Abstract

The discovery of natural radioactivity is one of the most fruitful development of Physics.In 1896 Becquerel discovered radioactivity,After this Lord Rutherford and Villard discovered α rays ,β- rays and  γ- rays .In this chapter brief introduction about radioactivity and  its units, Laws of radioactivity  is given. 

Introduction: 

The phenomenon of spontaneous emission of energy is known as Radioactivity.it was discovered by French Scientist Henri Becquerell in the year 1896. Becquerel performed his experiment by taking Uranium.

After Becquerel Madam Curie and her husband Pierrie Curie started work on radioactivity.Apart from Uranium they started work on other elements.They observed that the nature and the intensity of the radiations emitted by radioactive substances were not influeneced by any Physical or Chemical change.Applications of heat or pressure had no effect on radioactivity.

Madam Curie and Pierre Curie discovered the new radioactive elements Polonium (Z= 84) and radium (Z= 88) which were more active than uranium.

Apart from above scientists in France, another group of scientists started investigations in Radioactivity under the guidance of Ernest Rutherford.Rutherford showed early that uranium emitted two kinds of rays. One of these was absorbed which Ruterford named α – rays.The other was more penetrating called β – rays . Later a third type of rays called γ – rays was discovered by French scientist P.V. Villard in 1900 and was found to be more penetrating than α – rays and β – rays.

Radioactivity was also discovered in thorium (Z=90) and radium (Z = 88)These are called radioactive elements.

Together with her husband, Marie curie was awarded half of the Nobel Prize for Physics in 1903 for her research of radiation phenomena for their study into the spontaneous radiation discovered by Becquerel, who was awarded the other half of the Prize. In 1911, she won a second Nobel Prize in chemistry for the discovery of polonium and radium. 

Laws of Radioactive transformations:

From experiemnets it was concluded that radioactive substances emit three types of radiations

1)    Alpha radiations consisit heavy positively charged particles that move with velocity 10⁹ cm/s and they are absorbed by aluminium foil few microns thick.later it was shown that alpha particles are helium nuclei ₂He⁴

2)    Beta radiations consisits of light negatively charged particles movin g with velocity closer to velocity of light and absorbed by aluminium foil about 1mm thick.These particles are electrons

3)    Gamma radiations highly penetrating radiations that is not deflected by an electric or magnetic field.These are hard electromagnetic radiations with wavelength shorter than X-rays .Atomic nucleus is the source of all three types of radiation.

Law of Radioactive decay

Radioactivity is a property of the nucleus .it is impossible to affect the process of radioactive decay without changing the state of the nucleus. Consequently, the probability λ of radioactive decay per unit time is constant for given nucleus in a given energy state

The number dN of act of radioactive decay in time dt is determined only by the number of radioactibe nuclei. 

 dN = - λNdt                                       -----(1)

where λ is called disintegration constant and negative sign shows that amount of the substance decreases with time

dN/dt = Rate of decrease of nuclei with time

 experimentally it is found that the activity at any instant of time t is directly  proportional to the number N of parent type nuclei  present at that time . 

-dN/dt  ∞ N

-dN/dt = λN                                  --------(2)

Where λ> 0 is proportionality constant

Negative sign shows that N decreases as t increases

For unit time  eqn 2 becomes 

Λ = (-dN/N)                                    ------(3)

Λ is fractional change in N per sec it gives us the probability of decay per unit time so it is called probability constant , decay constant or disintegration constant

dN is the number of parent nuclei that decay between times

t and t + dt

eqn 2 can be written as

dN/N = - λdt  ---------------------------(4) 

at time t =0 , Number of atoms   = N₀                                                                                                                        

at time  = t , Number of atoms    = N

 Now after integrating eqn. 4  we get

   = - λ

  = - λ(t)

 = - λ( t -0)

 = - λt 

 = - λt

N/N₀ =

N= N₀        ------ --------------(5)

Half Life Period (T) :

It is defined as the time in which the quantity of radioactive substance is reduced to half of its original value

As we know

N/N₀ =   ----------------- (1) 

At  t= T  we have   N= N₀/2                     -----------------(2)

N₀/2 = N₀    

 λT =

T   =   λ 

T = 0.6931/λ            ------------------(3)

Eqn  3 gives relation between decay constant and half lifetime

Activity of a Radioactive Substance :

The activity of a radioactive substance is the rate of disintegrartion /sec and it is denoted by A 

A  = dN/dt

     = d/dt (N₀  )

A =  λN                                  ----------(1)

At t=0 the activity of substance is A₀

A₀ = λN₀                                        -------------(2)

From 1 and 2 we get

 A/A₀    =  N/N₀    = N₀    =

A = A₀                      ----------     (4)

Units of Radioactivity: 

The unit of radioactivity is called Curie and Rutherford  and was =

Curie (C_i)

In radioactivity curie is the standard unit and is definedas the quantity of any radioactive material giving 3.70X 10¹⁰  disintegrations/sec.

1C_i  = 3.70X 10¹⁰  disintegrations/sec.

Rutherford (rd)

It is defined as the activity of a radioactive substance having 10¹⁰ disintegrations/sec.

1rd = 10⁶ disintegrations/sec

1C_i   = 3.7 x 10⁴ rd

Properties of    α , β   and    γ  Particles

Αlpha  Particles

1 These particles ejected from radioactive source with large velocities ranging from 1.4 x 10⁷  to 2.3 x 10⁷ metres per second.

2 They produce ionisation in the gas through which they pass

3 They affect photographic plate.

4 They produce fluorescence in substance like zinc sulphide etc.

5 The range of alpha particles depend upon the nature of the medium through which they pass and it is also proportional to the cube of velocity of emission

R ∞ v³

6 When exposed to these ray’s body suffers burns

7 These rays are deflected by electric and magnetic fields that shows that they are charged particles.

Properties of β Particles

1 These particles travel with high velocity

2 They affect a photographic plate .

3 They can penetrate through large thickness of matter .

4 They are easily scattered when pass through matter .

5 They are also affected by electric and magnetic fields .

6 These rays produce fluorescence in calcium , tungsten etc .

Properties of γ – Rays

1 They travel with velocity of light .

2 They produce ionisation in gas through which they pass but their effect is very small as compared to alpha and beta particles

3 They affect photographic plate and their effect is greater than beta rays .

4 They produce fluorescence in barium platinocyanide etc.

5 These rays are not affected by electric and magnetic fields.

6 These rays can be absorbed by matter in three ways

i. Photo electric absorption

ii. Compton effect

iii. Pair Production.