Friday, March 19, 2010

Palm trees and millisieverts

"It is verging on the ridiculous to be worried about them." Richard Wakefield from Manchester University's Dalton Nuclear Institute, quoted in the Guardian. As major UK airports roll out the use of full-body scanners in the war against would-be plane bombers, Wakefield says that the dose of radiation from them - about 0.02 mSv per scan - will pose no risk to travellers. (Physics World, March 2010).

Holy mother of all that's radiologically moly! 0.02 millisievert per scan? That's 20 microsieverts! The UK limit on ionising radiation doses to the public (excluding medical radiography) is 1 millisievert (1 mSv) per annum, and at 20 microsievert per scan, you'd only need to pass through a scanner on the outward and inward legs of 25 flights per annum to breach this limit.

One's thoughts immediately turn to the nomadic denizens of the UK's Formula One community, who already receive an annual cosmic-ray dose which could be as high as 2 mSv per annum, (assuming a pessimistic dose-rate of 10 microsievert per hour at commercial jet altitudes).

Fortunately for Mr Ecclestone's plucky flying circus, a perusal of the original article in The Guardian reveals that the dose from a certain type of airport scanner is actually just 0.02 micro­sieverts, not 0.02 millisieverts. It seems that in this case, at least, The Guardian is able to transcribe scientific information more reliably than Physics World.

Except, that is, for one further problem. There is no radiation expert at Manchester University's Dalton Nuclear Institute called Richard Wakefield. There is, however, one called Richard Wakeford. The erroneous spelling seems to originate from The Guardian article, and to have been transcribed, without verification, into Physics World.

The Health Protection Agency report that "even in the case of frequent fliers the doses are unlikely to exceed 20 micro Sv/year." Moreover, these doses of ionising radiation only arise from scanners which use x-ray backscatter techniques. Terahertz scanners, in contrast, and by definition, use non-ionising radiation.

There was plenty of time during last Sunday's Bahrain Grand Prix to analyse the spacing of the palm trees at the Sakhir circuit, and to conclude that they possess an unnatural uniform spacing, in stark contrast to the Poisson clustering of real trees. Hence, for those running up junior jet-club miles en route to such exotic Formula One destinations, the greatest risk to health this year may come not from ionising radiation, but from the possibility of being bored to death.

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