7/27/2019 Radioactivity Slides
1/3
Radioactivity and its Uses
Susheem Kanwar
The discovery of radioactivity was purely accidental.
A scientist named A.H. Becqueral kept in his drawer photographic
plates which turned out to
be foggy upon developing.He traced this to a uranium ore kept
close by.This phenomenon was
called radioactivity.
It was found that radiation flowed at a rate proportional to the
amount of Uranium in the
sample.
These studies led to Marie Curie suspecting that radioactivity
was due to a more active element
than U288 present in Uranium in small quantities.She found these
to be 2 highly radioactive
elements which she named Radiam(Z=88) Polonium.
*/new slide*/
Becquerel and his followers found that radioactive elements
consisted of 3 different
components. If we drill a small hole in a block of lead a well
defined beam will be emitted. If
this beam is passed through a strong electric or magnetic field
the beam will be split into its 3
components. Not knowing what these radiations were the
scientists named these radiationsalpha(),beta(),gamma().
Alpha rays were found by Rutherford to consist of fast moving
nuclei of Helium atoms.
Beta rays were found to be energetic fast moving electrons.
Gamma rays are the pure energy of electromagnetic radiation.
Decay Energies
The velocities of alpha particles emitted by various radioactive
elements range from 0.98 into
10(to the power nine) cm/sec for Sm up to 2.06 into 10(to the
power 9) cm/sec for Th, which
correspond to kinetic energies from 3.2 to 14.2 into 10(to the
power -6) erg. The energies of
beta particles and gamma photons are somewhat smaller but of the
same general magnitude.
These energies are enormously higher than the energies
encountered in ordinary physical
7/27/2019 Radioactivity Slides
2/3
phenomena eg. The kinetic energy of atoms in thermal motion at a
temperature of 6000 degree
Kelvin is only 1.25 into 10(to the power -12) erg ,i.e. several
million times smaller than the
energies involved in radioactive decay.
Half Lifetimes
The process of natural radioactive decay is ascribed to some
kind of intrinsic instability of the
atomic nuclei of certain chemical elements which results from
time to time in a violent break up
and the ejection from the nucleus of either an alpha particle or
an electron The nuclei of
different radioactive elements possess widely varying degrees of
internal instability.
In some cases(such as U) radioactive atoms may remain perfectly
stable for billions of years
before they are likely to break up,in other cases(such as
polonium) they can hardly exist longer
than a small fraction of a second. The breakup process of
unstable nuclei is a purely statistical
process and we can speak of the mean lifetime of any given
elements in just about the same
sense as insurance companies speak of the mean life expectancy
of the human population. The
difference is in case of humans the chance of decaying is low up
to a certain age radioactive
atom have the same chance of breaking up no matter how long it
has been since they were
formed.Since radioactive atoms begin to die at the very moment
of their birth, the decrease in
their no. with time is diff. from the corresponding decrease of
the no. of living individuals.
The no. of radioactive atoms per unit of time is proportional to
the no. of atoms available,but is
quite independent of the age of these atoms.
Uses of Radioactivity
Uranium-Lead Dating
Uranium is the father of all radioactive elements and the final
product of itsdisintegration is a stable isotope of lead.
Consider a rock of Jurassic times. If it has a small amount of
Uranium in it, as rocks oftendo, the uranium would decay steadily
and the lead resulting from that decay would be
deposited at the same spot.
The longer the time since the solidification of the rock, the
larger would be the relativeamount of lead with respect to leftover
uranium. Thus by measuring the ratio of
uranium to lead in various igneous rocks we can determine the
time of their origin and
the age of the geological deposits in which they were found.
