Radioactivity Flashcards
Who suggested the plum pudding model?
J.J Thomson in 1897
What did J.J Thomson discover?
Electrons could be removed from atoms, so atoms must be made up of smaller bits
What did the plum pudding model suggest?
Atoms were spheres of positive charge with tiny negative electrons stuck in them like fruit in a plum pudding
What did Rutherford realise after the firing alpha particles at thin gold foil?
Most of the mass of the atom was concentrated at the centre in a nucleus
Most of the atom is empty space
The nucleus has a positive charge as it repelled the positive alpha particles
What happened during Rutherford and Marsden’s experiment?
They fired alpha particles at thin gold foil expecting them to pass through or to only be slightly deflected
Most particles went through, some were deflected more than expected and a few were deflected back the way they had come
What did Rutherford’s discovery lead to?
The creation of the nuclear model of the atom
How did Niels Bohr tweak Rutherford’s nuclear model?
He proposed a model where the electrons were fixed in orbits at set distances from the nucleus. These distances were called energy levels
What did Bohr suggest?
Electrons only existed in fixed orbits (shells) and not anywhere inbetween
What does our current model of the atom show?
A positive nucleus surrounded by negative electrons
Mass contained in nucleus in neutrons and protons
The rest of the atom is mostly empty space
Electrons go round nucleus in fixed orbits called energy levels or shells
Why are atoms neutral?
The number of protons is the same as the number of electrons
The protons and electrons have an equal but opposite relative charge
What happens if an atom loses an electron?
It becomes a positive ion (cation)
What happens if an atom gains an electron?
It become a negative ion (anion)
What are molecules?
Joined up atoms
Where do electrons sit in an atom?
Energy levels or shells
Each one is a different distance from the nucleus
How can an inner electron move up to a higher energy level?
By absorbing electromagnetic (EM) radiation with the right amount of energy
What happens when electrons move up?
They move to an empty or partially filled shell
It is said to be ‘excited’
The electron will then quickly fall back to its original energy level. It will emit (lose) the same amount of energy it absorbed.
This energy is carried away by EM radiation
What does the energy of the radiation emitted from the atom determine?
What part of the EM spectrum the radiation emitted from the atom is from
What does the energy of the radiation emitted from the atom depend on?
The energy levels the electron moves between
What does a higher amount of energy mean?
A higher frequency of EM radiation
Eg visible light
What happens as you move further out from the nucleus?
The energy levels get closer together
The difference in energy between two levels next to each other gets smaller
What does a decrease in the difference of energy between two levels next to each other mean?
An excited electron falling from the third energy level to the second would release less energy than an excited electron falling from the second energy level to the first
What happens to the frequency of the generated radiation as you get further away from the nucleus?
It decreases
What does changes within the nucleus itself lead to?
The production or high energy, high frequency gamma rays
How is an atom ionised?
If it loses an electron
How can an outer electron leave the atom?
If it absorbs radiation with enough energy it can move so far out that it leaves the atom
What is ionising radiation?
Any radiation that can knock electrons from atoms (ionise atoms)
What are the different types of ionising nuclear radiation?
Alpha particles
Beta particles
Gamma rays
Neutrons (n) can also be emitted
What are isotopes?
Atoms of an element that have the same number of protons (the same atomic number) but a different number of neutrons (a different mass number)
What happens to unstable isotopes?
They decay into other elements and give out radiation as they try to become more stable.
What is radioactive decay?
The process where unstable atoms decay into other elements and give out radiation as they try to become more stable
What happens when radioactive substances decay?
They spit out one or more types of ionising radiation
What are Alpha particles?
Two neutrons and two protons (like a helium nucleus)
What is alpha radiation?
When alpha particles are emitted from the nucleus
Characteristics of alpha particles
Don’t penetrate far into material and are stopped quickly- they can only travel a few cm in air and are absorbed by a thin sheet of paper
What does the size of alpha particles mean?
They are strongly ionising
What are the types of beta particles?
Beta- minus particle
Beta- plus particle
What is a beta minus particle
A fast moving electron released by the nucleus
Characteristics of a beta minus particle
Virtually no mass
Relative charge of negative 1 (-1)
What is a beta plus particle?
A fast moving positron
What is a positron and what does this mean?
The antiparticle of the electron, this means that it has the same mass as the electron but a positive one charge (+1)
What range do beta minus particles have?
A few metres
Absorbed by a sheet of aluminium
What range do positrons have?
Smaller than beta minus particles
Why do positrons have a smaller range than beta minus particles?
When they hit an electron the two destroy each other and produce gamma rays
This is called annihilation
Where is annihilation used?
Medical imaging
What are gamma ray?
EM waves with a short wavelength released by the nucleus that carry away this energy
What happens to a nucleus after it has decayed?
It undergoes nuclear rearrangement and releases some energy
Characteristics of gamma rays
Penetrate far into materials without being stopped
Travel a long distance through air
Why are gamma rays weakly ionising?
They tend to pass through materials rather than colliding with atoms. Eventually they hit something and do damaged
How can gamma rays be absorbed?
Thick sheets of lead or metres of concrete
What show radioactive decay?
Nuclear equations by using element symbols
What form are nuclear equations written in?
Atom before decay—> atom after decay + radiation emitted
What is the rule when writing nuclear equations?
The total mass and atomic numbers must be equal on both sides
What do alpha particles do to the charge and mass of the nucleus ?
Decrease it
Why do alpha particles decrease the charge and mass of the nucleus?
When a nucleus emits an alpha particle, it loses two protons and two neutrons
The mass number decreases by 4
The atomic number decreases by 2
What happens during beta minus decay?
A neutron changes into a proton and an electron
The mass number doesn’t change but the atomic number increases by 1
Why doesn’t the mass number change during beta minus decay?
It has lost a neutron but gained a proton
In alpha and beta emissions, why is a new element formed?
The number of protons (atomic number) changes
What does beta minus decay do to the charge of the nucleus?
Increases it
What decreases the charge of the nucleus?
Positron emission
What happens during beta plus decay?
A proton changes into a neutron and a positron so the mass number doesn’t change but the atomic number decreases by 1
Why doesn’t the mass number change in beta plus decay?
It has lost a proton but gained a neutron
Why does the atomic number decrease by 1 in beta plus decay?
It has one less proton
What decreases the mass of the nucleus?
Neutron emission
What happens when the nucleus emits a neutron?
Mass number decreases by 1- it has lost a neutron
Atomic number stays the same
What are gamma rays a way of getting rid of?
Excess energy from an atom
What happens to the nucleus when it emits a gamma ray?
It goes from an excited state to a more stable state
What happens to the mass, charge and atomic numbers after gamma rays have been emitted?
They stay the same
What do radioactive sources contain?
Radioactive isotopes that give out radiation from the nuclei of their atoms
True or false
Radioactivity is totally random
True
How can you predict how many nuclei will have decayed in a given time?
It’s the half life of the source
If there are lots of nuclei
What is activity?
The rate at which a source decays
What is activity measured in?
Becquerels Bq
1 Bq is 1 decay per second
What is activity measured with?
A Geiger-Müller tube
How does a Geiger-Müller tube work?
It clicks when it detects radiation
The tube can be attached to a counter which displays the number of clicks per second (the count rate)
How can you also detect radiation (apart from a Geiger-Müller tube)?
Photographic film
How does photographic film detect radiation?
The more radiation it’s exposed to, the darker it becomes
What happens to radioactivity of a source over time?
It decreases
Why does reactivity of a source decrease over time?
Each time a radioactive nucleus decays, one more radioactive nucleus disappears
As the unstable nuclei all steadily disappear, the activity as a whole will decrease
What is the problem with trying to measure the reactivity of a source?
The activity never reaches zero
Half life is used
What does half life measure?
How quickly the activity drops off
Half life
The average time taken for the number of radioactive nuclei in an isotope to half
What does a short half life mean?
The activity falls quickly because the nuclei are very unstable and rapidly decay
Why are sources with a short half life dangerous?
The high amount of radiation they emit at the start
What does a long half life mean?
The activity falls more slowly because most of the nuclei don’t decay for a long time
The source just sits there, releasing small amounts of radiation for a long time
Why are sources with a long half life dangerous?
Nearby areas are exposed to radiation for millions of years
How is half life measured?
A graph
Time (s) in X
Activity (Bq) in Y
What is background radiation?
Low level radiation around us all the time
Where does background radiation come from?
- naturally occurring unstable isotopes
- space/ cosmic rays
- human activity
Where are unstable isotopes found?
Air, foods, building materials, rocks
Where does radiation from cosmic rays come from?
The sun
Earth’s atmosphere protects us from this
Where does radiation from human activity come from?
Fallout from nuclear explosions or nuclear waste
What is the absorbed radiation dose?
The amount of radiation you’re exposed to (and the amount your body absorbs)
What varies your absorbed radiation dose?
Where you live
If you have a job involving radiation
What is irradiation?
Exposure to a radioactive source
What are ways of reducing irradiation?
Keeping sources in lead lined boxes Standing behind barriers Being in a different room Using remote control arms Photographic film badges
What is contamination?
Unwanted radioactive atoms getting into an object
What might happen as a result of contamination?
The contaminating atoms might decay releasing radiation
Why is contamination dangerous?
Radioactive particles could enter your body making you at risk of harm until the contamination is removed or until all the radioactive atoms have decayed
What should be used when handling radioactive sources?
Gloves and tongues to avoid particles getting stuck to your skin or under nails
Some industrial workers wear protective suits to stop them breathing in particles
What leads to tissue damage?
Radiation entering living cells and ionising atoms and molecules within them
What do lower doses of radiation do to cells?
Minor damage without killing them
This gives rise to mutant cells which divide uncontrollably- a tumour
If they attack surrounding cells, this is a cancer
What do high doses of radiation lead to?
Killing cells
Radiation sickness- vomiting, tiredness, hair loss
How do hospitals limit staff and patients exposure to radiation?
Shielding
Tracers with short half lives
Outside the body, which forms of radiation are the most dangerous?
Beta and gamma
They can penetrate the body and get to delicate organs
Why is alpha radiation less dangerous outside the body?
It can not penetrate the skin
Which radiation is least dangerous inside the body?
Alpha sources
Strongly ionising so do damage in a localised area
This means contamination, rather than irradiation, is the major concern
