What is radioactivity?
The BTEC students need to understand the basics of radioactivity before they investigate how it can be used in medicine.
In order to understand what radioactivity is we need to know what an atom looks like. The simplest model is a nucleus consisting of positive protons and negative neutrons with negative electrons whizzing around the outside.
Radioactive decay occurs if the nucleus is unstable. The instability comes about either because there are relatively too many protons or neutrons in the nucleus for a particular atomic number or the nucleus has too high an energy state.
There are three types of radioactivity:
Alpha particles that consist of two protons and two neutrons (large mass);
Beta particles, which are like electrons (small mass), but come about when a proton turns into a neutron (beta + is emitted) or a neutron turns into a proton (Beta – emitted)
Gamma radiation, which is a high energy electromagnetic wave (zero mass)
Alpha radiation can be identified by the fact that it only travels a few cm in air and is stopped by pair. This is because it is so highly ionising due to the fact that it is the most charged of all the types of radioactivity (+2).
Beta radiation can be stopped by a few cm of aluminium. It is not as penetrating as alpha due to the fact that it is less charged (-1 or +1)
Gamma radiation, due to the fact that is poorly ionising due to being neutral, requires large thicknesses of lead (often with very large thickness of concrete as well) to reduce it to safer levels.
Radioactive decay is completely random. That is, you cannot predict which nucleus will decay. What you can say, providing you have a large enough sample, is that after a certain time half of the nuclei will decay. This time is known as the half-life.
Radioactive decay is exponential. This means the original amount (N) or activity (A) of the sample is reduced by a consistent rate over a period of time.
The formula(s) for radioactive decay can be seen below
N is the number of radioactive nuclei and A (number of decays per second) is the activity of the sample.
E is the exponential symbol and the minus sign means that there is decay occurring
t is the time over which the decay is being measured and l is the decay constant. The decay constant is the constant ratio for the number of atoms of a radionuclide that decay in a given period of time compared with the total number of atoms of the same kind present at the beginning of that period. Also called disintegration constant, radioactive constant.
The activity of the sample (A) is also equal to lN where l is the decay constant and N is the number of the radioactive nuclei at the time of measurement.
How does radiation affect the body?
In large doses, ionising radiation can be dangerous and can cause burns to the skin and sickness. In medical imaging radiation doses are too low to cause serious damage, though there are still risks. Radiation can damage the DNA in cells. This could cause cancer, inflammation, cell death and damage to genes can lead to mutations in offspring
Cells are more sensitive to radiation during cell division than at other times and cells which divide frequently (e.g. the gut walls) are more sensitive than those which rarely divide (e.g. nervous tissue).
Sometimes the cells can repair the damage, though sometimes they cannot and die. This fact is useful in the treatment of cancer where radiotherapy is used to kill the cells in a tumour. Another problem is that radiation can change the DNA without the cell dying. This is called a mutation, and mutations can lead to the person being more likely to develop cancer later in life, or can be passed on to their children, sometimes leading to genetic abnormalities.
Radiation can be detected by a Geiger counter. These can click every time either an alpha or beta particle, or a gamma ray, is detected.
Each particle or gamma ray is seen as a reading on the meter and can be heard as a click. In the above right picture the detector is responding to the radiation being emitted from the uranium ore.
In hospitals it is not really helpful to rely on Geiger counters to monitor the staffs’ exposure to radioactivity so they wear film badges instead.
The film badge dosimeter, or film badge, is a personal dosimeter used for monitoring cumulative radiation dose due to ionizing radiation.
The badge consists of two parts: photographic film or dental X-ray film, and a holder. The film is removed and developed to measure exposure. The film badge is used to measure and record radiation exposure due to gamma rays, X-rays and beta particles.
The diagram shows a typical radiation badge when it is closed and what the inside looks like when it is opened.
There is a light-proof packet of photographic film inside the badge. The more radiation this absorbs, the darker it becomes when it is developed. To get an accurate measure of the dose received, the badge contains different materials that the radiation must penetrate to reach the film. These may include aluminium, copper, lead-tin alloy and plastic. There is also an open area at the centre of the badge.