RADIOACTIVITY

For B.Sc Chemistry (General), Part- I

By

Dr. Kakoli Banerjee

Department of Chemistry

Aims and Objectives

Definition of Radioactivity Discovery of Radioactivity- The Pioneers Cause of Radioactivity- Nuclear Instability Discussion on the three most important

types of emissions- Effects of α, β and γ rays

Radioactive decay & half-life Nuclear Reactions- Fission & Fusion Where is Radioactivity encountered? Hazards of Radioactivity

Radioactivity- A spontaneous phenomenon

A natural and spontaneous process by which the unstable atoms of an element emit or radiate excess energy in the form of particles or waves

After emission the remaining daughter atom can either be a lower energy form of the same element or a completely different element

The Pioneers Roentgen:

(Discoverer of X-rays 1895)

Becquerel:

(Discoverer of Radioactivity 1896) Rutherford:

(Discoverer Alpha and Beta rays 1897)

The Curies:

(Discoverers of Radium and Polonium 1900-1908)

n/p ratio in stable isotopes

Comparison between types of radiation

Radioactive Kinetics-1st order reaction

Radioactivity or decay rate A is the rate of disintegration of nuclei. Initially (at t = 0), we have No nuclei, and at time t, we have N nuclei. This rate is proportional to N, and the proportional constant is called decay constant .

dNA = – ––––– = N Integration gives

d t

ln N = ln No – t or N = No e – t

Units of Radioactivity

The curie (Ci): The activity of a radioactive substance is said to be one curie if it undergoes 3.7 x 1010 disintegrations per second

1 curie = 3.7 x 1010 disintegrations / second

The rutherford (Rd): The activity of a radioactive substance is said to be one rutherford if it undergoes 106 disintegrations per second

1 rutherford = 106 disintegrations / second

The becquerel (Bq): The activity of a radioactive substance is said to be one becquerel if it undergoes 1 disintegration per second

1 becquerel = 1 disintegration / second

The half-life (t½) of a radioactive substance, is the time required for one half of it to decay

When N = No / 2 , t = t ½ So t ½ = 0.693 /

After 5 lifetimes more than 99% of the initial particles have decayed away!

Packing Fraction & Nuclear Binding Energy Mass Defect, Δm = [ Zmp + (A – Z) mn ] - M

The difference between the rest mass of the nucleus and the sum of the masses of the nucleons composing a nucleus is known as mass defect

Mass defect per nucleon is called Packing Fraction

Binding Energy, B.E = Δm c2

It is the energy required to break up a nucleus into its constituent parts and place them at an infinite distance from one another

Sources of Radioactivity Primordial - from before the creation of the Earth radioisotopes

34 primordial radionuclides represent isotopes of 28 separate elements.-Cadmium,tellurium, neodymium and uranium 

Cosmogenic - formed as a result of cosmic ray interactions

tritium, carbon-14 and phosphorus-32

Human produced - enhanced or formed due to human actions (minor amounts compared to natural)

Nuclear reactor - thallium-201 and iridium-192 Radionuclide generators-  technetium-99m generator

 used in nuclear medicine

Nuclear Fission Vs Fusion

Application & Hazards of Radioactivity

Ionizing radiation can damage living tissue in the human body. It strips away electrons from atoms breaks some chemical bonds.

Uses of Radioactivity Preservation of food grains and seeds 

Some of the isotopes are used in the treatment of cancer

Some of the isotopes are used to study the proper functioning of internal organs. 

Gamma radiations are used to sterilize the surgical instruments. 

Radio phosphorous is used for studying the rate of phosphorous assimilation by the plant. 

Radiocarbon Dating