Radioactivity is all about unstable isotopes of an element which give out radiation, and the way in which we can make use of it as well as protect ourselves from harm.

In this guide, we are going to learn more about radioactivity, its sources, its uses, its types and hazards.

Important terms

Term Meaning
Radioactive substance A substance that decays by emitting radiation from its atomic nuclei.
Radiation Energy spreading out from a source carried by particles or waves
Radioisotope A radioactive isotope of an element
Radioactive decay The decay of a radioactive substance when it’s atomic nuclei emit radiation
Radioactive tracing A technique that uses a radioactive substance to trace the flow of liquid or gas, or to find the presence of a cancerous tissue in the body
Radiocarbon dating A technique that uses the known rate of decay of radioactive carbon 14 to find the approximate age of an object made of dead organic matter

When an object has acquired some unwanted radioactive substance

(for example, if you eat something containing radioactive substances, you are said to be contaminated)

Irradiated When an object has been exposed to radiation

Sources of radiation

There are 2 sources of radiation:

  1. Natural (Background) radiation
  2. Artificial radiation

Sources of natural (background) radiation

  • Radon and thoron in the atmosphere: Radon and thoron seep up from the earth’s surface from radioactive uranium rocks underground. When we breathe air in, some of the radon and thoron gas enter our bodies, again exposing us to radiation.
  • Ground and buildings: The ground contains radioactive substances. Often substances from the ground are used in construction, so we are exposed to radiation from these.
  • Food and drinks: Plants grow and take in minerals from the ground. Some of the radioactive substances are taken up as well from the ground; when we consume them as food, we are again exposed to radiation.
  • Cosmic rays: Radiation reaches us from space in the form of cosmic rays. Most of it is stopped by the earth’s atmosphere; if you live at a higher altitude, you shall be exposed to more cosmic radiation than at ground level.


Candidates often make mistakes in questions where radiation is asked to be measured. It is extremely important to measure and subtract background radiation from experimental results or else, you’ll lose marks! Be careful!

Sources of artificial radiation

  • Medical sources: Most radiation we are exposed to comes from medical treatments such as using X-Rays and Gamma rays to:
    • See inside our bodies
    • Destroy cancerous cells
  • Nuclear weapons: In the past, nuclear weapons were developed and tested on land and in the air. This lead to an increase in the exposure of radiation
  • Air travel: The higher altitude you are on, the greater the exposure of cosmic rays and hence the greater the exposure to radiation.
  • At nuclear power stations: People working at nuclear power stations need to be careful while working.
  • Nuclear discharges: Radiation levels need to be constantly checked and measured around nuclear power stations in order to keep the area safe for people to live.

Detecting radiation

Radioactivity was discovered by Henry Becquerel, a French physicist in 1896.

There are several ways in which it can be detected:

Photographic films While investigating phosphorescence in Uranium using photographic films, Becquerel noticed that instead of glowing in the dark, uranium was emitting something that darkened the photographic film: invisible radiation!
Geiger counter Unlike the photographic film, a Geiger counter can quickly detect radiation.

A Geiger-Muller tube is pointed towards the object and radiation imparting from it can be measured when it gives a ‘click’ or a ‘beep’

The number of ‘clicks’ or beeps’ are the number of particles decaying

Radioactive decay

The decay of a radioactive substance when its atomic nuclei emit radiation is defined as radioactive decay.

I know you’re a bit confused! Don’t worry we’ve simplified it all!

  • Radiation is emitted by the nucleus of an atom
  • When this is observed in an atom, it is said to be unstable
  • An unstable nucleus emits radiation in an attempt to become more stable and less radioactive.

Radioactive decay is a random process

If you hear the ‘beeps’ or ‘clicks’ of a Geiger counter, you will notice that you cannot actually predict when the next click will come; this means that radioactive decay is a random process.

Radioactive decay and half life

The half life of a radioactive substance is the average time taken for half of the atoms in a sample to decay.

For example, if a substance is said to have a half life of 10 minutes, it means that the substance will have half of its atoms undecayed. Moreover, it means that every 10 minutes, half of the atoms will decay.

Calculating half-life

Please see the worked example below for more details:

Alpha, beta and gamma

Alpha, beta and gamma are three types of radiation. Here are tables that distinguish between their characteristics.


Name Symbol Made of Mass Charge Speed/ m/s
Alpha <symbol here> 2 protons and neutrons Mass of proton x 4 +2 ~3 x 107
Beta <symbol here> An electron Mass of proton/ 1840  


~2.9 x 108
Gamma <symbol here> Photon of electro magnetic radiation 0 0 3 x 108



Alpha <equation of alpha decay here>
Beta <equation of beta decay here>


Behaviour in electrical and magnetic fields

Name of particle Electrical fields Magnetic fields
Alpha (+) Get deflected towards the negatively charged electric plate Get deflected towards the south pole
Beta (-) Get deflected towards the positively charged electric plate Get deflected towards the north pole
Gamma (0) Are not deflected as they don’t have any charge Are not deflected as they don’t have any charge


Penetrating and ionising ability

Name of particle Penetrating power: the ability to enter a material Ionising power: the ability to ionise (charge) materials
Alpha a Extremely poor penetrating power

b Can only travel a few centimetres

c Can be stopped by a thin sheet of paper

Are the most ionising
Beta a. Better penetrating power than alpha particles

b Can travel longer lengths

c Can be stopped by a thick block of aluminium

Are less ionising than alpha particles; greater ionising than gamma rays
Gamma a. Extremely strong penetrating power

b Can travel the longest distances

c Can only be stopped by a very thick layer of a dense element such as lead

Are the least ionising



Isotopes that are radioactive are called as radioisotopes. For example, carbon-14 is a radioisotope.

Effects of radioisotopes on cells

If you are exposed to a lot of radiation, here is what will happen!

  • The intense radiation will cause a lot of ionisation in a cell and this will kill the cell by burning it down. This is called as a radiation burn and is fatal if treatment is not given soon.
  • If the radiation alters the sequences of DNA in the cell’s nucleus, the mechanisms that control the cell may break down. This may lead to the cell dividing uncontrollably, forming a tumour.
  • If the affected DNA is a gamete such as a sperm or an egg cell, and if it gets fertilized, it may lead to mutations that can either be beneficial for the child or harmful, resulting in some genetic disorder.

Using Radioisotopes

Uses related to penetrating power
Application (use) How it works Type of radiation used
Smoke detectors a Radiation from the source (containing Americium-241) falls on a detector

b As alpha radiation is charged, a small current flows through the detector.

c The output from the processing unit is OFF

d When the smoke enters the gap between the source and the detector, it absorbs all the alpha radiation.

e Now no current flows through the detector and the processing circuit switches ON, sounding the alarm.

Alpha radiation
Thickness measurements a Beta radiation is directed through the paper

b A detector measures the amount of radiation coming through

c If the paper is too thick, the amount of radiation getting through will be lesser

d The paper production system will then automatically adjust the paper’s thickness.

Beta radiation
Medical diagnosis a The patient is injected with a radioactive chemical that targets the problem area.

b The detector detects the radiation coming from the chemical and an image is produced of the tissue under investigation.

Gamma rays
Fault detection A photographic film is strapped to the outside of the pipe and a radioactive substance is place inside it.

When the film is developed, it looks like an X-ray image and shows any faults in the pipe.

(The photographic film was also used by Becquerel when he discovered radioactivity!)

Gamma rays


***This is the end of this guide. Hope you enjoyed it! Thanks for using! We hope you will give us a chance to serve you again! Thank you!