Detection of Radioactivity
and its measurement, units and ionising radiation sources
The radiation can be detected and measured in several ways
Geiger-Muller (GM) tube and counter
set up in the laboratory may
record a background radiation of 25
counts per second.
- By use of a Geiger-Muller (GM) tube and counter.
- This electronically amplifies the ionising effect of the radiation and is used for very accurate measurements of
radioactivity and it can detect a single radioactive event.
reacts to radiation in the same way as it does to light. It is used in film badges by workers in the nuclear industry and hospitals to monitor how much radiation people are exposed to in their potentially harmful environment. The film is developed after specified time interval, and the amount of 'exposure'
or darkening of the film is a measure of how much radiation has 'hit' the person.
The activity of a radioactive source
is measured in ...
- That means 25 individual, mainly gamma
rays, and some beta particles (probably no alpha particles) are 'hitting'
the approximately 1cm2 detector area every second.
- So, think how many must hit your body!, but
don't worry, we seem to have survived millions of years of evolution so far, and the body's
repair system can deal with a few hits!
- Just out of curiosity, look up how many
neutrino's we survive from passing through our body from the Sun every
second! its scary!!!!!
Doses of radiation
are measured in
gray, sievert or roentgen.
units (Bq, s-1), 1
Becquerel = 1 disintegration of an unstable nucleus per second.
- or in curie, 1 curie = 3.7 x
1010 disintegrations per second (Bq).
- A disintegration means the decay or
breakdown of an individual unstable nucleus,
- so 1 curie = 3.7 x 1010
Becquerel of unstable nuclei decaying per second.
Radioactive contamination in a
material e.g. its activity in food or fluids, might be measured in. Bq/Kg
for solids or
Bq/litre for liquids.
Biologically significant levels of
- Gray units (Gy, J kg-1)
are based on the absorbed dose of ionising radiation energy in joules per
kilogram of absorbing material.
- Sievert units (Sv, J kg-1) are based on the
dose equivalent of ionising radiation and these units seem to the most
important when dealing with health and safety issues.
- Röentgen units are based on the ionising effect of the
(charge produced in
coulombs per kilogram of material)
- 1 Röentgen = 2.58 x 10-4
Other examples of radiation doses
- Maximum dose allowed for general public:
5 mSv/year (mSv = millisievert = Sv/1000, 1 mSv = 100 mRem)
- Maximum dose allowed for radiation
workers (medical, industrial, nuclear power): 50 mSv/year
- Natural background dose rate: 1.25
- Maximum dose due to atmospheric atomic
weapon testing 1954-61: 12µSv/year (µ = micro = 10-6)
- Maximum dose due to medical and
industrial use: 120µSv/year
- Average dose due to nuclear reactors:
- Threshold for nausea ('radiation
sickness'): 1 Sv in a few
- Threshold for death: 1.5-2.0 Sv in a few
hours (not 100%, but fatalities start to occur in the days or weeks after
exposure to the radiation)
Dangers of ionising radiation,
precautions when dealing with radioactive materials are dealt with in section 4b
- In the UK your background radiation dosage is around 2.2
mSv/year (2.2 millisieverts/year), though this can vary and is greater in areas
of granite rocks containing isotopes of uranium.
- Radiation doses are an important factor in designing and
applying radiotherapy for cancer treatment or diagnostic techniques using
radioactive tracers e.g. the radiation dose of a single PET scan (see
uses of radioisotopes) is ~7 mSv (seven
millisieverts), over three times what you receive naturally from the
- One dental X-ray 0.20 mSv, 1 chest X-ray 0.30 mSv, 1 C-T
scan 4 mSv
Sources of ionising radiation - emissions from radioactive sources
Typical relative % of background radiation
sources - typical values
|radon gas from rock minerals
|building materials, rocks and
|cosmic rays from the sun
|radioisotopes used in medicine
|food and water
|nuclear power industry
- Dangers of ionising radiation,
precautions when dealing with radioactive materials are dealt with in section 4b
- If a Geiger counter (a radioactive emission detector) is set up anywhere in the world it will register (hopefully!) a
very low level of radioactivity.
- Every second of the day you will be hit by some
particle from a radioactive source or the sun, but don't worry, under normal
circumstances, the does is far to low to cause you any harm.
- Low-level radiation is all around us and passing through
- Your body can
take care of a little radioactive emission.
- This is called the background radiation, it is
always around in the environment, and there are two sets of sources.
- When doing accurate experiments this
background radiation must be taken into account.
background radiation is measured and subtracted from any experimental
results using radioisotopes.
Natural sources of radiation
- Radiation from outer space e.g. cosmic rays from the Sun.
- Fortunately, the Earth's upper atmosphere absorbs some of
the Sun's high energy radiation and the Earth's magnetic field deflects cosmic
rays from us.
- Radioactivity from naturally occurring unstable radioisotopes in rocks at the surface
e.g. there are traces of radioisotopes of uranium in granite rocks.
- There is geological factor involved here.
- The background radiation from soil and rocks is quite
variable, depending on their chemical composition.
- The radioactive gas radon is formed in the process, and can build up to harmful levels in cellars,
which in certain areas of granite rocks e.g. Cornwall and Scotland., such
cellars should be well ventilated, or the radon gas will build up.
- Uranium miners are exposed to far more than the average
background radiation and should wear protective clothing.
- Radioactivity from naturally occurring radioisotopes deep in the Earth's core, the energy released keeps the core very hot and heats the magma in the Earth's mantle.
Radiation sources due to human activity
- Emissions from nuclear power stations
are governed by health and safety legislation, and the
nuclear industry is allowed to emit tiny amounts of radioactive material into the environment).
- Safe storage
from power stations is a current problem that is yet to be
solved for the long-term future. It is very contentious issue for obvious
health, safety and environmental reasons and no satisfactory solution has been
found to the problem of safe waste disposal.
- The used radioisotopes and nuclear fuel
most be processed into a safer form e.g. a glass solid. This solid waste is
stored in long-term and leak-proof containers which could be buried in a
deep and well shielded storage area underground.
- BUT even before this long-term process,
nuclear reactor/weapon waste is particularly and exceptionally dangerously
radioactive due to radioisotopes with short half-lives. So initially it is
stored in containers under water until it has 'cooled off' and safer to
- Some idea of the
and problems in handling radioactive materials are mentioned
in section 4.
in the notes on half-life data in section 6.
- Radioisotope tracers are used in industry and hospitals
(see later) and so their use and disposal must be carefully controlled.
- Nuclear accidents, the worst being at Chernobyl power station in
the Ukraine. Parts of the Lake District in England are still slightly contaminated from the 'fallout' in the rain.
- Atomic weapons testing in the 40's, 50's and 60's. The 'super powers' were testing their latest nuclear bombs in the air or on the surface, producing contaminated dust in the atmosphere. Some of the radioisotopes formed in the explosions, like strontium-90, are still around
in the environment.
- Dangers of ionising
radiation, precautions when dealing with radioactive materials are dealt with in
RADIOACTIVITY and NUCLEAR PHYSICS INDEX
Atomic structure, fundamental particles and radioactivity
is radioactivity? Why does it happen? What radiations are emitted?
3. Detection of radioactivity, measurement,
dose units, ionising radiation sources, background radiation
properties and dangers of alpha, beta & gamma radioactive emission
uses of radioactive Isotopes emitting alpha, beta or gamma radiation
6. Halflife of radioisotopes, how
long does material remain radioactive? Uses of decay data & halflife values
changes in radioactive decay? how to write nuclear
equations? Production of Radioisotopes
Nuclear fusion reactions and the formation of 'heavy elements'
9. Nuclear Fission Reactions, nuclear power energy resources
multiple choice QUIZZES
word-fills on radioactivity
puzzle on radioactivity