PHYB5 - Nuclear Radiation and it's Uses

Revision notes for AQA Physics B: Physics in Context. Covers a section of the unit 5 exam.

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Nuclear Radiation and its Uses
In the treatment of cancer, malignant cells are severely damaged by radiation so they cannot
reproduce and will die. Normal tissue is also damaged, but these cells can repair themselves
once treatment is finished. To select the appropriate nuclide for a particular purpose it is
necessary to know; the type of radiation the nuclide emits, the ability of the different types
of radiation to penetrate matter, the effect of the different types of radiation on the
material it passes through, and the rate of decay of the source.
Types of Radiation
Alpha-particle scattering experiments showed the structure of the atom to be a small,
positively charged nucleus surrounded by negatively charged electrons. Further experiments
showed that the nucleus of an atom consists of protons and neutrons. A neutral atom has
as many electrons as protons.
When considering the use of radioactive sources, it is important to remember that the body
uses the atoms in chemical processes. Nuclides with the same proton number but a
different number of neutrons are isotopes. They may be stable or radioactive. The chemical
behaviour of the atom is identical whether it is stable or unstable, therefore they will be
used by the body in the same way. This can be helpful as it means the body will deliver
certain atoms to certain parts of the body.
When radioactive decay occurs, unstable nuclei decay spontaneously without any outside
influence into a more stable nucleus. Energy is released in doing so. The energy released by
the nucleus appears as kinetic energy of the particles formed as a result of the decay or as a
gamma ray photon. Some of the energy becomes the kinetic energy of the daughter
nucleus as it recoils to conserve momentum
The charge of the radiation can be identified by applying a magnetic field perpendicular to the
direction of travel. This would cause the alpha to bend in the opposite direction to the beta .
Alpha would bend less due to its greater mass, making it harder to move. Gamma would
remain unaffected:
Alpha ()

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In alpha decay, a stable helium nucleus 2He is emitted with a charge of +2. The A number
decreases by 4 and the Z number decreases by 2. Alpha decay can be very damaging as it is
very ionising.
A A-4 4
Z X Z-2 Y + 2
Alpha particles can travel around 5cm in air at normal atmospheric pressure, and are stopped
completely by a sheet of paper. This is because it interacts easily with other particles,
meaning a collision is more likely.…read more

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Radioactive Decay Rate
A patient undergoing radiation treatment has to receive an appropriate dose. Too little and
the treatment will be ineffective, whereas too much and the radiation will do more harm
than good. The dose used depends on the ionising effect of the radiation and the rate at
which the source is emitting. This means that the dose used depends on the nuclide used
and the number of radioactive nuclei that decay each second.…read more

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This shows that the rate of decrease of the radioactive atoms is proportional to the number
of radioactive atoms present.
Changes in Radioactive Decay Rate
Decay is an exponential decay, meaning it will take the same time to halve each time. The
graph of `N' against `t' shows this. The gradient will be the rate of the decay and will
decrease as the number of radioactive nuclei decrease.
Half Life ­ Time taken for the number of radioactive nuclei to halve
ln 2 0.…read more

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If not the values will all be too high, leading to an incorrect
half life.
Radiation in Medicine
Isotopes used in medicine have short half lives, and this affects where and how they are
produced. Some are made by nuclear reactors and some by particle accelerators. They then
need to be transported to the hospitals. If this is a large difference a significant amount of
their activity may be lost. This has to be accounted for when determining the dose that will
be given.…read more

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T1 = T1 + T1
e 1b 1p
2 2 2
Pacemakers are used to control heart beats where there is an irregular heart beat by sending
short pulses of electric current to the heart. To avoid the need for frequent operations, long
life batteries are essential. A pacemaker has two parts; a sealed battery which provides the
energy, and an electronic circuit which produces pulses.…read more

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V = Potential Difference (Voltage)(V)
Energy Stored by a Capacitor
When a battery charges the capacitor, energy is transferred from the battery to the
capacitor. When a charge flows between the two points which have an electrical potential
difference between them, the energy transferred is equal to the voltage multiplied by the
charge transferred. The addition of the charge increases the potential difference between
the plates, so more energy is transferred when the next unit of charge is transferred.…read more

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Capacitors as Timing Devices
The voltage across a capacitor varies with time in a predictable way when being charged or
discharged through a resistor. This means that it can be used for measuring time intervals.
Capacitor Discharge
A capacitor will discharge quickly at first due to the great repulsion between the great
numbers of electrons on the plate. As electrons are pushed off the rate will slow due to less
repulsion. This takes the shape of an exponential curve.…read more


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