Radioactive elements are naturally found in the environment and are continually emitting radiation. This
naturally occurring radiation is called background radiation, which we are all exposed to throughout our lives.
Background radiation comes from a number of sources. (Note that these are averaged across the population
and may differ for different groups, for example depending on any medical treatment you may have, or whether
you make many aeroplane flights.)
One of the major sources of background radiation
is radon gas. This is produced by minute amounts
of uranium, which occurs naturally in rocks, and
is present in all parts of the country. It disperses
outdoors so is only a problem if trapped inside a
building. Exposure to high levels of radon can
lead to an increased risk of lung cancer.
Types of Nuclear Radiation
There are three types of radiation emitted by radioactive materials. They are all emitted from unstable nuclei:
Most nuclei never change; they are stable. Radioactive materials contain unstable nuclei. These can break up
and emit radiation. When this happens, we say the nucleus has decayed. The result for alpha and beta decay is
the nucleus of a different element. For gamma decay, it is the same element but it has less energy.
Mass number decreases by 4 (2 protons + 2 neutrons lost). Atomic number decreases by 2 (2 protons lost).
Often after either alpha or beta decay the nucleons have an excess of energy. By rearranging the layout of
their protons and neutrons, they reach a lower energy state and the excess energy is emitted in the form of a gamma ray.
- Most types of nuclei never change; they are stable. However, radioactive materials contain unstable nuclei. The
nucleus of an unstable atom can break up (decay) and when this happens, it emits radiation.
- A nucleus of a different element is left behind.
- As time goes by radioactive materials contain fewer and fewer unstable atoms and so become less and less
radioactive and emit less and less radiation.
- There is no way of predicting when an individual nucleus will decay; it is a completely random process. A
nucleus may decay in the next second or not for a million years. This means it is impossible to tell how long it
will take for all the nuclei to decay.
- Like throwing a die, you cannot predict when a six will be thrown. However, given a very large number of dice
you can estimate that a certain proportion, 1/6th, will land as a six.
- We define activity as the number of nuclei that decay per second (N.B. 1 decay per second = 1 Bq). The time it
takes for the activity of a radioactive material to halve (because half of the unstable nuclei that were originally
there have decayed) is called the half-life.
- We see the activity falling as there are fewer nuclei available to decay. However, note that the time taken to
halve is independent of the number of nuclei, in this case 2 seconds. Half-lives are unique to each individual
isotope and range from billions of years to fractions of a second.
- The half-life of a radioactive isotope is formally defined as:
- ‘The time it takes for half the nuclei of the isotope in a sample to decay, or the time it takes for
the count rate from a sample containing the isotope to fall to half its initial level.’
N = Amount of radioisotope particles after nth half life.
N0 = Initial amount of radioisotope particles.
n = number of half life
A graph of activity vs. time can be plotted from experimental measurements. We must
remember to subtract the background count from the actual count to find the count due to the source alone. We
call this the corrected count rate.
Nuclear radiation never completely dies away, but eventually drops to a negligible level, close to the
background. At this point, a source is considered safe. Consideration of half-life therefore, has importance when
considering which isotopes to use for various applications and the disposal of radioactive waste – see section on
applications of radioactivity.
Nuclear Energy - Einstein Formulam = mass change (kg)
c = speed of light (m s-1 )
E = energy changed (J)
Rules for nuclear equations
The total mass number must be the same on both sides of the equation.
The total atomic number on both sides of the equation must be the same.
The total charge must be the same on both sides of the equation.
Nuclei have positive charge due to the protons in them. All the protons repel, so why does the nucleus not explode?
There is another force acting called the strong nuclear force.
This acts between all nucleons, both protons and neutrons.
- A fundamental particle is one that cannot be split into anything simpler.
- The word atom
fired at protons
they are made
up of smaller
In beta decay, one of the up quarks changes to a
down quark or vice versa.
Protons and neutrons are made of just
two types of quark, the up and the
down. Other particles have to be
created in special machines called
Is Radiation Dangerous?
- All nuclear radiation is ionizing. It can knock electrons out of atoms, or break
molecules into bits. If these molecules are
part of a living cell, this may kill the cell.
- Radiation dose is measured in Sieverts. This unit
measures the amount of energy deposited in the tissue
by the radiation, and takes account of the type of
radiation, because some particles are more effective at
damaging cells than others. It is a measure of the
possible harm done to your body.
- Nuclear fission is the splitting of an atomic nucleus.
- A large parent nucleus, such as 235-uranium or 239-plutonium, splits into two smaller daughter nuclei, of approximately equal size.
- This process also releases energy (heat) which can be used to generate electricity. Normally, this will happen spontaneously but can be speeded up by inducing fission.
- If uranium were burned chemically to uranium oxide, it would release about 4500 J/g. The equivalent energy release from nuclear fission is 8.2 × 1010 J/g.
- The daughter products themselves are radioactive because they still tend to be neutron rich (i.e. lying above the N/Z curve), and decay, releasing more thermal energy and nuclear radiation. They have a wide range of half-lives. These factors need to be taken into account when considering their disposal.
Nuclear fusion is the joining of two light nuclei to form a heavier
nucleus. It is the process by which energy is released in stars.
Bonus: History of Radioactivity