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Radioactivity

The phenomena of disintegration of heavy elements into comparatively lighter elements by the emission of radiations is called radioactivity. This phenomena was discovered by Henry Becquerel in 1896.

Radiations Emitted by a Radioactive Element

Three types of radiations emitted by radioactive elements

(i) α-rays

(ii) β-rays

(iii) γ – rays

α-rays consists of α-particles, which are doubly ionised helium ion.

β-rays are consist of fast moving electrons.

γ – rays are electromagnetic rays.

When an α – particle is emitted by a nucleus its atomic number decreases by 2 and mass number decreases by 4.

Radioactivity | Physics Class 12 - NEET

When a β -particle is emitted by a nucleus its atomic number is Increases by one and mass number remains unchanged.

Radioactivity | Physics Class 12 - NEET

When a γ – particle is emitted by a nucleus its atomic number and mass number remain unchanged

Radioactive Decay law

The rate of disintegration of radioactive atoms at any instant is directly proportional to the number of radioactive atoms present in the sample at that instant.

Rate of disintegration (– dN / dt) ∝ N

– dN / dt = λ N

where λ is the decay constant.

The number of atoms present undecayed in the sample at any instant N = No e-λt

where, No is number of atoms at time t = 0 and N is number of atoms at time t.

Half-life of a Radioactive Element

The time is which the half number of atoms present initially in any sample decays, is called half-life (T) of that radioactive element.

Relation between half-life and disintegration constant is given by

T = log2e / λ = 0.6931 / λ

Average Life or Mean Life(τ)

Average life or mean life (τ) of a radioactive element is the ratio of total life time of all the atoms and total number of atoms present initially in the sample.

Relation between average life and decay constant τ = 1 / λ

Relation between half-life and average life τ = 1.44 T

The number of atoms left undecayed after n half-lives is given by

N = No (1 / 2)n = No (1 / 2) t/T

where, n = t / T, here t = total time.

Activity of a Radioactive Element

The activity of a radioactive element is equal to its rate of disintegration.

Activity R = (– dN / dt)

Activity of the sample after time t,

R = Ro-λt

Its SI unit is Becquerel (Bq).

Its other units are Curie and Rutherford.

1 Curie = 3.7 * 1010 decay/s

1 Rutherford = 106 decay/s

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FAQs on Radioactivity - Physics Class 12 - NEET

1. What is radioactivity and how does it occur?
Ans. Radioactivity is the spontaneous emission of radiation from the nucleus of an unstable atom. It occurs when the nucleus of an atom is not in a stable state and undergoes decay by emitting radiation.
2. What are the different types of radiation emitted during radioactivity?
Ans. There are three main types of radiation emitted during radioactivity: alpha particles, beta particles, and gamma rays. Alpha particles consist of two protons and two neutrons, beta particles are high-energy electrons or positrons, and gamma rays are high-energy electromagnetic radiation.
3. What are the potential health effects of exposure to radioactivity?
Ans. Exposure to radioactivity can have various health effects depending on the level and duration of exposure. These effects can include radiation sickness, increased risk of cancer, genetic mutations, and damage to organs and tissues. However, the severity of these effects greatly depends on the individual's level of exposure.
4. How is radioactivity measured and monitored?
Ans. Radioactivity is measured using units such as becquerel (Bq) or curie (Ci), which quantify the rate of radioactive decay. Monitoring of radioactivity involves the use of radiation detectors and dosimeters to measure the amount of radiation present in a given area or on a person.
5. What are some sources of natural and artificial radioactivity?
Ans. Natural sources of radioactivity include radioactive elements present in the Earth's crust, such as uranium and radon gas. Artificial sources of radioactivity can be created through nuclear power plants, nuclear weapons testing, and medical procedures using radioactive materials.
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