Topic 5: Nuclear Physics

5.1.1 The Atom

What Is an Atom?

An atom is the smallest particle of an element that still has the properties of that element. Every atom has two main parts:

• A tiny, dense, positive nucleus at the centre
• Negative electrons orbiting (moving around) the nucleus

The nucleus is incredibly small compared to the whole atom. Imagine a cricket stadium -- like the M. Chinnaswamy Stadium in Bengaluru. If the atom were the size of that stadium, the nucleus would be a small cricket ball sitting right at the centre of the pitch. The electrons would be like tiny flies buzzing around way up in the stands. Most of the atom is just empty space!

Here are some important facts about atom structure:

• The nucleus is about 10,000 times smaller than the whole atom.
• Despite being tiny, the nucleus contains almost all the mass of the atom (more than 99.9%).
• Electrons are held in orbit by the electrostatic attraction between the positive nucleus and the negative electrons.
• In a neutral atom, the number of electrons equals the number of protons, so the positive and negative charges balance out exactly.

Size and Scale of the Atom

To truly appreciate how small the nucleus is compared to the atom, consider these comparisons:

• An atom is about 10^-10 metres across (0.0000000001 m). That is one ten-billionth of a metre!
• The nucleus is about 10^-15 metres across -- about 100,000 times smaller than the atom itself.
• If the atom were the size of the M. Chinnaswamy cricket stadium in Bengaluru (about 150 metres across), the nucleus would be about 1.5 millimetres -- smaller than a grain of rice!
• If you removed all the empty space from the atoms in every human on Earth, all 8 billion people would fit into a space the size of a sugar cube. That is how much empty space atoms contain!

Despite being so tiny, the nucleus contains more than 99.9% of the atom's total mass. All those heavy protons and neutrons are packed into an incredibly small space, making the nucleus extremely dense. If you could pack matter as densely as a nucleus everywhere, a teaspoon of that material would weigh about 6 billion tonnes -- roughly the same as Mount Everest!

The Electric Force in Atoms

What keeps the atom together? The electrostatic force (or Coulomb force) between the positive nucleus and the negative electrons. This is the same force that makes a balloon stick to a wall after you rub it on your hair -- opposite charges attract each other.

This force also explains why:

• Electrons stay in orbit around the nucleus (they are attracted to the positive nucleus)
• The nucleus is hard to break apart (protons are held together by an even stronger force called the strong nuclear force)
• Alpha particles are deflected in the scattering experiment (the positive alpha particle is repelled by the positive nucleus)
• Fusion is difficult (both nuclei are positive and repel each other -- you need extreme temperatures to overcome this repulsion)

What Are Ions?

A normal atom is electrically neutral -- it has the same number of protons (+) as electrons (-), so the charges cancel out. But sometimes atoms gain or lose electrons:

• Positive ion: An atom that has lost one or more electrons. It now has more protons than electrons, so the overall charge is positive. Example: Na+ (sodium ion -- lost 1 electron).
• Negative ion: An atom that has gained one or more electrons. It now has more electrons than protons, so the overall charge is negative. Example: Cl- (chloride ion -- gained 1 electron).

Let's see this with a real example. A magnesium atom has 12 protons and 12 electrons. When it forms an ion (Mg2+), it loses 2 electrons. Now it has 12 protons but only 10 electrons, so the overall charge is +2.

5.1.2 The Nucleus

What's Inside the Nucleus?

The nucleus contains two types of particles, collectively called nucleons:

• Protons -- positively charged
• Neutrons -- no charge (neutral)

The Subatomic Particles

Some important things to remember about these particles:

• Protons and neutrons are found inside the nucleus. Together, they are called nucleons.
• Electrons are found in orbits (or shells) around the nucleus.
• Protons and neutrons have roughly the same mass (relative mass = 1 each).
• An electron is about 1/1836 times the mass of a proton -- so small that we call it "negligible" (meaning we can usually ignore it when calculating atomic mass).
• The number of protons defines what element an atom is. Change the protons, and you change the element.
• The number of electrons can change (making ions), and the number of neutrons can change (making isotopes) -- but as long as the number of protons stays the same, it is still the same element.

Proton Number (Z) and Nucleon Number (A)

• Proton number (Z) = the number of protons in the nucleus. Also called the atomic number. This defines what element an atom is. All carbon atoms have Z = 6. All oxygen atoms have Z = 8.
• Nucleon number (A) = the total number of protons + neutrons in the nucleus. Also called the mass number.

Nuclide Notation

Scientists write atoms in a special shorthand that tells you everything about the nucleus at a glance. For example, carbon-12 is written as:

Isotopes

Isotopes are atoms of the same element (same number of protons) but with a different number of neutrons. This means they have the same proton number (Z) but different nucleon numbers (A).

For example, carbon has three common isotopes:

• Carbon-12 (¹²₆C): 6 protons, 6 neutrons -- the most common
• Carbon-13 (¹³₆C): 6 protons, 7 neutrons -- stable, found in nature
• Carbon-14 (¹⁴₆C): 6 protons, 8 neutrons -- radioactive, used for carbon dating

All three are still carbon (they all have 6 protons), but they have different masses because of the different numbers of neutrons.

Key points about isotopes:

• Isotopes have the same chemical properties (because they have the same number of electrons, and chemical reactions depend on electrons).
• Isotopes have different physical properties (different masses, different densities, and some may be radioactive while others are stable).
• Some isotopes are stable (like carbon-12) and some are unstable/radioactive (like carbon-14, which is used for carbon dating to find the age of ancient objects).

Don't Confuse Isotopes and Ions!

Students often mix up isotopes and ions. Here is the clear difference:

5.2.1 Detection of Radioactivity

Background Radiation

You are being hit by radiation right now. Don't worry -- it is a very small, natural amount called background radiation. It is always present around us, everywhere on Earth. Background radiation comes from both natural and man-made sources:

Sources of Background Radiation

The amount of background radiation you receive depends on where you live and what you do:

• People living in areas with granite rocks (like parts of Karnataka and Kerala) receive more radiation from the ground.
• People living at high altitudes (like in Ladakh or Shimla) receive more cosmic radiation because there is less atmosphere above them to act as a shield.
• People who fly frequently receive more cosmic radiation (aircraft fly at 10,000+ metres altitude).
• People who have many medical X-rays or CT scans receive more man-made radiation.

But here is the important thing: background radiation is normal and unavoidable. The levels are generally too low to cause any health problems. It has been present throughout all of human history, and our bodies have evolved to handle it.

Measuring Radiation: The Geiger-Muller Tube

We detect ionising radiation using a Geiger-Muller (GM) tube connected to a counter (or ratemeter). Here is how it works:

1. Radiation enters the GM tube through a thin window.
2. The radiation ionises the gas inside the tube (knocks electrons off gas atoms).
3. The free electrons and ions create a brief pulse of current.
4. The counter detects this pulse and records it as one "count."
5. Each count represents one particle or ray of radiation detected.

• Count rate is measured in counts per second (counts/s) or counts per minute (counts/min).
• Even with no radioactive source nearby, the GM tube will still register counts from background radiation.
• The GM tube makes a clicking sound with each count -- the faster the clicking, the higher the radiation level.

Important: A GM tube does not tell you what type of radiation it is detecting. It simply counts all ionising radiation that enters the tube. To identify the type, you need to do absorption tests (place different materials between the source and the detector).

5.2.2 The Three Types of Nuclear Emission

What Does "Ionising" Mean?

Before we look at the three types of radiation, we need to understand what "ionising" means. Ionising radiation is radiation that has enough energy to knock electrons off atoms, turning them into ions. This is what makes radioactive radiation dangerous to living things.

When radiation passes through a material, it can collide with atoms and knock their electrons loose. The atom becomes a positive ion (it lost an electron), and the freed electron becomes a negative ion. This process is called ionisation.

All three types of nuclear radiation (alpha, beta, and gamma) are ionising, but they differ in how strongly they ionise matter.

Spontaneous and Random

Radioactive decay is:

• Spontaneous -- it happens by itself, without any outside trigger. You cannot make a nucleus decay by heating it, cooling it, or doing anything to it. It just happens.
• Random -- you cannot predict which particular nucleus will decay next, or exactly when it will decay. It is like popcorn in a pan -- each kernel pops at a random moment, and you cannot predict which one will pop next!

The Three Types of Radiation

Where Does Each Type Come From?

It is important to understand where each type of radiation originates:

• Alpha particles come from the nucleus. They are emitted when a heavy, unstable nucleus needs to lose mass. The nucleus ejects a package of 2 protons and 2 neutrons (a helium nucleus). This is common in very heavy elements like uranium, radium, and plutonium.
• Beta particles also come from the nucleus -- NOT from the electron shells. Inside the nucleus, a neutron transforms into a proton and an electron. The newly created electron is ejected at high speed. This happens in nuclei that have too many neutrons compared to protons.
• Gamma rays come from the nucleus. After emitting an alpha or beta particle, the nucleus may still have excess energy. It releases this energy as a gamma ray (a high-frequency electromagnetic wave). Gamma emission often accompanies alpha or beta decay.

Notice: all three types originate from the nucleus, not from the electron shells. This is why they are called nuclear radiation.

How to Identify the Type of Radiation (Absorption Test)

In the lab, you can identify what type of radiation a source emits by placing different materials between the source and the GM tube:

1. Measure the count rate with nothing between the source and detector.
2. Place paper between them. If the count rate drops to background, the source emits alpha.
3. If the count is still above background, place aluminium (a few mm). If the count drops to background now, the source emits beta.
4. If the count is still above background after aluminium, the source emits gamma. You would need thick lead or concrete to significantly reduce it.

Remember: some sources emit more than one type of radiation. If the count drops with paper but does not reach background, and drops further with aluminium, the source may emit both alpha and beta (or alpha and gamma).

Summary: Comparing Alpha, Beta, and Gamma

Here is an easy way to compare the three types of radiation. Think of it as a spectrum from "heavy and slow" to "weightless and fast":

• Alpha: Heaviest, slowest, most ionising, least penetrating. Like a bowling ball rolling across the room -- it knocks everything over but stops quickly.
• Beta: Light, fast, moderately ionising, moderately penetrating. Like a tennis ball thrown hard -- goes further than the bowling ball but does less damage per hit.
• Gamma: No mass, speed of light, weakly ionising, highly penetrating. Like a laser beam -- passes through most things, rarely interacts, but can still cause damage if it does.

5.2.3 Radioactive Decay

What Is Radioactive Decay?

Radioactive decay is the process by which an unstable nucleus changes by emitting radiation (alpha, beta, or gamma). The key points are:

• It is spontaneous -- it happens by itself without any external cause. You cannot make it happen faster by heating, cooling, crushing, or doing anything to the sample. It is a nuclear process, completely independent of physical or chemical conditions.
• It is random -- you cannot predict which particular nucleus will decay next, or exactly when any individual nucleus will decay. However, for a large number of nuclei, you CAN predict roughly how many will decay in a given time (that is the concept of half-life).
• When an atom emits alpha or beta radiation, the element changes -- it becomes a different element. This is because the number of protons changes! This is sometimes called transmutation (changing one element into another).
• When an atom emits gamma radiation, the element does NOT change -- the nucleus simply loses excess energy.

Think of radioactive decay like a bowl of popcorn kernels in a hot pan. Each kernel pops (decays) at a random, unpredictable moment. You cannot predict which specific kernel will pop next. But if you have 1000 kernels, you can predict that after a certain time, roughly half will have popped. That is exactly how radioactive decay works!

Summary of Decay Types

How to Balance Nuclear Decay Equations

Nuclear equations must always balance on both sides. Here is a step-by-step method:

1. Write down what you know on each side of the arrow.
2. Add up all the nucleon numbers (A) on the left side. The total on the right side must be the same.
3. Add up all the proton numbers (Z) on the left side. The total on the right side must be the same.
4. If there is an unknown particle, use the balance to work out its A and Z values.
5. Use the Z value to identify the element (look it up on the periodic table).

Common particles you need to know:

5.2.4 Half-life

What Is Half-life?

Half-life is the time it takes for half the undecayed radioactive nuclei in a sample to decay. It can also be defined as the time for the activity (count rate) of a sample to fall to half its original value.

Think of it like the popcorn analogy: if you have 1000 unpopped kernels and after 2 minutes, 500 have popped, the half-life is 2 minutes. After another 2 minutes, half of the remaining 500 will have popped (250 more), leaving 250 unpopped. And so on.

Here is the pattern:

Notice that each time, exactly half of whatever remains decays. After one half-life, 500 remain. After the second half-life, half of those 500 (which is 250) decay, leaving 250. It is not "250 decay each time" -- it is always "half of whatever is left."

The Dice Experiment Analogy

In class, your teacher might demonstrate half-life using dice. Here is how it works:

1. Start with 100 dice (these represent 100 undecayed radioactive nuclei).
2. Roll them all. Remove every die that shows a 6 (these have "decayed").
3. Count how many remain. Roll those again. Remove all the 6s.
4. Keep going until very few dice remain.

You cannot predict WHICH specific die will show a 6 on any roll (random!). No one forces the dice to show 6 -- it happens by itself (spontaneous!). But you can predict that roughly 1/6 of the dice will be removed each round. This is exactly how radioactive decay works -- random for individual nuclei, but predictable for large numbers.

Important Things About Half-life

• Half-life is a constant for any given isotope. It does not change with temperature, pressure, chemical reactions, or any other external condition.
• Different isotopes have wildly different half-lives: uranium-238 has a half-life of 4.5 billion years, while polonium-214 has a half-life of just 0.000164 seconds!
• After one half-life, 50% of the original nuclei remain undecayed. After two half-lives, 25% remain. After ten half-lives, less than 0.1% remain -- effectively the sample is no longer significantly radioactive.
• Half-life also applies to the activity (count rate) of a sample: the activity halves with each half-life too.

Step-by-Step Guide: Finding Half-life from Data

Here is a reliable method for half-life questions in the exam:

1. Step 1: If background radiation is given, subtract it from all count rate values first to get the corrected count rates.
2. Step 2: Write out the corrected values in order.
3. Step 3: Find where the value halves. Note the time taken for each halving.
4. Step 4: Check that the halving time is consistent (it should be the same each time). If it varies slightly, this is normal -- take the average.
5. Step 5: The consistent halving time is your half-life.

Alternatively, if given start and end values:

1. Count how many times you need to halve the starting value to reach the ending value.
2. Divide the total time by the number of halvings.

5.2.5 Safety Precautions

Dangers of Ionising Radiation

Ionising radiation is dangerous because it can knock electrons off atoms in living cells. This can:

• Kill cells -- high doses destroy cells, causing radiation sickness
• Cause mutations -- damage to DNA can cause genetic changes
• Cause cancer -- mutations in DNA can lead to uncontrolled cell growth

Safe Handling Rules

• Always use tongs or remote handling tools -- never pick up radioactive sources with your hands
• Store sources in lead-lined containers when not in use
• Keep exposure time as short as possible
• Point sources away from people
• Never eat, drink, or smoke when working with radioactive materials

Summary: Uses and Their Radiation Requirements

Let's consolidate everything we have learned about applications. For each use, think about: (a) what type of radiation is needed and why, (b) what half-life is needed and why.

Which Type Is Most Dangerous?

This is a tricky question because the answer depends on whether the source is outside or inside your body:

• Outside the body: Gamma is the most dangerous because it penetrates skin and reaches internal organs. Alpha is least dangerous because it is stopped by your dead outer layer of skin.
• Inside the body (if you swallow or breathe in a radioactive source): Alpha is the most dangerous! All of its ionising energy is absorbed by your internal tissues, causing maximum damage. Gamma is less dangerous inside because much of it passes straight through without interacting.

Radiation Exposure in Everyday Life

We are all exposed to small amounts of radiation every day. Here are some typical annual doses (in millisieverts, mSv):

• Average natural background: about 2.4 mSv per year
• Kerala monazite sands: up to 70 mSv per year in some spots
• A single chest X-ray: about 0.02 mSv
• A CT scan: about 7 mSv
• Maximum allowed for nuclear workers: 20 mSv per year

The key principle is: keep your total exposure as low as reasonably achievable (this is called the ALARA principle).

Safety Precautions in a School Lab

If you ever handle radioactive sources in your physics lab at school, here are the rules your teacher will follow:

1. Always use long-handled tongs to hold the source -- never use bare hands.
2. Keep the source at arm's length and point it away from your body and other people.
3. Keep exposure time to a minimum -- do your measurements quickly and return the source to its lead-lined container.
4. Never eat, drink, or put your hands near your face when working with radioactive sources.
5. Store all sources in lead-lined containers in a locked, labelled cupboard.
6. Wash your hands thoroughly after any practical work involving radioactive materials.
7. The teacher should have a radiation monitoring badge (dosimeter) to track their total exposure over time.

Half-lives of Some Important Isotopes

Nuclear Energy in India: A Quick Overview

India has a significant nuclear energy programme. Here are some key facts:

• Kudankulam Nuclear Power Plant (Tamil Nadu) -- India's largest, with two 1000 MW reactors and more under construction. Uses pressurised water reactors with enriched uranium fuel.
• Tarapur Atomic Power Station (Maharashtra) -- India's first commercial nuclear power station, operational since 1969. It was built with American assistance.
• BARC (Mumbai) -- India's premier nuclear research centre, named after Homi Bhabha. Develops reactor designs, isotope production, and nuclear medicine technology.
• India has a unique three-stage nuclear programme designed to eventually use thorium (which India has in abundance in Kerala's monazite sands) as nuclear fuel.
• Nuclear power currently provides about 3% of India's electricity, but this is planned to increase significantly.

All Indian nuclear reactors use nuclear fission. Fusion power is still being researched worldwide (India is a partner in the ITER project in France).