Answer to Question #1466 Submitted to "Ask the Experts"

Let us respond with more information than might be necessary. The four radon decay products in air are ²¹⁸Po (alpha emitter, 3.05 min half-life), ²¹⁴Pb (beta emitter 26.8 min half-life), ²¹⁴Bi (beta emitter, 19.7 min half-life), and ²¹⁴Po (alpha emitter, 164 microsecond half-life). If these four were present in the air in secular equilibrium (that is, they all have the same concentration), the beta-to-alpha ratio in air would be 1 to 1. As a rule, they are not in equilibrium. The ratios between the decay products are variable and depend, among other things, on the ventilation system.

Nevertheless, a typical indoor ratio of ²²²Rn/²¹⁸Po/²¹⁴Pb/²¹⁴Bi would be 1.0/0.5/0.3/0.2. ²¹⁴Po is not included in these ratios because it is always present at the same concentration as ²¹⁴Bi. The ratio for the decay products would thus be 5 (²¹⁸Po)/3 (²¹⁴Pb)/2 (²¹⁴Bi)/2 (²¹⁴Po). This gives a typical beta-to-alpha ratio for the decay products in indoor air of 7 to 5 (that is, slightly more than 1 to 1).

However, this is in air, and the ratios in air are not what one gets on the filter used to collect the decay products. During sampling, the radionuclides collected on the filter are decaying, and the activity of the short-lived ²¹⁸Po decays more than the rest of the decay products. As an example of what this means: at the end of a 10-minute air sample, the ²¹⁸Po activity on the filter will be less than half the ²¹⁴Pb activity even if their concentrations in air are identical. For a four-hour sample period like yours, the relative amount of ²¹⁸Po would be even lower. The result is that the beta-to-alpha ratio on the filter at the end of sampling should be greater than 1 to 1. Then of course, the activity of the ²¹⁸Po (three-minute half-life) on the filter decays faster during the wait time between sampling and counting than the activities of the ²¹⁴Pb, ²¹⁴Bi, and ²¹⁴Po. After a ten-minute wait, the ²¹⁸Po has effectively disappeared. This leaves, for all practical purposes, only ²¹⁴Pb, ²¹⁴Bi, and ²¹⁴Po on the filter. If these decay products were in secular equilibrium on the filter, the beta-to-alpha ratio would be 2 to 1. However, the relative activities of ²¹⁴Pb and ²¹⁴Bi on the filter, and hence the beta-to-alpha ratios, change during the wait period. In part this is because ²¹⁴Pb and ²¹⁴Bi have different half-lives. In addition, the beta-to-alpha ratios can change because the decay of the ²¹⁴Pb results in the ingrowth of ²¹⁴Bi—there is almost no corresponding ingrowth of the ²¹⁴Pb because of the near absence of ²¹⁸Po.

Another factor that can increase the beta-to-alpha ratio is the attenuation of the alpha particles in the filter—the beta attenuation would be negligible. Attenuation will be greatest for cellulose filters (for example, Whatman 41) and lowest for membrane filters (for example, Millipore). The numerous studies indicate that only 50 percent or so of the emitted alpha particles will escape a cellulose filter.

Given all this, your beta-to-alpha ratio of 3 would not be totally unexpected if you are sampling with a cellulose filter. However, it would strike me as being a bit high if you are using glass fiber or membrane filters.

Several things to keep in mind: Standardize the sampling to a particular type of filter, sampling time, and wait time (between sampling and counting). Longer sampling and wait times can lead to higher beta-to-alpha ratios. Even then, the equilibrium of the radon decay products in air can vary from one location to another and from one time to another. It is best to take multiple background samples so as to get an idea as to what kind of variation to expect.

Finally, this issue is discussed, at least to some extent, in Moe's Operational Health Physics Training. See page 14-9.

Paul Frame, CHP, PhD

I wish to add a few comments to Paul Frame's reply. As he noted, the situation is somewhat complex. Without knowing exactly the type of counting system being used, and how it is used, it is impossible to predict what ratio of observed beta counts to alpha counts would be expected. For example, if a grab sample is being counted several minutes after the end of the sampling then, as Paul noted, the ²¹⁸Po might be totally gone, thus losing one source of alpha particles. (This is why for good grab measurements of radon decay products, the counting is started within one or two minutes of the end of the sampling period.)

If, on the other hand, this is some type of continuous monitoring system where the counting and sampling are occurring simultaneously, then the ²¹⁸Po should be present on the filter, but the counting efficiency for detecting the alpha particles from ²¹⁸Po and ²¹⁴Po-214 may be something less than ideal due to the filter medium and the geometry of the filter and detector and therefore may affect the ratio of observed beta counts to observed alpha counts. Also, in a continuous monitoring situation, if air is sampled through a filter for long periods of time (days/weeks), there could be a measurable/significant contribution due to the long-lived decay products of radon, for example, ²¹⁰Pb (22 yr half-life), which is a beta emitter.

So, in summary, there are numerous variables:

1. the equilibrium condition of the short-lived radon decay products in the sampled air,
2. the sampling time,
3. the decay time, if the sample is counted sometime after the sampling has stopped (grab sample),
4. in the continuous monitoring mode, how long the sampling occurred prior to the count,
5. the counting time,
6. the collection efficiency of the filter, and
7. the counting efficiencies for the various energy betas and alphas being detected.

If there are other variables, I cannot think of them at the moment. If someone is truly interested in pursuing this further, somewhere I have equations describing the activities of the radon decay products on a filter as a function of sampling time and the decays of each of the radon decay products during a counting time after the sampling has ended (grab sample mode). These equations could easily be modified to take into account the collection efficiency and the counting efficiencies for the various beta and alpha particles, to then produce the number of beta and alpha counts. The equations could probably also be easily modified to apply to the continuous monitoring mode rather than the grab sampling mode.

Phil Jenkins, CHP