Environmental Physics
Lecture 13
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web: http://people.physics.anu.edu.au/~jps107/
Radiation
What is radiation?
Energy which travels through space
Can be photons – light energy
Energy can also be carried by particles
Can have any energy
Roughly divided into two categories
Ionising radiation – high energy
Non-ionising radiation – low energy
Even low energy radiation can cause damage
Non-Ionising Radiation
There are two main types of non-ionising radiation
Low energy neutron radiation
Electromagnetic radiation
Low energy neutron radiation
Neutrons are a particle usually found in the nucleus of an
atom
No charge
Can be emitted in radioactive decay or in nuclear reactions
As they have no charge, they do not usually ionise atoms
or molecules
They can turn atoms radioactive if they are absorbed
Non-Ionising Radiation
Electromagnetic radiation
We have talked about this a lot in the course
This is made of photons – packets of light
Can have different energy
Depends on wavelength
Energies lower than the first part of the ultraviolet
spectrum (near ultraviolet) are non-ionising
Visible light
Infrared
Radio waves etc…
Ionising radiation
There are a number of different kinds of ionising radiation
Alpha radiation
Beta radiation
Gamma Radiation
X-Ray radiation
High energy neutron radiation
Ionising radiation has higher energy than non-ionising
radiation
Can remove electrons from atoms or molecules
Can break molecular bonds
Causes direct damage to living things
Ionising radiation
Alpha radiation
Made up of alpha particles
Like a small (helium) atomic nucleus
Two protons + two neutrons
Has a charge of +2
Is “heavy”
Come from the radioactive decay of a nucleus
238
U → 234 Th + α
Can be stopped fairly easily
Readily ionises anything and loses all its energy
Will be stopped by a sheet of paper
or even the dead layer of skin on your body
Ionising radiation
Beta radiation
Made from beta particles
Can be either electrons or positrons
Positrons are a form of antimatter
Much lighter (smaller) than an alpha particle
Also come from the decay of a radioactive nucleus
137
Cs → 137Ba + e − +ν e
22
Na → 22Ne + e + +ν e
Can have high energy
Able to ionise more strongly than alpha decay
More ionisation events per particle
Ionising radiation
Beta radiation
Because it is smaller, beta radiation is also able to
penetrate further
Can usually be stopped by a thin sheet of metal (few
centimetres)
Positrons can also annihilate
They are the antiparticle of the electron
Same mass, different charge
When they are combined, they turn into energy (photons)
E = mc2
Ionising radiation
Gamma radiation
Made of high energy photons
Light from the decay of an atomic nucleus
Can accompany other forms of radiation in the decay process
(alpha and beta)
Co → 60Ni + e − +ν e + γ
60
Ni → 60Ni + γ
60
Have no mass, so can penetrate deeply
Higher energy means they can travel further through a substance
May need many centimetres of lead to stop gamma radiation
Ionising radiation
X-Rays
Also a high energy photon
In this case the photon does not come from nuclear decay
Comes from electrons
Can be from an atom – decay of an excited state
Can also be from a machine
Hospital X-Ray machine
Synchrotron or other scientific machines
High energy neutrons
Same as low energy, but can now ionise atoms or molecules
Can penetrate large distances
Best shielding is something with many H atoms
Water, concrete are most common
Where do we find radiation?
It’s everywhere!
Radiation is a part of our everyday environment
Background levels can vary depending on location
Ionising background radiation comes from many sources
Cosmic radiation – comes from space
Can cause other kinds of radiation when it gets here
This is how 14C is created
Naturally occurring radioactive elements
Uranium, thorium, carbon-14
Even from inside our bodies – radioactive elements are absorbed into the
food chain
Non-ionising radiation is also present
Light
Radiowaves, microwaves
Radiation – good and bad
Radiation can have both good and bad effects
Radiation can cause damage to materials or living things
In extreme cases it can kill living things
Even non-ionising radiation can cause damage
Radiation can also be beneficial
Used in medical scans and technological processes
Can cause good side effects in living things
Want to be able to measure how much radiation
there is
Can then see what level is safe and what level will be
dangerous
Measuring radiation
There are different ways to measure radiation – it can
become confusing
Radioactive isotopes
Can measure the activity of radioactive isotopes
Number of decays per second – 1 becquerel = 1 decay per second
This does not tell us how much damage can take place
Depends on the type of radiation
Can measure the energy deposited into a material
1 gray = I joule of absorbed energy per kilogram
Different types of radiation can cause different amounts of
damage
Can try and measure the equivalent damage, or equivalent dose
Equivalent dose is measure in sieverts
Radiation amounts
Background radiation
Average background dose per year worldwide is around 2.4
millisievert
Varies from location to location
Highest recorded background dose in a year was 260 mSv
Ramsar, Iran
Consistently high background levels of radiation
Nuclear testing and nuclear accidents have left a human made
addition to background radiation
Also radiation released from coal fired power plants
This amounts to 5 µSv/year on average
Radiation exposure also occurs due to medical tests
Average of 0.04 to 1 mSv/year
Also have increased radiation when flying in a plane - ~0.1 mSv for an
eight hour flight
Radiation damage
What effect does radiation have?
Damage occurs to molecules within the body
Radiation can break bonds and cause damage to important body functions
In particular, DNA can be broken and not repaired properly
DNA damage can lead to cancers
High doses in a small time (acute) can make you sick immediately
Very high doses will cause death
Linear no-threshold model
Used to calculate how much damage is caused by radiation
Assumes that ANY amount of radiation is dangerous
Many small doses are the same as one big dose
Acceptable dose limits are then calculated
Current limits are 1 mSv/year for the general public
Above background levels
50 mSv/year for radiation workers
Radiation limits – alternative pictures
It may be that there is a safe threshold for exposure
Average exposure is larger than the nominated safe limit
for the general public
No evidence that areas with higher background levels have
higher cancer risks
For example, Ramsar, Iran
Some scientist argue that a small amount of radiation
may even be helpful
Radiation hormesis
Small exposures may cause a response in the body that
helps to protect from larger amounts of radiation
exposure
Radiation shielding
Radiation can be stopped by using absorbing materials
For alpha and beta radiation, relatively thin layers can be
effective
Sometimes, even a layer of air can be enough to prevent the radiation
from reaching living things
For gamma and x-rays, high density materials are used
Lead is very common, also tungsten
Thickness required depends on the energy of the photon
For neutron radiation, need materials with a lot of H atoms
Water is common in nuclear reactors
Some polymer compounds
Cement is also used – cheap, can make it very thick very easily
Thickness is calculated to reduce radiation exposure to
safe levels
Uses for radiation
Radiation is also used in many applications
Hospitals
Everyone is familiar with x-rays
High energy photons are transmitted through the body
More absorbed by high density areas (bones)
Take a picture, just like a camera
Absorption gives the required contrast
Positron Emission Tomography
Imaging technique using antimatter
Radioactive isotope attached to a glucose molecule
Introduced to the body
What happens to glucose?
Positron Emission Tomography
Glucose is absorbed at sites where
there is a lot of energy being used
Cancers are fast growing groups of cells
Brain uses a lot of energy
Positron is emitted at this point
Annihilates with an electron to give two
gamma rays
Gamma rays are detected and the
point at which the annihilation took
place is determined
Gives the location of increased brain
function
Or of a tumor site
Uses of radiation
Also used for many other things
Check your bags at the airport
X-rays
Also longer wave radiation – body scanners
Metal detectors
Smoke detectors
Food and medical instrument sterilisation
High doses of radiation can kill harmful bacteria without doing
significant damage
Final word
Radiation can be good or bad
It only depends on the context
Safety needs to be taken seriously
If it is, there is virtually no danger
Still a lot to learn about the effects of radiation on the
human body
Is there a safe level of exposure?
Is the LNT model good?
Can radiation exposure actually be good for you?