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The RAD7: How Technology Translates to Performance


When “sniffing” for radon, if you find 100 pCi/L in the basement, how long must you wait to measure 3 pCi/L in the next basement—or even upstairs!? With a typical radon detector you are out of business for six hours or more, but when using the RAD7, 30 minutes is all you need. While there are many instruments available on the market for measuring radon, you will find the RAD7 to be the best. Here’s why.

All radon instruments detect the radon atoms that decay inside the instrument. After the initial decay, to Polonium-218, there is a chain of further radioactive decays, all of which occur inside the instrument. A series of different elements are formed. Each has a different half-life and each emits either an alpha, beta or gamma particle/ray when it decays. Polonium-218 has a half-life of three minutes, and emits an alpha particle when it decays to Lead-214. Later on there are a couple of beta particles and then another alpha particle from the decay of Polonium-214, typically around 45 min later. The Lead-210 that forms has a half-life of 22 years, so it builds up continuously. The Lead-210 contributes a steadily increasing count to the background, and this can severely degrade the Lower Limit of Detection of electronic radon detectors—except for the RAD7.

Radon detectors, in general, count all the alpha particles and sometimes the beta particles and gamma rays, without any means of discriminating between them—except for the RAD7. In particular, other radon detectors cannot identify the alphas arising specifically from the decay of Polonium-218. This means that

(a) they are measuring radon from around 2 - 10 minutes ago (the Polonium-218 decay) plus radon from around a half to three hours ago (the Polonium-214 decay), and

(b) they suffer a steady increase in Lead-210 background, especially if they are ever exposed to high radon levels such as may be found in a radon gusher.

Thus, with other instruments

(a) you cannot take a truly independent reading at a new position, after making a measurement in one place, unless you move the instrument into the new position and wait three hours or more, and

(b) you have to avoid, or quickly move away from, high radon areas to prevent background buildup.

After three hours away from one position, the remnant radon reading of these other instruments will have dropped to less than 5% of the original measured value—but this remnant reading may still be unacceptable if the first position has a radon concentration several times the level at the next measurement position. In addition, regardless of any remnant reading from earlier measurements, it takes close to three hours for the reading at the new position to reach more than 95% of its final value. In effect, these radon detectors are simultaneously accumulating several rolling averages (one for each decay product counted), each one with a different time scale.

In sharp contrast, the RAD7 actually measures the energy of every alpha particle detected. In SNIFF MODE it looks at the 6 MeV alpha particles from the Polonium-218 decay and ignores all the rest. Thus there is a 95% recovery from high readings in 15 minutes. You also reach better than 95% of your new final count rate after 15 minutes of measurement at any new location. These times contrast with the 3 hours or more it takes to get your first approximate reading with other technologies.

When sniffing for radon gushers with the RAD7, you can quickly tell if you have found one: You hear the count rate rising fast. Normally you move away as soon as the gusher is identified, and continue the search for other gushers. But if you need an actual measurement of the gusher’s radon concentration, sample for just 20 minutes and you have a reading better than 90% of the final value. The RAD7 also tells you if thoron is there and gives a thoron measurement independent of the radon. Thoron can act as a very-fast-response tracer for radon fresh from the soil, see SNIFFING for thoron.

Even if you do monitor the gusher for 20 minutes, another 20 minutes later, after taking the RAD7 away, you are almost back to normal, and you can continue sniffing for radon and thoron. Any other instrument would need many hours to recover from exposure to the gusher, and in those other instruments such high exposure would also cause a significant increase in the Lead-210 background.

Radioactivity measurements involve counting random events. The uncertainty in a reading depends on the number of events counted: The more events, the more precise the reading. With 5-minute cycles, low radon concentrations will have few counts—there could even be zero—so that low radon levels will have a large uncertainty in the reading. The RAD7 in SNIFF MODE protocol will tell you, in 15 minutes or so, that you have low radon levels—but the statistical uncertainty in the reading will be large. To be confident the level is less than 4 pCi/L, monitor for at least 50 minutes and average the readings of the last eight 5-minute cycles. Note that the result will, in any case, just be a spot reading and not necessarily representative of the daily average.

For accurate measurement of low levels, count over long periods. There are, built into the RAD7, 24-hour and 48-hour EPA protocols, which give you a bar chart of the variation of radon concentration during the measurement period, and also a final average far more accurate than is available from passive devices. You can control the length of each cycle and the total number of cycles. You can also preset your own choice of protocol and save it to use whenever you wish. You may print a spectrum of the alpha particle energies at the end of each cycle. This enables you to observe the different radon and thoron daughters participating in the count and shows you when they reach equilibrium. Anytime during the run you can print out a bar chart of the radon variation. At the end of the run, after a complete bar chart of radon levels, the RAD7 prints an accumulated energy spectrum of all the counts for the entire run.

With its built-in pump, and inlet and outlet ports on the face plate, the RAD7 is well adapted for a number of special applications. First is the ability to make measurements of radon in water. The RAD H2O accessory provides an elegant technique that gives you an accurate radon in water concentration in less than an hour after taking the sample, in contrast to the typically three or more days delay when a lab does the analysis. The new RAD AQUA accessory gives a continuous measurement of the radon concentration in a water supply.

As a research tool, the RAD7 is outstanding. The pumped sample path means that emission measurements from soil, building materials and surfaces are simple to make. An enclosed box containing the emitting material is hooked up to the RAD7 as a totally sealed volume. The RAD7 then monitors the increasing radon concentration with just a 15-minute response time. Data stored in the RAD7 is easily downloaded into a computer for later analysis.

To measure radon emission from a surface, seal a known-volume enclosure against the surface and hook it up to the RAD7. The rising radon (and thoron) concentration in the enclosed volume is directly related to the surface emission.

The measurement of soil gas with the RAD7 is straightforward, and free from problems that may plague other techniques. A hollow perforated sample probe is pushed into the soil. For very damp soil, or where there is a chance of penetrating below the water table, a water trap, upstream of the standard drier, is recommended. Insert the probe to different depths to obtain a vertical profile of soil-gas radon concentration. (Purchase probes as an optional accessory.) With the drier in place, in the sample stream, the reading is independent of the moisture content of the soil gas. Again, even though the radon concentration of soil gas is typically extremely high, the RAD7 is resistant to the Lead-210 background accumulation.

The DURRIDGE RAD7 is truly a unique instrument with exceptional technology. It can meet your needs in ways that are completely and fundamentally beyond the capabilities of other radon monitors. Indeed, the RAD7 represents outstanding value in radon instrumentation, unmatched at any price.

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