[Physics Class Notes] on Radioactivity and Gamma Decay Pdf for Exam

Radioactivity is the property in which a heavy element disintegrates itself without being forced by any external agent to do so.

A French physicist, Henry Becquerel, in 1986 was investigating the newly discovered X-rays. By accident, he discovered uranium salts.

It led him to study how uranium salts are affected by light.

He observed that uranium salts possessed a peculiar attribute of affecting a photographic plate even when it was in a light-proof package.

He thought that it must be due to certain active radiations emitted by uranium salts.

These radiations were called Becquerel rays, and the phenomenon of emission of active radiations by an element was termed radioactivity, and the element exhibiting this property is called a radioactive element.

At present, the total number of radioactive elements is 40 in number.

They have unstable nuclei.

Some examples are Radium, Polonium, Thorium, Actinium, etc.

Alpha Gamma Beta Particles

In the years 1899 and 1900, physicists Ernest Rutherford (working at McGill University in Montreal, Canada) and Paul Villard (working in Paris) did experimental investigations and separated radiation into three kinds.

Rutherford named them as alpha, beta, and gamma, based on penetration of matter and deflection by a magnetic field.

1. α-Particle

An alpha particle carries double the positive charge of a proton, equivalent to the helium nucleus.

The mass of the α-particle is approximately four times that of hydrogen atom i.e., equal to the mass of the helium atom.

2. γ-Particle

• γ-rays don’t get deflected by electric or magnetic fields.

• They travel with the speed of light.

• They have a very large penetrating power.

Gamma rays can travel several centimeters of iron and lead.

3. β-Particle

• A β-particle carries a 1.6 × 10^-19 C of unit negative charge.

• Mass of β-particle is 9.1 × 10^-31 kg = mass of electron.

• In beta decay, a high-speed electron or positron emitted by the radioactive decay of an atomic nucleus. 

Alpha Beta Gamma Decay

The radioactive elements are unstable and emit radiations to achieve states of greater stability.

The parent nucleus emits α, β, and γ particles during its disintegration into daughter nuclei.

This daughter further disintegrates to form a stable nucleus.

Let’s talk about the three kinds of decay:

1. α-Decay

α-decay is the phenomenon of emission of α-particles from the radioactive nucleus. For example, 92U238 一undergoes α-decay ⇾  90Th234 + 2He 4..(1)

As you can see, the nucleus of uranium emits an α-particle, where its mass number reduces by 4,i.e., 238-4 = 234 and charge number: 92-2 = 0. New element thorium (Th) is formed.

In general,  equation(1) can be represented as:

z XA → z YA-2 + 2He4 + Q

After a spontaneous process of α-decay, the total mass of 90Th234 and  2He 4 became less than 92U238. Therefore, the total mass-energy (Q) also reduces, which is equivalent to the difference between the Initial mass-energy and the final mass-energy, given by,

Q = (mx – m – mHe)

Q = (mx – my – mHe)c²

This Q is also called the disintegration energy and it is shared by the daughter nucleus Y and an alpha particle.

2. β-Decay

β-decay is the phenomenon of emission of an electron from a radioactive nucleus. For example, when Thorium  90Th234 emits a β-particle, the mass number of the daughter nucleus remains unchanged, i.e., (234 – 0 = 234), while its charge number becomes 91 (90+1).

A new element called Palladium 91Pa234 is formed.

90Th234 ⇾  91Pa234 + -1e0 (β-particle)

In general, we can write it as:

zXA ⇾  z+1YA + -1e0 + Q 

Where Q is the energy released in β-decay.

3. γ – Decay

It is a phenomenon of emission of gamma-ray photons from a radioactive nucleus.

After α, β-decay, the daughter nucleus is in an excited state, for achieving its greater stability, it emits one or more gamma-ray photons.

General representation for γ – decay:

zXA ⇾ zYA + γ

For example, the β-decay of 27Co60 transforms into an exciting 28Ni60 nucleus. It reaches the ground state by emitting γ -rays. The representation for the same is:

27Co60  ⇾ 28Ni60** + -1e0

28Ni60** ⇾ 28Ni60* + Eγ ( = 1.17 MeV)

28Ni60* ⇾ 28Ni + Eγ (= 1.33 MeV)

What is Gamma Decay?

γ-ray emission is called the gamma decay.

In this process, an excited daugh
ter nucleus releases a high energy photon in the range of a Mega-electron volt called the γ-rays. 

The daughter nucleus is an isotope. The number of protons and neutrons remains the same in this process. However, the energy state of the atom is lowered to reach a stable state.

Gamma Decay Example

The gamma decay of Technetium-99m to Technetium-99. Where m stands for metastable. It means an atom is in an excited state.

Applications of Gamma Rays

• Medicine: Radiotherapy: To treat tumors and cancers, sterilizing medical equipment.

• Industry: Sterilization and disinfection.

• Astronomy: To look for distant gamma-ray sources.

• Nuclear Industry: To develop nuclear reactors and bombs.