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" that 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 - quick and easy to build. Then I said - let's 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 drop the pressure and 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 materials is always a problem and you have a narrow window to use the chamber.
Phase-change cooling requires huge, noisy refrigeration units and is pretty complex and difficult to build at home, so this type was also out of our list of considerations!
We decided on Thermo-Electric Cooling (TEC) (also known as the "Peltier effect") - it is not really efficient method, 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 really portable, 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 own electronics junk box.

The most common size "Peltier" cooler is 40mm x 40mm thus two side-by-side units will give us a cooling area of 40mm x 80mm. The final design has a slightly larger 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 requires 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 cascaded TEC stages, 1 stage liquid cooling and 1 stage air cooling (both for the main heat-exchanger and also 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 rectangular "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 under the stacks 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" x 13" 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 made another rectangular "window" (approx. 4" x 3" 3/8), this time slightly smaller than the aluminum top plate's narrow side but larger than the copper cold plate. The aluminum plate was centered over the window and attached with countersunk 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 protruding 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 heat 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 energy burden, generated by the TEC modules in addition to the heat removed from 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 sides 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 much enhanced) so additional help by the heatsink improves the overall heat-removal process. Copper components were used exclusively due to their outstanding thermal conductivity.

The cascaded TEC stacks are surrounded by tightly fit Closed-Cell Rubber Foam insulation ("Neoprene"). We made 2 foam insulation "frames" (glued together) with a 40mm x 80mm cutout and notches for the lead 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-copper plate assembly and the TEC modules but also to prevent any heat, rising from the liquid cooling blocks & heatsink and heating up the cold plate so any air convection should be prevented.
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, countersunk, oxide plated, stainless steel screws to minimize heat transfer between the hot and cold sides. These screws must have minimal thermal conductance and their length to diameter ratio and material help a lot, but are also exposed to airflow around the hot side to further cool. The screws go through the aluminum plate and the copper heatsink (3" 5/8 x 3" 5/8 x 1" 1/16) essentially "squeezing" in-between the two plates the rest of the cooling stack - the cold plate, all 6 TEC modules and the 2 water cooling blocks. A dab of RTV sealant was used to seal the wire exit openings in the surrounding foam insulation.

This is the final design of the Cooling Unit for the Cloud Chamber.
Two small holes were drilled in the side of the cold plate - one hole was used to attach screw with wire 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 lined with fiberglass tape 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.

No comments: