P5 - Radioactive Materials Flashcards
Describe the structure of a nucleus
The nucleus is positively charged and is
made of protons (positive) and neutrons
(neutral).
What are the relative charges of protons,
electrons and neutrons?
Protons: +1
Electrons: -1
Neutrons: 0
What are the relative masses of protons,
electrons and neutrons?
Protons: 1
Electrons: 0 (0.0005)
Neutrons: 1
Give an estimate for the radius of an
atom
1x10^−10m
Describe the arrangement of electrons in
an atom
Electrons are arranged at different
energy levels, different distances from
the nucleus.
How can electrons arrangement
change?
If an atom absorbs EM radiation,
electrons can move to higher energy
levels or leave the atoms entirely.
Describe Dalton’s atomic model
Everything is made of tiny spheres
(atoms) which could not be divided into
anything smaller.
(1800)
Describe JJ Thompson’s atomic model
The Plum Pudding Model: the overall
charge of an atom is neutral, so it
consists of a positive sphere (“pudding”)
with embedded negative electrons.
(1897)
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).
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.
Describe Rutherford’s atomic model
There is a positive nucleus at the centre
of an atom, with negative electrons
existing in a “cloud”/region around the
nucleus.
(1913)
Describe Bohr’s atomic model
Bohr’s model was very similar to the Rutherford model, but he described electrons as existing in fixed orbitals/shells/energy levels around the nucleus.
What is an isotope?
Atoms of the same element, with the
same number of protons, but a different
number of neutrons so therefore different
masses.
What do all atoms of the same element
share?
The same number of protons (atomic
number).
When is an atom neutral?
When proton number = electron number.
When does radioactive decay occur?
When an atomic nucleus is unstable; it
gives off radiation to become more
stable.
Define activity
The rate of decay of a source of unstable
nuclei; the number of decays per
second.
What is activity measured in?
Becquerels, Bq
Define count rate
The number of decays per second.
How can count rate be measured?
Using a Geiger-Muller Tube.
Describe α radiation
● 1 particle is 2 protons, 2 neutrons (same as a helium nucleus). ● Highly ionising. ● Weakly penetrating (blocked by ~5cm of air).
Give the equation for an α particle
α4/2
Describe ß radiation
● 1 particle is a single electron.
● Medium ionising effect.
● Medium penetration (blocked by
~50cm of air or a sheet of metal).
Give the equation for a ß particle
ß0/-1
Describe γ radiation
● Weakly ionising. ● Highly penetrating (blocked by several cm of lead). ● Electromagnetic radiation (no particles).
Give the equation for γ radiation
γ0/0
γ radiation is an EM wave.
Define half life
Half life is the time taken for the number
of radioactive nuclei to halve, or the time
taken for the count rate/activity to halve.
Why can the decay of a single nucleus
not be predicted?
Radioactive decay is a random process.
How is net decline calculated?
Net decline = initial number - number after n half
lives
Or
Net decline = initial number - (initial number x (1⁄2)n)
What are the risks of ionising radiation to
people?
It can damage living cells, causing them
to die or mutate and become cancerous.
What is contamination?
Unwanted radioactive atoms found on
materials.
What is irradiation?
Exposing an object to nuclear radiation,
without making it radioactive.
What is a use of irradiation?
Sterilisation; medical tools are irradiated
to kill bacteria.
Give 3 practical applications of
radioactive materials
● Medical tracers
● Radiotherapy
● Smoke alarms
Give an example of a medical tracer
Technetium
Why is technetium used as a medical
tracer?
It has a half life of 6 hours and decays into a safe
isotope which can be excreted, making it safe for
use in the body.
It releases only gamma radiation so is weakly
ionising and can pass easily through body tissue
without damaging it.
What kind of radiation is used for
radiotherapy?
Gamma radiation
Explain the process of radiotherapy
● Gamma emitters direct gamma rays
onto specific areas with cancerous
cells.
● The cells absorb the radiation and die.
What kind of radiation is used in smoke
alarms?
Alpha radiation
Give an isotope used in smoke alarms
Americium
How do smoke alarms work?
Alpha radiation is emitted into the air,
reaching a detector and completing the
circuit. If smoke is present, it blocks alpha
radiation so it does not reach the detector
and the circuit is broken, causing an alarm
to sound.
What factors need to be considered in
choosing radiation type?
● Ionising ability. ● Penetration (alpha should not be used in the body, as it is blocked by skin so will be trapped inside). ● Half life.
What is a nuclear fuel?
A radioactive material that releases
energy during changes in the nucleus.
What is nuclear fission?
The splitting of a large and unstable
nucleus.
How does fission occur?
When a nucleus absorbs a neutron.
What is produced by fission?
2 smaller nuclei, 2-3 neutrons and gamma rays (energy).
Explain how a fission chain reaction
occurs
When a nucleus absorbs a neutron and
decays, it produces neutrons which can
cause other nuclei to decay.
Give 2 examples of nuclear fuel for
fission
● Uranium
● Plutonium
What other form of energy is released by
fission?
Kinetic energy; the daughter nuclei move
away with kinetic energy.
What is fusion?
When two small nuclei fuse to form a
heavier nucleus and release energy.
How do the masses of the reactants and
products compare in fusion?
The sum of the masses of the two nuclei
which react is more than the mass of the
heavier nucleus that is formed.
(total mass decreases)
Why does mass decrease in nuclear
fusion?
Some mass is converted into energy,
released as radiation.
Give an example of where fusion occurs
In stars (e.g. the sun)
Why is fusion currently not a viable
energy source?
There is no design yet which
accomplishes positive net energy; so far,
fusion always uses more energy than it
gives out.
Give an example of a nuclear fusion
reaction
Two hydrogen nuclei fusing to form a
new helium nucleus.
