Scintillation Gamma Spectroscopy: Setup

My DIY Gamma Spectroscopy setup is finally completed! 

A Gamma Spectroscopy system is comprised of 6 major components:

1. Spectroscopy-Grade Scintillation Detector - this is the single most important component and the "heart" of the system.  

The detector is an RF and magnetically shielded (with Mu-Metal) and light-tight assembly of a scintillation crystal or scintillation plastic (for example NaI(Tl) - Thallium doped Sodium Iodide, CsI(Tl), CeBr3, LaBr3, Bicron plastic, etc). The crystal itself is surrounded with MgO reflector, except for one of the sides interfacing the Photomultiplier Tube and encapsulated in an air-tight aluminum canister with a glass window. NaI is an extremely hygroscopic compound, and it must be perfectly sealed to prevent moisture from destroying the crystal. 

The canister's window is is coupled via an optical interface (optical grease) with a Photomultiplier Tube (PMT) and a Dynode Voltage Divider for the PMT bias is usually mounted on the PMT's socket. 

The low-energy light photons produced by NaI(Tl) crystal as a result from high energy gamma photon excitations are in 320-550 nm wavelength range with a maximum emission at 415 nm.

In my setup, I use a Scionix Holland 38B57/1.5M-E1 detector with NaI(Tl) crystal 1.5" x 2.25", Hamamatsu R980 10-stage PMT and a dynode voltage divider, I custom built with 2M for R or total impedance of 24M.

My modified scintillation detector.

 I took apart a commercial Sodium Iodide (NaI(Tl) Scionix-Holland) detector from an Exploranium GR-135 and reworked the whole Dynode Voltage divider circuit. In addition, I installed a bulkhead BNC connector on the cap of the housing - the original detector just had cables coming from a rubber grommet on the side of the cylinder.

These are older but quite nice, all-around, spectroscopy-grade detectors that can be found used as part of surplus sales and are not outrageously expensive as most such detectors - mine came from a "Border Patrol contract" GR-135 unit.

I was lucky to find a unit with ~7.5% resolution at 662 keV - while this is typical of these detectors, if the device was abused, crystal was cracked or moisture got inside, performance will greatly deteriorate, or it could be rendered completely useless for Spectroscopy.

The scintillation detectors are very expensive and fragile devices and should be protected at all times (!!!). The housing must be sealed air-tight as ambient moisture will destroy the crystal, any ambient daylight will destroy the PMT when bias voltage is applied! Tiny light leaks will spoil the measurements, mechanical shock (even a simple drop) can damage both, the crystal and break the glass-envelope PMT (which is a vacuum tube in an essence), bias over-voltage will shorten the PMT's life, etc.

 I am storing my detectors in a well-padded Pelican case with some Silicagel bags to absorb moisture.

One should pay attention at the cosmetic condition (as everything is factory sealed anyways) when buying a used and/or untested detector as this usually can show evidence of rough handling. The bottom, aluminum part where the crystal is encapsulated should not have any dings (!) or deep scratches. 

Considering how fragile these devices are, the risk of buying a damaged / dropped detector is very real! 

The original detector as just removed from a GR-135 unit cannot be used directly with GS-USB-PRO - the Dynode Voltage Divider must be completely reworked. 

The role of the PMT voltage-divider is to provide HV for the tube's Cathode-to-Anode bias and gradual voltage to all of the dynodes in-between, as each dynode receives higher voltage than the preceding one, thus progressively increasing the gain with each consecutive dynode stage (Hamamatsu R980 PMT has 10 dynode stages). This process achieves multiplication of the electrons in an "avalanche effect" until all electrons are collected by the anode. The overall multiplication factor follows the level of the High Voltage bias input.

The original divider in 38B57 is a transistorized circuit and has a number of active components - it is designed to work with the firmware in GR-135 where the linearity is compensated for in the software and tailored for a specific energy range. Such divider will provide very poor linearity if used directly, in a conventional way - a "classic" PMT dynode voltage divider is needed.

This is the original circuit on the back of the PMT. Cables are fed thru a sealed rubber grommet on the side. Easiest way to rework the divider is to completely remove the board. As there is no socket for the PMT and the PMT itself is equipped with wire pins which are not suitable for a socket, all 12 pins must be de-soldered from the PCB. The PMT is a glass-envelope vacuum tube and caution must be exercised when de-soldering.  If the stainless steel outer housing is not removed (and it is nearly impossible to be removed non-destructively as it is glued extremely well), installing the board onto the PMT leads can be quite difficult but not impossible - it took me about 20 min to re-align the stiff lead wires with the PCB holes. I just didn't want to even try to remove the main detector housing - it is glued and sealed to the crystal housing with a very strong adhesive, and I just didn't like the idea of disturbing the assembly.

Caution!  The PMT in 38B57 is glued and sealed directly to to the front of the crystal inside the lower aluminum housing and it can not be replaced if damaged without destroying the crystal. Basically, other than the voltage divider, nothing else in these units is serviceable. It is an all-in-one type design with no separate encapsulation for the crystal. 

Alternatively, one can work on and rebuild the voltage divider even without removing the original board - I did it for another detector and it was not difficult at all.

I removed all of the old components and had to figure out way how to apply the new divider circuit using the old PCB, traces and pads. 

The board was cleaned from solder and old flux before soldering the new voltage divider components - all of them are SMDs. The pin numbering on this picture is wrong!  This was a temp numbering as if using 14pin socket - this PMT has 12 pins - 10 stage dynodes, Anode and Cathode.

The reworked Dynode Voltage divider, using metal-film, high-voltage SMD resistors and HV ceramic 3kV capacitors. R=2M, 2R=4M for a total of 24M. All resistors are within 1% tolerance and actually better - I purchased extra resistors and used a LCR bridge to select and match 10 resistors as close as possible to the designated value and to each-other. The reason for using 2MOhm resistors in the divider instead of 1M, often used in lab setup PMTs is to minimize the voltage drop as the GS-USB-PRO's HV Power Supply is not incredibly "stiff" as a lab supply so a bit higher impedance helps . 

I was able to use the old pads and traces with only one additional small jumper-wire. BNC connector was installed on the cap, sealed and connected via small diameter short coax to the PCB. Unfortunately there is not enough space for an SHV type connector with its longer, insulated center conductor lead. The PMT's Cathode, Mu-metal tube shielding and the detector housing were all grounded together.

The resistors values in the voltage divider are specific for use with GS - the overall impedance is too low if used as a simple scintillation probe with a battery operated Ludlum-type meter for example - in such case 120M divider is expected with R=10M and 2R=20M correspondingly.

 The PMT's dynode voltage divider - a pretty standard circuit with the exceptions of the values in my case R is 2M and R1 is 2R or 4M. 

If this detector is modified for use as a standard scintillation probe (with a survey meter like Ludlum for example), R should be 10M with 2R = 20M - just like on the schematics. Higher impedance is needed in such case for these battery operated meters as their voltage supply is not as "stiff".

One thing I was very pleased to see is that the PMT inside the stainless steel housing had a separate Mu-Metal magnetic shield - both, shield and housing are also internally connected to GND.

My Gamma Spectacular GS-1525 NaI (Tl) detector. 

This detector offers better resolution (~6.5% FWHM for Cs-137 / 662 keV) then the Scionix-Holland 38B57 (7.5%) using EPIC 1.5" x 2.5" NaI(Tl) crystal and ADIT PMT but at a much steeper price tag (and it ships from Australia). Same bias voltage - 650V is producing higher level pulse signals. This detector is equipped with SHV connector for both bias voltage and signal.

The beauty of this detector is the encapsulated crystal with an optical window and a separate PMT which allows for service and experimentation with various detector components. 

2. Detector Driver - This device is the electrical interface between the Detector and the MCA - it usually contains a High-Voltage Bias power supply for the PMT, coupling/decoupling circuit, an adjustable gain pre-amplifier, ADC and a Sound Card USB chip (in my specific case).

I use Gamma Spectacular GS-USB-PRO (www.gammaspectacular.com) as a Detector Driver and this device is great - I love it and highly recommend it! The unit connects to the USB port and powers the detector as well as it provides the analog (external sound card needed) and digital (built-in sound card in GS) interface to the MCA software. It is a highly configurable, flexible and adjustable device. GS-USB-PRO also provides a very nice stable HV bias supply /w voltmeter and allows for super-easy adjustment of the high-voltage to the PMT - range is 0 to 2000V. 

I run my detector at 650V - it is a good compromise - lower voltage gives you better linearity but low gain, while high-voltage affects the linearization but gives more gain to pick up weak gamma-emissions (and noise of course). The exact optimal voltage should be determined experimentally because it is tied to the particular condition of the crystal, PMT and voltage divider as well as to the targeted energy range.

All active components of my Gamma Spectroscopy Setup.

The GS-USB-PRO Detector Driver - compact and attractive design. SHV and BNC connectors on the front panel alongside the USB port, analog audio jack and all trimmer adjustments for the high-voltage, pulse shape and volume.  On the bottom of the housing there are 2 switches for selecting detector wiring mode and the analog jack mode. The analog jack can serve as an input to connect two units for coincidence measurements. My GS unit is the latest v. 3.2 hardware revision with all modifications to accommodate the Schmitt trigger daughterboard for use with Neutron Detectors.

There is a bright digital voltmeter on the top side of the unit. The trimmer pot on the front is adjustable with a small screwdriver for a range of 0 to 2000V. The voltage readout is absolutely stable and adjustable to 1V resolution.

3. MCA or Multi-Channel Analyzer - this can be either a stand-alone external hardware unit or a software component running on a PC. I use the software version - actually a few of them, running together on my laptop - PRA, Theremino MCA and BecqMoni (all free software btw.). Each application has its own pros and cons but fortunately they can run simultaneously on the same machine and listen to the same detector.

Theremino MCA displaying a typical Thorium-232 spectrum.

PRA displaying the results from a Lu-176 (LYSO Crystals)  Gamma-Spectrum Analysis.

If the Detector is the "heart" and the Driver is the "spine", then the MCA is the "brains" of the operation.

What the MCA does is pulse detection and acquisition thru the sound card input, pulse filtering to reject malformed and distorted pulses by evaluating the pulse shape, classifying the pulses by placing them into "bins"/channels based on their energy and building a number of histograms and graphs out of the processed data.

Energy calibration is done using calibration sources, allowing the software to correlate known gamma energies to specific channels. The software will also perform Gaussian detection over the Pulse Height Histogram and detect peaks as potential Regions Of Interest (ROI). A variety of tools such as Detector Resolution Compensation and filtering can be used to process the acquired spectrum.

 The software is not extremely complex but requires a good bit of understanding and knowledge in Gamma Spectroscopy and pulse detection and acquisition. Very little is automated and manual evaluation of the spectrum with the help of reference sources is generally needed to identify unknown radio-nucleoids.  

4. Interconnects - these are mainly the cables connecting the Detector to the Detector Driver and cable connections for audio and USB to the computer. 

GS-USB-PRO supports "Single cable" configuration (which is what I use) where HV bias and signal pulses share the same coax cable) as well as "Dual Cable Mode"  - 2 separate lines - one for the PMT HV bias and one for Signal. There are 2 connectors on the front - in a "Single cable mode" only the SHV connector is used. In "Dual Cable mode", configured by a switch, the signal is received over the BNC connector while SHV is used for the PMT bias only. 

The critical part is the Signal cable as it should be of a high-quality coaxial type with a fairly low capacitance - best is less than 60pF total (with both connectors installed on). Higher cable capacitance widens the pulses (which normally are very short) and makes their shape "mushy", especially the trailing edge.

The high-voltage cable should be done with a properly shielded coax cable, able to withstand up to 2000V without arcing. For anything above 1000V I would use SHV type connectors (rated at 5000V/5A) - regular BNC are not really meant for high voltages. I installed a female BNC connector on my Detector housing so my single coax is SHV-to-BNC as the detector runs on only 650V and there is not enough space for the extra-long SHV type connector..

Cables are part of the calibration chain - i.e. changing cables WILL require a re-calibration.

5. Detector Shielding - for more details on making the Lead shielding for my detector, please see here!

Portable, field deployment

Lab setup

Part of the inner copper shielding is clearly visible inside the main shielding shells. There are additional, removable copper-tin cylindrical inserts (not pictured here). They are made by rolling a "sandwich" of thick copper foil and real tin (pewter) foil into a tube with wall thickness of 1 mm copper on the inside - 1 mm tin and another  0.5 - 1 mm copper on the outside.

Main detector shield (top) and the "large sample holder" (bottom) mated together - the lead thickness is 22mm around the crystal in the main shield or up to 35-50 mm once the outer sleeve shield and (optional) bricks are installed. In the wooden shield-carrier cradle, the main shield assembly is situated over 3/4" thick base of lead bricks and the outer shield (sleeve) is placed over the assembly on lead raisers.

A stack of bismuth, cadmium, tin and copper disks is lining the bottom of the sample chamber. Pictured here are the tin and copper disks.

This is called "graded shielding". A coper-tin-copper sleeve insert is placed on top of the stack lining the walls of the sample chamber. Similar but longer sleeve insert is located in the upper shell, wrapping around the detector assembly.

A 1/16" thick Bismuth metal disk (on the left) and the Cadmium metal disks as part of the graded shielding for the very bottom of the sample chamber. The goal is to absorb as much as possible of the low energy XRF induced by the gamma rays in the lead shielding. 

Outer lead shielding - 12.7 mm of lead thickness.

 Nothing beats the looks of an OD Green-colored "canister" resembling device with a bright yellow "Radioactive" sign 😁.

Look inside the detector cavity of the main shield.

The little removable copper cup on the bottom is used to hold small high-activity samples or calibration sources or it can be used as a "raiser" (when flipped) to bring the sample closer to the scintillation crystal. 

6. Calibration Sources - these are extremely important aids for the proper energy calibration of the instrument. Without accurate calibration the measurements are more or less meaningless.

In theory, any strong radioactive source with a known spectrum and activity can work. 

By now, I should have approximately 0.736 uCi of activity left in my Cs-137 Calibration Source made in April 2007. 

The 1uCi Eu-152 and Co-60 were freshly made in the beginning of August 2020.

 The uncertainty of these sources is generally +/- 20% of the specified activity.

Decay Scheme of Cs-137

Decay Scheme of Co-60

The "classic" Cs-137 Gamma-spectrum (peak @661.6 keV) is a good mid-range starting point for the calibration process.

Eu-152 Gamma-spectrum with multiple peaks nicely spaced up to 1408 keV.

There are other energies emitted by Eu-172 but their intensities are just a few percent.

This spectrum was produced with "Sum Quantity" feature in PRA

One of my Lutetium calibration sources - 1 gram of 99.9% pure natural Lutetium sealed in the center of 1" plexiglass disk. 

Natural Lutetium contains 2.6% (or 26 milligrams in my sample) of the primordial Lu-176 isotope and it produces two very distinctive gamma photopeaks at 201.83 keV and 306.78 keV and an X-Ray (Kα1 Hf) peak at 55.79 keV. There is another Lu-176, weaker photopeak at 88.34 keV. The natural Lutetium metal has an activity of ~51.6 Bq/gram.

This is a fairly inexpensive calibration source with a price of around $8-10 per gram for the pure rare-earth metal.

Lutetium and Potassium Chloride are excellent "air-travel" calibration sources for when bringing the spectroscopy setup to the field - their radioactivity is extremely weak (Geiger Counters will not even register activity), making them completely safe to carry on-board of an airplane. (attempting to board a plane with a Cs-137 or Co-60 source will likely result in a lot of troubles)

11 hours spectrum scan of Lu-176 from 2 LYSO crystals in a Lead castle (with 9 hours background spectrum subtracted). The plot also shows the Sum peak at around 508.61 keV as well as the 55.79 keV Kα1 Hf-176 X-Ray peak.

The 509keV Sum peak occurs due to the coincidence emissions and detection of the 202kV and 307keV photons.

A Detector Escape peak from the NaI in the detector crystal is usually observed at around 26 keV in the Lu-176 spectrum. This Iodine Escape Peak comes from the K-shell of the Iodine when it interacts with higher energy X-rays and the resultant emission escapes the detector. it is located ~28keV (the binging energy of the K-shell electron of the Iodine) lower than the incident photon causing it - in this case the Hf-176 Ka emission at 55keV.

The escape peak is prominent when the energy is below 200keV - because the difference is so small (only 28keV) for higher energies it falls within the main photopeak and it is usually masked.

Spectrogram-wise, there is no discernible difference between LYSO crystals and metal Lutetium.

LYSO crystals are used in PET detector arrays can be found from a number of sources.

In the absence of a proper strong calibration source, for two-point calibration one can use Potassium (K) which has a small amounts (0.012% of all Potassium) of the primordial isotope K-40 with gamma @ 1.46 MeV (actually, I use about 100g of Potassium Chloride (KCl) as my K-40 source) - such calibration takes fairly long time in order to get a good, measurable peak as the activity is pretty low but while doing this, I can also observe the positron annihilation peak at 511 keV as another point of reference for the calibration. Generally it is good to have a good separation between the 2 peaks and to encompass the targeted energy range. Sources like Eu-152 and Na-22 give very good results for energy calibration. 

Normally, my first step is calibration with a Lu-176 source (using Lutetium metal or Lutetium-Yttrium Oxyorthosilicate (LYSO) crystals) with gamma photopeaks at 201.83 keV and 306.78 keV, followed by the "classic" Cs-137 (1.0 uCi) disk source at 661.66 keV and the X-ray peak at 32.19 keV.  Next is calibration of the high-energy region using 1uCi of Co-60 disk source at 1.173 MeV and 1.332 MeV. As a final step and confidence check and to fine tune the linearity of the detector, I use 1.0 uCi disk source of Eu-152. Europium-152 produces a number of peaks throughout the spectrum and it is a good way to check the overall calibration accuracy. 

Thorium (Thorium Dioxide (ThO2) from a gas mantel) also works well to check the linearity of the detector as it has many peaks from daughter products which are spread throughput the useable gamma spectrum to as high as 2614.5 keV (Tl-208)

Natural Uranium test-source.

1" plexiglass disk with a center hole, housing Autunite crystals. The crystals are completely and safely sealed inside epoxy resin. The resin block all Alpha particles - the Autunite activity was around 6000 CPM but after filling the cavity with epoxy, sealing the mineral, it dropped to 2200 CPM. 

This source gives a nice "classic" uranium spectrum with Photopeaks of various daughter products.

Collecting background spectrum for subtraction and energy calibration should be done before AND after main spectrum acquisition to minimize effects of thermal drift and changes in the background - this is especially important for low-activity samples.

Conclusion

I love the portability and compactness of my setup - everything fits in 2 Pelican cases and can be carried in the field. One case contains the detector, driver, interconnects, etc. The other case holds all of the lead shielding components (and weights over 70 Lbs!). The calibration sources are stored inside the shielding for transportation.

High-activity sample setup - less lead shielding is generally needed.

Low-activity sample setup - additional lead is added to suppress the natural radiation background and the detector is fully encased (4xPi) in lead.

It is amazing to think that once available only in Scientific Laboratories and Universities, Gamma-Spectroscopy is now affordable and enjoyable for hobbyists and Nuclear / High-Energy Physics  enthusiasts!  

Special Thanks to Steven Sesselmann for hosting the Gamma Spectacular Science Forum.

Scintillation Gamma Spectroscopy: Shield

Since I was working with and using a lot of lead for my "Lead Pigs", I decided to prepare the shield of my Gamma Spectroscopy (GS) setup.
Shielding the GS detector from the environmental radiation is extremely important for low activity samples and detection of trace isotopes. Some weak peaks will be otherwise masked by the background radiation noise. The scintillating detector is a supper-sensitive device producing hundreds of pulses per second just from the background so the more Lead for shielding, the better! You cant really over do it with the shielding. There is a parameter called "halving thickness" for Pb and shielding can be easily 50lb or 150lb of Pb - based on what are the limitations - space, weight, cost, portability etc. 

Generally speaking, the weaker and smaller the sample is, the more shielding is needed. For example - energy calibration with 1uCi of Cs-137 requires no shielding, while looking for traces of Uranium or Thorium in a small mineral sample or soil could require hundreds of pounds of lead to bring the peaks out of the background. Traces of Co-60, producing high energy gamma in 1330 keV range could "hide" under the broad 1460 keV K-40 peak and at these energies a lot of lead is required.

Shielding for Spectroscopy, on the other hand is not that simple - "just pile up lots of lead" and you are done! Besides the environmental background, one should worry about what is taking place inside the shielded volume itself - samples emitting beta particles or strong gamma will cause secondary X-rays. This is also the case of low-energy X-Rays induced in the shield by the primary gamma rays as part of the Pb X-Ray fluorescence (XRF). 

Geometry and distance between sample and the detector crystal is also a factor in the efficiency.r Backscatter will produce additional peaks as well.

Additional shielding from different materials is needed to slow down the beta particles before they hit the lead in the main shielding and also to absorb / suppress the secondary x-rays - tin, bismuth, cadmium and copper are normally used.

My detector shield is constructed from 3 layers - outer shielding, which has a total thickness of 12.7 mm rolled lead sheet on a 4" form, a 22mm Lead cast main shield around the detector (made of two parts) and inner shield inserts made of thick copper foil and tin / pewter foil.

A final consideration is the mechanical constructions and geometry - the best ratio between shielding efficiency and amount of lead used (read: weight and cost) is achieved when the sample chamber is kept small and most lead is placed as close as possible around the detector.  As the radius increases, more lead is needed to achieve the same attenuation as when the lead is right around the sample volume and detector..

These are the tools I used for the casting process in addition to about 20+ kg of "soft" Lead ingots

The key component is this "5kg Casting Clay Graphite Crucible", purchased on eBay for $30.

The Lead ingots were melted down using a large propane "Weed torch". 

I constructed a very simple furnace out of concrete blocks. After all of the lead liquified, I scooped out the floating oxides and impurities with a steel spoon and turned off the flame. The crucible keeps the lead molten for at least 7-8 min with no flame going so there was a plenty of time to work with it. There is a little value in getting the molten lead very hot - it just oxidizes faster and one must wait longer for it to cool down.

To make the cavity /hole for the detector, I used an 8.4 oz can of Red Bull, tightly packed with fine sand and clay. I also added some cement just to harden a bit the contents - as the lead cools down it exerts uneven pressure on the walls of the can and it will wrinkle them if the walls are not supported. 

The can was inserted and centered into the molten lead and held in place until the Lead cooled and solidified. 

A word of caution (aside from the dangers of working with a large volume of molten metal) - nearly everything floats in molten lead so inserting the aluminum can all the way down to the bottom of the crucible, displacing the liquid lead,  centering it and holding it in place until the lead cools down - all this, while fighting with the can's buoyancy was a bit of a chore - thick casting gloves and a steel rod, inserted in the can as an improvised "handle" helped a lot. (it took 2 attempts until I got it right in terms of procedure and exact lead quantity). The bottom of the aluminum can had to be submerged,  initially, at an angle to allow the air to escaped from the concave cavity on the bottom of the can. This picture is from my first attempt - l kept this first cast and turned it into a "lead pig"/ sample holder-chamber.

 The cast shield, after emptying the can of sand and peeling it off the internal wall of the cast. A few taps with a small hammer punched the hole where the bottom of the can was - the lead is very thin in this area due to the concave bottom of the can and its sharp edge. It worked perfectly, leaving a nice round edge hole.

The thickness of the shield wall where the crystal of the detector is situated is more than 22mm and it tapers off  gradually in a "bullet shape" (after approx. 60mm height) to 6mm wall thickness at the very top.

As the lead cools down, it shrinks before solidifying completely, leaving an uneven void on the top of the cast. I used a propane torch to melt another 500g ingot and "top it off" filling all of the gaps and smoothing it out while the cast was fairly hot.

The inner wall will be covered with 3-layer "sandwich" of the inner shield insert - rolled tin pewter, 2" copper pipe and aluminum foil on the inside.

 All it took was just a few gentle taps for the cast to come out of the graphite crucible and the result is nearly perfect! The crucible's shape and material are optimal for an easy release of the cast.

The inside diameter is 55mm and overall height is 110mm (the dimensions of the crucible would have probably allowed me to add yet another 10mm of height)

 Here is the shield, in place on top of the sample holder.

The outside diameter of the shield matches perfectly the aluminum flange of my sample holder.

A strip of lead sheet 1/8" thickness, 6" wide and 60" long (~23 lbs of Lead) was rolled onto a 4" ID Schedule 40 PVC pipe form as a 12.7 mm  (1/2") thick Lead "sleeve" (4+ complete turns). Before rolling the lead, I also added 2 turns of copper foil over the PVC. The sleeve (pvc form + copper + lead) is held together with duct tape in a nice, tight, easy to handle package.

The entire sample holder is to be placed in a small lead castle made out of 1" thick lead bricks in a couple of layers - all around, overlapping a bit the bottom of the middle shield.

Start to finish it took about an hour to complete the cast.

There is a plenty of space between the shield and detector for the copper, aluminum and tin insert around the probe and lining also the inner lower volume, holding the samples.

I used the short cast from my first attempt and filled the bottom of the opening with about 1 lbs. of molten lead, creating an improvised lead "bowl" / chamber. This is my "large sample holder". 

I also cast a small 1 cm thick lid for when it is used as a stand-alone "Lead Pig" for temporary desktop storage of hot samples.

This is what the main shield looks when both pieces are put together. The bottom of the detector crystal sits just above the seam. 

With small samples, it is possible for the detector to be completely inserted, moving the crystal into the lower part. In this case, the top cap of the detector sits flush with the top of the shield and the detector is completely recessed.

I had to build a wooden box / cradle so I can keep everything together and transport the entire shield assembly. The shield is pretty heavy - over 35 kg of pure Lead. 1" Lead bricks create a base for the bottom portion of the main shield  (placed in a polyethylene foam spacer - later I replaced the polyethylene with wood). 

Also, visible is the copper foil lining the inside of the two parts of the main lead shield - self-adhesive copper tape was used to cover the inner walls. Inserts of much thicker rolled copper foil, as well as 2" copper pipe section are placed in the detector shield and the sample holder. All this is surrounded with tin-pewter sheets. The bottom of the sample holder is lined with a stack of 6 copper disks and 6 tin-pewter disks.

Some of the components of the modular lead shield.

Four Lead bricks are placed around the base of the sample holder . These bricks serve as a raiser for the outer sleeve.

There are 4 more bricks, laying flat, lining the bottom of the cradle, right underneath the sample holder.

The top of the outer shield sleeve sits flush with the top of the inner cast shield.

Using 4" PVC pipe, I made a "collar" shield for the top portion of the detector. 

8 Lead Ingots were placed on the inside of the form. I had to carefully select the 8 ingots from a pool of  about 20, until I found the combination that will fit in - still, some force and the use of a mallet was required to jam them up on the inside.

The two outer shield components.

The collar, placed around the top portion of the protruding detector. 

A lead cap with a hole for the coaxial cable is then placed on top.

The complete "Large sample" version of the shield in all of it's glory - almost 70 lbs of pure Lead.

This version is not as convenient to use as the tray sample holder and it requires partial disassembly / assembly to access the sample chamber but shielding is much better.

The shield is modular, easy to transport, it takes only a couple of minutes to put it together and can be used in different configurations.

The 1"+ thick lead cap on top of the shielding. The lead cap was spray-painted with "Plasti Dip" rubberized coating to protect from lead exposure as it is handled often. It was casted in the same manner as the main shield but using a steel rod to displace the lead and make the hole in the center so no drilling was needed.

The counts per second rate of the background without and with the shield, The unshielded probe is reading an average of around 110-130 cps. After inserting it in the shield, the rate drops down to less than 5 counts/ sec. This is a factor of over 10 times reduction in the natural background.

Histogram of the shielded detector background count rate - average 3.2cps is not bad at all!

As I mentioned before, you cant really over-do the shielding part when dealing with weak activity - the more lead, the better it is for reducing the radiation background noise and bringing out the weak peaks. 

I found my amount of shielding to be adequate for most of my experiments as I don't usually do "super-low" activity testing as in food, water or soil samples. 

The current setup IMHO is a good compromise between amount of lead shielding and weight / cost / portability. The whole shield can be dismantled and transported quite easily and it doesn't require a special, sturdy support table as it will be with 200-300 lbs lead castle. 

DIY: Making a large "Lead Pig" container for Radioactive Minerals and Materials

  As my collection of Uranium and Thorium Minerals grew, I was in a need for properly shielded, large storage containers for my hottest specimens - I, certainly don't keep under my bed samples of torbernite, autunite or vials of Uranyl Nitrate but shielding is important factor in addition to distance.

Small "Lead Pigs" are readily available from a variety of sources, but these offer generally very limited space and are good enough to store small samples / vials with fairly low activity - mainly used in Nuclear Medicine. I normally use them to store vials with Uranium, Thorium or Radium compounds and very small mineral samples (unmounted, thumbnail size) or just as temporary desktop storage during experiments.
Any larger than an inch mineral specimens would not fit and often the thin lead walls simply would not provide sufficient gamma shielding - some rich minerals exhibit activity, well in the excess of 600K CPM. Furthermore, this is an extremely inefficient way (in terms of space and cost) to store large number of specimens even if they can fit in - not to mention that minerals are usually mounted and displayed in "Perky" boxes, which will never fit in these small containers. 

Large containers on the other hand get very costly to procure.

These small Nuclear Medicine "Lead Pigs" are fairly inexpensive and are available on eBay or from other online sources but the wall thickness can be as thin as 3/32" for the small ones although, most of them are with 1/8" lead wall and some of the better ones exceed 1" wall thickness. A nice feature of these containers is the secondary outer plastic shell container or epoxy paint which prevents you from handling bare, toxic lead metal. They are fairly light and easy to move around but the main issue I have, is the relatively small volume - they are really designed to contain small vials.

Thanks to eBay, I picked up a number of Lead sheets (99.9% pure Lead Metal) with size 12" x 12" x 1/8" and 8" x 15" x 1/8". Lead itself is not very expensive metal but shipping, due to it's density can run up the cost quite high - I've seen large, commercial lead-lined containers where the shipping cost is sometimes two times the cost of the actual container due to the enormous weight. Buying lead as sheets seems to be a good deal as often the shipping is just a flat rate regardless of the number of sheets, since the sellers use "USPS Flat Rate" boxes and can fold the lead sheets to fit.

Constructing the box is fairly easy - just like making a box out of cardboard in kindergarten - 4 cuts/slots, each approx. 3" long into the sheet on two opposing sides and 3" from each edge, then forming flaps by bending and folding the sheet of lead into a box. 

Using a sheet of 12" x 12" results in a box 6" x 6" x 3" and zero leftover. Pure lead is an extremely soft metal and easy to cut with a pair of  "aviation snips" - especially thickness of 1/8" or less. An added benefit of this size box, is that after folding it, two of the side walls will become actually 1/4" thick because of the folded flaps on the inside are doubling the wall thickness (later, I had to put 2 small inserts to fill the gaps because the internal flaps will get a bit shorter due to the bend radius)

To make the folds, I used two thick aluminum plates and a few clamps, sandwiching the lead sheet in the middle and using the edge of the aluminum plate for a nice, sharp bend, folding it by hand and tapping it with a rubber mallet.

The lid was made out of another 12" x 12" lead sheet with an overlapping lip of 3/4" and size of approx. 6" 1/4 x 6" 1/4. I just flipped the box over the stock sheet and traced the outline, then added the dimensions of the lip overlap. The folding process for the lid is almost identical to the one for the box but it was a bit more challenging, using bare (gloved, really)  hands, due to the narrow strip of thick material to fold.. Gentle taps with a rubber mallet along the sides did the job quite nicely. One can even do a Telescope-style box where the cover extends all the way down, essentially doubling  the side walls but I was going to use a larger hard shell and needed to be able to center the box inside.

Once the lid was cut and roughly formed, I folded the lid flaps (this time, on the outside) and using a hammer and series of gentle taps, I did the final fitting, so the lid closes nice and smooth. The Lead metal is so soft that hand forming it with a little bit of tool help (small rubber mallet and hammer) and one can achieve absolutely perfect and smooth fit. The total weight of this inner container when empty is about 12 Lb. and provides a volume of 108 cubic inches. 

I didn't use any sealant on the inside seams (corners / flaps) as I don't want it airtight and to trap any Radon gas generated by the Radium as a daughter product.

Such lead container can really benefit from a hard shell - the soft lead is fairly easy to accidentally deform and damage if the box is dropped, handled too roughly or just stacked.

I constructed a wooden box using 9" x 3"1/2 x 3/4" ("4 x 1 lumber") pine for the sides, 9" 3/4 x 9" 3/4 plywood for the lid and bottom and coated decking screws. 

The handles were made of strong Dacron rope and pieces of vinyl tubing for added comfort. The extra long handles afford more distance from your body when carrying the container. 

I made the outer wooden box slightly larger (8" x 8" on the inside) and lined the walls and bottom with the same 1/8" thickness lead sheet material. The top is covered with yet another, secondary lead lid - a removable 8" x 8" x 1/8" plate placed over the inner box lid (this plate can be setup as a portable shield, once the container is open).

Each of the side plates for the lining are sized 1/8" shorter the inside space and they lock each-other in place by being offset. The bottom square plate has dimensions with 1/4" shorter than the inside space and it is installed after the side plates, locking them into place so no additional fasteners are needed for the lining.

The holes for the rope handles were drilled thru the wood sides and the lead lining.

 

The height of the side plate lining is 1/8 shorter than the depth of the box, so when the top plate / cover is placed, it  sits flush with the to wooden box edge. The top lead plate (cover) is with the exact dimensions of the inside space and it rests on the edge of the side plates.

 I can add more plates or bricks of lead around the walls as more scraps become available from other projects or just fill the gap between the lining and the inner box with lead shot. Currently the total lead thickness of the inner box + lining varies between 1/4" (6.3mm) (top and bottom) up to 3/8" (9.5mm) (sides)

The whole thing is quite heavy in it's final configuration - 30 lb /13.6 kg. of lead.

My ultimate goal is to have at least 1/2" or more Lead metal all around. Lead sheets can even be nailed on the outside of the wooden box if need be - when it comes to gamma ray shielding there is nothing wrong with over-doing it (except for the weight). 

1/2" Lead provides over 50% attenuation of Gamma Rays (also known as "halving-thickness") at up to 2 MeV (this is the equivalent of 1" steel or 7.2" water shielding). There is a point of a diminishing returns though - the amount of lead and the weight would not justify the amount of attenuation for low amounts of activity - you'll need for an example 3 cm of Lead (~1"  3/16) to attenuate the intensity to 1/8th of the original intensity. 

I also added HDPE (High-Density Polyethylene) plates (polyethylene is a very hydrogen-carbon rich plastic, which works well as a neutron shield / moderator, not that is needed here) to fill the gap between the inner lead container and the outer lead sheets, lining the inside walls of the wooden box. This acts as "padding" and keeps the inner box centered so the lid can be easily removed. 

Natural Uranium contains 0.7% of  ²³⁵U, known for it's spontaneous but neutron yield is absolutely not a concern. Solid blocks of paraffin (candle wax) would work OK for those planning to store strong neutron sources (²⁵²Cf anyone? 😀) 
This HDPE filler between the inner and outer box can also be substituted with small lead bricks or lead shot (from a local hunting supplies store) , if one is willing to deal with the extra weight. 

Future plans include to line the inside of the inner container with a "sandwich" liner of aluminum and copper foil - this will slow down the beta particles before they reach the lead shield and are abruptly stopped, thus reducing the generation of secondary low-energy X-Rays (the Bremsstrahlung effect caused by the rapid deceleration of a beta particle).

These X-rays can not make it thru the lead but can be an exposure factor when one is digging thru the contents. 

The Lead Box and my Infab Revolution Maxi-Flex X-Ray Lead Gloves (0.50mm Pb equivalent) for lengthy handling of extra hot samples or just digging thru the contents of the lead containers. 

I intentionally left the lead surface as bare metal and didn't paint it or cover it with duct tape, self-adhesive vinyl or rubber spray-on coating (all, good options btw.) for two reasons - one day I might need to re-use it for a different project (and don't want to deal with sticky residue) and also the bare lead metal lid (as weird as it might sound) is a good reminder for me: *GLOVES ON* when handling the contents of box (and honestly I do like the bare metal look).

All mineral samples inside, are organized by activity - the ones with highest activity are positioned towards the center of the volume and the lower activity rocks are placed around the perimeter as a natural shield.

I also installed a plywood cover with two small hinges to complete the outer box - all from Home Depot.

The wood surface was later covered with clear polyurethane varnish after applying the graphics. 

I built two of these "lead pig" containers.

I thought, at this point the containers were worthy of some stenciled graphics! 😀 

This way, it is easier to identify them as well.

After painting on the graphics, I applied 3 coats of clear polyurethane varnish. 

Yet another option is a Lead lined Ammunition steel can - in this case I used a .50 BMG Ammo Can which provides an ample space for large specimens - volume is approx. 11" x 6.5" x 5.5"

 The main drawback is the container's shape - "tall and narrow" so things have to be piled on top of each-other - not as optimal as the shallow boxes I made previously. In addition, a single handle is not the most convenient way to transport it if the lead lining is more than 1/4" - it gets awfully heavy to carry with one hand. 

On the positive side - lining with lead is very easy and straightforward - 1 x U-shaped piece of lead sheet for the large side walls and the bottom combined, 2 pieces of lead sheet for the front and back walls and yet one more for the lid. The front and back lining is the exact size of the walls and it is placed first. The U-shaped large piece is placed second and locks the other two pieces of lining in place.

The steel container is also very sturdy and air-tight due to the rubber gasket.

Stencil and Airbrush and the final look is pretty cool!

A probably unnecessary safety warning: use gloves when working with Lead, don't eat / drink while doing while dealing with lead metal and wash hands very well afterwards. Methods of cutting (like a saw or Dremel tool) which create lead dust are bad idea and will require respirator and cleanup.

Actual Lead Pig 😀

How Diffusion Cloud Chamber Works?

Theory of operation of the Cloud Chamber is fairly simple yet brilliant.
The Diffusion Cloud Chamber is basically a sealed volume, filled with Alcohol Vapors.
Conditions are created for a steep vertical temperature gradient inside the volume.
Alcohol is evaporated from a reservoir near the top of the volume where the temperature is higher and vapors fall down as they cool, towards a very cold plate at the bottom.
Just above the cold plate, the alcohol vapors reach a supersaturated state due to the deep cooling (colder than -26℃). 

In this state, the air above the cold plate is so saturated with alcohol vapors and they are cooled beyond the threshold of condensation (dew point) so the vapors only need a trigger - a "seed" around which to start the process of condensing into small liquid droplets, just like a normal fog.
The ionization trail of a charged particle provides these condensation "seeds".

(c) 2020 Andrey Stoev

(please note that the graphical representations of the elements above are not to scale with each-other in order to improve the clarity of the diagram)

When a charged particle enters the volume (or it is created inside) and passes thru the Cloud Chamber's Active Zone, it leaves an ionization trail. 

Gas atoms of oxygen and nitrogen from the air become Ions in the path of the energetic particle as the particle collides with the gas atoms and deposits some of its energy while knocking off electrons.

Once the atoms are stripped from even a single electron, the charge balance is no longer present - the atom becomes positively charged due to the positive charge of its nucleus.

Since the Alcohol and Water molecules are slightly polar, the molecules are then attracted to these ions. The ions serve as "seeds" / condensation centers and droplets of fine mist form around these condensation centers, tracing the path of the particle.

The condensation trails appear only in the active zone where conditions are right (alcohol vapors are supersaturated) and are white in color just like the normal water-based fog.
Black background and intense tangential lighting helps to improve the contrast, bringing them out and making them more visible to the observer.

Improved observation conditions are achieved by deeper cooling which extends the thickness of the Active Zone layer by creating a steeper temperature gradient.

Another feature (optional) which greatly improves the performance of the Cloud Chamber is the Ion Scrubber. "Ion Scrubber" is the scientific term for High-Voltage Electric Field applied to the chamber's volume. Typically, between the cold plate (-) and an electrode (+) suspended above the cold plate. 

CERN recommends about 100V/cm field potential.
The electrostatic field improves the performance of the Cloud Chamber in the following ways:
- clears the volume from airborne dust particles and contaminants at startup - this removes unwanted condensation centers
- deflects the con-trails in the Active Zone for a better observation
- "resets" the chamber by quickly removing (scrubbing) the ions so condensation trails don't linger for too long and new ones can be observed
- prevents ions from creating an alcohol "rain" over the active zone.
- improves sensitivity of the Cloud Chamber and increases the definition of the con-trails, making them crisp and well defined by quickly stripping off the "fuzzy" outer part of the con-trail.

It is very important that the air inside of the chamber is perfectly still as the fine vapor trails are very "fragile" and they will quickly disperse in the presence of air movement / turbulence. Any air leaks from outside will also bring in warmer air which will interfere with the operation of the Active Zone - a good, air-tight seal of the volume should always be maintained during operation!

Large Cloud Chambers usually have a special port to bring in radioactive samples or inject radioactive gas (Radon) without much air disturbance. Such specimens can be introduced also by gently opening the volume (lifting the cover) but it will take a few seconds for the conditions inside to normalize and the con-trails to reappear.

Trivia (credit: Wikipedia)
The invention of the cloud was Charles Thomson Rees Wilson. This was his signature accomplishment, earning him the Nobel Prize for Physics in 1927.  The Cavendish laboratory praised him for the creation of "a novel and striking method of investigating the properties of ionized gases". The cloud chamber allowed huge experimental leaps forward in the study of subatomic particles and the field of particle physics, generally. Some have credited Wilson with making the study of particles possible at all.
Cloud chambers played a prominent role in experimental particle physics from the 1920s to the 1950s, until the advent of the bubble chamber. In particular, the discoveries of the positron in 1932 and the muon in 1936, both by Carl Anderson (awarded a Nobel Prize in Physics in 1936), used cloud chambers. Discovery of the kaon by George Rochester and Clifford Charles Butler in 1947, also was made using a cloud chamber as the detector. In each case, cosmic rays were the source of ionizing radiation.

The diffusion cloud chamber was developed in 1936 by Alexander Langsdorf.

Diffusion Cloud Chamber (Conclusion)

The Diffusion Cloud Chamber in operation!

 

The complete Cloud Chamber setup, ready for power up!

The clips bellows are from the very first test of the completed Cloud Chamber.

Best viewed in a full-screen mode!

The Cloud Chamber is capable of displaying only charged particles such as α-particles (Helium nuclei), β^- particles (electron), β⁺ particles (positron), μ (muon), Cosmic Rays / proton clusters, etc..

It will not display neutrons, neutrinos and any other sub-atomic particles without a charge.

When I say "display", you don't see and you cannot see the actual particle but rather the ionized path and the resulting  condensation trails left behind a charged particle going thru the volume. 

I injected the Cloud Chamber with "Thoron" which is a short-lived isotop of Radon - ²²⁰Rn has a half-life of only 55.6 sec and it is an α-emitter (6405 keV). It is a daughter product of ²²⁴Ra, which, on the other hand is further down the decay chain of Thorium (²³²Th).
My son's reaction to this α-particle fireworks display was pretty rewarding!

Here is another example of Radon-220 ("Thoron") in the Cloud Chamber. To obtain the "Thoron", I placed a Thorium Gas Mantle (Thorium Dioxide - ²³²ThO₂) inside a large syringe and capped it for a couple of days - no point of waiting more than that as it is a very short-lived isotop (55.6 sec) and it will decay quickly after it is produced by Radium-224 (one of the Thorium series decay daughter products) so no substantial accumulation will occur. (In a couple of days, inside the syringe, near an equilibrium state will take place - the concentration of ²²⁰Rn will become stable as new ²²⁰Rn will be generated to replace the one which is decaying at that moment.)

I lifted the edge of cover and gently injected the air and Radon mixture from the syringe into the Cloud Chamber.
Similar experiment can be done with another Rn isotop - ²²²Rn generated by Radium Watch hands but then it is worth waiting at least 3-4 days longer for more Radon-222 to accumulate inside the syringe - ²²²Rn has a half-life of 3.82 days.

As a side experiment, after removing the Thorium Mantle from the syringe and expelling the gas contents, the "empty" syringe was still producing an activity of around 150 CPM on the outside due to β^- decay of other solid daughter products deposited after the Radon decay - namely ²¹²Pb and ²¹²Bi.
 This β^- activity, as expected died out quickly, over the next few hours and normalized to background levels as the decay chain reached the stable ²⁰⁸Pb. 

The clip shows some high energy β^- particles and some low energy β^- are also visible. Around 0:08 there is nice Y-trace, showing an interaction with an atom by a high-energy β^-. One leg of the Y is sort of zigzagging probably from the knocked off low-energy electron, the other leg of the Y is straighter, created by the original particle. There is also a nicely deflected low-energy β^- at around 0:07.

An α-particle, then a low-energy β^- particle and at the very end a high-energy β^- or a muon (μ).

Some really nice particle interactions!

This is the equivalent of a "Sub-Atomic Particles Disco-Club". On the left is a Natural Uranium doped glass marble, emitting mainly alpha particles and some beta by daughter products. In the center is a pellet of ²⁴¹Am (a very strong α emitter) and on the right is a vial filled with Tritium (³H) emitting very-low-energy β^-.
This video is from an early test before the Light bars were fully constructed.

A tiny piece of Uranium Ore (Uraninite infused matrix) attached to a Lego square. This piece produces only around 1000 CPM @ 1 cm from my GMC-600+ Geiger Counter. By the time this clip was filmed, the mineral was already covered with liquid alcohol condensate so not that many Alphas are able to escape but plenty of Betas can be seen, emitted by Uranium decay daughter products. The Cold Plate has been running at a steady -35.5℃ for over an hour -  the room temperature is +23.5℃.

This clip shows the Ion Scrubber in action and the difference it makes during operation! On the left side there is a tiny piece of Uraninite (UO₂) attached to a Lego square. There is a distinctive loud "click" in the audio from the switch when the High Voltage (HV) is turned OFF or ON. The first 10 sec. of the video is with HV turned ON, then 10 sec. of HV turned OFF and then again 10 sec. of HV turned ON. One can note that when HV is OFF, the tracks become "fuzzy" and less defined. Currently, the Ion Scrubber operates at 4.3 kV but I am planing to add a circuit for voltage adjustment.

The background clicking is from a nearby Geiger Counter.

A word of Caution: Aside from Nuclear / Radiation Safety (dealing with radioactive materials is not something, I will encourage without the proper understanding and training!!!) one should be very careful not to contaminate the volume of the Cloud Chamber - this can affect later observations of the background / cosmic radiation!

I must say, this was a real fun project to work on and a true kitchen table / garage project! My son and I had great time designing and working on the Cloud Chamber and the results absolutely exceeded my expectations. It is a nice portable setup and I have the feeling my son will be bringing it to school more than once.

Unfortunately, the Science Fair, we built the Cloud Chamber for was postponed due to the COVID-19 Pandemic but this will give him time to prepare all of the supporting materials and diagrams.

On the bright side, besides being a Cloud Chamber, this instrument is a good platform to demonstrate, study and experiment with multi-stage Thermo-Electric Coolers.

The High-Voltage Power supply and adjustable electric field strength is also useful for experimentation with electric fields.
The Liquid Cooling Plant is quite useful too for various physics, chemistry or electronics experimentation.

Just as with a Telescope - now I wish the Cloud Chamber was a bit bigger :-)

Next project on the table - DIY Gamma-Spectroscopy!

Diffusion Cloud Chamber (Part 4)

The Liquid Cooling Plant (LCP) was constructed on the same 1/2" plywood material (10" x 17") as the Cloud Chamber's top surface. 

One of the design goals for this project was a modular system approach. Usually, desktop space is very limited during a Science Fair at my son's school and we wanted to preserve it for supporting materials. By having a physically separate Liquid Cooling Plant, this cooling module can be placed on the floor or somewhere else, out of the way, keeping the focus at Cloud Chamber's Main Unit. The Static Head Lift of the coolant pump is listed as 5 meters which is more than enough if placed on the floor!
Another benefit of a stand-alone LCP is that this module can be used for other experiments requiring liquid cooling - all is needed is another pair of male Quick Connect/Disconnect adapters. For example, it can be used in our Chemistry Lab for distillation procedures, without the need for constantly running water.

The Liquid Cooling Plant for Stage 4 cooling is using a large, densely finned Heat-Exchanger / Radiator with 18 channels (normally used for Laser or PC water cooling), 3 large fans, a centrifugal water pump /w coolant tank, visual flow meter, two digital thermometers and Quick Connect/Disconnect ports.
All liquid coolant interconnects are done with 3/8" ID 1/2" OD vinyl tubing. 

I had to replace the G1/4 to 5/16" barb fittings for G1/4" to 3/8" fittings on the water pump and flow meter to accommodate the larger diameter tubing. The tubing management is not fantastic - the 1/2" OD tubing and the small footprint of the base made it a bit difficult to route all tubing in a neat manner while avoiding small radius bends which can "pinch" the tubing.

The fans are configured to suck the air thru the radiator and can be used at the same time to blow air at and cool down the high current Stage 3 (18A) Power supply when placed in front of the fans.
A flow-meter with a spinning propeller gives a visual feedback about the speed of the coolant flow. This housing is also used to accommodate the inlet temperature sensor.

The Centrifugal Water Pump is 12V / 19W unit with an alleged specified flow of 800L/h and Static Head Lift of 5 meters. The pump is gravity-fed from a coolant tank just above it. It is configured to pump water into the cooling water blocks and the return line goes thru the flow meter, into the heat exchanger and the output of the radiator is fed back into the coolant tank.

Two digital thermometers display the output and returned water temperatures. The output water temperature is measured at the water tank just before the pump inlet. The sensor for the returned water temperature is located at the flow-meter housing.
The female Quick Connect/Disconnect ports were a bit tricky to mount. We had to fabricate special aluminum L-brackets to mount the assembly firmly in place. I used 3 x 1/4" Female-Female brass couplers soldered together side-by-side in order to mount the Inlet/Output ports on one side and the adapters MIP to 3/8" barb to the other. The whole QC assembly was mounted by threading a long brass screw thru the middle 1/4 adapter (acting as a spacer) and the two supporting L-brackets already mounted on the plywood.
Using Quick Connect ports makes it extremely easy and fast to put the whole setup together and to drain the coolant when done.

The heat-exchanger / radiator  (393 mm  x 120 mm x 32 mm) is attached with L-shape aluminum angle bracket to the plywood base. It has 18 internal finned channels side-to-side.

The entire cooling system, including tubing and water cooling blocks takes about 600 mL of coolant and it is fully sealed.

The water lines 3/8" ID 1/2" OD and male Quick Connect/Disconnect adapters on the Cloud Chamber side.

Due to the fast coolant flow and overall efficiency, the temperature differential at the heat-exchanger is not large but the system cools the water efficiently, maintaining a constant temperature of about 3-4°C above the ambient air temperature.

The Digital Thermometers are powered from the +5V line on the main wire harness. The 3 cooling fans and the water pump are powered with the +12V line.

It takes about 10 minutes for the temperature to drop down to the nominal -36°C due to the large thermal mass of the copper Cold Plate - thickness of the plate is 6.3mm. Again, due to the same large mass the temperature is very stable and rises slowly when power is turned off.  First particle tracks appear when the cold plate reaches about -28°C. Due to the plate thickness, I am probably loosing a couple of degrees at the minimum temperature end but on the other hand, temperature is very evenly distributed across the active zone surface.

Using high concentration of Isopropyl Alcohol (Isopropanol) is absolutely critical for proper operation. The higher the concentration is, the better the Cloud Chamber will work. 

We use 99.9% Lab Grade pure Isopropyl Alcohol and results are excellent. In theory, everything above 91% should work but using >95% is highly recommended! 

Isopropyl Alcohol has the lowest Ionization Energy (IE) level - 10.10 eV of the 3 popular types of alcohol, and it is the best choice! Ethanol (IE ~10.48 eV) or Methanol (!Poison!) with IE of 10.85 eV will also work.
A couple of Squeeze Alcohol Wash-Bottles are a very handy accessory - one bottle, filled with alcohol to saturate the felt reservoir of the Alcohol Evaporator Unit when priming the Cloud Chamber for startup and an empty bottle to periodically collect the alcohol condensate from the Cold Plate and recycle it.
A "must-have" accessory is a "Lead Pig" (Lead Lined Container) to store radioactive samples - Americium-241 pellet from a Smoke Detector, Thorium Gas Mantles, Thoriated Welding Rods, Uranium Glass, Tritium Vials, Ores, Minerals etc., as well as pair of tweezers to handle the samples. I have a few different lead container sizes for storing radioactive samples. Plastic covered Lead Pigs are the best choice for small, "Cloud Chamber ready" samples - due to the lead's toxicity one should avoid handling bare lead metal.

For larger specimens, I prepared lead lined boxes.

This is complete Main Unit of the Diffusion Cloud Chamber!

Building the Cloud Chamber took us about a week of build time, but it was actually stretched over a month due to delays with some parts and issues with the eBay shipments.