Radioactivity - Definition, Nuclear stability, Units of Radioactivity, Types of Radiations, Alpha and Beta Decay

Most of you must have seen the smoke detectors which are placed in residential apartments or commercial buildings and you must be aware of their use, but do you know what is the constituent compound that is there in the smoke detectors and how it saves our life? Well! Most of the smoke detectors placed in our residential apartments contain a low amount of americium-241element which is radioactive in nature. This compound emits alpha particles (one type of radioactive radiation) which ionise the air making the air conductive in nature. When the smoke enters the unit it disrupts the flow of ions and activates the alarm. The radioactive nature of uranium was discovered by French physicist Henri Becquerel and is considered one of the most accidental discoveries in history. Let’s discuss more about radioactivity.

Table of content

Definition of radioactivity
Nuclear Stability
Unit of Radioactivity
Types of Radiation
Alpha Decay
Beta Decay
Practice Problems
Frequently Asked Questions-FAQs

Definition of Radioactivity

Some elements like Uranium, Radium, Polonium, Thorium etc. are unstable due to the nuclear instability, nuclei present in the elements undergo radioactive decay to form stable nuclei. The spontaneous breakdown of unstable nuclei of elements to generate stable nuclei is known as radioactivity. The stability of nuclei of an element is calculated by the $\frac{n}{p}$ ratio. Element upto calcium (Z=20) are stable and with $\frac{n}{p}$ ratio equal to 1. When the atomic number increases, repulsion between protons increases and it requires more neutrons to maintain this np ratio for stability. So, $\frac{n}{p}$ ratio keeps on increasing for stable nuclei.

Nuclear Stability

Nuclear stability is determined by $\frac{n}{p}$ ratio. The stable nuclei lie within the shaded area which is known as the zone of stability. Nuclei which fall above the stability zone have an excess neutron while those nuclei which fall below the stability zone have excess proton. The unstable nuclei emit either neutrons or protons to achieve stability.

Case 1: When the nuclei lie above the shaded area (i.e number of neutrons is in excess) then it emits radiation to achieve stability.

For example in the case of , emission takes place as it lies above the shaded area of the zone of stability.

Case 2: When the nuclei lie below the zone of stability (i.e., the number of protons is in excess) then emission or K-electron capture or positron emission takes place.

Note: When the nuclei of naturally occurring element are present then generally emission takes place. While K-electron capture or positron emission takes place in the case of an artificially prepared radioactive element.

Unit of Radioactivity

The unit of radioactivity is called Curie (Ci).

One curie is defined as that quantity of any radioactive substance which has a decay rate of 3.71010 disintegrations per second.

The S.I unit of radioactivity is known as Becquerel which is also referred to as one dps.

Another unit of radioactivity is Rutherford.

One Rutherford is defined as the amount of a radioactive substance which undergoes 106 disintegrations per sec.

Conversion of units of radioactivity.

$1 c u r i e = 3.7 \times 10^{4} R u t h e r f o r d = 3.7 \times 10^{10} B e c q u e r e l = 3.7 \times 10^{10} d p s$

Types of Radiation

There are three types of radioactive radiation namely- rays, rays and radiation. These radiations differ from each other in various properties which include ionising power, penetration power, nature of radiation and velocity etc.

Comparison in the properties of different types of radiation

Properties of Radiation	Alpha Rays	Beta Rays	Gamma Rays
Nature of radiation	It consists of a stream of particles.They are doubly ionised helium nuclei with the charge of +2 and represented as .	It consists of a stream of particles. They are identical to electrons with a charge of -1 and represented as	rays is the electromagnetic radiation which has a shorter wavelength as compared with theX-rays.
Ionisation power	Due to higher kinetic energy it has the highest ionisation power and can easily ionise a gas through which it passes.	Due to comparatively lower kinetic energy, it has lower ionisation power as compared with the particles.	It generally does not ionise a gas when allowed to pass through it.
Penetration Power	It has minimum penetration power due to its larger size and can be stopped by a sheet of paper, 0.01 mm thick aluminium foil.	It has moderate penetration power due to high velocity and negligible mass and can be stopped by 1 cm thick aluminium foil.	It has maximum penetration power due to its higher velocity and non-material nature. It cannot be even stopped by 5 cm thick lead chamber.
Velocity	It travels with a velocity approx one-tenth of the speed of light, but the actual velocity depends upon the nucleus from which it is emitted.	It travels about 10 times faster than particles	It travels with a speed of light as it is the form of an electromagnetic radiation

Alpha Decay

When the radioactive nucleus decay by the emission of particles from the nucleus it is termed as alpha decay.

Alpha particles have 4 unit of atomic mass and 2 units of positive charge.

For example,

Here,

M represents the symbol of parent nucleus

Z₁ represents the atomic number of parent nuclei
A₁ represents the mass number of parent nuclei
N represents the symbol of daughter nuclei
Z₂ represents the atomic number of daughter nuclei
A₂ represents the mass number of daughter nuclei

Atomic mass of the daughter nucleus decreases by 4 and atomic number decreases by 2.

Therefore,

Z₂=Z₁-2

A₂=A₁-4

Beta Decay

When the radioactive nucleus decay by the emission of a particles from the nucleus it is termed as beta decay. It cannot exist such as in the nucleus but it is produced by conversion of neutrons into protons with the emission of electrons (Beta particle).

Emission of -particles does not alter the atomic mass of the daughter nucleus.

Here,

R represents the symbol of parent nucleus

Atomic mass of the daughter nucleus remain unchanged and atomic number increases by 1

Therefore,

Z₂=Z₁+1

A₂=A₁

Note: Daughter element is an isobar of parent element in case of -emission.

Practice Problems

Q. Select the correct option with respect to the type of radiation emitted from a radioactive sample.

During emission both the mass number and the atomic number of the parent atom changes
In radiation there is no deflection in the electric field
In case of emission, the daughter element is an isobar of the parent element.
All of the above

Answer: (D)

Solution: When particle is emitted mass number of the daughter nucleus decreases by 4 and the atomic number decreases by 2. In the case of radiation there is no charge present and is non-material in nature, so there will be no deflection in the electric field in the presence of radiation. In case of emission, the mass number of the daughter nucleus remains unchanged and the atomic number increases by 1.Therefore Daughter element is an isobar of the parent element in case of -emission. So, all (A), (B) and (C) options are correct.

Q. What will be the atomic number and mass number respectively of the daughter nuclei if uranium emits one particle?

94, 238
90, 234
92, 234
90, 238

Answer: (B)

Solution:

When particle is emitted, the mass number of the daughter nucleus decreases by 4 and the atomic number decreases by 2.

Therefore,

Mass number of daughter nucleus = 238-4=234

Atomic number of daughter nucleus = 92-2=90

Q. What will be n/p ratio (approx) of the daughter nucleus if emits one particle?

1.48
1
2.53
1.58

Answer: (D)

Solution:

When particle is emitted mass number of the daughter nucleus decreases by 4 and the atomic number decreases by 2.

Therefore,

Mass number of the daughter nucleus = 226-4=222

Atomic number of daughter nucleus = 88-2=86

We know that;

atomic number = number of protons

Number of protons of daughter nuclei = 86

Mass number = number of protons + number of neutrons

Number of neutrons of daughter nuclei = mass number - number of protons

Number of neutrons of daughter nuclei = 222-86=136

$\Rightarrow \frac{N u m b e r o f n e u t r o n s (n)}{N u m b e r o f p r o t o n s (p)} = \frac{136}{86} ≃ 1.58$

Q. Select the correct option with respect to properties of alpha, beta and gamma particles.

$α > β > γ$ (penetration power)
$γ > β > α$ (ionisation power)
$γ > β > α$ (velocity)
Both A and C

Answer: (C)

Solution: Penetration power of $γ > β > α$ whereas ionisation power of $α > β > γ$ . The velocity of radiation is maximum as it travels with the speed of light followed by rays and then radiation.

Frequently asked questions - FAQs

Q. What is an ionisation chamber?
Answer: It is the simplest device which is used to measure the strength of radiation. An ionisation chamber is fitted with two metal plates that are separated by air. When the radiation passes through this chamber it knocks the electron and a positive ion is formed. The ion formed migrates toward the anode and produces a current which is measured by an ammeter and the strength of radiation is calculated.

Q.Which form of hydrogen isotope is radioactive in nature?
Answer: There are three isotopes of hydrogen (protium, deuterium and tritium) that occur naturally in nature. Among these three forms of hydrogen, “tritium” is a radioactive and unstable form of isotopes of hydrogen.

Q. What is the half life of radioactive substances?
Answer: It is defined as the time required for the decay of half of the radioactive sample by emitting the radiation in different forms and converting it into stable nuclei.

Q. What is radioactive contamination?
Answer: Radioactive contamination occurs when radioactive material gets deposited on an object or any living being. It can cause the environment, air, water, soil, plant, surface, building, animals and human beings to get contaminated which can cause immediate, serious illness including death to human beings, animals and plants. It also has a long-term effect which depends upon the strength of the radiation.