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:

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 1,000 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 1,000 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,

Thoron sniffing to locate radon entry points

If there is much uranium in the soil then usually there is also some thorium. Therefore if the soil gas contains lots of radon, it will usually contain significant amounts of thoron as well.

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 radon entry point.

The RAD7 is 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 or so. 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 when you sample close to the entry point.

Note that you can monitor the thoron level by listening to the beeps, produced by the RAD7, with the tone set to Geiger, and monitoring the LCD status display, which shows if the beep was in the B Window, which represents thoron. In addition, you may set the cycle time to as little as 2 minutes, and, with THORON on, print the thoron concentration every cycle. Or you can monitor the count rate in Window B directly. 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.

If it turns out, in a particular case, that there is very little thoron entering the house, then SNIFFing with the RAD7 still gives the radon level, with the rapid (15 min) response and recovery possible only with the RAD7 technology.

*The RAD7 performs real-time spectral analysis of the energy of each alpha particle detected. It can therefore distinguish between decays of 218Po, 216Po, 214Po, 212Po and 210Po inside the instrument.

218Po is the first daughter of 222Rn, and has a 3.05 min. half life. By counting only 218Po decays, the RAD7, in SNIFF mode, gives a rapid response to and recovery from high radon levels.

216Po is the first daughter of thoron, 220Rn. The 150ms half life gives the RAD7 its instantaneous response to thoron gas.

210Po is much further down the decay chain of 222Rn. 210Pb, a parent of 210Po, has a half-life of 21 years, and so builds up continuously as the instrument is used. By NOT counting 210Po decays, the RAD7 maintains its vanishingly small intrinsic background, even after long exposure to high radon levels.

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