Uses of radioisotopes
Atoms of an element come in different forms called isotopes, with each isotope having the
same number of protons and electrons but different numbers of neutrons. Some of these
isotopes have an unstable nucleus, and at some point will randomly decay into a more stable
atom. They do so my releasing energy or particles called radiation, and are therefore called
radioisotopes (= radioactive isotopes, see Figure 1). While this radiation can be very dangerous
to humans, it can also be put to many different uses, including carbon dating, nuclear power
and medical imaging.
Figure 1: Unstable atoms
spontaneously decay into more
stable isotopes by releasing
radiation. This radiation can
take 3 forms: alpha (helium
nucleus), beta (high energy
electron)
or
gamma
(electromagnetic wave). Decay
of uranium-235 produces alpha,
decay of carbon-14 produces
beta and decay of technetium99 produces gamma.
Half-life and carbon dating:
Throughout their lives organisms consume
different isotopes of carbon, primarily the stable
Carbon-12. However, small amounts of unstable
Carbon-14 is also consumed and build up in the
body. When the organism dies it has a certain
amount of Carbon-14 in its body, which will slowly
decay over time into more stable Nitrogen-14
atoms, releasing β radiation in the process (Fig 1).
Because the decay of radioisotopes is random,
scientists describe the process using the term
“half-life”. The half-life is the length of time
needed for 50% of the atoms to decay. As
scientists know the half-life of Carbon-14
(approximately 5000 years), if they measure the
amount of Carbon-14 in a dead organism they can
determine how long ago it died.
Mass number = no. protons + no. neutrons
A
Z
X
Atomic number = # protons
Element
symbol
For example, if scientists find a skull and measure
that there is 2.5g of Carbon-14 and 2.5g of
Nitrogen-14 in it they know the organism died
approximately 1 half-life ago i.e. about 5000 years
(Fig. 2).
Figure 4: The three types of radiation differ in
their penetration power. As alpha (α) is a large
particle it cannot penetrate tissue and is only
dangerous if ingested/inhaled. Beta (β) is smaller
and thus can penetrate further, causing buns to
skin and more damage if ingested. Because
gamma (γ) is a high energy electromagnetic wave
it is the most dangerous as it can penetrate into
the body and cause internal damage to cells.
Medical imaging:
Radioisotopes are commonly used in medicine,
both to produce images of parts of the body or
treat different diseases. The most common form of
imaging is X-rays, where high energy radiation is
released, penetrating through part of the body,
and is then detected on the other side.
However, other forms of imaging also use
radioisotopes. Doctors can have patient ingest,
inhale or be injected with a radioisotope. Certain
types of cancers will consume large amounts of
the isotope, which then releases gamma radiation.
This radiation can be detected by a gamma
camera, showing the cancer as a bright area
(Fig 4). This allows doctors to diagnose cancer and
identify where it is in the body.
Nuclear power:
Nuclear reactors use radiation released by
radioisotopes to provide power to vehicles and
cities. The reactor consists of a radioisotope fuel,
control rods, a cooling system and a turbine (Fig 5).
The most common fuel is uranium-235, with a half
life of around 700 million years. Within the reactor
it undergoes a process called nuclear fission,
releasing several types of radiation. This radiation
has large amounts of energy, heating up the water
surrounding the fuel, which in turn makes steam
that drives a turbine. The movement of the turbine
is then converted into electricity, and can be used
to power submarines, factories and cities.
To reduce the harm caused by radiation an isotope
with a short half-life is used. The most commonly
used isotope is technetium-99 with a half life of
only 6 hours (Fig 1).
Reactors need to be carefully managed. The heat
from the decay needs to be controlled using rods
that absorb the radiation and a cooling
system. A meltdown occurs if the control rods or
cooling system fails, as occurred in Fukushima after
an earthquake in 2011.
Figure 5 : radioisotope imaging, with bright areas showing
high levels of radiation released by technetium-99. The
brain and kidneys normally take up large amounts, but the
bright spot on the chest is abnormal, suggesting cancer.
Figure 6: major components of nuclear reactor, including
fuel elements (uranium-235), control rods and a cooling
system.
Figure 2: decay of Carbon-14, with only 50% remaining after
each half life (5,000 years)
Figure 3: isotopes are represented using isotope notation, which
includes the element symbol (X), mass number (A) and atomic
number (Z) of the atom. Each isotope is referred to by joining the
symbol X and mass number A in the form X-A (e.g. a Hydrogen
isotope with mass number of 2 is called Hydrogen-2).