Gamma Radiation

Of the three types of nuclear radiation discussed at GCSE level, gamma is definitely the unsung hero. Alpha and beta get plenty of limelight because they have both mass and charge, so they feature prominently in nuclear decay equations (as explained previously, here). But gamma radiation is a wave rather than a particle, so it has no mass and no charge – and that means it is often left out of nuclear decay equations. You could almost say, at GCSE level, that gamma radiation doesn’t matter in nuclear decay but that definitely isn’t true in the bigger scheme of things.

More importantly, gamma radiation doesn’t exist just because of nuclear decay; there are also gamma rays that reach us from space. Therefore, some of the gamma radiation we experience here on Earth comes from radioactivity that is part of our planet and some of it is cosmic radiation. This contrasts with the alpha and beta radiation that we experience, all of which comes from radioactive decay events happening here on Earth.

The two origins of gamma rays can be deduced by taking radiation measurements at different altitudes. If all the radiation that we experience came from the Earth then it would be reasonable to expect radiation levels to decrease as altitude increases. But that isn’t the case. In fact, the opposite is true.

I recently bought a Polimaster 1208M gamma radiation dosimeter, which is a small GM detector inside a (rather heavy) wristwatch casing. The dosimeter can display either cumulative exposure or the current rate of exposure, called Dose Equivalent Rate, in micro-Sieverts per hour (μSv/h).

The international Commission on Radiological Protection (ICRP) advises that the maximum annual exposure for the public is 1 mSv (1000 μSv). Dividing this down by the number of hours in a year (365.25 days) we get an average exposure rate of 0.11 μSv/h.

The exposure rate figures I’ve been recording for Jersey have been typically in the range 0.10 – 0.14 μSv/h. If that sounds a little high compared to the ICRP’s advice then it’s worth knowing that the occupational exposure limit is about 20 times higher than the public limit (according to a World Nuclear Association factsheet) so there’s nothing much for us to worry about, despite living on a rock of mildly radioactive granite.

The annual permitted dose for flight attendants is higher than for the general public (but nothing like as high as for nuclear workers) and this is because of that second source of gamma radiation, cosmic rays. I checked the dose rate on a recent off-Island trip and found that it peaked at just over 0.5 μSv/h during level flight at 18 000 ft. It’s also worth noting that at my destination, in the north of England, the background dose rate was lower than in Jersey, generally being in the range 0.06 – 0.10 μSv/h.

To add a further layer of reassurance, we can refer to the Banana Equivalent Dose (yes, this really does exist). It is well known that bananas have an above-average level of radioactivity owing to being rich in potassium, including the radioactive isotope potassium-40. Eating one banana gives you a dose of about 0.1 μSv (according to radiation-dosimetry.org) which is about the same as the average exposure per hour for members of the public.

You could say that if you ate a banana every hour then you would be doubling your exposure, and exceeding the ICRP recommended maximum dose, but I’d say you would simply get fed up of (and on) bananas very quickly and you would probably want to change to a more varied diet.

Of course, if you are worried about bananas then by all means don’t eat them. And stop taking flights, especially high-altitude long-haul flights, as well. But in the real world, radiation is all around you and it (probably) isn’t doing you too much harm at the sort of levels we usually encounter. The chart below, which is an extract from a larger chart posted on Wikipedia by XKCD webcomic creator Randall Munroe, provides a bit more perspective.