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What is a radioactive substance?
A radioactive substance contains unstable nuclei that become stable by emitting radiation.
What are the three main types of radiation given out?
alpha , beta, gamma
alpha particles
Positively charged particles
Is radioactive decay random or systematic?
Random - you can't predict which nucleus in a sample will decay next
Write two differences between radiation from uranium and radiation from a lamp
radiation from U consists = particles, radiation from lamp = electromagnetic waves,
radiation from U is ionising, radiation from lamp is non-ionising
Write two differences between radioactive atoms compared with the atoms in a filament lamp
radioactive atoms have unstable nuclei whereas atoms in lamp filament do not,
decay of radioactive atom cannot be stopped whereas atoms in lamp filament stop emitting radiation when
filament current switched off
Write four features of every nucleus of every atom
nucleus much smaller than atom,
nucleus positively charged,
mass of atom concentrated in nucleus
all positive charge of atom concentrated in nucleus
How can you measure radiation levels?
Geiger counter
History of the atom
Before the discovery of the electron, atoms were thought to be tiny
spheres that could not be divided.
Plum Pudding Model
The discovery of the electron led to the plum pudding model of the atom. The plum pudding model suggested that the atom is a ball of positive charge with negative electrons embedded in it.
Alpha particle experiment
The results from the alpha particle scattering experiment led to the conclusion that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged. This nuclear model
replaced the plum pudding model.
Results of alpha scattering experiment
most passed straight through the metal foil
some were deflected back towards the source
Why was the experiment carried out in a vacuum chamber
To prevent air molecules absorbing the alpha particles
Bohr Model
Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances. The theoretical calculations of Bohr agreed with experimental observations.
Protons
Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge.
The name proton was given to these particles.
Chadwick
The experimental work of James Chadwick provided the evidence
to show the existence of neutrons within the nucleus.
Nuclear Model of the Atom
A conceptual model of the atom in which a
small positively - charged nucleus is
surrounded by planetary electrons.
Differences between nuclear model and plum pudding model
nuclear: all positive charge concentrated in nucleus much smaller than atom, plum pudding: positive charge spread out throughout atom,
nuclear: most mass concentrated in nucleus, plum pudding: mass spread out throughout atom
nuclear: most atom empty space, plum pudding: no empty space
Why did the alpha scattering experiment lead to the rejection of the plum pudding model
nuclear model explains why some alpha particles scattered through large angles,
in plum pudding model such large-angle scattering should not be observed
How big are atoms?
Atoms are very small, having a radius of about 1 × 10-10 metres
What is activity and what is it measured in?
Activity is the rate at which a source of unstable nuclei decays.
Activity is measured in becquerel (Bq)
What is count rate?
Count-rate is the number of decays recorded each second by a detector (eg Geiger-Muller tube).
Describe the three types of radiation?
• an alpha particle (α) - this consists of two neutrons and two protons, it is the same as a helium nucleus
stopped by paper
• a beta particle (β) - a high speed electron ejected from the nucleus as a neutron turns into a proton
stopped by a thin sheet of metal
• a gamma ray (γ) - electromagnetic radiation from the nucleus stopped by thick led
Which part of an atom emits a beta particle?
nucleus / neutron
How does the composition of an atom change when it emits an alpha particle?
its atomic number goes down by two
its mass number goes down by four
How does the composition of an atom change when it emits a beta particle?
neutron changes to a proton or number of neutrons goes down 1
and the number of protons goes up by 1
its atomic number goes up by one
mass number is unchanged
Describe the structure of alpha particles.
two protons plus two neutrons
What are beta particles?
(fast moving) electrons (from the nucleus)
Ionisation
Any process in which atoms become charged
The radiation from a radioactive substance can knock electrons out of the atoms
Irradiated
exposed to radiation, not radioactive
Peer review
A process by which the procedures and results of an experiment are evaluated by other scientists who are in the same field or who are conducting similar research.
What is a half-life?
The half-life of a radioactive isotope is the time it takes for the number of nuclei of the isotope in a sample to halve, or the time it takes for the count rate (or activity) from a sample containing the isotope to fall to half its initial level.
What is meant by the term 'half-life'?
(average) time taken for number of nuclei to halve
What is the activity of a radioactive source
The number of unstable atoms in the source that decay per second
Explain the risk to the patient of using a radioactive substance as a medical tracer
radiation causes ionisation that may then harm / kill healthy cells
Even though there may be a risk, doctors frequently use radioactive substances for medical diagnosis and treatments.
Suggest why.
benefit (of diagnosis / treatment) greater than risk (of radiation)
Radioactive tracers
Radioactive tracer graph
Why is radioactive iodine used for this test
Gamma cameras
Gamma radiation - cancer
Radioactive implants
What type of radiation would be used to obtain an image of an internal organ?
beta or gamma,
can be detected outside body
What type of radiation would be used to find out weather a kidney is blocked
gamma,
radioactive source injected into patient to enter organ to be imaged so needs to do least damage whilst in body,
gamma radiation passes through body tissue and detected using gamma camera
Two sources of background radiation
any two from:
food,
drink,
radon
Describe how nuclear radiation is used to destroy a tumor using a radioactive implant
small 'seeds' of radioactive isotope placed in tumour,
radiation from isotope destroys cancer cells,
use isotope with half-lives not long enough to damage normal cells surrounding tumour,
half-life not too short or unstable nuclei decay before radiation destroys tumour
Importance of using the right isotope
too short: radioactive isotope decays too much before scan completed,
too long: patient exposed to ionising radiation unnecessarily
Which type of radiation has the greatest range in air?
gamma
Which of the three types of radiation is the most strongly ionising?
alpha
What is radioactive contamination?
Radioactive contamination is the unwanted presence of materials
containing radioactive atoms on other materials. The hazard from
contamination is due to the decay of the contaminating atoms. The
type of radiation emitted affects the level of hazard.
What is irradiation?
Irradiation is the process of exposing an object to nuclear radiation.
The irradiated object does not become radioactive.
Where does background radiation come from?
Background radiation is around us all of the time. It comes from:
• natural sources such as rocks and cosmic rays from space
• man-made sources such as the fallout from nuclear weapons
testing and nuclear accidents.
The level of background radiation and radiation dose may be
affected by occupation and/or location.
What are nuclear radiations used in medicine for?
Nuclear radiations are used in medicine for the:
