Page 180 - Physics 10_Float

ATOMIC AND NUCLEAR PHYSICS

18.5 HALF-LIFE AND ITS MEASUREMENT
Process of radioactivity is random and the rate of Alpha ( ) Particles 
radioactive decay is proportional to the number of unstable Positively charged particles
nuclei present. In the process, a constant fraction of large (helium nuclei), ejected at high
number of unstable radioactive nuclei decays in a certain speed with a range of only a few
centimetres in air. They can be
time. So the life time of the unstable nuclei is unlimited and stopped by an ordinary sheet of
is difficult to measure. We can get the idea about decay thin aluminium foil.
rate by the term half-life.
Beta ( ) Particles 
Remember
S t re a m s o f h i g h - e n e rg y
Three types of Radiations electrons, ejected at various
Alpha Particle Beta Particle Gamma Ray speeds as high as close to the
speed of light. Beta particles may
Charge +2 Charge – 1 No charge
be able to penetrate several
Least penetration Moderate penetration Highest penetration
millimetres of aluminium.
Transmutes nucleus: Transmutes nucleus: Changes only energy
A A – 4 A A A A Gamma ( ) Rays
Z Z – 2 Z Z + 1 Z Z Electromagnetic radiation of very
N N – 2 N N – 1 N N short wavelenght. Their
wavelengths and energies can
Half-Life vary. High-energy gamma rays
can penetrate at least 30 cm of
The time during which half of the unstable radioactive
lead or 2 km of air.
nuclei disintegrate is called the half-life of the sample of
radioactive element. For your information
Every radioactive element has its own characteristic half- i. Nuclear radiation is
life. For example, radium-226 has a half-life of 1620 years, measured in units of roentgen
equivalent man (rem), a unit of
which means that half of a radium-226 sample will be
equivalent dose.
converted to other elements by the end of 1620 years ii. Patient should be exposed to
(Fig.18.7). In the next 1620 years, half of the remaining X-rays with the limit of 0.1 to
radium will decay, leaving only one-fourth the original 1.0 rem.
amount of radium, and so on. iii. Safe limit of radiation
N exposure is 5.0 rem per year.
Number of
nuclides Physics insight
A half-life is the time a
radioactive element takes for
1/2N half of a given number of
nuclei to decay. During a
second half-life, half of the
1/4 N
remaining nuclei decay, so in
1/8 N two half-lives, three-quarters
of the original material has
1620 3240 4860 Years decayed, not all of it.
Fig.18.7: Radioactivity of radium

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