Radioactivity

An atom has a positively charged nucleus, consisting of protons on neutrons, surrounded by negatively charged electrons, with the nuclear radius of 1 × 10-15m compared to the size of an atom at 1 × 10-10m. JJ Thomson found that atoms have electrons with a negative charge and a negligible mass and incorporated them in the plum pudding model. Ernest Rutherford carried out the gold foil experiment, shooting alpha particles through gold foil. Most went through while a few were deflected, meaning there must be a concentrated area of positive charge – the nucleus. His was similar to the current model, which Niels Bohr improved to convey that electrons only exist in fixed orbits

Protons and neutrons are nucleons with relative mass 1, with the protons having a positive charge. Electrons have a negative charge and a mass of 1/1836. In each atom the number of protons equals the number of electrons and is therefore neutral. The electrons orbit the nucleus at different set distances from the nucleus.

Isotopes are forms of an element with the same atomic number but different mass numbers due to varying number of neutrons

shows that this isotope has 6 protons and electrons and 6 neutrons

Electrons change orbit when there is absorption or emission of electromagnetic radiation. If an atom absorbs energy, an electron can move to a higher orbit. When an electron returns to a lower orbit the atom emits energy in an EM wave

Sometimes an atom gains so much energy that one or more electrons can escape from the atom altogether, becoming an ion. Radiation that causes electrons to escape is called ionising radiation. If an atom has lost an electron it has an overall positive charge and is now a cation

Background radiation is radiation we are constantly exposed to from space and naturally radioactive substances in the environment. Radon gas from rocks, medical procedures, cosmic rays and food and drink are the main sources.

Alpha	Helium Nucleus	Travel a few cm in air	Very ionising	Stopped by paper	Change in element	Mass -4 Atomic -2
Beta Minus	High energy, high speed electron	Travel a few m in air	Moderately ionising	Stopped by Al 3mm thick	Neutron into proton and electron (ejected)	Mass +-0 Atomic +1
Positron	High energy positively charged particle	Travel a few m in air	Moderately ionising	Stopped by Al 3mm thick	Proton into neutron and positron	Mass +-0 Atomic -1
Gamma Ray	EM Radiation	Travel a few km in air	Weakly ionising	Stopped by Pb cm thick	Neucleons are rearranged for stability	No change

Radioactivity can be detected using photographic film, which becomes darker as more radiation reaches it. However, the film has to be developed to measure the amount of radiation. A Geiger-Müller Tube is used to measure radioactivity. Radiation passing through the tube ionises gas inside it and allows a short pulse of current to flow. A GM tube can be connected to a counter to count the pulses of current, or the GM click may give a click when radiation is detected. The count rate is the number of clicks per second / minute. When finding the radioactivity of a source, subtract the mean background count from the result

Alpha, beta minus, positron, gamma rays and neutron radiation are emitted from unstable nuclei in a random and spontaneous process. Alpha, beta minus, position and gamma rays are ionising radiations.

The change in the mass and atomic number is shown on the previous page and can be used for nuclear equations. These equations must be balanced, meaning total mass number must be the same on each side and total charges must be the same. Use the symbols in the table above and remember that the element may change

Over time the activity of a radioactive source decreases as more of the nuclei in the sample become stable after undergoing decay. The activity of a radioactive source is the number of decays per second and is measured in Becquerel (Bq). The half-life of a radioactive isotope is the time taken for half of the undecayed nuclei to decay or the activity of a source to decay by half. It cannot be predicted when a particular nucleus will decay but half-life enables the activity of a large sample to be predicted during the decay process. Using a graph of activity against time, take a reading of activity and the time. Take the time reading for activity of half this value. Find the different between these two times – this should be the half-life.

Uses of Radioactivity

Gamma rays are used to irradiate food or sterilise surgical equipment
Radioactive tracers can be used in tracers. A gamma source added to water is used to detect leaks in water pipes. When there is a leak, water flows into surrounding earth and a Geiger-Müller tube will detect higher levels of radiation
Beta particles are used to control paper thickness. When paper is too thin, beta count will be too high, which will reduce the force applied to the rollers and cause thicker paper
Smoke alarms contain alpha particles from americium-241. The detector has an electrical circuit with an air gap between two charged plates. The source emits alpha particles which ionise molecules. These ions are attracted to oppositely-charged plates and so allow a small current to flow. Smoke molecules would slow down the ions, decreasing the current and causing the alarm to sound
Radioactive materials are used in medical diagnosis. A tracer which emits gamma rays is put into the patient with molecules that are taken by a particular organ. The tracer is injected or swallowed. The location of the tracer is detected using gamma cameras. Tracers find sources of internal bleeding as the area of highest radiation is where bleeding is occurring. Gamma cameras detect tumours when radioactive glucose molecules are taken by active cancer cells which take up glucose more quickly for division. These radioactive isotopes must be made nearby as they must have a very short half-life to minimise the time of exposure and are therefore used very quickly
Tracers that emit positrons are also used for detection of medical issues. The tracer emits a positron and when it meets an electron, it is destroyed with the electron and two gamma rays are emitted in opposite direction. The detector in a PET scanner moves and builds a picture of where gamma radiation is coming from
Internal radiotherapy uses a beta emitter in the body, near the tumour, and does not always require surgery. External radiotherapy used beams of gamma rays directed at the tumour. Several lower strength beams may be directed from different directions so that only the tumour absorbs a lot of the energy and the surrounding tissues are harmed less

Dangers of Radioactivity

Ionising radiation can cause tissue damage such as radiation burns. Radiation can also cause mutations which could lead to cancer. Gene mutations that occur in gametes can be passed on but some mutations are not harmful. Radiation is a hazard as it can be harmful. Background radiation is low so is not harmful. However, when working with radioactive sources, tongs are always used and sources are stored in lead-lined containers and are never pointed at people. Medical staff monitor exposure using dosimeters. Isotopes with short half-lives are used so that the time for which the person is exposed to harm is minimised.

Contamination occurs when particles of radioactive material attach to skin or enter the body. They will be exposed until the material decays or is removed. Contamination with a longer half-life is a greater hazard. Irradiation occurs when a person is exposed to a radioactive source. Irradiation stops once the person moves away.

Nuclear Energy

Nuclear reactions, including fission, fusion and radioactive decay, can be a source of energy. Nuclear fuels store a huge amount of energy and are very efficient. They do not burn and do not produce carbon dioxide or other greenhouse gases. However, uranium is a non-renewable fuel and nuclear waste must be sealed carefully for thousands of years. Power stations are very expensive to decommission, and accidents can be deadly

Nuclear Fusion

Uranium-235 nucleus absorbs a neutron
Splits into two daughter nuclei and two or more neutrons are released
Both nuclei and neutrons store high KE due to high speeds
When other uranium-235 nuclei absorb the neutrons, they too become unstable and split up, releasing more neutrons
This is an uncontrollable chain reaction which must be controlled
In a nuclear reactor the fuel is made into fuel rods
As fission occurs, neutrons leave the fuel rods at high speed and are slowed down to increase the chance of absorption by the moderator
Control rods contain elements which absorb neutrons. These rods are placed between fuel rods in the reactor core
If the rate of fission needs to be increased, the rods are moved out so fewer neutrons are absorbed
When the control rods are fully lowered, the chain reaction stops
Energy released from the core is transferred to the coolant, which is pumped through the reactor. The coolant can be water, gas or liquid metal
The coolant is pumped to a heat exchanger where it is used to make steam
The steam drives a turbine which turns a generator to produce electricity

Nuclear Fusion

Fusion occurs when small nuclei combine to form larger ones. The mass of the new nucleus formed is slightly less than the total of the masses of the two smaller nuclei as the lost mass is converted to energy. Fusion is the source of energy for stars. The possibility of using nuclear fusion is being explored. However, the conditions required to force deuterium and tritium together, overcoming the electrostatic repulsion, so that they are within 10-15m of each other are very difficult to replicate at the pressures must be incredibly intense and the temperature must be high enough for nuclei to move fast enough to fuse. Essentially, the conditions of the Sun must be replicated. It is very difficult to sustain the extreme temperatures and pressures so currently fusion is unviable. However, huge amounts of energy can be produced and no harmful waste products are made.