Radioactivity and Radioactive decay

Radioactivity and its invention, Properties of gama rays, half life periods, si unit of radioactivity, Applications of radio isotopes .Radioactivity and Radioactive decay General science


Radioactivity and its invention (discovery) :

Naturally occurring substances, elements various compounds emit certain invisible rays by
the process of self disintegration is called radioactive substance and this phenomenon is called radioactivity.
The invisible rays emitted from the radioactive substances are called radioactive rays. The radioactivity is a nuclear phenomenon and it occurs due to the nuclear unstability of the atoms.

Henery Bacquerel in 1896 invented the phenomenon of radioactivity and observed that through Uranium and its compound of salts certain invisible rays emit. In the early time these invisible
radiations were called Bacquerel rays. Later Madam Curie and Pierre Curie asserted that the emission of invisible radiations from Uranium and its compounds are totally a nuclear phenomenon and this specific characteristic of Uranium and its compound doesn't depend upon physical and chemical parameters.
Radioactivity and Radioactive decay

In 1898 Madam Curie and Schimide also detected that thorium and its compounds also exhibit the
phenomenon of radioactivity. Again in 1902 Madam Curie and Pierre Curie observed that a miniral of
Uranium called Pitch Blende whose radioactivity is nearly more than four times than Uranium. Later these Curie's Couple invented Radium from the pitch Blende which was also radioactive. All natural elements from Bacquerel in 1896 invented the atomic number 1 (Hydrogen) to 83 (Bismuth) are stable because their nuclei are stable. Elements from atomic number 84 (Polonium) to the last element (at. no. 105) have unstable nuclei and these are radioactive. Today about 40 natural isotopes of the clements and other compounds exhibit the characteristics of radioactivity.

Radioactive rays and its properties : Radioactive elements and their compounds split into smaller
fragments by the process of nuclear spontaneous disintegration and emit invisible radiations which
were called Bacquerel rays. Bacquerel rays consists of positively charged alpha-rays (a-rays), negatively charged beta-rays (B-rays) and electrically neutral gamma-rays (y-rays). The radioactive rays a, B and y were pronounced their name by Rutherford.
Radioactive rays and its properties

Properties of alfa-rays
(a) a-rays are the streams of He++ ions which have mass of 4 a.m.u. and charge of 2 units that's why
a-rays are called a-particles.
(b) When a-particles are passed through an electric and a magnetic field then these are deflected.
(c) It has maximum power of ionisation through the gases.
(d) Its velocity is less than that of light and it is equal to 1/10th of the velocity of light in vacuum
300000000 m/ second)
(e) It has least penetrating power as compared to that of B and y-rays.

Properties of ß-rays
(a) B-rays are streams of fast moving electrons.
(b) Each B-particle is an electron having mass of 1/1836 a.m.u. and the charge of -1 unit.
(c) It has less power of ionisation through the gases as compared to a-rays.
(d) Its velocity is equal to (33-92%) of the velocity of light.
(e) It has more penetrating power than a-rays and less penetrating power than y-rays.

Properties of y-rays
(a) Y-rays are electro-magnetic radiations of high energy.
(b) It is composed of photons (rest mass zero) of high energy.
(c) It is electrically neutral and it is an electromagnetic wave. Thus it has a velocity which is equal to the velocity of light in vacuum (3 x 10 m/second).
(d) It has the largest (maximum) penetrating power and it can be passed through 8 cm thick lead block and 25 cm thickened iron sheet.

Types of radioactive elements : Elements which exhibit the phenomenon of radioactivity are called
radioactive elements and these are of two types;
(a) Natural radioactive elements : The elements Po (84), At (85), Rn (86), Fr (87), Ra (88), Ac (89), Th (90), Pa (91) and U (92) are naturally occurring radioactive elements.
(b) Artificial radioactive elements : The elements Np (93), Pu (94).... to Habnium, Ha (105) are radioactive elements which have been synthesized inside the nuclear laboratory. These are called artificial radioactive elements and also called transuranic elements.

The idea of artificial radioactivity firstly came in the mind of F. Juliot in 1934 which was immediately supported by I. Curie. Various attempts were taken to produce radioactive elements artificially by lighter non-radioactive elements. Rutherford was working in this regard very rigorously and succeeded in disintegrating nitrogen nuclei by bombarding ordinary nitrogen gas with a-particles. Thus a nuclear reaction occurs as given below :
Rutherford was the first person to knowingly split the nucleus. In 1919, while studying at Manchester University, Rutherford bombarded nitrogen with naturally occurring alpha particles from radioactive material, which resulted in a proton emitted with higher energy than the original alpha particle. This happened because nitrogen was converted to oxygen, essentially splitting the nucleus. Although chemists had been doing this unknowingly for years when carrying out chemical reactions, Rutherford was the first person to actually understand what was happening, and purposefully induce this change.

Thus Rutherford transformed an ordinary nitrogen into a rare isotope of oxygen. This was the first
artificial (man made) radioactive element produced by nuclear transformation.

Out of all the radioactive elements, Radium is the most powerful radioactive element and it was
discovered by Madam Curie.

Half life period of a radioactive element : 

Half life period of a radioactive elementis the time during which half of the total number of atoms of the radioactive element disintegrate and it is represented by T,

Half life period of an element-T 1/2
lamda  is called disintegration constant or decay constant.

Characteristics of half life period

(a) Every radioactive element has its own half life and thus different radioactive elements have
different half lives.
(b) Half life period of a radioactive element is independent of all external conditions such as
temperature, pressure, mass etc.
(c) A radioactive element can be detected by means of its half life period.
(d) Smaller the half life period of a radioactive element, larger is its radioactivity and vice-versa.
half life period of some radioactive elements

Radioactive disintegration:  Radioactive substances emit spontaneously either a-particles or B-particles and some y-rays but both a and B-particles are never seen to be emitted simultaneously. As radioactivity is a nuclear phenomenon and its disintegration stops when unstable nuclei become stable and all the natural radioactive elements give the final product lead (Pb) as the end-product which is non-radioactive.

Rutherford-Soddy theory of radioactive disintegration : Rutherford and Soddy studied the
radioactive disintegration -and formulated a theory which is based on the following facts-

The radioactive emission is a characteristic of the isotope and it varies from one isotope to another of
the same element. The emission occurs spontaneously and can not be speeded up or slowed down by any external factors like temperature, pressure etc. The disintegration occurs at random and which atom would disintegrate first is simply a matter of chance.

On the basis of these facts they gave a law which is called Rutherford-Soddy law of radioactive
disintegration (decay) given as below :

The rate of disintegration of a radioactive element at a given instant of time is proportional to the number of atoms of the radioactive element present at the moment.

If N be the number of atoms present in a radioactive substance at any instant t and dN be the number that disintegrates in a short interval dt, then the rate of  disintegration is -dN/dt and it is proportional to N.
Rutherford-Soddy theory of radioactive disintegration

This is the reason why the radioactivity of a substance is being never zero and it will be zero only at infinite time.

Average or Mean life: The atom of a radioactive substance disintegrates constantly but which particular atom will disintegrate at any tìme is quite uncertain. Thus life span of every atom of a radioactive element is different. The atom which disintegrates earlier has a very short life and others disintegrating at a later have a large life. Thus to ascertain the life of a radioactive element the average life or mean life of all its atoms is taken and it is represented by T The average life is
thus defined as below ;
average life or mean life

Units and Measurement of Radioactivity There are various units of radioactivity-

Curie : The activity of 1 g of pure radium is called
1 curie = 3.7 x 1010 disintegration/second (decay/ second)

Rutherford : The amount of a radioactive substance which gives 10° disintegration per second.
- 1 Rutherford = 10 disintegration/second

Bacquerel (S.I. Unit) : It is defined as the amount of a radioactive substance which gives 1 disintegration per second.

1 Bacquerel=1 disintegration/second (decay/ second)

Incidently one micro rutherford equals one bacquerel mass.
Radioactivity of the radioactive substance is
measured by an instrument which is called Geiger
Muller Counter.

Radioactive Series : All the natural radioactive elements lie in the range of atomic numbers from Z=
83 to Z = 92, The nuclei of these elements are unstable and disintegrate by ejecting either an a-particle or a B-particle along with sometimes y-rays. The ejection of an a-particle lowers the mass number A by 4, the atomic number Z by 2. The ejection of a B-particle has no effect on mass number but increases the atomic number by 1. The atomic number is the characteristic of an element and a change in it implies the formation of an atom of a new element. The new atom so formed is
also radioactive and further disintegrates into another new atom and so on. Thus a series of new radioactive elements is produced by successive disintegration until a stable element is obtained. Such a series is called radioactive series.
There are four important radioactive series- (UTAN)

Uranium series, Thoriumm series, Actinium series and Neptunium series.

Soddy Fajan's group displacenment laws: Every radioactive substance emits either an a-particle or a S-particle and often some y-rays. The emission of these radiations change the original nucleus (called parent nucleus) to a new nucleus (called daughter nucleus). Soddy-Fajan's group displacement law states that the emission ofone a-particle reduces the mass number by 4 units and atomic number by 2 units, while the emission of one B-particle is caused by the decay of a neutron into proton and atomic number in its emission is increased by 1 unit and mass number remains same. The emission of y-rays do not affect the mass nunmber and atomic number but its emission changes the nucleus from an excited state (high energy state) to a less excited (lower energy state) state
Soddy Fajan's group displacenment laws

Radioactive dating or Radio isotope dating :
Naturally occurring radioactive isotopes have been very useful in dating (estimating age) the geological events. Thus the technique of detecting the amount or quantity of a radio isotope in the sample of the rock, dead plants or organism or in a bio residue to estimate (measure) its actual (exact) age is called Radioactive or radio isotope dating.

Carbon dating is one of the best example of a radio isotope dating. The idea of carbon dating was
suggested by Prof. Libby a Nuclear physicist of Chicago. Our atmosphere has a large number of stable isotopes. When cosmic rays strike these isotopes a number of radio isotopes are produced. One of these radio isotopes is carbon -14 (CH) which is produced by the bombardment of atmospheric nitrogen with a high energy neutron.

Radioactive dating or Radio isotope dating

Radio carbon (CH) is unstable and decays (by emitting 6-particle) into nitrogen which has a half life
period of 5600 years. By measuring the ratio of the concentration of Cto Cof an ancient organism like
fossil, dead tree or plant one can measure (estimate) the exact age.

The ages of non-living ancient geological substances like old rocks, earth etc are estimated by
the use of uranium or its most suitable mineral pitch blende in which both uranium and thorium are
present. This technique is called uranium dating. But for the most ancient geological rocks Potassium-Argon dating technique is used.

Most important applications of these radio isolopes:

  1. The radio-isotopes are used in the form of tracer in mecdicines and hy the Tracer technique tumours (unwanted growth of cells) in human hody are detected.
  2. The leakage in the pipe (tube)planted for the flow of underground water or oils are today detected by the use of radio-isotopes
  3. The cancerous cells are destroyed completely by the use of radio-isotopes. For example, cobalt isotope Co) is today freqquently used in the therapy of cancer and in destroying brain tumours, The element radium (Ra) has been used for buning and destroving cancerous cella,
  4. The radio-isotope (radio sodium) is used to detect any resichue or unwanted circulatory system.
  5. The radio-phosphorus is used today in curing bone diseases,
  6. The radio-iodine is used to detect any aide effect appears inside the thymid gland.
  7. The radio-sodium is used to measure the speed of blood flow in the human body.
  8. The radio-iron is used to detect the disease like anaemia, tuberculosis and other malnutrient diseases.
  9. The radio-uranium (U) is used to estimate the age of the earth.
  10. Mutation occurs in plants and seeds with
intense radioactive radiation, resulting in the
development of new and improved plants. More
effective insecticides have also been developed
with the help of radio-isotopes.
to the nuclear
transformation in any radioactive subatance there is
always a loss in mass which appears in the form of
nuclear energy Thus nuclear energy is produced by
the simple conversion of lost mass into the energy by
Einstein's mass energy equivalent relation AE Ame,
where e- velocity of light in vacuum.
There are two sources of nuclear energy;
I. Nuclear Fission
In 1939 two German scientists-Otto Hahn and
Fritz Strassman discovered a strange and new type
of nuclear reaction. They found that when Uranium
nucleus ,U) is bombarded with a slow neutron
the nucleus splits up into nearly two equal fragments
with the release of some free neutrons and tremendous
energy of about 200 MeV per U nucleus. Such a
nuclear reaction was termed as nuclear fission.
Thusthe process (nuclearreaction)in whichaheavy
nucleus splits up into two nuclei of nearly comparable
masses with tremendous release of energy and some
free neutrons is called nuclear fission. Elements having
a higher value of neutron to proton ratio are more likely
to undergo fission.

Neutron induced fission of uranium has been
represented as below:
(Slow neutron) (compound nucleus)
Bal41 + Kr2 + 3,n' + 200 MeV.
The average number of neutrons released in a
uranium atom fission is 2.5. These neutrons produced
in nuclear fission under favourable circumstances
cause further atoms of uranium to undergo fission and
in turn emit more neutrons which will cause further
fission explosion. Thus a chain reaction is established
in a short time releasing enormous sum of energy. One
evolves upon fission about 2 x 10' kilo-
gm of
calorie of energy.
Types of nuclear fission : Fission chain reaction is
of two types, namely;
1. Explosive/Uncontrolled chain reaction.
2. Controlled chain reaction.
1. Explosive/Uncontrolled chain reaction : A
nuclear chain reaction in which fission neutrons keep
on increasing till the whole of fissionable material is
consumed is called explosive or uncontrolled chain
Such a process (reaction) proceeds very quickly
with the liberation of tremendous amount of fission
energy in a very short span of time. The atom
bomb is a practical example of an/a explosive/
uncontrolled fission. The nuclear fission is used in
the manufacturing of atom bomb using U235 isotope.
The first atom bomb (núclear bomb) was dropped on
the two cities of Japan-Hiroshima and Nagasaki in
August 1945 by USA during 2nd world war. These two
industrial cities Hiroshima and Nagasaki completely
destroyed by high explosion and radioactive fall out
of a, B and y-rays.
2. Controlled chain reaction
A fission chain
reaction which proceeds slowly without any explosion
and in which the energy released can be controlled is
called controlled chain reaction.
The nuclear reactor is a practical example of a
controlled chain reaction. In the nuclear reactor the
energy released through the fission is used to generate
the electricity.
Basic Components of a Nuclear Reactor
(a) Nuclear fuels : The elements used to activate
the controlled fission in the nuclear reactors are called
nuclear fuels. Some common nuclear fuels are uranium
isotopes - U²33, U235, thorium isotope -Th232 and
Plutonium isotope - Pu239 etc.
(b) Moderators : Moderators are used to slow
down the emitted neutrons. Heavy water, graphite,
beryllium, beryllium oxide, some organic liquids etc.
are used as moderators. But heavy water is the best
moderator because of its very small cross section and
larger slowing down power capacity.
(c) Coolant : A coolant removes the tremendous
heat developed in the reactor core. This heat is evolved
from the K.E. of the fission fragments when these
are slowed down in the fissionable substance and

moderator. Through a heat exchanger, the coolant
transfers heat to the secondary thermal system of the
reactor. Water, steam, helium, CO, air, molten metals
etc. are used as coolants,
(d) Control rods
reaction, control rods are used. Due to large absorption
cross-section area Cadmium and Boron rods are used
as control rods. When control rods are inserted in
the reactor unit, they absorb the neutrons and chain
reaction ceases.
To start and stop a fission
(e) Radiation protective arrangement
nuclear reactor larger amount of penetrating radiations
like y-rays, in addition to neutrons are also generated.
These radiations pose danger to the technicians
working around a reactor. Hence a reactor is always
surrounded by a thick shield, in the form of a concrete
wall many metres thick (lined with lead) to absorb
these radiations and prevent them from leaking out
into the adjacent area.
In the
Nuclear reactors in India:The first nuclear reactor
in all over the world was built by an Italian nuclear
physicist Enrico Fermi in Chicago university at USA in
1942. In India the first nuclear reactor was built in 1956
at Trombay and it is a swimming pool reactor with U235
rods hanging in a tank containing heavy water. Other
nuclear reactors are Cerus, Zerlina, Purnima and R-5
at Trombay.
II. Nuclear Fusion
The process of combining two light nuclei to form
a heavy nucleus with tremendous release of energy is
known as nuclear fusion.
Similar to fission, in fusion an appearance of small
mass difference takes place between the reactants and
product and this mass difference transforms into
nuclear energy by Einstein's mass energy equivalence
relation; AE - Amc?. In nuclear fusion lighter nuclei
such as hydrogen, deutron, tritium and helium etc are
involved. A typical nuclear fusion reaction occuring in
sun is given as below :
H' +,H' +,H' + ,H' → „He + 2e* + 24.7 MeV.
The main difficulty in carrying out the fusion
reaction is that when two light nuclei are brought
together, they strongly repel each other due to the
identical nature of charges on them. Thus for the
occurrence of the fusion, the K.E. of colliding nuclei
must be high enough to overcome this repulsion. The
order of this energy is about 0.1 MeV.
To impart energies to nuclei as high as 0.1 MeV,
the temperature would have to raised to about 10°K.
Without achieving the temperature of this order nuclear
fusion is not possible and that's why, the fusion is also
called thermo-nuclear reaction. At temperature of this
order the fusion materials are found in an ionised state
and behave like a swirling mass of high density and it
is called plasma. At present the plasma is now assumed
as fourth state of matter. The fusion is the source of
stellar energy (energy released by sun and other stars).
Sun is radiating energy at a rate of about 1026 Joule/
second and thus losing about 4 x 10° tonnes of matter/
second. Owing to very large mass (nearly 100 kg) sun
will continue to exist for several billion years.

The hydrogen bomb is a practical example of the
nuclear fusion. The hydrogen bomb is about 1000
times more powerful than the atom bomb of same
mass which is based on nuclear fission. The essential
conditions for the operation of hydrogen bomb are
extremely high temperature and pressure required
for the fusion to start. Once started, the fusion itself
maintains the temperature to keep the process going on
and for this purpose the atom bomb is used as a primer.
By exploding the atom bomb the high temperature and
high pressure is achieved which is necessarily required
for the operation of hydrogen bomb.



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Advanced Chemistry: Radioactivity and Radioactive decay
Radioactivity and Radioactive decay
Radioactivity and its invention, Properties of gama rays, half life periods, si unit of radioactivity, Applications of radio isotopes .Radioactivity and Radioactive decay General science
Advanced Chemistry
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