What Are the Differences Between Alpha, Beta, and Gamma Decay?
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.
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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.
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.
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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.
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Gamma rays can travel several centimeters of iron and lead.
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.
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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.
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Let’s talk about the three kinds of decay:
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.
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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.
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)
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In general, we can write it as:
zXA ⇾ z+1YA + -1e0 + Q
Where Q is the energy released in β-decay.
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:
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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 daughter 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.
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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.
FAQs on Radioactivity and Gamma Decay: Concepts, Types & Examples
1. What is radioactivity and what fundamentally causes it?
Radioactivity is the natural phenomenon where an unstable atomic nucleus spontaneously disintegrates, or decays, to form a more stable nucleus. This process involves the emission of energy and particles. The fundamental cause is an imbalance in the ratio of neutrons to protons within the nucleus, which creates an unstable state that the atom seeks to resolve by releasing radiation.
2. What are the three main types of radioactive decay explained in the CBSE syllabus?
The three principal types of radioactive decay are:
- Alpha (α) Decay: The emission of an alpha particle (a helium nucleus) from the parent nucleus.
- Beta (β) Decay: The emission of a beta particle (an electron or a positron) from the nucleus.
- Gamma (γ) Decay: The emission of a high-energy photon (gamma ray) from an excited nucleus.
3. What happens to the nucleus of an atom during alpha decay?
During alpha decay, the parent nucleus emits an alpha particle, which consists of two protons and two neutrons. As a result, the mass number of the nucleus decreases by 4, and its atomic number decreases by 2. This transforms the original element into a new, different element. For example, Uranium-238 decays into Thorium-234.
4. How does beta decay change the atomic and mass number of a nucleus?
In beta decay, the mass number of the nucleus remains unchanged. However, the atomic number changes. In beta-minus (β⁻) decay, a neutron transforms into a proton, and an electron is emitted. This increases the atomic number by 1. For example, Thorium-234 undergoes beta decay to become Protactinium-234.
5. What is gamma decay and when does it typically occur?
Gamma decay is the process where an excited atomic nucleus releases excess energy by emitting a high-energy photon, known as a gamma ray. It does not change the number of protons or neutrons in the nucleus. Gamma decay typically occurs immediately after an alpha or beta decay event, as the resulting daughter nucleus is often left in a high-energy, excited state and needs to transition to a more stable, lower-energy ground state.
6. What are some practical applications of gamma rays in medicine and industry?
Gamma rays have several important real-world applications due to their high energy and penetrating power. Key examples include:
- Medicine: Used in radiotherapy to destroy cancerous cells (e.g., in a Gamma Knife) and to sterilise medical equipment.
- Industry: Used for sterilising foodstuffs to prolong shelf life, and for industrial radiography to inspect welds and detect flaws in metal castings.
- Astronomy: Helps astronomers study high-energy phenomena in the universe, such as supernovae and black holes.
7. How is gamma decay fundamentally different from alpha and beta decay?
The fundamental difference lies in what is emitted. Alpha and beta decay are processes where the nucleus emits particles (a helium nucleus or an electron/positron). This changes the composition of the nucleus, altering its number of protons and/or neutrons and thus transforming it into a different element. In contrast, gamma decay involves the emission of pure energy in the form of a photon. It does not change the particle composition of the nucleus, but simply lowers its energy state.
8. Why is energy (known as Q-value) released during radioactive decay?
Energy is released during radioactive decay due to the principle of mass-energy equivalence (E=mc²). The total mass of the daughter nucleus and the emitted particle(s) is always slightly less than the mass of the original parent nucleus. This 'missing' mass, called the mass defect, is not lost but is converted into a large amount of energy, which is carried away as the kinetic energy of the decay products. This released energy is the Q-value.
9. If gamma rays are a form of light, why are they so much more dangerous than visible light?
While both are forms of electromagnetic radiation, the key difference is energy. Gamma ray photons have millions of times more energy than photons of visible light. This extremely high energy allows them to penetrate deeply into materials, including biological tissue. When they interact with atoms in the body, they can knock electrons out of orbit, a process called ionisation. This ionisation can damage or destroy critical molecules like DNA, leading to cell death or cancerous mutations.
10. What is the difference between the phenomenon of radioactivity and the process of radioactive decay?
These terms are closely related but describe different aspects of the same subject:
- Radioactivity is the overall property or characteristic of an unstable atomic nucleus. It is the inherent tendency of an element like Uranium to spontaneously emit radiation.
- Radioactive Decay is the specific process or event through which that emission occurs. It describes the actual transformation of a parent nucleus into a daughter nucleus, for example, the specific act of a Uranium-238 nucleus undergoing alpha decay.
