Reading Background Radioactivity Levels

Ok, so let us talk about a subject that most people have an opinion on but is often misunderstood… Nuclear radiation! Do not get me wrong, large doses of this stuff can make you very sick and in some cases will be lethal. However, we are exposed to radioactivity, at a low level, on a daily basis. In fact, we even have radioactive nuclei within our body. Our body’s immune system, if we are fit and healthy, can deal with this low level of nuclear radiation without us even realising that we are fighting it. In fact, our clever system of defence is constantly fighting not only these low levels of nuclear radiation but also viruses and other nasty microbes, effects of ultra-violet radiation, chemicals that we digest in our food and many other sources of not so healthy things. That is one of the amazing characteristics of us humans. We are designed to survive as best as possible!

So, back to nuclear radiation; firstly this is the type of radiation that is released from the nucleus of an unstable atom and there are three types. The largest, least energetic but most ionising is alpha radiation. Fortunately our skin and even 5cm of air will absorb the energy of an alpha particle, basically meaning that we are safe from this type of nuclear radiation unless it enters our body. The second type is beta radiation, a much smaller particle that has higher energy but is less ionising so will interact with our bodies less unless in very high doses. And finally, there is gamma radiation, the most energetic of the three types but the least ionising so again, we need to be exposed to large doses before we should experience any ill effects.

The next fact about nuclear radiation to bear in mind is its randomness. What this means is that we cannot predict when an unstable atom of a radioactive substance, like uranium, will decay and release nuclear radiation. So any measurements that we make of nuclear radiation exposure is an average over a certain length of time, meaning that we may have been exposed to slightly more or slightly less than this average. Any measurements will seemingly jump around depending on the amount of nuclear radiation the tube sensor in a radioactivity meter encounters each minute.

The final important fact we need to discuss is background radiation. As mentioned above, we are exposed to background radiation all of the time, be it from the buildings that we live or work in to the food that we eat and the amount of time we spend flying to different destinations. We receive nuclear radiation from basically two sources; the Earth and anything that we use or derive from it, and solar radiation from the Sun and space.

So with all this in mind, we can explain why were so interested in studying this on Dartmoor and doing a comparison with its neighbouring National Park, Exmoor. Despite the similarities of the moorland on the surface of each park, geologically they are vastly different. Dartmoor primarily sits over a gigantic granite intrusion called a batholith, squeezed into the country rock around 300 million years ago, and now famously exposed as tors. Besides these iconic outcrops, it is clear to see the importance of this rock to earlier generations since it makes up the main component of most buildings, drystone walls and bridges.

In contrast Exmoor’s geology is about 100 million years older; a combination of predominately sedimentary rocks with some volcanic rocks towards the west. Like with Dartmoor, these local rocks have also played a key part in the shaping of settlements in this region. In both parks, humans have been fortunate not to have needed to look far for suitable building materials. In Exmoor, the abundant shales and sandstones have been put to good use to build a range of structures for shelter and protection.

Granite and other igneous rocks are good sources of background radiation since they contain very small amounts of highly radioactive minerals. Sandstones and shales typically contain significantly less radioactive minerals so would be expected to give out much lower readings of nuclear radiation. With both of us having backgrounds in geophysics, this was one of our Scientific focuses that we were most looking forward to carrying out; to compare the nuclear radiation readings for Exmoor and Dartmoor National Parks, with possibly additional information from the Peak District National Park to further compare. We agreed to take 10 readings (Time constraints allowing) along our respective routes to try to map out a simplistic picture of the nuclear radiation readings for each park. We were hoping to use the measurements to look for any variation within each park and also a comparison between the two. The stopping to take measurements also gave us a little breathing time and recovery before pushing on through the mileage!

Armed with a couple of radioactivity meters and lots of useful geological information from Dave Gurnett and Orlando Rutter, the educational representatives for the Exmoor and Dartmoor National Parks respectively, we had high expectations of getting some useful data. Physics investigations have a habit of not always working in the classroom so we were really keen to get some success in the field. Excitingly, it worked! Even though readings could only be relative due to the limited sensitivity of the meters, there was a clear difference between our findings from each park. The presence of the granite in Dartmoor produced significantly more elevated average reading of around 0.17 micro-Sieverts, compared to an average of 10 micro-Sieverts for Exmoor. Fantastic, a physics experiment that not only worked but was great fun in collecting!

However, before we finish off this blog, there are some important notes to remember. Firstly, nuclear radiation is measured in several units, including Bequerel (Bq) to measure the rate at which nuclear radiation is given out by a source and Sievert (Sv) to measure the estimated dose absorbed by the body. It is impossible to mathematically convert from Bequerel to Sievert since one is an actual measurement and one is estimation. The map below shows the geographical variation of measured radiation from radon in the UK. Equivalent radiation dose measurements from geology range from approximately 0.400mSv per year in the darkest regions to 0.200mSv per year in the lightest

Secondly and most importantly, is that you would actually need to be exposed to around 1000mSv in a single dose to experience any radiation poisoning symptoms and a single dose of at least 5000mSv to cause death!!! Also, since our biggest exposure to nuclear radiation comes from solar radiation, with average values of 230mSv for anyone living at sea level and adding on an extra 0.010mSv for every 100m above sea level, you can see that a much bigger source comes from above. If you are a regular flyer, you can also add on 0.040mSv per hour of flying. This means that it is far better for your health to make regular visits to our National Parks than it is to fly off to far-away destinations for holiday!

Sources:

http://webarchive.nationalarchives.gov.uk/20130123

www.nirs.go.jp/ENG/