KS4 Atomic Structure:
Marie Curie didn't realise for a long time that the radioactive isotopes that she and her colleagues had discovered were dangerous. She used to carry test tubes of radium in her pocket! In the end her prolonged exposure brought about her relatively early death.
Hazards and uses
In this third section we learn just a little about the hazards presented to us by radioactive materials, but also about their great uses.
It usually comes as a surprise to most people to learn that sources of radioactivity are all around us!
Fortunately for us and all living creatures, the level of activity from these sources is relatively low.
Since the sources are all around us but their activity is relatively low, we call this Background Radiation.
Background radiation comes:
1) mainly from natural sources such as rocks (eg granite), soil and cosmic rays from space (which are actually high energy charged particles rather than true "rays")
2) man-made medical sources such as CT scans, radiotherapy and X-rays
3) other man-made sources such as the fallout from nuclear weapons testing and nuclear accidents (although, both of these account for less than 1% of the total background radiation)
In the UK almost 50 % of the background radiation that an average person might receive comes from radioactive radon gas from the rocks in the ground.
It is formed by the decay of the small amounts of uranium that occur naturally in rocks (particularly granite) and soil.
Exposure to background radiation can depend on a person's location and on their job.
For example, an airline pilot or airline steward who regularly flies high in the atmosphere at say 30,000 feet (a common cruising height for modern aircraft) will be exposed to a higher dose of radiation due to cosmic rays. Down at ground level, the majority of cosmic rays have been absorbed by the gases of the atmosphere.
Another example: a doctor or nurse who uses radioactive isotopes to treat cancer patients may recieve a higher dose of radiation due to their job (they will, of course, take precautions to limit the danger but their dose of radiation is inevitably going to be higher than that of a person who does a job at ground level, not involving such materials).
Since we have mentioned "radiation dose", it is worth pointing out that a person's radiation dose is measured in a unit called sieverts (Sv).
According to the organisation "Public Health England", on average people in the UK are exposed to about 2.7 millisieverts (2.7 mSv) of radiation per year.
But for people who live in Cornwall, where there is greater risk of exposure to radon gas, the average annual dose is 10 mSv.
The annual safe exposure limit for those working in the nuclear industry is 20 mSv, though the actual average exposure whilst working is only 0.18 mSv.
The level at which changes in blood cells can be readily observed is 100 mSv.
The dose of radiation that might kill half of those recieiving it in a month is 5000 mSv.
NB You don't need to recall the Sv unit for the exam, though it can be mentioned in questions so you must be aware of it.
In section 188.8.131.52 we introduced "half-life" and commented on the very wide range of half-lives that we find amongst radioactive materials. But to repeat it here; the half-lives of radioactive materials range from tiny fractions of a millisecond, up to billions of years (the age of the earth itself, in the case of one isotope of uranium). So, the range is enormous.
What we need to add to the discussion in this section is - how does knowledge of the half-life of a radioactive material affect our judgement of it as a hazard?
Well, it is pretty obvious that a radioactive material with a long half-life is potentially more hazardous than one with a very short half-life, because quite simply, it will be active for much longer.
For example, the main reason why current Nuclear Power stations are considered dangerous by some people is because when their nuclear fuel is used up, it leaves behind some by-products (known as "fission products") which include some that are highly radioactive and have very long half-lives (thousands of years!). The hazard is the safe containment of these highly radioactive products for thousands of years.
On the other hand, in the world of medicine, technicians have to prepare radioactive materials every day for use in radiotherapy, CT scans etc, because the material they use has a quite short half-life (hours, half a day). These materials present a reduced hazard compared to the fission products but for the duration of their active life they are still treated with great care.
Is length of half-life the only factor determining the scope of the "hazard"?
No, the Activity (measured in Bq) of the source is at least as important.
For example Uranium-238, which has a half-life of over 4 billion years, presents a very small hazard due to its low activity, but Iodine-131 is very active during its half-life of only 8 days and presents a much bigger hazard during that time and for a month or more afterwards.
Also, the type of radioactivity being emitted (α, β, or γ) is very important to consider. For example, if a radioactive source is outside a body then a gamma source is considered the most dangerous since it is more likely to penetrate through the outer parts of the body to the more important inner organs, possibly damaging them. On the other hand, if the source is already inside the body then an alpha source is more dangerous since it is the most strongly ionising and is more likely to damage or kill living cells.
1. Control/destruction of unwanted tissue
Since the ionising power of radioactive sources can damage or kill living cells, such sources can be put to positive use in the hands of highly trained doctors, nurses and radiotherapists.
They can carefully use the cell killing ability of these sources to target unwanted cancer cells and heal cancer patients.
The equipment used for precisely targeting the radioactive sources at the cancer locations is improving all the time. There are at least two techniques currently in use:
1) From outside the body a beam of gamma radiation is targeted through the body towards the cancer area, but not just from one direction; multiple trajectories or beams are used so that the damaging effect to the surrounding tissues is reduced. A patient will generally lie down whilst a device "firing" the beam will move, stop "fire" then move to a new position, "fire" etc.
2) A small amount of radioactive material is placed inside the body up close to the cancerous tissue. In this case, the radioactive material will be an alpha source with very high ionising power but, crucially, very short range so as not to affect other surrounding tissues. This very clever technique means that the patient does not have to attend the hospital for, perhaps daily, radiotherapy treatments described in 1) and there is little damage to surrounding tissues. Great consideration, however, has to be given to the exact radioactive material being used, to its activity and to its half-life.
2. Exploration of internal organs
Doctors have long had the ability to see into our bodies using X rays and more recently using ultrasound scans, but there are now other procedures being used which allow them to examine parts of our bodies that were previously difficult to access. By injecting a small amount of a radioactive isotope into our blood stream, doctors can trace the flow of blood around our body and through all of our vital organs looking for blockages etc. The isotope being used for this is known as a "tracer" and it has to be a gamma source, doesn't it, since once inside our body it has to be able to penetrate the body in order to reach a detector outside for the doctor to monitor its presence. Its activity is probably quite low and its half-life needs to be just long enough for the procedure to be carried out, so a number of hours. One isotope made up in hospitals every day for procedures like this is Technetium-99 which has a half-life of 6 hours (its the most commonly used medical radioisotope).
Time for a few more questions