What’s the quickest way to locate a radon point of entry? SNIFF for Thoron.

The following is a letter from Thomas Lindhe, a RAD7 owner in Sweden:

Dear Derek,

I have been using the RAD7 now for quite a while, and I am very pleased with it. When searching for radon entry points in SNIFF mode I have discovered that Thoron often shows in the spectrum printout, where radon enters the house. Measuring one half meter away from that point Thoron doesn’t show. This is a very good help to find the entry points.

I got this idea when searching for radon entry points at my brother’s house. He has about 1000 Bq/m3 in his cellar, and is planning to make a guest room there. His 5 year old son sometimes plays down there. Some time ago I was trying to find the entry points with another electronic radon monitor. This monitor is marketed as a great “sniffer”. With this instrument I could not find any particular entry point. The only reading I got was around 1000 Bq/m3 in every room. My conclusion was that radon was leaking in between the slab and supporting and outer walls in general. This monitor was also affected by background from the high readings, which made it difficult to pinpoint the entry points.

With the RAD7 I found which room and particular walls where the radon leaked in. It was impossible to visually see the wall/floor joint because of the wall plaster. After 10 min. I got a reading of roughly 2.500 Bq/m3 radon and a lot of Thoron in the spectrum printout when measuring close to the wall/floor joint. I found another entry point in the inspection hatch for incoming water-1.500 Bq/m3 radon and Thoron. After purging I measured one half meter away from the first entry point. After 20 minutes the reading was roughly 560 Bq/m3 and no Thoron at all. I also measured 1 meter away from the incoming water hatch and found no Thoron there. I have done this kind of “entry search” in several other houses and I can confirm that watching the Thoron level is a good indicator of radon entry.

Maybe Thoron doesn’t always show in a radon entry point but when it shows one can be quite sure that this is a hot spot. Apart from just measuring radon, finding radon entry points is a business idea of mine. I can tell house owners where they should seal and where to drill the hole/s for sub-slab depressurization systems. The RAD7 is the perfect instrument for this purpose.

In my brother’s house we are going to remove the plaster in the wall joint and seal it. I will report to you the results.

Best regards, Thomas

Thoron sniffing to locate radon entry points

If there is much uranium in the soil then frequently, though not always, there is also some thorium. In many cases, therefore, soil gas, in the soil, containing lots of radon, will also contain thoron.

When the soil gas leaves the soil, both the radon and the thoron start to decay. Thoron, however, has a half-life of less than a minute, so soil gas which has been out of the soil more than a few minutes will have lost almost all its thoron. If the gas moves at one foot per minute, say, then by the time it has moved a yard or so, most of the thoron has decayed. If the radon entry point is a short path from the soil, and if the soil gas contains thoron, then there will be significant thoron entering the house. However, because of its short half-life, the thoron will be found only close to the entry point.

The RAD7 is the only commercially available radon detector capable of making a direct and specific measurement of thoron. The first daughter of thoron, polonium 216, has a half-life of just 150 msec. So the response of the RAD7 to thoron is virtually instantaneous – all that is required is to get the thoron into the measurement chamber, which takes maybe 30 seconds. So sniffing for thoron has two big advantages over other techniques to locate radon entry points. One is that the thoron is localized to the radon entry points. The second is that the instrument response is instantaneous – you see it immediately you sample close to the entry point.

Note that you can most quickly monitor the thoron level by listening to the beeps, with the tone set to Geiger, and monitoring the status display, which shows if the beep was in the B, thoron, window. In addition, you may set the cycle time to as little as 2 minutes, and, with THORON on, print the thoron concentration every cycle. As soon as you see thoron indicated, you know that you are within a foot or two of the radon entry point. You can move the probe around to find the highest count rate, and zero right in on the entry point.

Of course, if it turns out that, in a particular case, there is no thoron entering the house, then sniffing with the RAD7 still gives the radon level, with the rapid response and recovery possible only with the unique RAD7 technology.

We are developing a simple, do-it-yourself soil probe. The goal is to have all the parts available from any hardware store for a total of less than $10, and assembly to take no more than an hour. When a unit is built and tested, we will publish the design. It consists, basically, of a metal tube and a metal rod that fits inside it. The two, together, are pushed, or hammered, into the ground, the rod is removed and the tube connected to a RAD7. A small transparent container in the line acts as a water trap, just in case. With such a probe, you can take a quick (altogether no more than ten minutes) look at the soil gas around the foundations, to see if it, perhaps, contains sufficient thoron to locate the entry points.

Another approach: One mitigator drills several small test holes in the basement floor and, with the RAD7, determines the sub-slab gas radon concentration. The hottest spot may be close to the radon entry point, and is where the mitigation suction is applied. The technique is almost always effective in ensuring an efficient mitigation system.

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