be classified as particulate if
they are in motion and possess
sufficient kinetic energy.
Ernest Rutherford, an
English scientist, discovered alpha
particles in 1899 while working
with uranium. Rutherford's
studies contributed to our
understanding of the atom and its
nucleus through the RutherfordBohr planetary model of the atom.
An alpha particle can be considered as a helium
nucleus. Helium has 2 protons and 2 neutrons in its
nucleus. If both of its electrons were removed, the
result would be an alpha particle:
They are generally produced in the process of
alpha decay, but may also be produced in other
ways. Alpha particles are named after the first
letter in the Greek alphabet, α. The symbol for the
alpha particle is α.
Alpha decay is a
radioactive process in which
a particle with two neutrons
and two protons is ejected
from the nucleus of a
Alpha decay only
occurs in very heavy
elements such as
uranium, thorium and radium.
The nuclei of these atoms are
very “neutron rich” (i.e. have a
lot more neutrons in their
nucleus than they do protons)
which makes emission of the
alpha particle possible.
After an atom ejects an alpha particle, a new
parent atom is formed which has two less neutrons
and two less protons. Thus, when uranium-238
(which has a Z of 92) decays by alpha emission,
thorium-234 is created (which has a Z of 90).
Since there are two protons and no
electrons, alpha particles are positively charged. Alpha
particles are not very penetrating. Paper, clothing or a
few centimeters of air can effectively shield against
alpha particles. However, if ingested or inhaled, alpha
particles can be hazardous.
Henri Becquerel is credited
with the discovery of beta
particles. In 1900, he showed
that beta particles were
identical to electrons, which had
recently been discovered by
Joseph John Thompson.
Beta particles are high-speed electrons emitted
from the nuclei of decaying radioisotopes. Since these
are electrons, they have a negative charge and a small
mass, approximated as 0 amu.
The beta particles emitted are a form of ionizing
radiation also known as beta rays. The production of
beta particles is termed beta decay. They are
designated by the Greek letter beta (β). There are two
forms of beta decay, β− and β+, which respectively give
rise to the electron and the positron
Beta decay is a
radioactive process in which an
electron is emitted from the
nucleus of a radioactive atom,
along with an unusual particle
called an antineutrino (almost
massless particle that carries
away some of the energy).
Like alpha decay, beta
decay occurs in isotopes which
are “neutron rich” .
When a nucleus ejects a beta
particle, one of the neutrons in
the nucleus is transformed into a
Two forms of Beta decay
1. β− decay (electron emission)
- An unstable atomic nucleus with an excess of
neutrons may undergo β− decay, where a neutron is
converted into a proton, an electron and an electrontype antineutrino (the antiparticle of the neutrino):
n → p + e− + ν
+ Electron + Antineutron
2. β+ decay (positron emission)
-Unstable atomic nuclei with an excess of
protons may undergo β+ decay, also called positron
decay, where a proton is converted into a neutron, a
positron and an electron-type neutrino:
p → n + e+ + ν
5 protons + Neutrino + Positron
Since the number of protons in the nucleus has
changed, a new daughter atom is formed which has one less
neutron but one more proton than the parent. For example,
when rhenium-187 decays (which has a Z of 75) by beta
decay, osmium-187 is created (which has a Z of 76).
Beta particles have a single negative charge
and weigh only a small fraction of a neutron or proton.
As a result, beta particles interact less readily with
material than alpha particles. Beta particles will travel
up to several meters in air, and are stopped by thin
layers of metal or plastic.
Beta particles may travel 2 or 3 meters through
air. Heavy clothing, thick cardboard or one-inch thick
wood will provide protection from beta radiation.
X-rays – are produced outside the nucleus in the electron
Gamma rays – emitted from the nucleus of a radioisotopes
and are usually associated with alpha or beta emission.
called Photons (have no mass and no charge)
Travel at the speed of light (c= 3x10^8 m/s) and considered
energy disturbances in space.
Once emitted, they have an ionization rate approximately
100 ion pairs per centimeter, about equal to beta particles.
Physicists credit French physicist
Henri Becquerel with discovering gamma
radiation. In 1896, he discovered that
uranium minerals could expose a photographic
plate through a heavy opaque paper. Roentgen
had recently discovered x-rays, and
Becquerel reasoned that uranium emitted
some invisible light similar to x-rays. He
called it "metallic phosphorescence”.
- Gamma radiation is very much like x rays. It
has no charge, a very short wavelength and high energy.
Gamma radiation is the most penetrating form of
radiation considered in this section. It travels great
distances through air (500 meters). To be protected
from a gamma emitter, thick sheets of lead or concrete
The positron is represented by the symbol:
After a decay reaction, the nucleus is often in an “excited”
state. This means that the decay has resulted in producing a nucleus
which still has excess energy to get rid of. Rather than emitting
another beta or alpha particle, this energy is lost by emitting a
pulse of electromagnetic radiation called a gamma ray. The gamma
ray is identical in nature to light or microwaves, but of very high
Like all forms of electromagnetic radiation,
the gamma ray has no mass and no charge. Gamma
rays interact with material by colliding with the
electrons in the shells of atoms. They lose their
energy slowly in material, being able to travel
significant distances before stopping. Depending on
their initial energy, gamma rays can travel from 1 to
hundreds of meters in air and can easily go right
It is important to note that most alpha and
beta emitters also emit gamma rays as part of their
decay process. However, there is no such thing as a
“pure” gamma emitter.
paper or clothing
lead or concrete
-is any physical
law stating that a
quantity or intensity is
inversely proportional to
the square of the
distance from the
source of that physical
In equation form:
Where I is the intensity of
the radiation and
d is the distance.
If I1 and I2 are intensities of light at
distances d1 and d2 respectively. Then Inverse square
law is given by:
Inverse square law formula is useful in finding
distance or intensity of any given radiation. The
intensity is given in Lumen or candela and distance s
expressed in meters. It has wide applications in
problems based on light.
Example 1) Use Newton's Inverse Square Law to calculate the intensity of
a radioactive source at a different distance than the distance it was originally
measured. If the intensity of a Iridium 192 source was found to be 62
milliroentgen/hour 100 feet, what is the exposure at a distance of 1 foot.
Example 2) A source is producing an intensity of 456 R/h at
one foot from the source. What would be the distance in feet
to the 100, 5, and 2 mR/h boundaries.
Convert Rem per hour to mRem per hour
456R/h x 1000 = 456,000 mR/h
D2= 67.5 feet
Question 1: The intensity of a monochromatic light
are in the ratio 16:1. Calculate the second distance if
the first distance is 6m?
Given: I1 : I2 = 16 : 1,
d1 = 6m,
d2 = ?
Distance d2 = I1d21I2−−−√
d2 = 9.8 m.
Question 2: Calculate the intensity of radio
active source antimony 124 if it has intensity of
80 milliroentgen/hour for 50 feet. What will be
its intensity at 10 foot?
Given: I1 = 80 milliroentgen/hour,
d1 = 50 feet,
I2 = ?
d2 = 10 feet.
Intensity I2 = I1d21d22
= 40 milliroentgen/hour.
Inverse square law is
Gravitational strength with distance.
Electrostatic force with distance.
Light intensity from a point object.
Sound intensity from a point source.
Nuclear radiation from a point source.