Science

Radioactive substances

An atom of any given element consists of a nucleus containing a number of protons and neutrons. The nucleus is surrounded by electrons.

The half-life of a radioactive isotope is the time taken for half its radioactive atoms to decay.

There are three main types of radiation, called alpha, beta and gamma radiation, which all have different properties. Radiation can damage cells and make them cancerous. Very high doses of radiation can kill cells. It can be detected using photographic film or a Geiger-Muller tube. Radiation badges are used to monitor the level of radiation that people who work with radioactive sources are exposed to.

Radiation has many practical uses. It can be used in medicine to trace where certain chemicals collect in the body, indicating disease, and also in industry, where it can be used to control measuring equipment.

Atoms and isotopes

The nuclear model

the proton and neutron are within the nucleus which is within the centre of the atom, the elctrons are on the edges of the atom

Structure of the atom

Atoms contain three sub-atomic particles called protons, neutrons and electrons. The protons and neutrons are found in the nucleus at the centre of the atom, and the electrons are arranged in energy levels or shells around the nucleus.

Isotopes

All the atoms of a given element have the same number of protons and electrons. However, the number of neutrons can vary. Atoms of the same element that have different numbers of neutrons are called isotopes of that element. The diagram shows three hydrogen isotopes.

Hydrogen-1

1 proton, 0 neutron, 1 electron



Hydrogen-2

1 proton, 1 neutron, 1 electron



Hydrogen-3

1 proton, 2 neutrons, 1 electron



Radioactivity

The different isotopes of an element have identical chemical properties. Some isotopes, however, are radioactive. This means that they give out radiation from their nuclei. This happens all the time, whatever is done to the substance. For example, the radiation is still given out if the substance is cooled down in a freezer, or takes part in a chemical reaction.

Types of radiation

There are three main types of radiation emitted from radioactive atoms. These are alpha, beta and gamma radiation.

Alpha radiation

Alpha radiation consists of alpha particles. An alpha particle is identical to the nucleus of a helium atom, which comprises two protons and two neutrons.

Helium atom

Alpha particle

  • 2 protons
  • 2 neutrons
  • 2 electrons
  • 2 protons
  • 2 neutrons
  • 0 electrons

Beta radiation

Beta radiation consists of high energy electrons emitted from the nucleus. These electrons have not come from the electron shells or energy levels around the nucleus. Instead, they form when a neutron splits into a proton and an electron. The electron then shoots out of the nucleus at high speed.

Gamma radiation

Gamma radiation is very short wavelength - high frequency - electromagnetic radiation. This is similar to other types of electromagnetic radiation such as visible light and X-rays, which can travel long distances.

Penetrating properties of radiation

Radiation can be absorbed by substances in its path. For example, alpha radiation travels only a few centimetres in air, beta radiation travels tens of centimetres in air and gamma radiation travels many metres. All types of radiation become less intense the further the distance from the radioactive material, as the particles or rays become more spread out.

The thicker the substance, the more the radiation is absorbed. The three types of radiation penetrate materials in different ways.

Alpha radiation

Alpha radiation is the least penetrating. It can be stopped - or absorbed - by just a sheet of paper.

Beta radiation

Beta radiation can penetrate air and paper. It can be stopped by a thin sheet of aluminium.

Gamma radiation

Gamma radiation is the most penetrating. Even small levels can penetrate air, paper or thin metal. Higher levels can only be stopped by many centimetres of lead or many metres of concrete.

alpha radiation cannot pass through humans, beta is stopped by aluminium, gamma by lead

Penetrative properties of different types of radiation

Check your understanding by having a go at this animation. Click on each image of the rock to discover the reading on the radiation meter. Use the readings to confirm that the rock gives out beta radiation.

Deflecting radiation

Electric fields

Alpha particles are positively charged, beta particles are negatively charged and gamma radiation is electrically neutral. This means that alpha radiation and beta radiation can be deflected by electric fields, but gamma radiation is not deflected.

Remember that opposite charges attract. Beta particles are negatively charged so they will be attracted towards a positively charged plate. And positive alpha particles will be attracted towards a negatively charged plate.

Check your understanding of this by trying the animation.

Magnetic fields

Because they consist of charged particles, alpha radiation and beta radiation can also be deflected by magnetic fields. Just as with electric fields, gamma radiation is not deflected by magnetic fields.

Check your understanding of this by trying the animation.

Detecting radiation

Human senses cannot detect radiation, so we need equipment to do this.

Photographic film

Photographic film goes darker when it absorbs radiation, just like it does when it absorbs visible light. The more radiation the film absorbs, the darker it is when it is developed.

People who work with radiation wear film badges, which are checked regularly to monitor the levels of radiation absorbed. The diagram shows a typical radiation badge when it is closed and what the inside looks like when it is opened.

photographic film is sealed in thin plastic

A typical radiation badge

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.

Geiger-Muller tube

The Geiger-Muller tube detects radiation. Each time it absorbs radiation, it transmits an electrical pulse to a counting machine. This makes a clicking sound or displays the count rate. The greater the frequency of clicks, or the higher the count rate, the more radiation the Geiger-Muller tube is absorbing.

Check your understanding of this topic by having a go at the activity.

Hazards of radiation

Radiation and living cells

When radiation collides with molecules in living cells it can damage them. If the DNA in the nucleus of a cell is damaged, the cell may become cancerous. The cell then goes out of control, divides rapidly and causes serious health problems.

yellow circle with 3 black pie shaped segments inside

Radiation warning symbol

The greater the dose of radiation a cell gets, the greater the chance that the cell will become cancerous. However, very high doses of radiation can kill the cell completely. We use this property of radiation to kill cancer cells, and also harmful bacteria and other micro-organisms.

The hazard symbol is shown on containers of radioactive substances to warn of the danger.

Alpha, beta and gamma radiation

The degree to which each different type of radiation is most dangerous to the body depends on whether the source is outside or inside the body.

If the radioactive source is inside the body, perhaps after being swallowed or breathed in:

  • Alpha radiation is the most dangerous because it is easily absorbed by cells.
  • Beta and gamma radiation are not as dangerous because they are less likely to be absorbed by a cell and will usually just pass right through it.

If the radioactive source is outside the body:

  • Alpha radiation is not as dangerous because it is unlikely to reach living cells inside the body.
  • Beta and gamma radiation are the most dangerous sources because they can penetrate the skin and damage the cells inside.

Notice that these effects are opposites and make sure you get them the right way around.

Half-life

The nuclei of radioactive atoms are unstable. They break down and change into a completely different type of atom. This is called radioactive decay. For example, carbon-14 decays to nitrogen-14 when it emits beta radiation.

It is not possible to predict when an individual atom might decay. But it is possible to measure how long it takes for half the nuclei of a piece of radioactive material to decay. This is called the half-life of the radioactive isotope.

Two definitions

There are two definitions of half-life, but they mean essentially the same thing:

  1. the time it takes for the number of nuclei of the isotope in a sample to halve
  2. the time it takes for the count rate from a sample containing the isotope to fall to half its starting level

Different radioactive isotopes have different half-lives. For example, the half-life of carbon-14 is 5,715 years, but the half-life of francium-223 is just 20 minutes.

Graphs

It is possible to find out the half-life of a radioactive substance from a graph of the count rate against time. The graph shows the decay curve for a radioactive substance.

counts per minute drops from 80 to 5 in 10 days

The decay curve for a radioactive substance

The count rate drops from 80 to 40 counts a minute in two days, so the half-life is two days. In the next two days, it drops from 40 to 20 - it halves. In the two days after that, it drops from 20 to 10 - it halves again - and so on.

Using radiation

Here are some examples of how radiation is used:

  • in smoke detectors
  • for sterilising medical instruments
  • for killing cancer cells
  • for dating rocks and materials such as archaeological finds
  • in chemical tracers to help with medical diagnosis
  • for measuring the thickness of materials in, for example, a paper factory

Tracers

Doctors may use radioactive chemicals called tracers for medical imaging. Certain chemicals concentrate in different damaged or diseased parts of the body, and the radiation concentrates with it. Radiation detectors placed outside the body detect the radiation emitted and, with the aid of computers, build up an image of the inside of the body.

When a radioactive chemical is used in this way it is not normally harmful, because:

  • it has a short half-life and so decays before it can do much damage
  • it is not poisonous

Emitters of beta radiation or gamma radiation are used because these types of radiation readily pass out of the body, and they are less likely to be absorbed by cells than alpha radiation.

Monitoring the thickness of materials

Radiation is used in industry in detectors that monitor and control the thickness of materials such as paper, plastic and aluminium. The thicker the material, the more radiation is absorbed and the less radiation reaches the detector. It then sends signals to the equipment that adjusts the thickness of the material.

Check your understanding of this by watching the simulation.

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