International Year of Crystallography

Radioactivity


What is radioactivity? What is radioactive decay? Are there different types of radiation and how far can radiation penetrate? Find answers to your questions here.


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Radioactivity and radiation  


'Radioactivity' is the energy and mass released by spontaneous changes in the nucleus of an atom.

 

'Radiation' is energy that travels ('radiates') as waves or particles. Heat, light, sound, microwaves, radar, radio waves, X-rays, alpha and beta particles, and gamma rays are all forms of radiation.
 


 

Radioactive decay  

Radiation Decay
How Radiation Travels


'Radioactive decay' is the spontaneous radioactive disintegration of an atomic nucleus, resulting in the release of energy. Some atoms are stable. Others are unstable and 'decay', emitting radiation to achieve a stable state. The emissions from an unstable atom's nucleus, as it decays, can be in the form of alpha, beta or gamma radiation.

 

When an atom decays, it changes into another isotope, or form, of the same element or into a completely different element, in a process called transmutation. Different isotopes of the same element differ in the number of neutrons in their nuclei.

 

Some elements reach stability via a series of steps through several isotopes, or 'daughter products'.

 

One example is uranium-238 (U-238), which, through the process of radioactive decay, will eventually become a stable isotope of lead. However, this process takes billions of years. Along the way, as the U-238 isotope's initial energy declines, it will transmute via a series of elements, each more stable than the last - thorium, radium, radon, polonium and bismuth - before it stabilises as lead.

 

Alpha decay


In alpha decay, a positively-charged particle is emitted from the nucleus of an atom. This alpha particle consists of two protons and two neutrons (the same structure as a helium-4 nucleus). Although alpha particles are normally highly energetic, they travel only a few centimetres in air and are stopped by a sheet of paper or the outer layer of dead skin.

 

Beta decay
In beta decay, a particle is emitted from the nucleus of an atom. This beta particle is an electron with either negative or positive electric charge. Beta particles may travel metres in air and several millimetres into the human body. Most beta particles may be stopped by a small thickness of a light material such as aluminium or plastic.

 

Gamma decay


Gamma decay occurs because the nucleus of an atom is at too high an energy state. The nucleus 'falls down' to a lower energy state, emitting a high energy photon known as a gamma particle in the process. Gamma particles travel in a wave-like pattern at the speed of light. They can only be stopped by a dense material such as lead, steel, concrete or several metres of water.


 

Types of radiation   


There are two main types of radiation: non-ionising and ionising radiation.

  1. 'Non-ionising radiation' has less energy than ionising radiation but can still excite molecules and atoms causing them to vibrate faster. Near ultraviolet, visible light, infrared, microwave, radio waves, and low-frequency RF (longwave) are all examples of non-ionising radiation.
     
  2. 'Ionising radiation' has enough energy to change the chemical composition of matter by forcing an atom or molecule to give up an electron, therefore 'ionising' it. These electrically-charged particles are called ions. Ionising radiation sources include alpha and beta particles, gamma rays, neutrons and cosmic rays.


More about ionising radiation

 

There are three types of ionising radiation: alpha, beta and gamma. When ionising radiation hits any material or the human body, that material or body does not itself become radioactive, but the different particles will have different levels of penetration and effects.
 

Sources of ionising radiation include neutrons and cosmic rays. Neutron radiation occurs within a nuclear reactor. Cosmic radiation originates from stars in outer space, making up part of the 'background radiation' that we are all exposed to every day.


 

Penetration of radiation   


The discovery of radioactivity has delivered many benefits, but it must be handled with care. A sheet of paper, or even the skin of our bodies, will stop alpha particles, while a thin sheet of perspex or glass will stop beta radiation. However, the energy of both can cause damage to cells if they enter the body through inhalation, swallowing or wounds. Thick barriers of lead, water and concrete are necessary to stop much more penetrating and damaging gamma radiation.
 

EXT_IMG_
Penetration of radiation- a guide

 

Fission versus fusion


When atoms split apart or join together, energy is released. Splitting atoms is called nuclear 'fission', while joining atoms is called nuclear 'fusion'.
 

Nuclear fission


An atom's nucleus can be split apart. When this is done, a tremendous amount of energy is released in the form of both heat and light. The word fission means to split apart. Inside a nuclear reactor, such as OPAL, uranium atoms are split apart in a controlled chain reaction.
 

In a chain reaction, neutrons released by the splitting of the atom go off and strike other uranium atoms, splitting those and releasing another lot of neutrons to continue striking more uranium atoms. In nuclear reactors, design features and control rods are used to regulate the splitting so it does not go too fast.
 

If the reaction is not controlled and the fuel is almost pure uranium-235 or plutonium, you could have an atomic bomb. These conditions are not present in a nuclear reactor.
 

Nuclear Fusion


Fusion is when smaller nuclei join together to make a large nucleus. The sun is powered by nuclear fusion of hydrogen atoms into helium atoms. This gives off heat, light and other radiation.
 

Scientists have been working on controlling nuclear fusion for a long time, trying to make a fusion reactor to produce electricity. To date, prototype fusion reactors have consumed more energy than they have produced, but scientists believe that the future of fusion research is promising.