Air filter to catch radioactive particles - suggestions please

I've used an activated carbon filter to test rainwater indirectly for radon washout and so this is just an extension of that.

Thanks for all your ideas. At the moment I think the cheapest, least hassle option is to buy a home air purifier, leave it outside in a sheltered spot where the rain can't get at it, and then test its HEPA filter regularly for increases in radioactivity.

Just had another thought.

As my idea was not continuous detection, but rather testing the filter on a daily basis. Could I then use the much cheaper option of buying a handheld vacuum cleaner with a hepa filter?

The vacuum could be used to vacuum external surfaces of settled dust, which would stick to the hepa filter. Also the hepa filter would be the perfect size for testing purposes.

Any thoughts? Good plan or a rubbish idea?

That would probably work, but I think a continuous filter would more reliably detect a passing cloud of radioactive particles. I would worry about a settling area being blown clean before the vacuum gets there, or that the settling rate varies with wind speed. A continuous fan would probably catch more particles, and the filter could be sampled either periodically or continuously. Amazon advertises Sunbeam, Holmes or Honeywell tabletop fans that are pretty reasonable.

I've finally got my hands on an air purifier with a HEPA type filter (99% of particles 2 microns and larger), but which can also use a true HEPA filter (99.97% of particles 0.3 microns and larger). The air purifier is the Bionaire BAP9240 (just google it).

It's incredibly easy to check the filter. It's just a matter of unclipping a latch and then full access to both filters to check for radioactivity It came with a washable foam pre-filter, but I've ditched that and replaced it with a much more efficient activated carbon pre-filter cannabalised from a cooker extractor hood

It's only designed for interior use, so at the moment the best I can do is position it in front of an open window in order to sample the outside air just as it enters the building.

So far (thankfully), nothing above the background cpm