▪ Radioactivity also known as radioactive decay is a process
in which unstable atomic nucleus loses energy by emitting
▪ Radiation is released in form of particles or
▪ The rate of radioactive decay is described in half-lives.
▪ The main source of radiation in Earth is sun. Radiation is
going through everywhere and all the time.
Pioneers in Radioactivity
Discoverer of X-
Alpha and Beta
Type of Radioactivity
By the end of the 1800s, it was known that
certain isotopes emit penetrating rays.
Three types of radiation were known:
1)Alpha particles (a)
2)Beta particles (b)
Alpha Particles (a)
This is the
atomic weight, which
is the number of
protons plus neutrons
The alpha-particle (a) is a Helium nucleus.
It’s the same as the element Helium, with the electrons
stripped off !
A sheet of paper or human skin can stop Alpha Particles.
These are only hazardous to human health if they are
Beta Particles (b)
We see that one of the neutrons from the C14 nucleus
“converted” into a proton, and an electron was ejected.
The remaining nucleus contains 7p and 7n, which is a nitrogen
nucleus. In symbolic notation, the following process occurred:
n p + e ( + n )
Beta Particles (b)
They can travel a few feet in air but can usually be
stopped by clothing or a few centimeters of wood.
They are considered hazardous mainly if ingested or
inhaled, but can cause radiation damage to the skin
if the exposure is large enough.
Unstable Neutron decays into a proton.
Gamma particles (g)
In much the same way that electrons in atoms can be in an
excited state, so can a nucleus.
A gamma is a high energy light particle. It is NOT visible by your
naked eye because it is not in the visible part of the EM spectrum.
(in excited state)
(lowest energy state)
Gamma particles (g)
Occurs when an unstable nucleus emits electromagnetic
radiation. The radiation has no mass, and so its emission
does not change the element.
They penetrate matter easily and are best stopped by water
or thick layers of lead or concrete.
Gamma radiation is hazardous to people inside and outside
of the body.
Gamma rays have the lowest ionizing power, but the highest
The “half-life” (h) is the time it takes for half the atoms of
a radioactive substance to decay.
For example, suppose we had 20,000 atoms of a
radioactive substance. If the half-life is 1 hour, how many
atoms of that substance would be left after:
1 hour (one lifetime) ?
2 hours (two lifetimes) ?
3 hours (three lifetimes)
% of atoms
The “lifetime” of a particle is an alternate definition ofthe
rate of decay.
The lifetime of a free neutron is 14.7 minutes.
If there were 1000 free neutrons in a box, after 14.7
minutes some number of them will have decayed.
The number remaining after some time is given by the
radioactive decay law-
N N e
N0 = starting number
= particle’s lifetime
Its value is 2.718
Not all particles have the same lifetime.
Uranium-238 has a lifetime of about 4.5 billion
(4.5x109) years !
Some subatomic particles have lifetimes that are
less than 1x10-12 sec !
Given a batch of unstable particles, we cannot
say which one will decay.
The process of decay is statistical. That is, we can
only talk about either,
1) the lifetime of a radioactive substance*, or
2) the “probability” that a given particle will decay.
Uses of radiation
• Radioactive tracers are used to locate tumors, to study the
functioning of a particular organ, or to monitor the flow of
blood. Such as iodine-131 is used for thyroid problems.
• used to treat cancer may involve the use of implanted
radioactive isotopes such as gold-198 or iridium-192.
• Radioactive particles often used to measure the thickness of
• Radio isotopes often used for smoke alarms.
• Nuclear fission is used to generate Electricity.
• The age of fossil or rock or anything can be determined by radio
isotopes suck as carbon-14. though it can measure only 50,000
Geiger Muller Counter
• Is used to detect
• original operating
discovered in 1908
in early radiation
• However, there
are limitations in