Topic 6 - Radioactivity Flashcards
What is an atom
A positively charged nucleus, consisting of protons and neutrons. Surrounded by negatively charged electrons, with the nuclear radius much smaller than that of the atom and with almost all of the mass in the nucleus
What is the radius of an atoms
1x10-10 metres
What proportion of the total radius of an atom is the radius of the nucleus?
1/10,000
Give two ways than an atoms electron arrangement can be changed?
Absorbing electromagnetic radiation
Emitting electromagnetic radiation
Explain how an atoms electron arrangement changed when it absorbs em radiation?
Electrons Move to higher energy levels
They move away from the nucleus
Explain how an atoms electrons arrangement changes when it emits EM radiation
Electrons move to a lower energy level
They move towards the nucleus
Ratio of electrons to protons
They are the same 1:1 therefore no overall charge in the Atom
What is an isotope
Atoms of an element that has same atomic number but different number of neutrons
How do atoms turn into positive ions
They lose one or more of their outer electrons
Electrons are negatively charged, so the resultant charge of the atom is positive
Relative mass and charge of protons, neutron, electron
1 - proton + 1
1 - neutron - 0
1/1850 - electron - (-1)
State four types of nuclear radiation
Alpha particles
Beta particles
Gamma rays
Neutrons
What is meant by background radiation
Radiation that is Always present
It is in very small amount and so not harmful
Give 4 sources of background radiation
Rocks
Cósmic rays from space
Nuclear weapon testing
Nuclear accidents
How do you measure and detect background radiation?
Photographic film
Geiger-Müller counter
How is photogenic film used to measure radiation?
A photographic film turns dark when it absorbs radiation. This is useful for people who work on radiation as the more radiation they are exposed to, the darker the film becomes. Therefore the workers know when they have been exposed to too much radiation
How is Geiger-Muller tubes used to measure radiation?
When the Geiger-Muller tube absorbs radiation it produces a pulse, which a machine uses to count the amount of radiation. The frequency of the pulse depends on how much radiation is present. A high frequency would mean the tube is absorbing a large amount of radiation.
What constitutes an alpha particle?
Two protons and two neutrons
It is the same as a helium nucleus
What is the range of an alpha particle through air?
A few centimetres (normally in the range of 2-10cm).
Stopped by paper
What will block beta radiation?
A thin sheet of aluminium
Several metres of air
What will block gamma radiation?
● Several centimetres of lead
● A few metres of concrete
Which type of radiation is most ionising?
Alpha radiation.
Which type of radiation is least ionising?
Gamma radiation
How does gamma emission affect mass/charge of an atom?
Both mass and charge remain unchanged.
Describe the plum-pudding model of the atom
A sphere of positive charge, with the negatively charged electrons distributed evenly throughout it.
Prior to the discovery of the electron what was believed about the atom?
The atom was believed to be indivisible.
Which experiment led to the plum-pudding model being discarded?
Rutherford’s alpha-Scattering experiment.
What is the name given to the currently accepted model of the atom?
The Bohr model.
Describe Rutherford’s experiment
Alpha particles (charge +2) were fired at a thin sheet of gold foil
● Most particles went straight through
● Some particles were deflected by small angles
(< 90o)
● A few particles were deflected by large angles
(> 90o)
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What are the conclusions of Rutherford’s experiment?
● Most of an atom is empty space
● The nucleus has a positive charge
● Most of the mass is concentrated in the nucleus
What happens in the process of beta plus decay?
A proton turns into a neutron and a positron (in order to conserve charge).
What is the process called when a neutron changes into a proton and an electron?
Beta minus decay.
When alpha decay occurs, what happens to the atomic number and the mass number of the atom?
The atomic number decreases by 2
● The mass number decreases by 4
● A new element is made since the atomic number has changed
What effect does beta minus decay have on the mass number and atomic number of an atom?
The mass number stays the same as the total number of neutrons and protons hasn’t changed (one has just turned in the other).
The atomic number increases since there is one more proton.
Define the activity of an unstable nucleus.
Activity is the rate at which a source of unstable nuclei decays.
What is the unit of radioactive activity?
Becquerel (Bq)
What is count-rate?
The number of radioactive decays per second for a radioactive source.
Give an example of a detector that may be used to measure count-rate.
Geiger-Muller tube
Describe the nature of radioactive decay.
Random ● Which nuclei decays and when is determined only by chance ● It is impossible to predict which nuclei will decay and when
Define the half-life of a radioactive isotope
● The time it takes for the number of unstable nuclei in a substance to halve
● The time it takes for the count rate from a sample to fall to half its initial level
Give example uses of radioactivity.
Household fire alarms (smoke)
● Irradiating food
● Sterilisation of equipment
● Tracing and gauging thicknesses of materials ● Diagnosis and treatment of cance
How do smoke alarms work?
A radioactive substance is in the alarm which emits alpha radiation
The emitted alpha particle ionises the air in the detector and causes a current to flow between the plates
When smoke interferes with the radiation, the air is no longer ionised and so no current can flow
● This reduction in current flow triggers the alarm
State two uses of nuclear radiation in the field of medicine.
Examining of internal organs
- Radiotherapy in the treatment of cancer
What is the role of beta radiation in tracers?
The tracer is inserted in your body, and targets a specific part of the body
● The radioactive substance in the tracer releases beta radiation which can be detected
by external machines
How is beta radiation used to determine thickness?
A beta source is placed above the material and a detector is placed below it
● If there is an increase in radiation detected by the detector, too much radiation is passing through the material, and so it is too thin
● If there is a decrease in radiation is detected, then the material blocks too much radiation, and so it is too
thick
Why is ionising radiation dangerous?
It can damage tissue and kill cells
● It can cause cell mutations
What is a consequence of cell mutation?
Cancer.
What precautions should people take when using ionising radiation?
● Avoid handling the source directly (use tongs)
● Wear radiation protective clothing
● Keep the radiation in lead containers to reduce
the amount of radiation that can escape
● Keep exposure time to a minimum
Does a long half life or a short half life make a source more dangerous?
If it has a long half life then it would
remain highly radioactive for longer therefore making it more dangerous.
What precautions are taken to reduce harm for doctors and patients using ionising radiation?
Only a small dose is given to the patient so they are not exposed to too much.
● The radiation used has a short half life so it won’t remain highly radioactive for long. This reduces the risk to the doctors using it as well as the patient.
● Doctors and patients (when applicable) wear protective clothing
What is radioactive contamination?
The presence of unwanted radioactive nuclei on other materials.
What is irradiation?
● The process of exposing a material to nuclear radiation
● The material does not become radioactive
How is a radioactive tracer used in medicine?
The tracer is placed inside the body (it can be in a drink, eaten or injected)
● The tracer releases gamma radiation which is detected by a detector which moves around the body
● This can then be used to produce a picture of the patient’s body
How does a PET scanner work?
● PET (positron emission tomography) uses a tracer, which is injected into the patient’s body
● The scanner detects the gamma rays which are released by the trace
● Multiple images are taken and this is used to form a 3D image of the patient’s body
Isotopes are used in PET scanners. What is important about where they are produced and why?
They must be produced near the hospital because the isotopes used have a short half life so must be used soon after production.
Describe the advantages of nuclear power for generating electricity.
● Nuclear fuels do not produce carbon dioxide
The fuel is readily available, so would reduce strain on the fossil fuel supplies
● Less nuclear fuel has to be used to produce the same
amount of energy as burning fossil fuel
● Does not contribute to global warming
Explain some disadvantages of using nuclear power to generate electricity.
Unpopular - the public perceive nuclear power as very dangerous
● Security risks as the radioactive substances can be useful for
terrorists
● Expensive to commission and decommission the stations - which may
be bad for a country’s economy
● The radioactive waste can be difficult to dispose of and will remain
radioactive for many years - which can be dangerous to humans and
the environment
● Risk of nuclear accidents
What is nuclear fission?
The splitting of large, unstable nuclei to form smaller more stable nuclei (+ the emission of spare neutrons).
What usually needs to happen to induce fission?
● The unstable nuclei must absorb a neutron
● Spontaneous fission (where no neutron absorption occurs) is rare
Alongside two smaller nuclei, what else is emitted in a fission reaction?
Two or three neutrons
● Gamma rays
● Energy
Name a common fissile nuclei.
Uranium-235
What are the three main components of the core a nuclear reactor?
1) Fuel rods
2. Control rods
3. Moderator
What takes place during a chain reaction in a nuclear reactor?
● An unstable nucleus absorbs a neutron ● The nucleus undergoes fission and releases 2 or 3 further neutrons ● These induce more fission, which results in a chain reaction
What is the consequence of an uncontrolled chain reaction?
● The rate of fission eventually becomes too high and results in the production of too much energy
● This can lead to a nuclear explosion
How is the chain reaction in a fission reactor kept under control?
● Control rods are positioned in between the fuel rods
● The rate of fission is controlled by moving these rods up and down
● The lower the rods are inserted, the slower the rate of fission
What is the role of the moderator in a nuclear reactor?
To slow down the neutrons so they are travelling at speeds which allow them to be absorbed by fissile nuclei and cause fission.
How is electricity produced in a nuclear power station?
● The reactions release thermal energy
● The thermal energy is used to boil water and
then produce steam.
● This steam is then used to turn a turbine which
starts the generator.
What is nuclear fusion?
When two light nuclei join to produce a heavier nucleus and release energy
Name two isotopes of hydrogen which are commonly used in nuclear fusion.
Deuterium and Tritium
Which releases more energy, nuclear fission or nuclear fusion?
Nuclear fusion.
Explain the difficulty of generating energy through nuclear fusion
Fusion requires very high temperatures which in itself requires large quantities of energy. Currently, the production of fusion results in a net loss of energy
Give an example of where fusion occurs?
In the sun - stars use fusion as their energy source.
Why does nuclear fusion require so much energy?
This is because both nuclei are positive therefore a lot of energy is required to overcome the electrostatic repulsion between the two nuclei.
