Sciences - Co-ordinated (Double) (0654) | Nuclear physics

Welcome to Nuclear Physics: Exploring the Core of Matter!

Hello there! Nuclear physics might sound intimidating, but it's really the study of the powerful tiny engine at the heart of every atom: the nucleus. This chapter explains what makes some nuclei unstable (radioactive) and how they release energy. Don't worry if this seems tricky at first—we'll break down the atoms, radiation, and safety steps clearly, step-by-step!

Understanding nuclear physics is crucial because it governs how energy is produced in stars and nuclear reactors, how we date ancient artifacts, and how we use radiation safely in medicine and industry. Let's dive in!

P5.1 The Atomic Nucleus

Composition of the Nucleus

Every atom consists of a tiny, dense central core called the nucleus, surrounded by orbiting electrons.

• Protons: Positively charged particles found inside the nucleus.
  • Relative charge: +1
• Neutrons: Neutrally charged particles found inside the nucleus.
  • Relative charge: 0
• Electrons: Negatively charged particles orbiting the nucleus.
  • Relative charge: -1

The particles inside the nucleus (protons and neutrons) are collectively known as nucleons.

Did you know? Since the nucleus only contains protons (+1) and neutrons (0), the overall charge of the nucleus is always positive, and its magnitude is equal to the total number of protons.

Defining Key Numbers (A and Z)

We use two main numbers to identify any nucleus:

• Proton Number (Atomic Number, Z): This is the number of protons in the nucleus.
  • Z determines the chemical element (e.g., all atoms with Z=6 are Carbon).
• Nucleon Number (Mass Number, A): This is the total number of protons and neutrons in the nucleus.

You can easily calculate the number of neutrons (\(N\)) using these definitions:

$$ \text{Number of Neutrons} = A - Z $$

For example, if an atom has A=12 and Z=6 (Carbon-12), it has \(12 - 6 = 6\) neutrons.

Nuclide Notation and Isotopes

We represent a nucleus using the nuclide notation:

$$ _Z^A X $$

Where: X is the chemical symbol, A is the Nucleon Number, and Z is the Proton Number.

Example: Uranium-235 is written as \(_ {92}^{235} U\). This tells us it has 92 protons and \(235 - 92 = 143\) neutrons.

Isotopes

Isotopes are atoms of the same element (same Z, thus same number of protons) but with different numbers of neutrons (different A).

Example: Carbon-12 (\(_6^{12} C\)) and Carbon-14 (\(_6^{14} C\)). Both have 6 protons, but C-14 has 8 neutrons while C-12 has 6.

Some isotopes have stable nuclei, but others have unstable nuclei. These unstable isotopes are radioactive, meaning they decay to become more stable by emitting radiation.

P5.2 Radioactivity: Unstable Nuclei

P5.2.1 Detection of Radioactivity and Background Radiation

Ionising nuclear radiation is radiation that has enough energy to knock electrons off atoms, turning them into ions. This process can cause damage to living tissue.

We measure this radiation using a detector (like a Geiger-Müller tube) connected to a counter, which records the count rate (measured in counts/s or counts/minute).

Background Radiation

Radiation is all around us! Background radiation is the low level of ionising radiation present in the environment from natural and man-made sources.

Significant sources of background radiation include:

• Natural Sources:
  1. Radon gas (in the air): This gas seeps out of rocks and soil. It's often the largest single source of background radiation.
  2. Rocks and buildings: Trace amounts of radioactive materials found naturally in building stones.
  3. Food and drink: Contains naturally occurring radioactive isotopes (like Potassium-40).
  4. Cosmic rays: High-energy particles from space, largely blocked by the atmosphere but increasing at high altitudes.

Common Mistake Alert: Do not forget that radiation from the Earth (rocks, radon) contributes significantly to background radiation!

P5.2.2 The Three Types of Nuclear Emission

Unstable nuclei decay by spontaneously emitting three main types of radiation: alpha (\(\alpha\)), beta (\(\beta\)), and gamma (\(\gamma\)).

1. Alpha (\(\alpha\)) Particles

• Nature: Consists of two protons and two neutrons. Identical to a Helium nucleus (\(_2^4 He\)). They are relatively heavy and positively charged (+2).
• Penetrating Ability: Very low. Stopped by a sheet of paper or a few centimetres of air.
• Ionising Effect: Very high. Because they are heavy and highly charged, they cause intense ionisation over a short range.

Analogy: Think of an alpha particle as a slow, heavy bowling ball that hits whatever is in its path forcefully, but can't travel far.

2. Beta (\(\beta\)) Particles (specifically Beta-minus, \(\beta^{-}\))

• Nature: High-speed electrons. They are very light and negatively charged (-1).
• Penetrating Ability: Medium. Stopped by a few millimetres of aluminium.
• Ionising Effect: Medium. Less ionising than alpha, but travels further.

Analogy: A beta particle is like a fast golf ball—it travels further than the bowling ball but doesn't cause as much immediate damage to surrounding particles.

3. Gamma (\(\gamma\)) Rays

• Nature: High-frequency electromagnetic waves (photons). They have no mass and no charge.
• Penetrating Ability: Very high. Significantly reduced only by thick layers of lead or concrete.
• Ionising Effect: Very low. They pass through most atoms without interacting, only causing ionisation sporadically.

Analogy: Gamma rays are like light—they travel extremely far and fast, and usually pass straight through objects.

P5.2.3 Radioactive Decay and Nuclear Equations

Spontaneous and Random Decay

Radioactive decay is the process where an unstable nucleus changes into a more stable one by emitting \(\alpha\), \(\beta\), and/or \(\gamma\) radiation.

• Spontaneous: The decay cannot be influenced by external factors like temperature, pressure, or chemical bonding. It just happens on its own timetable.
• Random: You cannot predict *when* a specific nucleus will decay, only the probability of it decaying within a certain time period.

Changes in the Nucleus During Decay

During \(\alpha\)-decay or \(\beta\)-decay, the nucleus changes its composition, resulting in the nucleus of a different element. Gamma emission usually occurs alongside alpha or beta decay and does not change the element itself.

Beta (\(\beta\)) Emission Process

During beta decay, a neutron inside the nucleus transforms into a proton and an electron. The electron (the beta particle) is then instantly ejected from the nucleus at high speed.

$$ \text{Neutron} \to \text{Proton} + \text{Electron} $$

Because the neutron changes into a proton, the Proton Number (Z) increases by 1, but the Nucleon Number (A) stays the same.

Writing Decay Equations (Extended Syllabus)

In nuclear decay equations, the total nucleon number (A) and the total proton number (Z) must be conserved (must be the same on both sides).

Alpha Decay Example: Radium-226 decays into Radon-222 by emitting an alpha particle (\(_2^4 He\)).

$$ _{88}^{226} Ra \to _{86}^{222} Rn + _2^4 He $$

(Check: A: 226 = 222 + 4. Z: 88 = 86 + 2. It balances!)

Beta Decay Example: Carbon-14 decays into Nitrogen-14 by emitting a beta particle (\(_{-1}^0 e\)).

$$ _6^{14} C \to _7^{14} N + _{-1}^0 e $$

(Check: A: 14 = 14 + 0. Z: 6 = 7 + (-1). It balances!)

Gamma Emission: Does not change A or Z. It is usually represented as the parent nucleus dropping to a lower energy state.

$$ _Z^A X^* \to _Z^A X + \gamma $$

P5.2.4 Half-life

The Definition of Half-life

Because radioactive decay is random, we describe the decay rate using half-life.

The half-life (\(T_{1/2}\)) of a radioactive isotope is the time taken for half the nuclei of that isotope in any sample to decay.

It is also the time taken for the count rate (or activity) of a sample to fall to half its initial value.

Example: If a substance has a half-life of 2 hours, and you start with 100 grams:

1. After 2 hours (1 half-life): 50 grams remain.
2. After 4 hours (2 half-lives): 25 grams remain.
3. After 6 hours (3 half-lives): 12.5 grams remain.

This definition is key! You must be able to use this concept in simple calculations or interpret decay curves and tables.

P5.2.5 Applications and Safety Precautions

Applications of Radioactivity

Radioactive isotopes have many useful applications due to the specific properties of their emissions (penetration and ionisation ability):

1. Household fire (smoke) alarms: Use an alpha source (e.g., Americium-241). Alpha particles ionise the air, creating a small current. Smoke blocks the alpha particles, stopping the current and triggering the alarm. (Alpha is used because it has a very short range and is safe inside the casing.)
2. Irradiating food to kill bacteria: Use gamma rays. Gamma rays penetrate the food deeply without leaving any harmful residue.
3. Sterilisation of equipment: Using gamma rays (similar to food irradiation) to kill microorganisms on medical instruments.
4. Measuring and controlling material thickness:
   • For thin materials (like paper): A beta source is used. If the material becomes too thick, fewer beta particles get through, signalling the need to adjust the rollers.
   • For thick materials (like steel): A highly penetrating gamma source may be required.
5. Diagnosis and treatment of cancer: Use gamma rays. Gamma rays can be targeted at tumours (treatment) or used as tracers inside the body (diagnosis).

Safety Precautions

Ionising radiation can be very dangerous. The main effects of excessive exposure include cell death, mutations, and ultimately, cancer. We must follow strict safety protocols:

Radioactive materials must be moved, used, and stored safely, following the principles of:

• Time: Minimize the duration of exposure. Less time spent near the source means less dose received.
• Distance: Increase the distance from the source. Since radiation intensity decreases rapidly with distance (inverse square law), stepping back significantly reduces exposure.
• Shielding: Use appropriate barriers to absorb the radiation.
  • Alpha: Needs only paper or thin casing.
  • Beta: Needs aluminium or plastic shields.
  • Gamma: Requires thick lead or concrete shielding.

P5.1 (Supplement) Nuclear Energy Processes

The nuclear reactions responsible for generating vast amounts of energy involve rearranging the nucleus, usually either by splitting it or joining it.

Nuclear Fission (The Splitting Process)

Nuclear fission is the process of splitting a large, heavy nucleus (typically Uranium-235) into two or more smaller nuclei, usually initiated by bombarding it with a slow-moving neutron.

This process releases a tremendous amount of energy and often releases more neutrons, which can cause a chain reaction. This is the fundamental process used in current nuclear power reactors.

Nuclear Fusion (The Joining Process)

Nuclear fusion is the process of joining two small, light nuclei (typically isotopes of hydrogen like deuterium and tritium) to form a single, larger, heavier nucleus.

Fusion releases even more energy per kilogram than fission. This is the reaction that powers our Sun and all stable stars, where hydrogen nuclei fuse to form helium under immense pressure and extremely high temperatures.

Did you know? Scientists are trying to harness fusion power on Earth because the 'fuel' (hydrogen isotopes) is plentiful and the process produces far less long-lived radioactive waste than fission.