4.2 Nuclear radiation Flashcards
Absorbing and emitting radiation
When atoms absorb electromagnetic radiation, electrons move to a higher energy level further away from the nucleus.
When atoms emit electromagnetic radiation, electrons can drop to a lower energy level, closer to the nucleus.
Unstable nuclei
Some atomic nuclei are unstable.
This is because of an imbalance in the forces within the nucleus.
Forces exist between the particles in the nucleus.
Some isotopes are unstable because of their large size or because they have too many or too few neutrons.
Radiation
Unstable nuclei can emit radiation to become more stable.
Radiation can be in the form of a high energy particle or wave.
As the radiation moves away from the nucleus, it takes some energy with it.
This reduces the overall energy of the nucleus.
This makes the nucleus more stable.
The process of emitting radiation is called radioactive decay.
Radioactive decay is a random process.
This means it is not possible to know exactly when a particular nucleus will decay.
Activity
Objects containing radioactive nuclei are called sources of radiation.
Sources of radiation decay at different rates which are defined by their activity.
The activity is defined as, The rate at which the unstable nuclei from a source of radiation decays.
Activity is measured in Becquerels.
The symbol for Becquerels is Bq.
1 Becquerel is equal to 1 nucleus in the source decaying in 1 second.
Detecting radiation
Radiation that is emitted from an unstable nucleus can be detected in different ways.
For example, photographic film changes colour when exposed to radiation
A Geiger-Muller tube is a device used to detect radiation.
Within the Geiger-Muller tube, ions are created by radiation passing through it
The Geiger-Muller tube can be connected to a Geiger counter.
This counts the ions created in the Geiger-Muller tube.
Count-rate is the number of decays recorded each second by a detector.
Alpha particles
The symbol for alpha is α.
An alpha particle is the same as a helium nucleus.
This is because they consist of two neutrons and two protons.
Alpha particles have a charge of +2.
This means they can be affected by an electric field.
Beta particles
The symbol for beta is β.
Beta particles are fast-moving electrons.
They are produced in nuclei when a neutron changes into a proton and an electron.
Beta particles have a charge of -1.
This means they can be affected by an electric field.
Gamma rays
The symbol for gamma is γ.
Gamma rays are electromagnetic waves.
They have the highest energy of the different types of electromagnetic waves.
Gamma rays have no charge.
Penetrating power
Alpha, beta and gamma have different properties.
They penetrate materials in different ways.
This means they are stopped by different materials.
Alpha is stopped by paper, whereas beta and gamma pass through it.
Beta is stopped by a few millimetres of aluminium.
Gamma can pass through aluminium.
Gamma rays are only partially stopped by thick lead.
Ionising power
All nuclear radiation is capable of ionising atoms that it hits.
When an atom is ionised, the number of electrons it has changes.
This gives it a non-zero charge.
Alpha radiation is the most ionising form of nuclear radiation.
This is because alpha particles have a charge of +2.
Gamma radiation is the least ionising form of nuclear radiation.
Range in air
The more ionising a form of radiation is, the sooner it will react with the air it is moving through.
Strongly ionising radiation has the shortest range in air.
Alpha only travels a few centimetres in air.
Beta has a range of a few tens of centimetres.
Gamma is not absorbed by air and so has an infinite range, although it does get less intense with distance.
Uses of radiation
Radiation is used in a number of different ways, for example:
Producing electricity through nuclear fission
Medical procedures including diagnosis and treatment
Testing material
Determining the age of ancient artefacts
Checking the thickness of materials
Smoke detectors
The properties of the different types of radiation determine which one is used in a particular application.
Alpha decay
During alpha decay an alpha particle is emitted from an unstable nucleus.
A completely new element is formed in the process.
Alpha decay usually happens in large unstable nuclei, causing the overall
mass and charge of the nucleus to decrease.
Beta decay
During beta decay, a neutron changes into a proton and an electron.
The electron is emitted and the proton remains in the nuclei.
A completely new element is formed because the atomic number changes.
Beta decay often happens in unstable nuclei that have too many neutrons. The mass number stays the same, but the atomic number increases by one.
Gamma decay
During gamma decay, a gamma ray is emitted from an unstable nucleus.
The process that makes the nucleus less energetic but does not change its structure.
Gamma decay does not affect the mass number or the atomic number of the radioactive nucleus, but it does reduce the energy of the nucleus.
Random nature of radioactive decay
It cannot be predicted when a particular unstable nucleus will decay.
This is because radioactive decay is a random process, this means that:
There is an equal probability of any nucleus decaying.
It cannot be known which particular nucleus will decay next.
It cannot be known at what time a particular nucleus will decay.
The rate of decay is unaffected by the surrounding conditions.
It is only possible to estimate the probability of a nuclei decaying in a given time period.
Half life
It is impossible to know when a particular unstable nucleus will decay.
But the rate at which the activity of a sample decreases can be known
This is known as the half-life.
The time it takes for the number of nuclei of a sample of radioactive isotopes to decrease by half.
In other words, the time it takes for the activity of a sample to fall to half its original level.
Different isotopes have different half-lives and half-lives can vary from a fraction of a second to billions of years in length.
Measuring half - life
To find the half-life of a radioactive isotope, we must strip out the background radiation of the environment around us.
Substance radiation = total radiation − background radiation
We can work out the level of background radiation using a Geiger counter.
Contamination
The unwanted presence of materials containing radioactive atoms on other materials.
A substance is only radioactive if it contains radioactive atoms that emit radiation.
Contamination occurs when a radioactive isotope gets onto a material where it should not be.
This is often due to a radiation leak.
As a result of this, the small amounts of the isotope in the contaminated areas will emit radiation and the material becomes radioactive.
Irradiation
The process of exposing a material to alpha, beta or gamma radiation.
Irradiating a material does not make that material radioactive.
However, it can kill living cells.
Irradiation can be used as a method of sterilisation:
Surgical equipment is irradiated before being used in order to kill any micro-organisms on it before surgery.
Food can be irradiated to kill any micro-organisms within it.
This makes the food last longer without going mouldy.
Protecting against irradiation and contamination
It is important to reduce the risk of exposure to radiation.
Radiation can mutate DNA in cells and cause cancer.
Shielding is used to absorb radiation.
Lead lined suits are used to reduce irradiation for people working with radioactive materials.
The lead absorbs most of the radiation that would otherwise hit the person.
To prevent contamination an airtight suit is used by people working in an area where there may have been a radiation leak.
This prevents radioactive atoms from getting inside the person.
Peer review
It’s important for scientists to share their research on radiation and its effects. Scientists can check each other’s work. This is called peer review.
The more we understand about radiation, the more we can try to reduce its potentially negative effects.
