Transcript Distinguishing Radiation

Radioactivity and
radioisotopes
• The nature of radiation
• Distinguishing Radiation
• Background Radiation
Alpha particles
Certain radionuclides of high atomic mass (Ra226, U238,
Pu239) decay by the emission of alpha particles. These
alpha particles are tightly bound units of two neutrons
and two protons each (He4 nucleus) and have a positive
charge. Emission of an alpha particle from the nucleus
results in a decrease of two units of atomic number (Z)
and four units of mass number (A). Alpha particles are
emitted with discrete energies characteristic of the
particular transformation from which they originate. All
alpha particles from a particular radionuclide
transformation will have identical energies.
Radioactive
sample
Beta particles
A radioactive nucleus that undergoes beta emission has a
neutron in its nucleus converted into a proton and an
electron, then it ejects the electron with high KE. The
remaining nucleus has one more protons and one fewer
neutron: the atomic number increases by one and the
mass number stays the same.
For example: Tritium, a radioactive isotope of hydrogen,
undergoes beta decay. Its product is an isotope of
helium
3
1
Tritium 
Helium
H  He  e
3
2

High KE
electron
Gamma - rays
A nucleus which is in an excited state may emit one or
more photons (packets of electromagnetic radiation) of
discrete energies. The emission of gamma rays does not
alter the number of protons or neutrons in the nucleus
but instead has the effect of moving the nucleus from a
higher to a lower energy state (unstable to stable).
Gamma ray emission frequently follows beta decay,
alpha decay, and other nuclear decay processes.
Distinguishing radiations
Every type of radiation has characteristic charge,
mass and penetrative and ionizing power. The
combination of these different properties can be
used to detect and distinguish between a, b and g
radiation.
Penetrative power
a
b
g
Skin/paper
stops ALPHA
Thin Aluminium
stops BETA
Thick lead stops
GAMMA
Deflection by B fields
ALPHA: +ve
charge +
large mass 
small
deflection
GAMMA no
charge  no
deflection
B field into wall
BETA: -ve charge
+ small mass 
large deflection
Ionization by a-particles
Alpha particles can ionize atoms by snatching electrons off the atoms
when they pass by them. The alpha particles positive charge pulls the
outer electrons away from the atoms as they approach them. An ion pair
has been formed, where the electron is the negative member of the pair
and the positive ion the positive member.
Ion
a-particle
Ion pair
Electron
What is background radiation?
All around us there are radioactive substances
that give low level radiation. All this radiation is
called BACKGROUND RADIATION.
These substances can be found:
• in the earth's crust;
• emanation of radioactive gas radon from the
earth;
• cosmic rays from outer space;
• trace amounts of radioactivity in the body;
• Radioactive waste.