Detection of Radioactivity
and its measurement, units and ionising radiation sources
The radiation can be detected and measured in several ways
Radioactive contamination in a
material e.g. its activity in food, might be measured in Bq/Kg or
Biologically significant levels of
- 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.
Geiger-Muller (GM) tube and counter
set up in the laboratory may
record a background radiation of 25
counts per second.
- 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!!!!!
- Photographic film
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 ...
units (Bq, s-1), 1
becquerel = 1 disintegration of an unstable nucleus per second.
- or in curie (Ci, 3.7 x 10-10
s-1), 1 curie = 3.7 x
1010 disintegrations per second.
- 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.
- Doses of radiation
are measured in
gray, sievert or roentgen.
- 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
coulombs per kilogram of material)
- 1 röentgen = 2.58 x 10-4
- 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)
Sources of ionising radiation - emissions from radioactive sources
- If a Geiger counter is set up anywhere in the world it will register (hopefully!) a low level of radioactivity.
- This is called the background radiation and there are two sets of sources for it.
- 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 eg cosmic rays from the Sun.
- Radioactivity from naturally occurring radioisotopes in rocks at the surface eg there are traces of radioisotopes of uranium in granite rocks.
- The radioactive gas Radon is formed in the process, and can build up to harmful levels in cellars.
- 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 (governed by health and safety legislation, they are 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 eg 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 below
in the notes on half-life data.
- 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 Russia. Parts of the Lake District in England are still 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.
multiple choice QUIZZES and WORKSHEETS
word-fills on radioactivity
puzzle on radioactivity
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