Lesson Notes By Weeks and Term - Senior Secondary School 2








TOPIC: Radioactivity


Emission of Alpha and Beta particles and Gamma rays.

Properties and Peaceful uses of radioactivity, radio active hazards and safety precautions. Binding energy .

Radioactivity is the spontaneous decay or disintegration of  the nucleus of the atom of an element during which it emits α, β or γ rays or a combination of any or all the three and energy ( or heat).


If a small sample of radium is placed at the bottom of a small hole drilled in a block of lead.  The radiation emitted from this radium emerged from the hole in a narrow beam. if  the rays were subjected to a strong magnetic field placed at the side of a beam. A photographic plate situated at appropriate sides to receive the rays showed that the paths of some rays were bent to the right, some to the left and some went straight on, unbent.

Electrically charged plates placed at the side of the beam gave the same effect.  The radiations that was bent towards the negative electric plate or the south pole of the magnetic field are called the Alpha particles ( α –particles ) 

The radiations deflected towards the positive electric plate or the North pole of the magnetic field is called Beta particles (β – particles ). The radiation that was neither affected by the electric or magnetic field is called gamma rays (γ ). They are actually electromagnetic radiations.



Beta Particles

Gamma - rays


Helium nuclei 42He

High Energy electrons

Electromagnetic wave of short wavelength


5 – 7% speed of light

Travel at approx. speed of light

Travel at speed of light

Effects of magnetic field

Slightly deflected in a magnetic field (+ve)

Strongly deflected in a magnetic field (-ve)

No effects

Ionizing magnetic field

Large, cause heavy ionization



Penetrating power

Little penetrating power e.g thin sheet

Good penetrating power e.g aluminium

High penetrating power e.g leads


Radioactive Decay; Half life, Decay Constant

Radioactivity is a spontaneous process. It goes on independent of external control, it is not affected by temperature, or pressure or by chemical treatment. It is a random process as no one can predict which atom will disintegrate at a given time.


The half life of a radio active element is the time taken for half of the atoms initially present in the element to decay.  The rate of decay of radioactive elements is found to be proportional to the number of atoms of the material present.  If there are N atoms of a radioactive element present at a time, ti, then the probable number of disintegration per unit time or activity.


    N α   -   dN


The minus sign arises from the fact that N is decreasing with time 

        dN   = -λN


λ is a constant of proportionality called the decay constant.

    :.  λ  =  -  1          dN

                           N           dt

Hence, decay constant is defined as the instantaneous rate of decay per unit atom of a substance

 Λ =  No of atoms disintegrating per second 

        Np pf atoms in the source at that time

By integration


    N =Noe-λt

No = Number of atoms present at time t = o

N = Number of atoms present at time t 

 T= =  0.693



A certain radioactive element has a half life of 10years.

  1. how long will take to lose 7/8 of its atoms originally present.
  2. How long will it take until only ¼ of the atoms originally present remain unchanged.


If 7/8 of its atoms has been lost, 1/8 remains

  Half life = 10years

  N/4 remains after 20 years

N/8 remains after 30 years:. it takes 30 years to lose 7/8 of its atoms


N/2  remain unchanged after 10 years

N/4 remains unchanged after 20 years

Ans = 20years


Transformation of Elements

There are two types of radioactivity nature and artificial radioactivity . Natural  radioactivity is the spontaneous disintegration of the nucleus of an atom during which α particles,β particles or γ rays and heat ( or energy) are released. When a radioactive elements undergoes radioactive decay, it may emit either α ,B, or γ rays. This changes the atomic number of the element, hance a new element is formed.


226 88         α    42He + 22288Rn   + energy


22288            2B              2 -1e  + 22288Ra + energy\


238      2α, 2β       2 ( 4He) 2-1oe  + 230 th of energy

  92                                                              90


234           β           

  90                              -ie  = 23491Pa + energy


Generally we represent alpha (α) decay by


A    42 He+  A  -4    y  + energy      

Z                                                         Z  -2

 And  B decay by



Z    -1e  + A y



If the  radioactivity is induced in an element by irradiation with for neutrons, the process is known as artificial radioactivity.  By irradiation, it means exposure to radiation either by accident or by intent.


4 He  + 14N                         18 F 17  O  +  1 H  + energy

2             7                             9       8           1

in artificial radioactivity, an ordinary materials is made radioactive by bombarding itwith radioactive particles.

4He + 27 Al     30P 1n     30Si + ie + energy.


I n + 63Li        3H  + 42He + energy

I n  + 24Mg      2411Na  + 1P + energy

4He + 9Be         12C + 1n + energy

1n  +5927Co         6027Co + energy

Isotopes can also be made artificially by bombarding neutrons, or protons or deuterons at elements e.g.


34S  + 1n     3510S + energy

79Br   + 1n          3510Br + energy

such artificially produced isotope are unstable and decay with the  emission of α – particles, β –particles and γ – rays. They are called radio isotopes.


Reading Assignment

New School Physics pg 468-471


1.The phenomemon of radioactivity was first discovered by

  (A) Marie  Curie   (B) J.J. Thompson    (C) Henri Becquerel  (D) Nent Bohr (E) Enrico Fermi

  1. What are Beta particles

  (A) protons    (B) Neutrons      (C) Electrons   (D) Helium nuclei

  1. Alpha particles are

(A) not charged    (B) highly penetrating   (C ) Helium nuclei  (D) electromagnetic radiation

  1. A substance has a half life 30 mins after 6 ins the count rate was observed to be 400. What was its count rate at zero time

(A) 200           (B) 1200   (C) 1600(D) 2400

  1. The number of neutrons contained in the nucleus of 23892U  is

(A) 92         (B) 146        (C  ) 238     (D) 330.


1a.  Define radioactivity; half life and decay constant.

  1. Write down the relation between half life and decay constant 
  2. In 180 minutes, the acitivity of a certain radioactive substance falls to one –eight of its

original value.. Calculate its half life.


  1. The half  life of a radioactive iodine is measured to be 8.0 days. A solution containing

1.5mg of this iodine on a certain day. Write down on a table the mass of iodine remaining 

after 8, 16, 24, 32 days.  Plot a graph of mass against time and from it deduce the mass of 

iodine  that would remain at the end of 30 days.

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