Air filter to catch radioactive particles - suggestions please

People here toss out old computers all the time. I've scrounged several from computer stores. They usually strip them and toss most into recycle. I've made a couple of simple filters using scrap computer fans. Tear the power supply apart. Find a thrift store that has 12 volt wall wart power packs cheap. Use duct tape and attach the power supply fan to a short length of PVC or ABS plastic pipe with a wrap of duck tape. Clip off the power supply plug, strip the wires about a half inch, Get the polarity right so the fan runs, making sure it sucks air through the pipe.
twist the leads together and wrap with electrical tape. Place a circular paper coffee filter over the other end of the pipe with a big rubber band. Use some more duck tape to position the filter paper close to your detector window. Costs little to nothing and it works. An even faster source of radon daughter isotopes is a coffee filter over the nozzle of a vacuum cleaner. Let it suck air off the basement floor. Mind, it's a bit noisey.
I've set up the PVC pipe and fan arrangement on my window sill, the filter about a centimetre from the window of my RM-80 pancake detector. You should notice a gradual increase in counts for a little while. Theoretically, you can then shut off the fan and plot the radon daughters decay curve from the CPM... Right.
Have fun. That's the main thing.

I'm sure the molded plastic looks prettier than my PVC pipe/fan combo sitting on a windowsill. As long as it works.

Making a scintillation probe isn't too difficult with the right help. A GCE member, Robert, a ham in England sold me a small 1" diameter Hammamatsu photomultiplier and even soldered up a dynode string on the socket using tiny resistors. I was able to purchase a 1" by 3" NaI (Tl) Russian crystal from SovTube for around $200. It's in a sealed billet with a glass window. Another GCE member sent me a box of junk that had a few probe cases. One was a perfect fit for the PMT and crystal billet wrapped in a layer of mouse pad foam. A friend in New Mexico suggested a slick method for joining tube and billet. Make a cross with electrical tape. place the billet on the cross. Apply the optical coupling grease. Settle the PMT. Bring up the strips of electrical tape with a bit of stretch. Wrap more tape around the mu metal shield and billet to keep the joint tight. Most expensive part was buying an HV supply from Australia, $325.00. The GS1100A has a preamp and switchable HV from 600 to 1100 VDC in 50 volt increments. tTwo BNC coax connectors and a toggle switch. Bottom one is HV plus signal, the top BNC socket is just for a signal from PMT's that have separate leads. The GS1100A has a USB socket for supplying 5 VDC power, and an audio output 3.5mm jack that goes to the line-in on your PC.
Building is fun, but actually getting meaningful histograms is the problem. Still, a scintillation probe is one very sensitive detector.
A simple shield for the probe when attempting gamma spectroscopy is a foot long piece of hard, 2" diameter or 3" diameter copper drain pipe. Wrap it with at least 8 feet of 1 foot wide lead cladding, apply two large hose clamps and a coat of enamel...or just wrap it with duck tape.
The copper blocks X-rays produced in the lead by high energy cosmic particles. Mine cuts background down from over a thousand CPM to around 150 CPM with the large probe.
My problem is I can't eyeball the histogram graph displays. I've got to sift through numbers in an Excel column looking for trends that show energy peaks, mark them, then manually compare values with known isotopes. I use a TG-77 radar transmit/receive tube as a Cs-137 source for calibration.

Have fun..

Hi Sonarflash and welcome to the Forum

If you have the technical skills then these DIY projects you mention are great to get involved in. Having two left hands I need to rely on off-the-shelf technology.

This morning the indoor background count was 0.14 microsieverts and the air intake side of the HEPA type filter was averaging 0.32 microsieverts, so down from yesterday.

I would guestimate that in 10 hours the air filter has sucked through about 250 - 300 cubic metres of air.

A change in the weather seems to have brought about a change in the amount of radon in the air. As the weather is meant to settle down tomorrow back to the seasonal normal, I would expect the radioactivity detected on the filter to fade back to background levels too.

The weather is still unsettled - dry for a period, then wet for a period, and no wind, humidity on the high side.

Indoor background of 0.16 microsieverts. Air intake side of HEPA type filter averaging 0.41 microsieverts.

What I think is notable is that the air intake side of the activated charcoal pre-filter is very dusty, yet the radioactivity isn't significantly above background. So the radioactive particles are small enough to get past the charcoal mat filter.

Weekly (or so) update.

The weather has finally settled into its autumnal norm and so the twice daily check of the air filters is revealing nothing noticeably above background.