How do you know the hits aren't cosmic rays? It seems a few hits per hour could easily be cosmic rays, unless they are distinguishable somehow.
A typical camera target is about 3 mm on a side, for a total sensitive area of ~1e-5 m2. The muon flux at sea level is about 1 per cm2 per second, or 36 million per m2 per hour, or 360 hits per hour on a camera target, so even granted that the "active" area of the target is only a few percent of the physical area, muons more than account for the signal he is seeing, and muons deposit a lot of energy.
The electostatic concentrator is an interesting feature. Charged nuclei typically attach themselves to dust motes in the air, which are then swept along by the E-field. It isn't clear why he needs such high voltages to get 50 V/m over a few centimetres, and in my experience Cockroft-Walton generators are easy to build but hard to make work. With such tiny capacitors any stray leakage current would destroy the effect, and he does not anywhere measure the voltage produced.
The enhancement of the count rate could be accounted for by electrical noise from the CW driver circuit.
This is pure speculation on my part, based on a good deal of experience designing and building radiation detectors, but I'd like to see a lot more evidence before I said this was an effective radon detector.
An alternative, easier to debug and demonstrate, and more likely to work approach would be to use a commercially available activated zinc sulphide sheet with the camera target as the detector: http://www.eljentechnology.com/index.php/products/zinc-sulfi...