gamma spectroscopy probe is the “full width at half maximum” (FWHM) of the spectral peak:
With the resolution defined as a ratio:
Resolution(%) = FWHM (keV) / Epeak (keV)
Expected ranges for our probes would be:
We measure our probes and obtain the following resolution:
Our probes perform better than expected!
The full collection of radiation detection gear:
Background radiation measured with the 2.5" x 2.5" NaI(Tl):
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Background radiation measured with the 2.5" x 2.5" NaI(Tl) from inside our lead shield:
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