Radiation
Typesof Radiation
There are five main types of radiation:
Alpha (α) radiation
This consists of a slow moving helium nuclei. This type of radiation is emitted from large nuclei greater in size than Bismuth (atomic number 83), that have an excess number of neutrons.
Properties
● +2 electrical charge
● Slow moving
● Ionising (medium)
● Affected by magnetic and electrical fields
● Low penetration, can be stopped by a sheet of paper, or skin
Beta negative (β-) radiation
This consists of fast moving electrons emitted from smaller nuclei with an excess of neutrons. A neutron is converted to a proton with the release of an electron.
Properties
● -1 electrical charge
● Fast moving
● Ionising (high)
● Affected by magnetic and electrical fields
● Medium penetration power, can penetrate paper and skin, stopped by sheet of metal like aluminum.
Beta positive (β+) radiation
This consists of fast moving positrons (positive equivalent of electrons), emitted from smaller nuclei with an excess of protons. A proton is converted to a neutron with the release of a positron.
Properties
● +1 electrical charge
● Fast moving
● Ionising (high)
● Affected by magnetic and electrical fields
● Medium penetration power, can penetrate paper and skin, stopped by sheet of metal like aluminum.
Gamma (γ) radiation
This consists of electromagnetic radiation and is emitted from many forms of decay, including nuclear fission and fusion as a result of mass and energy changes in the nucleus.
Properties
● No electrical charge
● Moves at the speed of light (3x 108m/s in a vacuum)
● Ionising (low compared to others)
● Not affected by electrical or magnetic fields
● Very high penetration power, can be stopped by concrete and sheets of lead
● Highly energetic
Neutron (n or no) radiation
Neutrons are emitted during fission and fusion reactions.
Properties
● No electrical charge
● Slow moving
● Low ionisation
● Not affected by electrical or magnetic fields
● Generally confined to the decay reaction
Background Radiation
It is a misconception that radiation is confined to nuclear reactions or science labs, in fact radiation is all around us at all times. This natural and human produced radiation is known as the background radiation, and the dose of radiation is measured in millisieverts (mSv). This is a measure of the amount of energy absorbed by body tissues and is thus a measure of the harm that radiation can cause.
A typical dose of background radiation in the UK is 2.4 mSv per year, although this does vary from place to place. Cornwall for example is higher than the average due to the presence of granite rocks.
Some sources of background radiation are natural in origin, cosmic ray from space, Potassium-40 in the food we eat and the largest component is from radon. This is a naturally occurring gas in our atmosphere released from rocks.
Others are as a result of human activity, medical industrial and the fallout from nuclear weapons testing and use in the second world war.
Various occupations can increase a person’s background dosage per annum. Cabin crew and airline pilots, for example, have an increased exposure to cosmic radiation. This is due to the increased amount of time spent at altitude, the atmosphere absorbs cosmic radiation, so flying increases exposure.
Background radiation at normal levels of exposure has no detectable effect on human health.
Detecting Radiation
Radiation is measuring in the SI unit of the Becquerel (Bq). This is a measure of the number of radioactive particles detected per second, (decays per second).
The Sievert is a measure of the absorption of radiation by body tissues and is used in considering the effect of radiation on health. The Becquerel doesn’t distinguish between types of radiation. Given that each have different energy levels, different ionization ability and penetration of tissues, a simple count of decays per second does not indicate the impact on health. The Sievert is a large unit so in practice exposure is normally recorded in millisieverts (mSv).
Geiger Műller Tube
A Geiger-Műller Tube is a device for detecting radiation. As all radiation is ionizing, the tube is filled with a gas that will produce ions when radiation interacts with the gas. The ions as charged particles create an electrical current that can be detected and then activate a counter device.
These are used in research, teaching and in various radiological protection situations to monitor radiation levels.
They are reasonably large devices and not suitable for personal radiological protection. People working with radioactive sources, such as personnel in nuclear power stations, hospital radiotherapy departments and certain military establishments, need to monitor their level of exposure. To do this they can wear a small strip of film within a badge. As radioactivity turns film cloudy this can be used as a means to monitor individual levels of exposure.
Atomic Models
The current model of the atom, consisting of electrons orbiting a dense positive nucleus has not always been the model used. In common with many theories in science it has evolved over time as new discoveries have been made and theories proved, or disproved.
The original idea of an atom as a small, but indivisible building block of nature stems from the ancient greeks. Atom comes from the Greek Atomos (indivisible).
The chemical idea of small solid objects that interact comes from the work of John Dalton in 1803. Over the following century the understanding that atoms were made of other particles and that some of these had a charge developed. In 1904, JJ Thompson proposed the plum-pudding model of the atom. (A plum pudding is the US term for a christmas pudding). In this model the the newly discovered electrons were imagined to be embedded in a positive substance, like fruit embedded in a christmas pudding.
In 1905, Ernest Rutherford tested his theory, by firing alpha particles at thin sheets of gold leaf and analysing the way they scattered. From this he discovered that the positive part of the atom was concentrated in one place. Using this he proposed the concept of the nucleus. How the electrons fitted into this model was still unclear.
In 1913 Niels Bohr’s work showed that Rutherford was correct, but explained the idea we use today, that the electrons are orbiting the positive nucleus.
Since then lots of other subatomic particles have been discovered and we now know that electrons, protons and neutrons themselves are made of smaller particles, such as quarks.
- Name the five types of radiation and state their composition.
- Your answer should include: Alpha / Beta negative / Beta positive / Gamma / Neutron radiation
Explanation: Alpha: Helium nuclei, Beta negative: electrons Beta positive: Positrons Gamma: Electromagnetic radiation Neutron radiation: Neutrons from fission and fusion - How did Rutherford’s 1905 experiment change our understanding of the structure of the atom?
- Your answer should include: positively / charged / nuclei
Explanation: It provided evidence for the existence of a positively charged nuclei. - Name two forms of natural background radiation and the typical level of exposure in the UK.
- Your answer should include: cosmic rays / rocks / radon gas / food / 2.4 mSv
Explanation: Natural background radiation can come from: Cosmic rays, rocks, radon gas and our food. In the UK a typical level of background radiation exposure is 2.4 mSv.