9-1 Physics/Atoms and nuclear radiation

Radioactive decay and nuclear radiation

Although there are often several different isotopes for a particular element only a few are stable, meaning that they can exist for an indefinite period of time. Unstable isotopes, which can't exist that long, decay into stable isotopes by their atomic nuclei (plural of nucleus) emitting (giving out) radiation. This is known as radioactive decay which is a random process; scientist's cannot accurately predict exactly when it occurs.

Activity which is the rate at which a source of unstable nuclei decays is used to measure the rate of radioactive decay and is measured in Becquerel's ( ${\text{Bq}}$ ). Activity is also linked to count-rate which is the number of decays recorded each second by a detector, such as a Geiger-Muller tube and counter. A high count-rate would mean a high activity or vice versa.

The following table shows a comparison of the different types of nuclear radiation that may be emitted during radioactive decay:


Nuclear Radiation	What is it?	Range in air	(How far) do they travel through materials?	How much ionising power do they have?
Alpha particle	2 protons and 2 neutrons (the same as a helium nucleus)	A few centimetres	Absorbed by a sheet of paper	Strongly ionising
Beta particle	A high speed electron which is emitted as a neutron turns into a proton	A few metres	Absorbed by a 5mm sheet of aluminium	Moderately ionising
Gamma ray	Waves of electromagnetic radiation (energy)	A long distance	Able to travel a far way through most materials but absorbed by thick sheets of lead and concrete.	Weakly ionising
Neutron	A single neutron

The ionising power measures the strength of the nuclear radiation.The higher the ionising power the greater the size of the particle however the faster and more easily absorbed the particles are.

Nuclear equations

Nuclear equations use special symbols and mathematics to represent and calculate radioactive decay. They look very similar to chemical equations as they show atoms before decay and the atoms that are formed after the decay as well as the periodic symbol of any elements along with special symbols for radiation. Here is an example:

${\ce {^{238}_{92}U -> ^{234}_{90}Th + ^{4}_{2}\alpha}}$

This shows Uranium-238 decaying to form Thorium-234 and an alpha particle being emitted. Like in chemistry, the top number is the mass number and the bottom number is the atomic number. Like in all of mathematics, one side must be the same as the other; so the total mass number of one side equals the total mass number of the otherside (and the same applies for the atomic number) due to the fact atoms cannot be created or destroyed. The following list shows the symbols used for each different type of nuclear radiation:

An alpha particle could be represented as $_{2}^{4}{\text{He}}$ to represent a helium atom or using the greek symbol $\alpha$ (with the same mass number and atomic number).
A beta particle would be $_{-1}^{0}{\text{e}}$ since it is an electron, or, $\beta$ .
A gamma particle only be represented by $^{}\gamma$ . No mass or atomic number change would occur as energy is being emitted from the nucleus, nothing else.
A neutron would be $_{1}^{1}{\text{n}}$ .

Half-life

A graph showing the half-life from the number of radioactive nuclei by time plot graph

Because radioactive decay is random, scientists measure the half-life of an isotope to make predictions about the activity of a nuclei. The half-life is the time taken for the number of nuclei of the isotope in a sample to halve. This is because some radioactive nuclei will decay faster than others, so using a sample of many radioactive nuclei allows us to look at the average rate of decay. To find the half-life using a graph, all you need to do is look along to where half of the value is and record the time at that point.

Half-life also means the time taken for the activity or count-rate to half because the rate of decay will decrease at the same rate that radioactive isotopes have decayed to a different isotope.

A short half-life means that the activity falls quickly, because the rate of decay is much faster. However, they can be very dangerous as they emit high amounts of radiation but become safe fairly quickly. On the other hand, isotopes with a long half-life emit radiation at a much slower rate but this can be dangerous as areas can be exposed to radiation for millions of years.

We can calculate the total decline after multiple half lives, by reading from the graph the number of radioactive nuclei after adding on the time for another half life or more.Imagine using half-life as a unit of measure. We can then also calculate a ratio between the initial reading at the first half life and the reading at a few half-life distances away by dividing the former by the latter.

Radioactive contamination

Background radiation is low-level radiation that is around us all the time but it is harmless because the radiation dose (which is measured in Sieverts) is too low for it to damage human tissues.

Radioactive contamination is the unwanted presence of materials containing radioactive atoms or other materials, in other words when unwanted radioactive atoms get onto or into an object. Contamination is hazardous because of the radioactive decay of the atoms (releasing radiation). Alpha particles are the most dangerous form of radiation when an object is contaminated as they do the most amount of harm in a very localised area, as these particles have the greatest ionising power.

Irradiation is when an object is exposed to nuclear radiation. Although the object is exposed, it does not make it radioactive. High levels of irradiation from any type of radiation is harmful, but especially from beta and gamma particles which can travel further than alpha particles.

Because human exposure to radiation can cause death, it is very important for studies into the effects of radiation on humans to be written, published and peer-reviewed in scientific journals to share any life-saving findings with the world.

Quick Questions

What is the definition of Energy?

The maximum amount of change possible in a system (object or group of objects)

What are the 2 main types of energy store and what is the difference between them?

Energy stores that relate to movement (Kinetic) and Potential energy stores which relate to the position of an object in space.

What is energy measured in?

Joules

What is specific heat capacity?

The amount of energy it takes to increase 1 Kg of a substance by 1

{\text{°C}}

What is Power?

The rate of energy transfer