Most news stories involving radiation are, to be blunt, overblown. Radiation can be dangerous, but the risk it actually poses is usually far lower than what the media stories would have us believe. So my first inclination when I hear about another story involving “deadly radiation” is to be skeptical. And then every now and again there’s the exception – a story about radiation that’s not overblown and an incident in which there is a very real risk; sometimes an incident in which lives are put at risk or even ended. Last week we had the latter sort of radiation story, and it’s worth a little discussion.
First, a short recap. A cancer therapy clinic in Tijuana Mexico was shipping a highly radioactive radiation therapy source to Mexico’s radioactive waste disposal facility near the center of the nation – at the time of the theft the source consisted of over 2500 curies of cobalt-60. Auto theft is common in Mexico – the truck driver claims he was sleeping in the truck at the side of the road when armed thieves ordered him out of the truck and stole it, source and all. There is every indication that the thieves were unaware of the source itself – that they were after the truck. And recent history bears this out since there have been a number of similar thefts (albeit with lower-activity sources) in recent years. Anyhow, the thieves seem to have removed the source from the back of the truck; it was found at the side of the road several miles from where the abandoned truck was located. From here things get a little speculative – a Mexican official feels it likely that at least a few of the thieves were exposed to fatal doses of radiation, and a half-dozen people came forward to be tested for radiation sickness (the tests came back negative). At the present time, the source was under guard by the Mexican military with a perimeter about 500 meters (a little over a quarter mile) away. So with this as a backdrop, let’s take a look at the science behind all of this.
Dose and dose rates
First, let’s think about the radiation dose rates and doses – the most important question in any radiation injury situation is how much dose a person received.
Radiation dose is a measure of the amount of energy deposited in a receptor – in this case, the receptor would be the thieves, but it could just as easily be a radiation detector. Cobalt-60 has two high-energy gamma rays; one curie of Co-60 gives off enough energy that it will expose a person to a dose rate of 1.14 R/hr at a distance of a meter (about arm’s length). So 2500 curies of activity will give a radiation dose of 2850 R/hr a meter away. A radiation dose of 1000 rem is invariably fatal, so a person would receive a fatal dose of radiation in a little over 20 minutes. Without medical treatment a dose of 400 rem is fatal to half of those who receive it – a person would receive this dose in eight minutes a meter away. And radiation sickness, which takes only about 100 rem, would start to appear in only 2-3 minutes (although it might not manifest itself for a few weeks). No two ways about it – this was a very dangerous source.
Radiation dose rate drops off with the inverse square of one’s distance from a source, so doubling your distance reduces the dose rate by a factor of four (and tripling your distance, by a factor of nine). This means that distance is your friend – take a long step away and a source that can be fatal in 20 minutes at arm’s length will take 80 minutes to have the same impact – still dangerous, but a little less immediately so. At a distance of 100 meters dose rate will be almost 0.3 R/hr – about the same dose in one hour that most of us will receive in an entire year from natural sources. The perimeter was set up at a distance of 500 meters – the dose rate from an unshielded source here will be about 12 mR/hr – at least 500 times normal environmental radiation levels, but well within the realm of safety. I have some radiation detectors that will accurately measure radiation dose rates that are only slightly higher than natural background levels – to get to the point at which the stolen source would fail to show up on these more sensitive detectors I’d have to be close to ten miles away. This doesn’t mean that the radiation is dangerous at these distances – just that it would be detectable.
Of course, a good question to ask is why there was cobalt-60 on the truck in the first place. And this gets a little more involved than one might think, going back over a century.
It didn’t take long for people to realize that radiation can burn the skin – within the first decade after its discovery there was anecdotal evidence of its ability to cause harm, which was confirmed by experiments. And it didn’t take much of a leap of imagination to figure out that, if radiation can burn healthy skin then it can also be used to burn out unwanted tissue – such as cancers. So doctors began experimenting, settling quickly on radium as a cancer therapy. Radium, though, has its own problems, including the fact that it decays to radioactive progeny nuclides – with the advent of the nuclear age scientists found they could produce a highly radioactive nuclide of cobalt that emitted high-energy gammas that were ideal for reaching even those cancers buried deep within the body. Other nuclides were also discovered – Cs-137 and Ir-192 are among them – but cobalt does a great job.
For over a half-century these artificial radionuclides ruled the roost in radiation oncology, joined by iodine (I-131) for treating cancers of the thyroid. But radionuclides have their own problems, chief among them being that they can never be turned off (so they always pose a risk) and that they require a costly radioactive materials license. As technology improved many of the more advanced nations began using linear accelerators to produce more finely tuned beams of radiation – today Co-60 is rarely used for cancer therapy in the US, Japan, or Western Europe. On the other hand, linear accelerators are expensive and they need a fairly high level of infrastructure to maintain the precise power requirements these touchy machines require. So we still find cobalt irradiators in much of the developing world.
Mexico (among other nations) is in the process of swapping out their irradiators for linear accelerators, including the Tijuana cancer clinic where this source originated. But with a half-life of 5.27 years it’s not advised to just let the cobalt decay to stability, a process that could take two generations or longer. So at some point these obsolete sources must be shipped for disposal – that was (and apparently still is) the fate in store for the Tijuana source.
But wait – there’s more!
There’s more to this story than what I’ve gone into here, but space keeps me from getting into all the questions it raises. In particular, there have been a number of incidents over the last half-century or so in which radioactive sources such as this one have cost lives, contaminated consumer products, and they’ve contaminated scrap metal mills. Next week we’ll talk about some of these incidents as well as the risk posed by these sources should they go accidentally or deliberately astray. At the same time we’ll talk about radioactive materials security and what protective actions make sense.