On ANSTO's website are some more information about the detectors and where they are used: www.ansto.gov.au/radon-analyt...

ANSTO also has much a much bigger radon monitor. The one in the front has a source delay volume of 5000 L and is the largest and most sensitive in the world. It is stationed at Cape Grim Baseline Atmospheric Pollution Station. The 1500 L (in the back) looks small in comparison!
Photo: Scott Chambers

In our case, the thoron delay has a volume of 400 L and the sample delay 1500 L, so it needs some space – but the larger the sample delay volume, the more accurate the detector. You can see what the set up looks like in reality, fully set up and during assembly.
Photos: Scott Chambers, Aimon Tanvir

The decay of these progeny is then counted by the detector and used to calculate the amount of radon in the original air. The air moves back into the sample delay volume (this is the second flow loop).

The measurement head contains the detector’s second filter. The air from the radon delay volume enters the measurement head and flows through a very fine mesh filter, which captures the newly formed radon progeny.

All the while, there is a separate pump inside this main volume that guides air very quickly through the detector’s measurement head, which catches these new particles. Radon concentration is then determined by counting the decay of these radon progeny particles.

Air moves continuously in and out of this volume, but it stays inside for about 20 minutes. Once the air leaves this volume it is released back into the environment. During the 20 minutes that air is inside this main volume, some of the radon-222 gas will decay to form new particles.

Once the air reaches the detector, any particle decay products of radon and thoron are removed by the detector’s first filter before the air moves into the main delay volume. This volume has more than three times the volume of the thoron delay volume.

Next, the air moves through a thoron delay volume. Thoron is one isotope of radon (radon-220) that we do not want to measure. It has a half-life of less than 1 min. The sampled air takes about 5 min to move through the thoron delay volume and during this time almost all thoron (>97 %) decays.

The detector maintains a continuous airflow through all its segments at all times. As a first step, before it gets to the main detector, any large particles in the air (e.g. dust, soot or pollen) are removed using a cheap disposable filter, to keep the inside of the pump and detector clean.

We focus on radon-222 which has a half-life of about 3.8 days. Two other isotopes of radon (radon-219, radon-220) are relevant in atmospheric research but have much shorter half-lives (less than a minute). They are, therefore, not relevant for determining air mass movement over days to weeks.

Radon is the first (and only) gas in the uranium decay chain. As such it might dissipate freely into the atmosphere. However, the products of its decay are solids, which attach to other particles in the atmosphere and settle to the ground eventually.
Many isotopes of radon occur naturally.

Some measurements could be done right on site, but most samples were processed in a small lab setup at the accommodation. The prepared samples are frozen and can then be transported to the main lab at the University on Terceira for analysis.