7.3 Ionising radiation Flashcards
An ion is
an atom that has gained or lost electrons
If an atom gains electrons it is
negatively charged (anion)
If an atom loses electrons it is
positively charged (cation)
There are three main sources of man-made ionising radiation:
Medical diagnosis and treatment, for example: X-rays used in radiography and
radiotherapy.
Industrial uses, for example: Non-destructive testing (NDT) and electricity
production. (Note: both medical and industrial uses of radiation produce
radioactive waste).
Fallout from nuclear weapon explosions and nuclear accidents such as
Chernobyl and Fukushima.
Occupational exposures may be significant for workers who deal with radiation in the
following activities:
nuclear power industry
medicine and dentistry
research laboratories
general industry.
The stability of the nucleus depends on
the relative numbers of protons and neutrons
present
The main types of ionising radiation are:
Alpha particles Beta particles Neutrons Gamma rays X-rays.
The ability of ionising radiation to cause harm is a function of
mass and penetrating
ability. Alpha particles have the greatest effect, gamma rays the least.
Alpha particles
Alpha particles are emitted from the nuclei of the radioactive atoms
and consist of two protons and two neutrons. They are heavy, slow
moving and carry a double positive charge.
They are potentially the most damaging type of ionising radiation (if ingested or
breathed in, for example) but are fairly easy to stop with barriers. They will not
penetrate human skin and can only travel a few centimetres in air.
Alpha particle emitters are used in smoke detectors and as static eliminators.
Beta particles
Beta particles are high energy negatively charged particles. Each particle is
actually an electron emitted from the nucleus.
They have an electrical charge of -1 and a mass of approximately 1/2000 of the
mass of a proton or neutron.
Beta particle electrons do not come from the electron shells around the nucleus, they
are formed when the ratio of neutrons to protons in the nucleus is too highm, and an
excess neutron transforms into a proton and an electron. The proton stays in the
nucleus and the electron is ejected energetically.
Beta particles can travel further than alpha particles and can penetrate human skin,
but are not as harmful as alpha particles.
Examples of beta emitters include: phosphorous-32; tritium (H-3); carbon-14;
strontium-90; technetium-99; iodine-129 and -131; caesium-137 and lead-210.
Beta emitters are used in industrial thickness gauges and as medical radioactive
tracers; carbon 14 is used in carbon dating.
Neutrons
Neutrons may be emitted from nuclear fusion or nuclear fission, or from any number
of different nuclear reactions such as from radioactive decay or reactions from
particle interactions (such as from cosmic rays or particle accelerators). Large
neutron sources are rare.
Neutron radiation is termed ‘indirectly ionising radiation’. Because neutrons have no
charge they do not ionise atoms by exciting an electron. However, neutron
interactions can result in gamma emission and subsequent removal of an electron
from an atom, or a nucleus recoiling from a neutron interaction is ionised and causes
more traditional subsequent ionisation in other atoms.
Because neutrons are uncharged, they are not affected by electrical fields and are
therefore more penetrating than alpha radiation or beta radiation. They are also more
penetrating than gamma radiation in materials of high atomic number.
Gamma rays
Gamma rays are very high-energy electromagnetic waves. They have very short wavelengths ranging from 3/100 ths to 3/1,000 ths of a nanometer (nm).
Gamma photons have about 10,000 times as much energy as the photons in the
visible range of the electromagnetic spectrum. They travel at the speed of light and
can cover hundreds to thousands of meters in air before spending their energy. They
can pass through many kinds of materials, including human tissue. Very dense
materials, such as lead, are commonly used as shielding to slow or stop gamma
photons.
Gamma radiation is often emitted following release of a Beta particle, when the
nucleus still has too much energy and needs to release it to become more stable.
Gamma emitters such as Cobalt 60, Caesium 137 and Technetium 99m have a
range of medical and industrial uses including steel thickness testing, medical
sterilisation, medical diagnostics, food pasteurisation and cancer treatment.
X-rays
Whereas gamma rays originate in the nucleus, X-rays originate in the electron fields
surrounding the nucleus (See Figure 7.4) or are machine-produced. X-rays sit
between ultra violet and gamma rays in the electromagnetic spectrum. X-rays have a
wavelength in the range of 0.01 to 10 nanometers.
X-ray machines are used universally, for example: in airport security; in industry for
non-destructive testing (NDT); and in medicine for examinations and radiotherapy
treatment.
Routes of exposure
People can be exposed externally, to radiation from a radioactive material or a
generator such as an X-ray set, or internally, by inhaling or ingesting radioactive
substances. Wounds that become contaminated by radioactive material can also
cause radioactive exposure.
Acute exposure and effects
Acute exposure is exposure to a large, single dose of radiation, or a series of
moderate doses received during a short period of time. Acute exposure to radiation
may cause both immediate and delayed effects.
A large dose of radiation can cause rapid development of radiation sickness,
evidenced by gastrointestinal disorders, bacterial infections, haemorrhaging,
anaemia, loss of body fluids, and electrolyte imbalance.
An extremely high dose of acute radiation exposure can result in death within a few
hours, days, or weeks.
Delayed biological effects include: cataracts, temporary or permanent sterility,
cancer, mutagenic (inheritable genetic effects); or teratogenic (interferes with
embryonic development) effects.
Chronic exposure and effects
Chronic exposure is continuous or intermittent exposure to low doses of radiation
over a long period of time.
With chronic exposure, there is a delay between the exposure and the observed
health effect.
The effects of chronic exposure include: cancer, benign tumours, cataracts, and
mutagenic or teratogenic effects.
Somatic effects
are the symptoms produced in the irradiated person which result
from direct damage to body cells. They are divided into ‘early’ and ‘late’ effects which
broadly correspond to acute and chronic effects.
Genetic effects
are those arising from damage to reproductive cells. Irradiation of
reproductive organs increases the risk of genetic malformation and disease in
offspring and subsequent generations of offspring.
Stochastic effects
are associated with long-term, low-level (chronic) exposure to
radiation.
Stochastic effects are effects that occur on a random basis, independent of the size
of dose. The effect typically has no threshold and is based on probabilities, with the
chances of seeing the effect increasing with dose.
Cancer is a stochastic effect. Ionising radiation’s ability to break chemical bonds in
atoms and molecules makes it a potent carcinogen. Damage at the cellular or
molecular level can disrupt the natural processes which control the rate at which
cells grow and replace themselves. Cancer is the uncontrolled growth of cells.
Radiation can cause changes in DNA, or mutations, which the body may not be able
to repair. The mutations may be mutagenic – which can be passed on to future
generations, or teratogenic which affect the developing foetus in the uterus and
affect only the individual who was exposed.
Non-stochastic effects
(also known as deterministic or threshold effects) can be
related directly to the dose received. The effect is more severe with a higher dose,
i.e., the burn gets worse as dose increases. It typically has a threshold, below which
the effect will not occur. A skin burn from radiation is a non-stochastic effect.
Non-stochastic effects appear in cases of short term exposure to high levels of
radiation (acute exposure) and become more severe as the exposure increases.
Acute health effects such as burns and radiation sickness usually occur quickly.
Radiation sickness can cause premature aging or even death. If the dose is fatal,
death usually occurs within two months. The symptoms of radiation sickness include:
nausea, weakness, hair loss, skin burns or diminished organ function.
Medical patients receiving radiation treatments often experience acute effects,
because they are receiving relatively high ‘bursts’ of radiation during treatment.
Non-stochastic effects are specific to each exposed individual and are characterised
by:
A minimum dose being exceeded before the particular effect is observed (the
threshold may differ from individual to individual).
The magnitude of the effect increases with the size of the dose received by
the individual.
There is a clear relationship between exposure to radiation and the
observed effect on the individual.
Measuring exposure
There are two basic types of instruments used for its detection:
Particle counting instruments.
Dose measuring instruments.
Radioactivity is measured in units called
becquerel (Bq). One becquerel = one
atomic disintegration per second.
The half-life of a radioisotope describes
how long it takes for half of the
atoms in a given mass to decay
