Uses of radioactivity
Measuring the thickness of paper
In a paper rolling mill, the thickness of the paper is monitored by how much Radiation caused by beta particles (high-energy electrons). A beta particle is an electron ejected from a nucleus when a neutron becomes a proton. is received at the detector.
Question
Why use beta radiation?
Alpha wouldn’t penetrate the paper. Gamma is so penetrating that there would be little difference in the reading detected if the paper became thick. The count rate reduces with increasing thickness if a beta source is used.
Question
Would the beta source have a long or short half-life?
Long. You don’t want to keep replacing the source every few days, and a short half-life would mean that the count rate detected might be reducing if the paper stayed the same thickness.
Sterilizing medical instruments and prolonging the life of fruit
High frequency electromagnetic radiation emitted during radioactive decay. Many centimetres of lead or metres of concrete are needed to absorb it. kills microbes and can be used to sterilize medical instruments and kill the bacteria on fruit and vegetables so they stay fresh longer.
Smoke alarms
An isotope of americium which emits Subatomic particle comprising two protons and two neutrons (the same as a helium nucleus). is used in smoke alarms. Alpha radiation To ionise is to convert an uncharged atom or molecule into a charged particle by adding or removing electrons. the air and this allows a small current to flow between two electrodes. Alpha is weakly penetrating so smoke stops it, the current drops and the alarm goes off.
The isotope should be an alpha emitter with a long half-life. This means the smoke alarm will not need to be changed daily.
Blood and fluid tracers
A tracer is something that shows how an object moves. A radioisotope used to trace, eg the flow of blood through an organ. are added to liquids to show if they are flowing correctly. They can show the movement of pollution, eg sewage or waste oil from factories. However, they are used mainly in medicine to monitor blood flow.
Technetium-99m, which is a gamma source with a short half-life of about six hours, is injected into blood. Arterial blockages or internal bleeds cause a build-up of radioactivity that can be easily detected outside the body. The same principle is used in leak detection of underground pipes. A leak will cause a buildup of The emission of particles and waves from an unstable nucleus. The nucleus loses energy and becomes more stable in this process. that can be detected above the ground.
Question
Why choose a gamma source with a short half-life?
The source has to be very penetrating so that it can be detected outside the body or above ground. Internally, gamma is not as ionising as alpha or beta so there is less risk of cell damage. A short half-life is chosen so the radioactivity does not remain in the body or in the pipes for very long – again to minimise risk.
Cancer treatment
Externally gamma radiation can be beamed at cancer cells to kill them. The gamma source used should have a long half-life to maintain the dose of radioactivity delivered to the tumour. A long half-life also means that the source in the radiotherapy equipment will not need changing regularly.
Internally an alpha source with a short half-life can be injected directly into the tumour. This is called targeted alpha therapy (TAT). Alpha is strongly ionising – so will kill the cancer cells. It is also weakly penetrating so stays within the tumour and doesn’t harm the healthy cells outside the tumour.
Carbon-14 dating
Living things, such as plants and animals, absorb carbon-14 daily. When we die, we no longer absorb carbon-14. Carbon-14 can be used to date organic objects up to about 100,000 years old. The isotope decay follows the usual decay curve.
Carbon-14 has a half-life of 5,730 years. If we find a dead plant or bone that has 25 % of the carbon-14 its living version would have had, we’d know that the fossil is 11,460 years old because two half-lives would have passed.
100 % → 50 % → 25 %
Two half-lives × 5,730 years = 11,460 years