Radioactivity
The spontaneous emission of powerful radiation from the nuclei of the heavier elements is called radioactivity. The elements which emit such radiations are called radioactive elements.
Radiation emitted by Radioactive element
The radioactive elements emit 3 different types of radiation. Such radiation is demonstrated by a simple experiment. A radioactive radium(Ra)source is placed inside the lead block. A small hole is drilled in a lead block from which a narrow beam of radiation will emerge. The nature of radiation is studied by applying an electric field.
Fig: Emission of radiation
This experiment shows that
The component of radiation which bends towards negative pate is called α rays.
The component of radiation which bends toward positive plate is called β rays.
The component of radiation which goes straight is called γ rays.
Radioactive disintegration(decay)
The process of breaking the nucleus during the emission of α,β, and γ, radiation from the original atom is called Radioactive disintegration. The nucleus thus produced is called the daughter nucleus. It can be divided into three types.
1. α –decay
The disintegration (or decay) of an atom in which it loses a not positive charge of 2 units and mass of 4 units is called α – decay.
2. β-decay
The disintegration (or decay ) in which the new atom is formed has the same mass number but the atomic number 2 increased by one unit from these of the original atom is called β -decay.
3. γ -decay
The disintegration (or decay ) in which the new atom is formed has the same mass number as the atomic number of the original atom is called γ -decay.
Properties of Radioactive radiation
1. Properties of α-particles
a. α particles are nuclei of helium atoms
b. They are deflected in electric and magnetic fields
c. They affect a photographic plate
2. Properties of β-particle
a. They move with the velocity of 108 m/s
b. They can induce artificial radioactivity
c. They affect a photographic plate
3. Properties of γ-rays
a. They produce fluorescence on some materials
b. They produce heat on surfaces exposed to them
c. They can produce a nuclear reaction
Law of radioactive disintegration
(i) Radioactive disintegration is a random and spontaneous process that is not affected by temperature, pressure, and electric field.
(ii) During the disintegration of atoms it emits α, β, and γ particles.
iii) The rate of disintegration of radioactive substance at any instant of time is directly proportional to the number of atoms present at the instant of time
Derivation
iii) The rate of disintegration of radioactive substance at any instant of time is directly proportional to the number of atoms present at the instant of time
Suppose radioactive substance initially contains 'No' atoms and 'N' be the number of atoms present at that instant of time 't' then by decay law,
(where λ is decay constant and the negative sign show that number of atom decrease as time increase )
from equation (i)
Integrating equation (ii)
this equation is known as the decay equation, this equation shows that number of atom fall exponentially with time which is shown in the graph
Fig: Radioactive decay with time
Half-life, Mean-life, and Decay constant
Decay Constant
from decay law,
Thus the ratio of the number of disintegration per second to the number of atoms present in that radioactive substance is called the decay constant.
Also, we have,
t=1/λ
then equation (ii) becomes
Half time
The half-life of a radioactive substance is defined as the time required for ½ of the radioactive substance to disintegrate. It is denoted by T1/2.
Relation between half and decay constant
We have from decay law
Hence half time of radioactive substance is inversely proportional to the decay constant.
Mean life (Average life)
The ratio of the life of all atoms to the total number of atoms in the radioactive element is called mean life or average life of radioactive substance It is denoted by T or Tmean.
In other words the reciprocal of decay constant is called mean life.
Radiocarbon dating
A process to determine the age of geological and archeological objects is called Radiocarbon dating. Nitrogen found in the Atmosphere absorbs a neutron and forms Radiocarbon
Age of fossil (t) =?
From decay equation
Medical uses of Nuclear reaction and Health Hazards
Medical uses
1) Radioactive radiations are used in the production and modification of plastics.
2) Nuclear radiation like γ-rays have been utilized for the preservation of food.
3) Radiation mutation in plants has been practiced to produce new varieties of these plants.
4) Radio phosphorous is used for treating skin diseases and leukemia.
5) Radio iodine is used to determine the condition of the human thyroid gland.
Possible Health Hazards.
1) The strong α-ray exposure can cause lung cancer.
2) Exposure to fast and slow neutrons can cause blindness.
3) The exposure to neutrons, protons, and α-particles causes damage to RBC.
4) The strong exposures to proton and neutron can cause serious damage to reproductive organs.
Safety measures or precautions
1) The worker asked to wear lead aprons.
2) Radioisotopes are handled with the help of a remote control device.
3) Nuclear explosions should carry out far away from the public area.
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