Radioactivity

Some nuclei are more unstable than others. However, they wish to become stable and do this by radioactive emissions. We can also cause or stimulate nuclei to decay by causing it to absorb a neutron. This in turn makes it unstable causing it to decay.

We detect radioactive emission using a Geiger-Muller tube connected to a counter. The unit of radioactivity is the Becquerel (Bq); one Becquerel is one count per second.

Radioactivity is around us all the time from a variety of sources. This is called background radiation.

When measuring the radioactivity of any sample, we must always subtract a previously measured reading of background radiation from all measurements.

Types of Radiation

There are 3 types of radioactive emission:

1) Alpha (α) – this has a helium nucleus and consists of 2 protons and 2 neutrons. It is the most massive but also the most ionizing type and can travel only 1-2cms in air before they are stopped. It is stopped by thin paper or skin.

2) Beta (β) - this is an electron emitted from the nucleus when a neutron changes into a proton.

gamma

γ

Km’s

0

lead

Radioactive emissions from a sample are not affected by physical factors such as heat and pressure. What dictates how radioactive a sample is, is the number of unstable nuclei it contains. Thus, since each emission makes an unstable nucleus stable, the number of unstable nuclei will decrease with time and thus the sample would become less radioactive with time.

The Health Hazards of Ionising Radiation

1)

• Ionising radiation can damage the molecules that make up the cells of living tissue. Cells suffer this kind of damage all the time for many different reasons. Fortunately, cells can repair or replace themselves given time so, usually, no permanent damage results.

• However, if cells suffer repeated damage because of ionising radiation, the cell ay be killed or the cell may start to behave in an unexpected way. We can call this effect cell mutation. Some types of cancer can happen because damaged cells start to divide

uncontrollably.

2)

•  Different types of ionising radiation present different risks. Alpha particles may have the greatest ionising effect, but they have little penetrating power. This means that alpha source presents little risk, as alpha particles do not penetrate the skin. The problem occurs when alpha particles are taken into the body.

• Radiation will be close to many different cells and this may be damaged if exposure is prolonged.

• Alpha emitters can be breathed in or taken eating food.

• Radon gas is decay of radium and is an alpha emitter, therefore presents risk to smokers as they take smoke into their lungs.

3)

• Beta and gamma radiation to provide a serious health risk when outside the body.

• Both can penetrate the skin and flesh and can cause damage by ionisation.

• Gamma is the most penetrative.

• Damaged caused by gamma rays will depend on how much energy is absorbed by ionising atoms along their path.

• Gamma and beta emitters that are absorbed by the body present little risk than alpha emitters, because of their lower ionising power.

Gauging

In industrial processes, raw materials and fuel are stored in large tanks or hoppers. Radioactive isotopes are used to gauge, or measure, how much material there is in a storage vessel.

The coal absorbs a large amount of radiation so the reading on the lower detector will be small. As the upper part of the hopper is empty the upper detector will have a high reading.

This method of gauging has several advantages over other methods. There is no contact with the material being gauged. Also, coal dust might cause false readings with an optical gauging system. Coal dust is much less dense than coal so the gamma ray system still works properly.

Tracing and measuring the flow of liquids and gases

Radioisotopes are used to track the flow of liquids in industrial processes. Very tiny amounts of radiation can easily be detected. Complex piping systems, heat exchangers in power stations, can be monitored for leaks. Radioactive tracers are even used to measure the rate or dispersal of sewage.