The Science Book

194 MARIE CURIE

isotopes are named after the
total number of protons and
neutrons, so the most common
isotope of uranium, with 146
neutrons, is written as
uranium-238 (i.e. 92 + 146).
Many heavy elements, such
as uranium, have nuclei that
are unstable, and this leads to
spontaneous radioactive decay.
Rutherford named the emissions
from radioactive elements alpha,
beta, and gamma rays. The nucleus
becomes more stable by emitting
an alpha particle, a beta particle,
or gamma radiation. An alpha
particle consists of two protons
and two neutrons. Beta particles
can be electrons or their opposites,
positrons, emitted from the nucleus

when a proton turns into a neutron

or vice versa. Alpha and beta decay

both change the number of protons

in the nucleus of the decaying

atom so that it becomes an atom

of a different element. Gamma rays

are a form of high-energy short-

wave electromagnetic radiation

and do not change the nature of

the element.

Radioactive decay is different

from the fission process that takes

place inside nuclear reactors, and

the fusion process that powers the

Sun. In fission, unstable nuclei such

as uranium-235 are bombarded

with neutrons and break up to form

much smaller atoms, releasing

energy in the process. In fusion,

two small nuclei combine to form a

larger one. Fusion also releases

energy, but the great temperatures

and pressures required to start the

process explain why scientists

have only achieved fusion in the

form of nuclear weapons. So far,

attempts to use nuclear fusion to

generate electricity consume more

energy than is released.

Half-life

As a radioactive material decays,

the atoms of the radioactive

element change to other elements,

and so the number of unstable

atoms reduces with time. The

fewer unstable atoms there are, the

less radioactivity will be produced.

The reduction in activity of a

radioactive isotope is measured by

its half-life. This is the time it takes

for the activity to halve, which

is the same as saying the time for

the number of unstable atoms in

a sample to halve. For example, the

isotope technetium-99m is widely

used in medicine, and has a half-

life of 6 hours. This means that 6

hours after a dose is injected into a

patient, the activity will be half of

its original level; 12 hours after

injection, the activity will be one

quarter of the original level, and so

on. By contrast, uranium-235 has a

half-life of over 700 million years.

Radioactive dating

This idea of half-life can be used

to date minerals or other materials.

Many different radioactive elements

with known half-lives can be used

to do this, but one of the best

known is carbon. The most

common isotope of carbon is

carbon-12, with 6 protons and 6

neutrons in each atom. Carbon-12

makes up 99 percent of the carbon

found on Earth, and has a stable

nucleus. A tiny proportion of the

carbon is carbon-14, which has

two extra neutrons. This unstable

Radioactive decay can happen in three ways. Plutonium-240 (top left)
decays to make uranium and an alpha particle. This is an example of
alpha decay. During beta decay, sodium-22 decays to make neon, a beta
particle (in this case a positron), and a neutrino. With gamma decay, a
high-energy nucleus gives off gamma radiation but no particles.

Alpha decay

Beta decay

Gamma decay

Pu U

Na Ne

e

+ Electron neutrino

Beta particle (positron)+

240 236

94

4

22 22

11 10

2

92

He

Alpha particle