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iGCSE Physics - Radioactivity (The effect of Alpha and Beta Decay on…
iGCSE Physics - Radioactivity
The Atom
The basic atom consists of a
nucleus
surrounded by
electrons
going round the nucleus in orbit
Electrons are
negatively
charged.
A lithium atom
The nucleus consists of
protons
which are
positively
charged
neutrons
that have no charge
The protons and neutrons are the
nucleons
The protons and neutrons have very nearly the same
relative mass
with the neutron having slightly more mass than the proton
Their mass is about 1
The mass of a proton or neutron in kilograms is about
1.6 x 10^-27 kg
The mass of an electron is about 1/1800 the mass of a proton at about
9.1 x 10^-31 kg
Atoms and Ions
Elements are often written like this...
A is the total number of nucleons which is called the
mass number
or the
nucleon number.
Z is the total number of protons which is called the
atomic number
or the
proton number
The number of protons determines the element, if we change the number of protons in the nucleus from 6 to 7, we change the element from carbon to nitrogen
To work out the number of neutrons, we take away the number of protons from the number of nucleons
No of neutrons = mass number - atomic number
If the number of electrons is the same as the number of protons, the atom carries
zero
overall charge. It is described as
neutral
If we change the number of electrons, the atom is
charged.
It becomes an ion
Remove an electron, the overall charge is
positive.
We have a positive ion
Add an electron, we have a negative ion
Ions are
NEVER
made by adding or taking away protons
Isotopes
Isotopes
have the same number of protons but different numbers of neutrons.
If we change the number of protons, we change the element completely
Isotopes have the same
chemical properties
as the normal element
In some types of atom, the nucleus is unstable and will decay into a more
stable
atom. This
radioactive decay
is completely
spontaneous
and
random
When an unstable nucleus decays, there are three ways that it can do so.
It may give out
an alpha particle (symbol α)
They are made up of
2 protons
and
2 neutrons
They have a charge of
+2
and a mass of
4
Alpha particles are relatively
slow
and
heavy
They have a low penetrating power - you can stop them with just a sheet of paper
Because they have a large charge, alpha particles
ionise other atoms strongly
In an electric or magnetic field, they are deflects as positive charge
a beta particle (symbol β)
Beta particles have a charge of
minus 1
and a mass of about 1/2000th of a proton
Beta particles are the same as electrons
Beta particles are
fast
and
light
They have a medium penetrating power - they are stopped by a sheet of aluminium or plastics such as perspex
Beta particles
ionise atoms that they pass
, but not as strongly as Alpha particles do
In an electric or magnetic field, they are deflected in the opposite direction to alpha
a gamma ray (symbol γ)
Gamma rays are
electromagnetic waves
, not particles.
They have no mass and no charge
Gamma rays have a high penetrating power - it takes a thick sheet of metal such as lead, or concrete to reduce them significantly
Gamma rays
do not directly ionise other atoms
, although they may cause atoms to emit other particles which will then cause ionisation
Gamma rays move very fast (at the speed of light)
Many radioactive substances emit alpha particles and beta particles as well as gamma rays.
Particles that ionise other atoms strongly have a low penetrating power, because they lose energy each time they ionise an atom.
Radioactive decay is not affected by external conditions
The effect of Alpha and Beta Decay on Nuclei
Radioactive decay occurs in
unstable
nuclei. The parent nucleus ejects a particle to form a new daughter nucleus.
The new nucleus is
excited
and loses energy by giving off a gamma ray
Alpha Decay
When a nucleus decays be
alpha decay,
it ejects a
helium NUCLEUS
(not atom). The nucleus recoils just like a canon firing a canon ball
Most alpha emitters are heavy nuclei - proton number greater than 82
It is believed that an alpha particle is created some time before its emission in the nucleus
The decay of a parent nucleus into a daughter nucleus by alpha emission
Beta decay
When a nucleus decays be
beta decay, a
neutron
turns into a proton. A
high speed electron** is ejected from the nucleus
The
atomic number
goes up by 1, so a new element is formed, but the mass number stays the same.
The electron comes out of the nucleus NOT the electron shells
Gamma radiation
Gamma rays are very short wavelength and highly energetic electromagnetic radiation
They are given off by very energetic nuclei when some other decay has occurred
Cobalt-60 is a common source of gamma rays
Gamma radiation does not in itself alter the nucleon and proton numbers
Gamma rays are not affected by electric or magnetic fields
Geiger-Muller Tube
Ionising radiation can be detected using photographic film or a Geiger-Muller tube with some form of counter attached
The Geiger-Muller detector usually tells us the number of particles detected per second ("counts per second")
This is also known as the
activity
of the radioactive source with the unit of Becquerels (Bq)
1 Bq = 1 count (decay) per second
Half-Life
If you pointed a Geiger counter at a radioactive substance for a period of time, you'd notice that the reading on the meter
decreases
as you watch
This can be shown on a graph
The radioactivity from some substances dies away very fast - perhaps in a few microseconds.
Others take thousands of years before you'd notice that the radioactivity had decreased at all
Half-Life is the time it takes for the radioactivity to fall by half
If it takes 4 days for half the atoms to decay
after 4 days, 1/2 are left over
after 8 days, 1/4 are left over
after 12 days, 1/8 are left over
This is called exponential decay.
Each isotope has its own unique half-life
It is not possible to speed up or slow down the rate of decay
Background Radiation
Background radiation is all around us. We can do little to avoid it
Most background radiation comes from natural sources, while most artificial radiation comes from medical examinations, such as X-ray photographs
Uses and Hazards of Radiation
Alpha
Use - Used in smoke detectors
Hazard - If taken in to the body, alpha emitters can do immense damage to living tissues
Beta
Use - Checking the thickness of paper sheet in manufacturing
Use - Radioactive tracers in medical research and diagnosis
Hazard - Some risk of tissue damage, although nowhere near as dangerous as alpha
Gamma
Use - Medical research
Use - Non-destructive testing of castings
Hazard - Can cause genetic damage and cancer
Uses of radioactivity
Smoke detectors
Smoke alarms contain a weak source made of Americium-241
Alpha particles are emitted from here, which ionise the air so that the air conducts electricity and a small current flows
If the smoke enters the alarm, this absorbs the alpha particles, the current reducs=es, and the alarm sounds
Am-241 has a half-life of 460 years
Thickness control
In paper mills, the thickness of the paper can be controlled by measuring how much beta radiation passes through the paper to a Geiger counter
The counter controls the pressure of the rollers to give the correct thickness
With paper, or plastic, or aluminium foil, beta rays are used because alpha will not go through the paper
A source with a long half-life is chosen so that it does not need to be replaced often
Sterilising
Even after it has been packages, gamma rays can be used to kill bacteria, mould and insects in food. This process prolongs the shelf-life of the food but sometimes changes the taste
Gamma rays are also used to sterilise hospital equipment, especially plastic syringes that would be damaged if heated
Radioactive Dating
Animals and plants have a known proportion of Carbon-14 (a radioisotope of Carbon) in their tissues
When they die, they stop taking Carbon in, then the amount of Carbon-14 goes down at a known rate
(Carbon-14 has a half-life of 5700 years)
The age of the ancient organic materials can be found by measuring the amount of Carbon-14 that is left
Radioactive Tracers
Radioisotopes can be used for medical purposes, such as checking for a blocked kidney.
To do this a small amount of Iodine-123 is injected into the patient, after 5 minutes 2 Geiger counters are placed over the kidneys
Radioisotopes are used in industry, to detect leaking pipes.
To do this, a small amount is injected into the pipe
It is then detected with a GM counter above ground
Cancer Treatment
Because Gamma rays can kill living cells, they are used to kill cancer cells without having to resort to difficult surgery
This is called "Radiotherapy"
It works because cancer cells can't repair themselves when damaged by gamma rays, as healthy cells can
The Nuclear Atom
Rutherford thought a (Helium nuclei) particles would be the ideal particle to probe the atom
He developed his famous gold foil experiment to investigate the inner structure of the atom
This classic diffraction experiment was conducted in 1911 by Hans Geiger and Ernest Marsden at the suggestion of Ernest Rutherford
alpha particles were shot at a thin gold foil
A zinc sulfide detection screen surrounding foil would fluorescence whenever radiation struck the screen
Geiger and Marsden expected to find most of the alpha particles travel straight through the foil with little deviation, with the remainder being deviated a percent or two
This was based on the plum pudding model
What they found was that most of the alpha particles passed right through the foil
Implying the atom is mostly empty space,
Particles that get close to the gold nuclei are slightly deflected
A few particles were wildly deflected implying a large concentration of positive charge in the centre of the atom
Rutherford's model of the atom included a dense, positively charged nucleus containing protons
Electrons were though to orbit the nucleus like planets orbit the sun
The Use of Alpha Particles
Alpha particles are small (only two protons and two neutrons) and yet have enough mass to be a suitable missile
They are produced naturally by radioactive nuclei that are alpha emitters and so a steady supply was easy to obtain
Their properties had been under investigation for about a decade and Rutherford had been doing research into the fact that if he used high energy alpha particles, they were able to penetrate metal foil sheets and would be ideal as a probe to atomic structure
He was expecting tiny changes in trajectory as they met up with atomic substance.
The Gold Foil
A single atom is too small to look at. It would be impossible to get 'just one' to examine
Therefore, Rutherford decided to look at a metal foil consisting of many atoms in a very thin sheet
Gold was the ideal choice of thin sheets as gold can be rolled out into very find gold leaf sheets
These very fine sheets are only a few atoms deep
Therefore gold foil would produce results of interactions that could be best related to the interaction between a single alpha and a single nucleus
If the foil was too thick, the alpha articles would just be absorbed.
The Evacuated Chamber
It had to be performed in a vacuum because the air would absorb the alpha particles before they hit the foil or before they got to the screen
The Zinc Sulphide Screen
Zinc Sulphide fluoresces (gives out a photon of visible light) when it is hit by a charged particle
Covering the microscope lens with ZnS allowed the viewer to 'see' where the alpha particles hit (or at least count their impacts)
Nuclear Energy
The conversion of
nuclear energy
to
heat
is at the heart of nucleus science
Scientists have learned to control the process so that instead of an explosion, a steady heat source is achieved.
A
nuclear reactor
can boil water to steam to turn a steam turbine
Fission
Very large nuclei tend to be rather unstable. This means that they are very radioactive
Some nuclei, for example, Uranium-235 and Plutonium-239, can be made so unstable that they
split
into two ro more nuclei of more stable elements
This is called
fission
The nuclei are called
fissile
These fissile nuclei are
isotopes
of more stable elements. If left alone, they decay radioactively by emitting alpha particles
Fission is not a spontaneous process
It has to be started by inject a neutron into the nucleus and has to be injected at the right speed
Too fast, the neutron will pass right through, or knock out another neutron
Too slow, the neutron will bounce off the nucleus
Two or three (or more)
neutrons
are released. These can go on to be absorbed by other nuclei to cause a
chain reaction
If the chain reaction is not controlled, a nuclear explosion will occur.
In a nuclear reactor, only one neutron is allowed to pass on to be incorporated into one nucleus
Nuclear Power Stations
The nuclear power station is identical to a normal power station in most respects but the difference is in the boiler that produces steam, the reactor
The uranium is fed to the reactor inside fuel rods which are canisters of stainless steel which have fins to transfer the heat.
Fuel rods can contain either Uranium or Plutonium (metal, oxide or alloy)
The reactor harnesses the heat energy produced when the uranium nuclei split and controls the reaction so that two out of the three neutrons produced are absorbed
Only one neutron out of the three goes on to tickle another nucleus.
If any more neutrons are produced, the reaction would start to go out of control.
If fewer neutrons are produced, the reaction stops
All this is achieved by
Moderator
Slows fast neutrons from the fission to slow thermal neutrons by repeated collisions with the nuclei of the moderator material
Graphite or heavy water are commonly used as moderators
Control rods
Made of boron or cadium
These absorb excess neutrons and thus can speed up or slow down the rate of fission reactions
If the control rods are fully in, the neutrons are absorbed completely
At a certain level, the ideal is reached and the reactor is balanced
If the control rods are too far out, then more neutrons than needed can cause the chain reaction to go out of control
The coolant (carbon dioxide, helium or water) is at a high temperature, up to 650degrees C and then transfers the energy as heat to the heat exchanger which boils the water to turn the turbines
In a pressurised water reactor, liquid water at 320degrees C is taken to the heat exchanged.
The reactor is housed in a large steel vessel surrounded by several metres of concrete to stop the radiation from getting out.
Disposal of radioactive waste
Low level
Contaminated equipment, materials and protective clothing
They are put in drums and surrounded by concrete and put into clay lined landfill sites
Intermediate level
Components from nuclear reactors, radioactive sources used in medicine or research
They are mixed with concrete, then put in a stainless steel drum in a purpose-built store
High level
Used nuclear fuel and chemicals from reprocessing fuels
They are stored underwater in large pools for 20 years, then placed in storage casks in purpose-built underground store where air can circulate to remove the heat produced.
High level waste decays into intermediate level waste over many thousands of years.
