Note: All material here remains Copyright Uranium
Information Centre Ltd.
The following are four kinds of nuclear
Alpha particles: These are particles
(atomic nuclei) consisting of 2 protons and 2 neutrons. They are
intensely ionising but can be readily stopped by a few centimetres
of air, a sheet of paper, or the human skin. They are only dangerous
to people if they are released inside the body. Alpha-radioactive
substances are safe if kept in any containers sealed to air.
Beta particles: These are either
electrons or positrons (therefore of very low mass). They can be
stopped by a thin piece of wood or plastic and are generally less
dangerous to people than gamma radiation. Exposure produces an effect
like sunburn, but which is slower to heal. Beta-radioactive substances
are also safe if kept in appropriate sealed containers.
Gamma rays: These are high energy
beams almost identical with x-rays and of shorter wavelength than
ultraviolet radiation. They are very penetrating, and need substantial
thicknesses of heavy materials such as lead, steel or concrete to
shield them. They are the main hazard to people in dealing with
sealed radioactive materials. Doses can be detected by the small
badges worn by workers handling any radioactive materials. Gamma
activity in a substance (e.g. rock) can be measured with a scintillometer
or geiger counter.
Neutrons: These are mostly released
by nuclear fission, and apart from a little cosmic radiation they
are seldom encountered outside the core of a nuclear reactor. Fast
neutrons are very penetrating as well as (indirectly) being strongly
ionising and hence very destructive to human tissue. They can be
slowed down (or "moderated") by wood, plastic, or (more commonly)
by graphite or water.
(X-rays are also ionising radiation, virtually
identical to gamma rays, but not nuclear in origin.)
The amount of ionising radiation absorbed
in tissue can be expressed in grays, 1 Gy = 1 J/kg. However,
since neutrons and alpha particles cause more damage per gray than
gamma or beta radiation, another unit, the sievert (Sv) is
used in setting radiological protection standards. One gray of beta
or gamma radiation has one sievert of biological effect, one gray
of alpha particles has 20 Sv effect and one gray of neutrons is
equivalent to around 10 Sv (depending on their energy).
Total dose is thus measured in sieverts
(or millisieverts - mSv - one thousandth of a sievert, or microsieverts
- µSv - one millionth of a sievert). The rate of dose is measured
in milli or micro sieverts per hour or year. For instance, our natural
dose is around 2 mSv/yr, and maximum annual dose allowed for a uranium
miner is 20 mSv/yr, though that received in Australian and Canadian
mining operations is typically less than half of this.
(These levels contrast with those which are harmful
in a disaster scenario: with gamma radiation a short term dose of
1 Sv causes (temporary) radiation sickness, 5 Sv would kill about
half the people receiving it in a month and a burst of 10 Sv would
be fatal to all in a matter of days. The 28 radiation fatalities
at Chernobyl appear to have received more than 5 Sv in a few days,
those suffering acute radiation sickness averaged 3.4 Sv.)
The becquerel (Bq) is the unit or a measure
of actual radioactivity in material (as distinct from the radiation
it emits, or the human dose from that), with reference to the number
of nuclear disintegrations per second (1 Bq = 1 disintegration/sec).
Quantities of radioactive material are commonly estimated by measuring
the amount of intrinsic radioactivity in becquerel - one Bq of radioactive
material is that amount which has an average of one disintegration
per second, ie an activity of 1 Bq.
Older units of radiation measurement
continue in use in some literature:
1 gray = 100 rads
1 sievert = 100 rem
1 becquerel = 27 picocuries or 2.7 x 10-11 curies
One curie was originally the activity of one
gram of radium-226, and represents 3.7 x 1010 disintegrations
per second (Bq).
The Working Level Month (WLM) has been
used as a measure of dose for exposure to radon and in particular,
radon decay products (see Appendix 2). One "Working Level" is approximately
equivalent to 3700 Bq/m3 of Rn-222 in equilibrium with
its decay products. Exposure to 0.4 WL was the maximum permissible
for workers. Continuous exposure to 0.4 WL during working hours
would result in a dose of 5 WLM over a full year, corresponding
to about 50 mSv/yr whole body dose for a 40-hour week. In the underground
mine at Olympic Dam, and at Ranger, individual workers' doses are
kept below 1 WLM/yr (10 mSv/yr), and typically average half this.
A background radon level of 40 Bq/m3
indoors and 6 Bq/m3 outdoors, assuming an indoor occupancy
of 80%, is equivalent to a dose rate of 1 mSv/yr and is the average
for most of the world's inhabitants.
2 mSv/year Typical background radiation to Australian public.
3 mSv/year Typical background radiation to North American public.
2.9 mSv/year Average occupational dose to US nuclear industry employees.
5.0 mSv/year Average occupational dose to Australian uranium miners.
1.5 mSv/year Average incremental dose for aircrew.
10 mSv/year Maximum actual dose to Australian uranium miners.
20 mSv/year Current limit for nuclear industry employees (5 year
50 mSv/year Former limit for nuclear industry employees and U miners,
current maximum limit in a single year.
350 mSv in lifetime Criterion for relocating people after Chernobyl
1000 mSv as short term dose: likely to cause (temporary) radiation
10,000 mSv as short term dose: fatal within days or weeks.
Uranium Information Centre or
contact Australian Radiation Protection & Nuclear Safety Agency,
Yallambie, Vic, phone (03) 9433 2211 or e-mail email@example.com