An ingenious method of measuring radioactive gases on old mirrors or the glass of photograph frames has been developed by UCD scientists, writes Dick Ahlstrom
Irish researchers have developed a way to read the "fossil record" left by radon, the radioactive gas. It provides a way to measure what radon levels might have been years ago, long before any modern measurements were taken.
The technique was developed and refined by Dr James McLaughlin, a senior lecturer in experimental physics at University College Dublin, and PhD student Kevin Kelleher in collaboration with Dr Christer Samuelsson of the University of Lund, in Sweden.
McLaughlin, who is also head of the natural-radiation-studies group at UCD, is using the technique in a planned "look back" at homes with high radon levels in co-operation with the Radiological Protection Institute of Ireland (RPII). He has already measured historic radon levels in a home in Castleisland, Co Kerry, that had the highest radon levels recorded anywhere in Europe.
The ingenious method involves taking radiation measurements of glass surfaces found in the home, explains McLaughlin. Old mirrors or the glass in family pictures hanging on a wall are perfect targets for this measurement technique, which depends on recording polonium 210, he says.
Radon is a naturally occurring radioactive gas that seeps up from the ground. Outside it dissipates harmlessly, but it can become trapped in pockets under floorboards or subfloors. The gas decays in the air, in turn producing daughter particles that, if inhaled, can deliver a radiation dose to sensitive lung tissues.
The production of these daughter particles is what provides the fossil record of radiation levels in these homes, says McLaughlin. "Radon in an enclosed space produces decay products in the air, and the first of these is polonium 128," he says."If it lands on a surface, for example a mirror or picture frame, it has a very short half-life of just three minutes."
It then breaks down to produce the next step in the decay path, bismuth 214, but in the process releases an alpha particle. The alpha particle goes in one direction and the bismuth 214 in the other, says McLaughlin. "This recoil energy is enough to implant it up to 50 or 60 nanometres [billionths of a metre\] into the hard surface. They are deposited on all surfaces but in particular on hard surfaces."
They in turn go on to decay into lead 210 and polonium 210, isotopes that can be measured in the glass, explains McLaughlin. "In particular we measure polonium 210, because it is an alpha emitter and is long lived."
The researchers use alpha-particle detectors very much like those used to detect radon in the first instance. They measure alphas given off by the polonium 210 in the glass surface, providing a historical record of radon levels in the room as they were years ago.
Window glass can be measured, but sunlight and air currents near the window can affect levels. For this reason they prefer to test family pictures, which are easily dated and tend to remain in a fixed location over many years.
"The RPII has a large databank of radon measurements in Irish houses," he says. It recommends remedial action to reduce radon levels when they reach 200 becquerels per cubic metre of air (bq). The look-back programme involves requesting test access in any home that reached levels of 1,000 bq or more.
"We can measure as far back as Victorian glass," says McLaughlin. Radiation emitted by Radon daughters in glass older than this stabilises, ruling out measurements in older glass, he says.
New tests on glass from the Co Kerry home that recorded radon levels of 49,000 bq showed that these high levels have persisted there for as long as 20 years, McLaughlin says. Other high Radon homes show similar findings.
