Saturday, April 18, 2020

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 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 -26C). 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 a liquid.
The ionization trail of a charged particle provides these condensation "seeds".

(please note that the graphical representations of the elements above are not to scale 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 ionized trail. Regular gas atoms of oxygen and nitrogen from the air become Ions in the path of the energetic particle as the particle knock off electrons from them.
Since the Alcohol and Water molecules are slightly polar, the molecules are attracted to these ions. The ions serve as "seeds" / condensation centers and droplets of fine mist form around them, tracing the path of the particle.

The condensation trails appear only in the active zone where conditions are right and are white in color just like the normal water-based fog.
Black background and intense tangential lighting helps to improve the contrast, bring them out and make 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. The Ion Scrubber is a high-voltage electric field inside the chamber. Typically between the cold plate (-) and an electrode (+) suspended above the cold plate. CERN recommends 100V/cm field potential.
The electric field serves a number of beneficial purposes in the Cloud Chamber:
- it clears the volume from airborne dust particles at startup - this removes unwanted condensation centers
- pulls the con-trails towards the Active Zone for a better observation
- "resets" the chamber by quickly removing (scrubbing)  ions so condensation trails don't hover 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 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 presents 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 seal of the volume should always be maintained!

Large Cloud Chambers usually have a sealable port to bring in radioactive samples or inject radioactive gas (Radon). Such specimens can be introduced also by gently opening the volume but it will take a few seconds for the conditions to normalize and the con trails to appear.

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.

Sunday, April 12, 2020

Diffusion Cloud Chamber (Conclusion)

The Diffusion Cloud Chamber in operation! 

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 - Rn-220 has a half-life of only 55.6 sec and it is an α-emitter (6405 keV). It is a daughter product of Ra-224, which on the other hand is down the decay chain of Thorium.
My son's reaction to this α-particle fireworks display was pretty rewarding!

Here is another example of Radon-220 in the Cloud Chamber. To obtain the "Thoron", I placed a Thorium Gas Mantle (Thorium Dioxide) 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, an equilibrium state will take place - the concentration of Rn-220 will become stable as new Rn-220 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 isotop - Rn-222 and Radium Watch hands but then it is worth waiting at least 3-4 days longer for more Radon-222 to accumulate in the syringe - Rn-222 has a half-life of 3.82 days.

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 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 Uranium doped glass marble emitting mainly alpha particles and some beta. In the center is a pellet of Am-241 (a very strong alpha emitter) and on the right is a vial filled with Tritium (H³) emitting very-low-energy betas.
This video is from an early test before the Light bars were fully constructed.

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 tho :-)
Next on the table - 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 top. One of the design goals for this project was a modular system approach. Usually table space is very limited during a Science Fair 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 and out of the way from the Cloud Chamber's Main Unit. The Static Head lift of the water pump is 5 meters - more than enough!
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.

The Liquid Cooling Plant for Stage 4 cooling is using a large, densely finned radiator with 18 channels (normally used for Laser cooling or PC water cooling), 3 large fans, a centrifugal water pump, coolant tank, flow meter, two digital thermometers and Quick Connect/Disconnect ports.
All liquid interconnects are done with 3/8" ID 1/2" OD tubing. I had to replace the G1/4 to 5/16" barb fittings for G1/4" to 3/8" hose 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.

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 (30A) 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 a specified flow of 800L/h and 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 in order to mount the Inlet/Output ports and the adapters MIP to 3/8" barb. 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.
Using Quick Connect ports makes it extremely easy and fast to put the whole setup together and to drain the coolant when done.

The radiator / heat exchanger (393 mm  x 120 mm x 32 mm) is attached with L-shape aluminum 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 600mL 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, 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 min 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 but on the other hand, temperature is very evenly distributed across the surface.

Using the right concentration 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 - 10.10eV of the 3 popular types of alcohol and it is the best choice! Ethanol (IE ~10.48eV) or Methanol (!Poison!)  (IE 10.85eV) will also work.
A couple of Squeeze Alcohol Wash-Bottles are a very handy accessory - one bottle, filled with alcohol to saturate the felt of the Alcohol Evaporator Unit when preparing 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 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 sizes for storing uranium ore samples. Plastic covered Lead Pigs are the best choice - due to the lead's toxicity one should avoid handling bare lead. For larger specimens, I prepared a lead lined steel box.

This is complete Diffusion Cloud Chamber!
Building the Cloud Chamber took about a week of build time but it was actually stretched over a month due to parts delays and issues with the eBay shipments.

Diffusion Cloud Chamber (Part 3)

All cable interconnects for this project were salvaged from old PC power supplies. The only exceptions are the Teflon insulated corona-free HV cable (rated at 15kV) and all top-side wires (heater and light bars) which are silver-plated Teflon insulated. All rocker switches are rated for 20A but the only critical circuit in terms of max current is the Power B switch for the 3rd TEC Stage where the current is 18A.

This is the underside of the completed Cloud Chamber. Removing residual heat left after the liquid cooling stage helps a bit by lowering the temperature by another 2-3°C. To accomplish this, a system of fans was designed - two small fans blow air across the fins of the heatsink, while another exhaust fan picks up the hot air on the other side of the heatsink and blows it out in the open thru the back of the enclosure. This system works for the tight space we had to work with. The screws keeping together the Cooling Unit "sandwich" are used to support the brackets for the exhaust fan via a set of stand-offs and the thin plastic air guides.

The intake pair of fans and the blue exhaust fan. Sheets of thin transparent plastic form air channels to contain and direct the stream of air into the fins of the heatsink.
An aluminum L-bracket secures the two stainless steel T- splitter for the coolant - the bottom one on the picture is the inlet for the cold water and the top one is the warm water outlet. The splitters also act as adapters converting the 3/8" ID tubing to 2 x 1/4" ID silicone tubing connected to the copper water cooling blocks.

The front panel Industrial Digital Thermocouple Thermometer (Lascar DTM-995B) is on the right-hand side. DTM-995B supports T, J and K-type thermocouples, it has a switchable backlighting, °C/°F display units and switchable Current/Min/Max display (a little button, installed next to the display, not seen in the pictures, scrolls through the 3 readings). I installed a Cryogenic-optimized T-Type thermocouple and it was calibrated for this probe before the installation at the Cooling Unit. K-type thermocouple which is more common can also be used but it is not as accurate at low temperatures. There are two adjustments on the back of the thermometer for calibration - Offset to set the 0°C value and Calibration for 100°C

Due to the close proximity between the thermocouple probe cables / thermometer and the HV lines leading to the Discharge switch, additional insulation (Neoprene foam sheet) was added and zip-tied to the HV cables. Any arcing there can destroy the digital thermometer or the power supply.
I took apart an identical switch to the one used for the HV/Discharge circuit and made sure that the distance between the contacts is at least 2 mm in the opposite throw of "Discharge" when the switch is in HV ON position - this should be enough to prevent HV leakage / arcing thru the Discharge circuit inside the switch as Air breakdown Voltage is generally 3kV/mm.
I was toying with the idea of using a HV vacuum relay instead of the switch but it was going to be just over-engineering on my part for only 4kV.

Thin, transparent plastic sheets, salvaged from plastic packaging are used to create a "scoop" for the heatsink exhaust hot air, just above the exhaust fan. This guide directs the hot air coming out of the heatsink fins directly into the blades of the fan. The exhaust scoop is visible on the right, partially covering the 3 switches. The sheet separating the air intake and the air exhaust can be seen on the left/center of the picture and it is secured by the screws on the heatsink.

In this picture, on the right side, one can see the two power resistors (50 Ohm (2 x 100 Ohm /3W in parallel) and switch used to control the power levels to the heater. Heater power is switchable between 1W on LOW and 1.4W on HIGH.
The 3 terminal strips are used to distribute different PS voltages to various circuits of the Cloud Chamber - this makes it really easy to change the power configuration of the TEC modules.

Proper tangential lighting above the Active Zone is absolutely essential for good observation of the vapor trails left behind the  charged particles. We used 2 opposing light elements - each light bar contains 9 evenly-spaced Super-Bright White LEDs. The LEDs are mounted very low, just above the Active Zone, illuminating the supersaturated alcohol vapors at a steep angle. Standard 5mm LEDs with a built-in lens work best providing sharp bright light beams compared to large diffuse light LEDs.
The 18 LEDs are in electrically divided into 6 groups and each group of 3 in parallel has its own 150 Ohm current limiting resistor. There is a simple Brightness control facilitated by a potentiometer and DPDT light switch with OFF, HIGH and LOW beam settings. LOW brightness includes a 1 kOhm trimmer - potentiometer in the circuit, allowing for a variable brightness level. This can help when doing long exposure still photography or videography of the Active Zone of the Cloud Chamber.
Two PCB strips with width of 1cm are used to form each Light bar. The strips are attached to each other at 90 deg using heavy gauge solid wire. The PCB design is fairly simple and can be etched easily at home or the copper can be mechanically removed with a Dremel tool to form the pads and traces.
Visible on the picture is the closed-cell foam "Vapor Fence" intended to contain the supersaturated alcohol vapors above the cold plate so they don't spill over the entire surface of the volume.
Since the vapor trails are whitish, everything inside the Cloud Chamber should be colored black for maximum contrast and visibility of the white trails.
The Active Zone is covered with Black Self-Adhesive Aluminum foil - this type of foil tape is ideal for the purpose as it has a wide temperature range adhesive, it has alcohol-resistant paint and it is heat conductive. This kind of tape is normally used in Broadcast studios by the Lighting Technicians to modify / mask studio spot lights and in comes in 2" width.
Also visible here is the outer Neoprene foam seal for the chamber's acrylic cover.
The air inside the chamber must be perfectly still so the vapor trails are not disturbed by air turbulence when formed. This seal prevents any air from coming in or out when the acrylic cover is in place, eliminating any draft and possible air turbulence.
All seams on the bottom and around the Active Zone are sealed with Black RTV Silicone sealant to prevent alcohol from seeping thru and infiltrating into the Cooling Unit.

This is the finished top side of the Cloud Chamber setup without the acrylic cover.
The top surface of the chamber's enclosure is covered with a fairly thin (~ 2 mm) black rubber sheet (We picked up this in the flooring section of Home Depot). The rubber sheet was glued using non-foaming Gorilla Glue to the top of the plywood. Gluing this rubber sheet was a bit tricky though - I had to make a jig to provide even downward pressure on the entire surface while the glue was curing to avoid any wrinkles or air pockets but the result turned out to be excellent.
Aluminum L-shaped profiles are used as a trim for the side edges of the enclosure.
The rubber surface around the chamber's volume provides a nice, durable, non-slippery, alcohol-resistant work area which can be used for additional experimental equipment, specimens, photographic equipment, etc.

The Heater element of the Alcohol Evaporator Unit. The Cloud Chamber relies on a steep temperature differential / gradient between the top part and bottom part of the chamber's volume. Raising the temperature of the top side helps with the alcohol evaporation. We constructed the Heater element from 6 x 68 Ohm (0.25W) resistors organized in two parallel strings - each string has 3 evenly spaced resistors in series. The total resistance of the heater is approximately 100 ohms, dissipating 1.44W at High setting. At Low setting, the heater runs thru an additional 50 Ohm resistance and dissipates close to 1W.
The heater is powered with 12V from the main power supply. The resistor strings are connected to Teflon insulated wires and placed inside heat-shrink tubing, . A small pair of barrel connectors (M/F) is used for the electrical connection, making the heater replaceable / interchangeable in case we decide to experiment with different heaters / power levels.
The supply line for the Heater comes thru the top surface, goes into one of the Delrin rods thru a hole on the side and exits from the top, terminated by the heater jack.

The completed Alcohol Evaporator Unit. The Heater element is inserted between 4 sheets of black felt squares. The felt squares are thoroughly soaked with Isopropyl Alcohol before the Cloud Chamber is closed and started. Felt material is ideal for releasing the alcohol vapors as it provides a very large surface area and even evaporation. Different number of felt sheets can be used for different run times / amount of alcohol. Heat is absorbed by the top and bottom sheets and alcohol vapors seep thru the mesh supporting them above the Active Zone of the Cloud Chamber. With 2 felt squares the Cloud Chamber can run for about 1.5 Hrs before it requires removal of the alcohol condensate from the bottom plate. This can be done with a Squeeze-bottle, syringe or just soaked with one of the felt squares and then reused.

The High-Voltage Positive Supply line comes thru the top surface and connects to the Ion Scrubber mesh on top. The wire is inserted in a black heat-shrink tubing and spliced to a thinner and more flexible teflon insulated wire, which allows for the metal frame holding the mesh to be adjusted up or down as needed.
While the Cloud Chamber will operate without an Ion Scrubber (IOS), the IOS greatly improves the performance of the Cloud Chamber and IMHO it is a "must-have" feature. The electric field cleans the air from dust contaminants when the Chamber is just started and vastly improves the visibility and the definition of the vapor trails by quickly removing (scrubbing) the ions. With IOS turned off, tracks are much thicker, slow dissolving, less defined and much lower in number. When IOS is turned ON the effect is rather dramatic - the number of visible tracks jumps many times, tracks are very crisp and well defined and the chamber quickly resets itself, clearing old trails.
Total of 4 cables come from the bottom compartment to the top surface - 2 x for the two Light Elements, the High-Voltage IOS supply line and the Heater lines. The rubber top surface provides a very nice and tight seal around each cable as it tries to self-close.

The transparent acrylic Cloud Chamber cover. It is a 6" x 6" x 6" inches hollow cube and provides enough vertical height for a good temperature gradient and excellent viewing angles. We picked up the cover on eBay, sold as Trophy / Memorabilia cover and it is perfect for the purpose. This cover contains and seals the volume of the chamber and it can be made of thin glass as well.
 I designed a special "Neoprene" foam seal for the bottom edge of the acrylic cover. This seal is air-tight and completely seals off the volume of the Chamber when the cover is inserted. It is made of 2 different thicknesses Closed-Cell Rubber Foam material - the thin, bottom layer seals the under-edge of the acrylic cover - it has an inside lip on which the acrylic cover steps on, while the thicker outside foam rim seals the cover on the outside wall. The seal is glued with black RTV Silicone to the top surface and provides also mechanical stability of the acrylic cover as it is a tight-fit and locks the acrylic cover in place.

Diffusion Cloud Chamber (Part 2)

After the cooling element was constructed and installed we did a quick test run to prove the concept and to make sure that we can achieve our goal of -36°C.

The IR thermometer I used is not at all accurate at very low temperatures! I had a black tape over the metal for this measurement (in order to reduce the reflectivity and improve the emissivity of IR) and it still showed about 10°C error. The actual temperature, established later with a thermocouple was -37°C - still exceeding our design goal.

Wiring of the front panel. We used the rest of the 2RU aluminum blank for the front panel. All Cloud Chamber control switches, LEDs and the cold plate thermometer were mounted on the front panel. The back side was left open for the water feedlines, power connectors and air cooling.
The Cloud Chamber is powered with two individual Power Supplies.
The main Power Supply is a 450W PC ATX power supply. This type of power supply serves the purpose well as it provides enough power and variety of voltages for all of the necessary circuits - the heater element (+12V), Ion Scrubber HV supply (+3.3V), Light elements (+5V), Front Panel Thermometer (+5V), First stage TEC (+3.3V), Second Stage TEC (+6V (or 12V/2)) and 5th Stage cooling fans (+12V). Same power supply also powers the Liquid Cooling Plant (+12V and +5V).
The ATX power supply plugs in directly in the female connector of the main wire harness and it is switched on/off by the main power switch on the front panel.
The second power supply (Power B) is 15V/32A (Alinco DM-330MVT) and it is strictly dedicated to Stage 3 TECs. The voltage in Stage 3 should never exceed 15V! (maximum rated voltage for TEC1-12709 module is 15.1V). All wiring for Stage 3 is done 12AWG stranded wire due to the high current demand of  the dual TEC1-12709 modules.

To facilitate the wiring of multiple circuits, three 5-position terminal blocks were mounted. One block is all GND terminals, another block is dedicated to the TEC modules and the third block is used for all other circuits - HV PS, Lights, Heater element, etc. The ATX PS main wire harness was connected on one side of the terminal blocks. We used PC PSU Extender Cable for the main power harness - one side comes with a female ATX connector - the male connector on the other end of the extender cable is simply removed and all wires are terminated with spade terminals.

The top side of the Cloud Chamber under construction. Visible is the Cooling Unit, attached with countersink brass screws to the plywood. 4 x Black Delrin 5" x 1/2" rods are attached with stainless steel machine screws (1/4"-20 x 1") to create the "support pillars" for the Positive mesh of the High Voltage Ion Scrubber and the Alcohol Evaporator Unit / Heater.
Delrin (Polyacetal) is a good choice as it is resistant to the hostile environment of super-cold super-saturated alcohol vapors and it is easy to work with. Other possible choices are Ceramic or Teflon. Plexiglass will become brittle under these conditions so I don't recommend it.
The Delrin rods I got were solid so I had to drill and tap all rods on both sides for 1/4-20 thread. One rod was drilled end-to-end to act as a conduit for the heater wires. I drilled a small hole on the side about 3/4" from one of the ends. This hole was drilled at 45 degrees to aid with the cable routing.
The rods are placed very close to the inside wall and corners of the Cloud Chamber Cover (outlined with pencil marks on the top surface) while making sure there is enough clearance.

The fine aluminum mesh of the Positive terminal of the Ion Scrubber is stretched over a brass tube frame, using a very thin silver-plated wire. The frame is constructed out of 4 brass tubes, slightly larger in diameter than 12AWG wire. Each corner of the frame is a solid copper 12AWG wire loop formed around the Delrin rods with ends at 90 degrees. The ends of the loop are inserted in the brass tube segments and then soldered. This arrangement is allowing the frame / mesh to be moved up or down, sliding over the corner rods. We wanted to be able to adjust the height of the mesh in order to control the intensity of the electric field above the Active zone. A number of rubber O-rings were placed over the Delrin rods below and above the corner loops of Ion Scrubber mesh to act as "movable stoppers" suspending the mesh at an adjustable height.
Another brass frame made of flat brass stock supports the tin-plated steel mesh (not installed yet in this picture) of the Alcohol Evaporator Unit. This frame is fixed on the very top of the Delrin rods. Each corner is a copper washer to which we soldered the brass frame elements and the frame is then secured on top of the support pillars with stainless steel screws. The AEU support steel mesh is soldered on the bottom of this brass frame.

This is the completed support structure with the Ion Scrubber HV mesh, the Alcohol Evaporator Unit (the heater element is visible on the very top). The black and green O-ring stoppers and another green O-ring which acts as an "insulator rib" to lengthen the electrical distance between the high-voltage mesh and the Alcohol Evaporator.
(!) Since the Cloud Chamber is filled with a mixture of air and alcohol vapors during operation, an extra effort was made to prevent any remote possibility of arcing! The temperature in the top portion of the volume where the HV mesh is located is above the alcohol's flash point temperature. Last thing one wants is a fireball. The HV operates at 4.6kV and with Air breakdown voltage of 3kV per mm, so a minimum of 3 mm distance (2x the breakdown distance) to GND is absolutely mandatory! Any High-Voltage Arcing inside the Cloud Chamber can ignite the alcohol vapors and can also destroy the main Power Supply. This is one of the reason why the heater supply wires were conducted on the inside of the Delrin tube and not just wrapped around it when passing the HV mesh.

The almost finished control panel with all SPST (Power and Power B) switches, DPDT switches, LED indicators and the Industrial Grade Thermocouple Thermometer for the Cold Plate. A quick test showed a good temperature drop.

Visible on the right-hand side is the High-Voltage Ion Scrubber power supply. This was taken out of an Electric Fly Swatter /Zapper. The circuit is made of a DC pulse converter, high-voltage step-up transformer and a voltage multiplier. At 3.3V input voltage I measured output of 4.6 kV.
CERN's recommendation for the Ion scrubber is min 100 V/cm and 4.6 kV is more than enough to satisfy it.
The HV supply is housed in a poly-carbonate plastic box (from CCI .22 Cal Ammunition) to provide additional insulation and safety. A special high-voltage Teflon insulated corona-free wire (rated at 15kV) is used to connect the positive output to the top-side mesh. The negative pole is connected to both, the aluminum plate and to the copper cold plate of the cooling element.
The HV supply has a capacitor on the output and since this is a school project, I would imagine that no kid would like to be shocked by 4.6kV. I designed a dedicated Discharge circuit to prevent any unpleasant accidents. Half of the Ion Scrubber DPDT switch is used to control power to the HV PS. The other pole of the same switch (on the opposite throw), when the HV PS is in OFF position is used to short the output of the HV PS thru a 15 kOhm / 3W metal-oxide resistor to the negative side. The middle position of the switch is OFF, the UP position turns power ON to the HV PS and the DOWN position is DISCHARGE - it shorts the HV output capacitor (In this position power to HVPS is already turned off).
The safety procedure requires the Cloud Chamber to be opened and manipulated with the switch in Discharge position. This circuit can be avoided if enough time is afforded after HV is switched off, for the voltage to drop down due to HV capacitor and wire leakage - I measured this to be approx. 30 sec. Using a Discharge circuit just takes the guessing out and guarantees that the mesh is at 0V.

Saturday, April 11, 2020

Diffusion Cloud Chamber (Part 1)

One day in February, my son came from school, holding a Science Fair application and said "I am not doing the stupid baking soda and vinegar volcano everyone at school seems to like! I want to do an actual science project!".
Lately, my son has taken a great interest in the model of the Atom, subatomic particles and nuclear physics in general so we knew exactly which direction to go and we started to brainstorm a possible project. A Geiger Counter was what immediately came to mind but it is a simple straightforward device. Then I said - lets take it a step further - why just count charged particles when we can almost see them. Enters - The Cloud Chamber Particle Detector!
There are two main types of Cloud Chambers - The Wilson Cloud Chamber, which relies on a rapid volume expansion to achieve a vapor supersaturated state and it is only sensitive for a moment and the Diffusion Cloud Chamber (DCC) which relies on super-cooling alcohol vapors to get them to a supersaturated state. DCC is more complex than the Wilson Chamber but it is continuously sensitive so no danger of missing an event while you are blinking! We decided to go "Diffusion". Such detector requires a plate cooled to a very low temperature. Here is how it works.
A Dry Ice or Liquid Nitrogen (LN) cooled DCC was right out - we wanted to use it at will, without depending on a Dry Ice / LN source, plus storage for these is always a problem and you have a narrow window to use the chamber.
Phase-change cooling requires huge, noisy refrigeration units and are pretty complex and difficult to build at home, so they were also out of our list of possibilities!
We decided on Thermo-Electric Cooling (TEC) (also known as Peltier effect) - it is not efficient, requires a lot of power but it is small and easy to build.
The other design parameter to establish was the size - with Cloud Chambers bigger is always better and more spectacular, just like with a Telescope but using the inefficient TEC means that a large area DCC will be extremely power hungry, with huge power supplies and a lot of supporting equipment. Again, just like a Telescope - too big and you'll use it less often. We wanted something desktop sized that we can quickly setup, get ready and start experimenting with.
All materials for this project were sourced from eBay, Home Depot and my junk box.
The most common size "Peltier" coolers are 40mm x 40mm thus two side-by-side units will give us a cooling area of 40mm x 80mm. The final design has a copper cold plate of 2" x 4" - not really big but enough to observe well the particle effects and with a fairly small footprint at the same time. The final chamber volume is 6" x 6" x 6" with an Active Zone of 8 sq. in.
Diffusion Cloud Chambers require a maximum temperature of -26°C (this is the temperature at which alcohol vapor supersaturation occurs) and this means that the Active Zone should be cooled down to at least -35°C from the +25°C room temperature to work reliably - a ΔT of 60°C. To achieve this, a cascaded (multi-stage) cooling will be needed. Cascaded TEC modules are not very good at pumping a lot of thermal energy but they achieve the highest temperature differential, impossible to achieve with a single stage. The specification for TEC1-12706 shows ΔT of 70°C but this is more of a theoretical value than what my real-world tests showed.
Our chamber has 5-stage cooling - 3 TEC stages, 1 stage liquid cooling and 1 stage air cooling (both for the heat exchanger and for removing the residual heat of the TEC modules right after the water cooling stage). Having 3 TEC stages opens the platform to experimentation with different power levels to achieve best optimization and the lowest possible temperature.

2 cooling stacks, placed in a side-by-side configuration. Each stack has 3 TEC modules and a water cooling block. The stacks are cooling a Tellurium Copper (Alloy C145000) Plate (2" x 4" x 0.25")
The top side of the Cooling Unit is an aluminum plate (3 1/2" x 5 1/2" with a "window" cutout, slightly smaller (3 7/8" x 1 7/8") than the cold plate to ensure a slight overlap (1/16") on each side, while exposing most of the copper plate. This top aluminum plate was made out of a 2RU equipment rack blank (the gray plate on the picture)

The top surface of the enclosure was made of a plywood square 14" x 14" x 1/2" (in a retrospective, we could have gone 13"x13" or even a tad smaller) with two sides of pine planks (14" x 3 1/2" x 3/4") attached with glue and long deck screws. In the center of the plywood square we cut a "window" (approx. 4" x 3" 3/8), slightly smaller than the aluminum top plate's narrow side. The aluminium plate was centered over the window and attached with countersink brass screws to the top plywood surface. The copper cold plate was placed over the window of the aluminum plate and centered using closed-cell rubber foam "Neoprene" insulation spacers.
Brass screws were used to mount the aluminum plate (frame) so I can easily solder to one of the screws on the back side and establish an electrical connection between the aluminum plate and the negative terminal of the HV PS.

The cascaded TEC stacks were formed using 2 types of Peltier modules - lower power TEC1-12706 for Stage 1 and 2 and higher power TEC1-12709 for Stage 3.  All modules are in cold side-hot side/cold-hot/cold-hot sequence. The idea is that the first 2 stages are lower power modules and operated at lower than nominal voltage - stage 1 runs at 3.3V and Stage 2 at 6V. The heavy lifting is done by stage 3 (TEC1-12709 @15V). Each stage needs to pump out the heat, pumped by the previous stages AND the heat generated by that stage  as well. Running the first 2 stages at lower power prevents stage 3 from being overwhelmed beyond cooling capabilities. In our setup, stage 3 is powered by a separate power supply due to the high current requirements - 15V / 18A or 270W electric power (9A for each stage 3 module in 2 stacks).

In this arrangement, Stage 1 adds ~6W (2 x 3W) of thermal energy which needs to be removed in addition with the thermal energy pumped out from the Cold Plate, Stage 2 adds another ~22W (2 x 11W) for a total of 28W. These extra 28W must be removed by stage 3 in addition to all of the thermal energy pumped out from the Cold Plate.
The water cooling blocks and heatsink must remove almost ~298W of thermal burden, generated by the TEC modules in addition to the heat removed form the Cloud Chamber volume.
Cascading the TEC modules also allows for a more gradual heat distribution across the whole stack and brings stage 3 in an area of better efficiency.

All modules are stacked together with generous amount of aluminum-based thermal compound on both side in a thin and even layer. Stage 3 modules are mounted using silver-based thermal compound on both sides. The two stacks of 3 TEC modules are placed side-by-side along with the water cooling blocks on the last hot side. This whole assembly is "sandwiched" between the aluminum plate/frame (and copper cold plate) and a large copper heatsink. The heatsink is intended to dissipate any residual heat, not completely removed by the liquid cooling stage. The copper water cooling blocks are not 100% efficient (I suspect the internal surface area of the block was not enhanced) so additional help by the heatsink improves the overall heat-removal. Copper components were used exclusively due to their outstanding thermal conductivity.

The cascaded TEC stacks are surrounded by tightly fit Closed-Cell Foam insulation. We made 2 foam insulation "frames" (glued together) with 40mm x 80mm cutout and notches for the power wires. The top portion of the foam frame fills up the cutout cavity in the plywood under the aluminum plate.
It is important not only to insulate the bottom of the aluminum plate and the TEC modules but also to prevent any heat, rising from the liquid cooling blocks / heatsink and heating up the cold plate.
1/4" ID silicone tubing is connected to the water cooling blocks. Both cooling blocks run in parallel using stainless steel T-splitters. Silicone tubing is optimal for this stage as it seals nicely around the barb fittings without the need for additional hose clamps.

The Cooling Unit "sandwich" is held together with small diameter countersink oxide plated stainless steel screws to minimize heat transfer between the hot and cold sides. The screws go through the aluminum plate and the copper heatsink (3 5/8" x 3 5/8" x 1 1/16") "squeezing" in-between the two, the cold plate, all 6 TEC modules and the 2 water cooling blocks. A dab of RTV sealant was used to seal the cable openings in the surrounding foam insulation.

This is the final design of the Cooling Unit for the Cloud Chamber.
Two small holes were drilled on the side of the cold plate - one hole was used to attach screw with lug and wire to the Negative terminal of the High Voltage Ion Scrubber and the other hole was filled with thermal compound and accommodated a T-Type (Cryogenic-Optimized) thermocouple probe. A small brass bracket was used to fix (pinch) the thermocouple probe cable to the plywood enclosure and prevent it from pulling out of the cavity in the copper plate.