What is Pair Annihilation?

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Grade Level:

Class 12

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Definition

What is it?

Pair annihilation is a process where a particle and its antiparticle meet and destroy each other, converting their entire mass into energy. This energy is usually released in the form of high-energy photons (gamma rays). It's like two opposite forces cancelling each other out completely.

Simple Example

Quick Example

Imagine you have a positive cricket score of +5 runs and your friend has a negative score of -5 runs. If you combine your scores, they cancel out to zero, right? In pair annihilation, it's similar: a particle (like an electron) and its exact opposite (a positron) meet and disappear, turning into pure energy, not zero mass.

Worked Example

Step-by-Step

Let's calculate the energy released when an electron and a positron annihilate each other. We use Einstein's famous equation, E=mc^2.
--- Step 1: Identify the particles. We have one electron and one positron.
--- Step 2: Find the mass of each particle. The mass of an electron (and a positron) is about 9.11 x 10^-31 kg.
--- Step 3: Calculate the total mass involved. Total mass (m) = mass of electron + mass of positron = (9.11 x 10^-31 kg) + (9.11 x 10^-31 kg) = 1.822 x 10^-30 kg.
--- Step 4: Recall the speed of light (c). c = 3 x 10^8 meters per second.
--- Step 5: Apply E = mc^2. E = (1.822 x 10^-30 kg) x (3 x 10^8 m/s)^2.
--- Step 6: Calculate E. E = (1.822 x 10^-30) x (9 x 10^16) = 1.6398 x 10^-13 Joules.
--- Answer: The energy released is approximately 1.64 x 10^-13 Joules.

Why It Matters

Understanding pair annihilation is crucial in fields like medicine for imaging techniques and in space technology for understanding cosmic phenomena. Scientists and engineers use this knowledge to develop advanced medical scans like PET scans and to study high-energy events in the universe.

Common Mistakes

MISTAKE: Thinking that the particles become 'nothing' or 'disappear without a trace'. | CORRECTION: The mass of the particles is converted into energy, usually in the form of gamma rays, following E=mc^2. Energy is conserved, not destroyed.

MISTAKE: Confusing pair annihilation with radioactive decay. | CORRECTION: Radioactive decay involves an unstable nucleus emitting particles to become more stable. Pair annihilation is specifically about a particle and its antiparticle meeting and converting mass to energy.

MISTAKE: Believing that any two particles can annihilate. | CORRECTION: Only a particle and its *corresponding antiparticle* can undergo pair annihilation. For example, an electron can annihilate with a positron, but not with another electron or a proton.

Practice Questions

Try It Yourself

QUESTION: What type of energy is typically released during pair annihilation? | ANSWER: High-energy photons (gamma rays)

QUESTION: If a proton and an antiproton annihilate, will energy be released? Why or why not? | ANSWER: Yes, energy will be released. A proton and an antiproton are a particle-antiparticle pair, so their mass will be converted into energy according to E=mc^2.

QUESTION: An electron and a positron annihilate. If the total mass converted to energy is 1.822 x 10^-30 kg, and the speed of light is 3 x 10^8 m/s, what is the energy released in Joules? (Use E=mc^2) | ANSWER: E = (1.822 x 10^-30 kg) * (3 x 10^8 m/s)^2 = 1.6398 x 10^-13 Joules.

MCQ

Quick Quiz

Which of the following statements best describes pair annihilation?

Two particles combine to form a heavier particle.

A particle and its antiparticle convert their entire mass into energy.

An atom splits into smaller atoms.

Energy is converted into a particle and an antiparticle.

The Correct Answer Is:

Option B correctly states that pair annihilation involves a particle and its antiparticle destroying each other, converting their mass into energy. Options A and C describe different nuclear reactions, and Option D describes pair production, which is the opposite process.

Real World Connection

In the Real World

In India, doctors use a medical imaging technique called Positron Emission Tomography (PET) scans to diagnose diseases like cancer or brain disorders. PET scans work by introducing a radioactive tracer into the body that emits positrons. These positrons quickly meet electrons in the body, undergo pair annihilation, and release gamma rays that are detected by the scanner to create detailed images of organs and tissues.

Key Vocabulary

Key Terms

PARTICLE: A tiny piece of matter, like an electron or proton. | ANTIPARTICLE: A particle with the same mass but opposite charge and other properties of a corresponding particle. | GAMMA RAYS: High-energy electromagnetic radiation, a form of light. | MASS-ENERGY EQUIVALENCE: The principle that mass and energy are interchangeable, described by E=mc^2. | POSITRON: The antiparticle of an electron, having the same mass but a positive charge.

What's Next

What to Learn Next

Next, you can explore 'Pair Production', which is the exact opposite of pair annihilation! It's super interesting to see how energy can turn into matter and antimatter, completing the cycle. This will help you understand more about the fundamental laws of the universe.