Interesting..........I have just in the last two years been doing the same. I found a geographic "bowl" of trapped Thoron gas (Radon from thorium), here in Richmond!! Sadly, this hot zone is in a tiny public park. What is it with parks!!!! Do they find a hot zone not good for housing and make them parks??? It's like trailer parks and their attraction for Tornadoes!! So far we are the only ones to report this and the parks are batting a thousand! Check your local parks folks! LOL.
There is a link, (URL cited below)), that has recorded all radon levels in many local areas as reported by the nation's many radon detection businesses. I found this hot park area by punching in the zip codes of the Richmond area. Note*** this is not a geographic ground survey, but a home radon survey. This mimes a ground survey only in inhabited areas where the businesses involved have taken readings.
Note**** When I say radon/thoron, a gamma scintillator will not read the presence of those gases directly. Instead it will read the horrid, gamma ray emitting, fast decaying daughters deposited rapidly wherever the gas exists. Thorium is the worst of all natural radioactive elements. Its radon (thoron) has a nasty 54 second half life. Thus, the first solid daughter Ra228 is created nearly instantly which settles out and needs 35 years in the grounds soil to reach equilibrium and make the full compliment of Ac228 which emits the nasty near 1mev gammy ray. Thus all of these gammas will sail right through deep soil and car bodies!! From this is is realized that such radiation from thoron daughters has been around undisturbed for at least 40+ years!! Radon is radon and is emitted by any of the principal 4 known decay series. However, we only need worry about radon from Uranium and Thorium in massive mineral deposits.
It is cool how the hot gamma daughters betray the presence of Thorium and Radium alpha only emitting elements and their alpha only emitting gas first daughters to the gamma only detecting scintillator.
Using this reference, I found the zones with measured basement readings of 20pCi/l, and well above that, located in the area of the University of Richmond. This is a high-end value home area where the wealthy and upper end white collar live.
I set my superlative TSA scintillator to a 3 sigma detection level and zero'd it for my neighborhood, (2-3pCi/l). As I drove to the park, along the way, I would get a chirp of localized detection on occasion. However, once in the area, over a 9 square block area about "the bowl", through the steel body of my automobile, the alarm never stopped sounding, no need to exit the vehicle!!
At the tiny park at the bottom of the bowl with its sliding board, juggle gym, flat merry go round and wood chip pile, I noticed that a creek flowed through it. Exiting my vehicle, I had my scintillator on a long, leather sling that I have used in Colorado and Utah for hunting the U stuff that lay just below the surface. ....( Some of the best hot U stuff from the mines fell off the bouncing, over filled, ore trucks in the 50's and is now covered with a washed overburden of from 3 inches to 18 inches of dirt. You have to dig these gems up and only a top flight scintillator will find them. A GM counter is worthless.).....As I entered the park I let the scintillator swing back and forth just off the ground, (1-2 inches), and observed the digital meter. No matter where I went, all over the park, the alarm blared out constantly with little or no increase in reading. NO buried thorium rock! Over near the creek there were a number of giant multi-ton, matt-grey boulders. Placing the scintillator in contact all over many of them did not increase the digital count to any significant degree. Weird! I noticed a number of larger cobbles in the stream. A contact reading on them and just over the water, again, did not register an increase.
I realized I was in a pure thoron, (Radon), environment!
Piedmont Virginia is famous for its many massive pegmitite deposits, a source of thorium minerals. This thoron gas was rising through the ground from below and the bowl collected it and due to it's mass, preserved it and its recent daughters from all but strong winds. The nearby homes trapped it. Wow! The nasty solid daughters are readily buried into the soil as rains fall. This explains the rather invariant gamma ray levels in the area.
All of the uranium in Virginia is in the "Valley" over 150 miles west of Richmond. The giant multi-million ton, un-worked reserve deposit exists near Chatam, Virginia in Pittsylvania Co. It is referred to as the "Coles Hill" deposit. Current Virginia law prohibits any form of Uranium mining in Virginia of this major national reserve of Uranium.
Here is that URL for radon readings. It can be tough to make work for you. Follow the trail, you will get there. It will generate a google-like map of the areas chosen.