Radioactivity Revision Notes

Alpha (α) Decay

In alpha decay, an unstable nucleus emits an alpha particle. An alpha particle is composed of two protons and two neutrons, effectively a helium nucleus. This means the atomic number (Z) of the decaying nucleus decreases by 2, and its mass number (A) decreases by 4. For example, Uranium-238 (Z=92, A=238) undergoes alpha decay to become Thorium-234 (Z=90, A=234).

Beta-minus (β-) Decay

Beta-minus decay involves a neutron within the nucleus converting into a proton, emitting an electron (beta particle) and an antineutrino. This transformation increases the atomic number (Z) by 1, as a new proton is formed. The mass number (A) remains unchanged because a neutron is replaced by a proton, keeping the total number of nucleons constant. For instance, Carbon-14 (Z=6, A=14) transforms into Nitrogen-14 (Z=7, A=14).

Gamma (γ) Decay

Gamma decay occurs when an atomic nucleus in an excited energy state releases excess energy in the form of a gamma ray (high-energy photon). Gamma rays are electromagnetic radiation and have no mass or charge. Therefore, during gamma decay, both the atomic number (Z) and the mass number (A) of the nucleus remain unchanged. The nuclide simply transitions to a lower, more stable energy state.

Neutron Emission

Neutron emission is a decay mode where a nucleus directly releases a neutron. This process reduces the number of neutrons in the nucleus by one. Consequently, the mass number (A) decreases by 1, while the atomic number (Z) (number of protons) remains unchanged. This type of decay is common in very neutron-rich fission products, for example.

Conditions for Nuclear Fusion

Nuclear fusion occurs when two light atomic nuclei combine to form a heavier nucleus, releasing a large amount of energy. To achieve this, several extreme conditions must be met:

1. Extremely High Temperatures: Nuclei are positively charged, causing them to repel each other (electrostatic repulsion). To overcome this, nuclei must move at very high speeds, which corresponds to extremely high temperatures, typically millions of degrees Celsius. At these temperatures, matter exists as a plasma.
2. High Pressure/Density: To increase the likelihood of collisions and fusion events, the plasma must be sufficiently dense or confined under high pressure. This ensures that nuclei are close enough for interactions to occur frequently.
3. Sufficient Confinement Time: The plasma must be confined for a long enough duration to allow a significant number of fusion reactions to take place and produce more energy than was used to initiate and maintain the reaction.

Difficulties in Building Economic Fusion Power Stations

The extreme conditions required for fusion pose significant challenges for developing practical and economic power stations:

1. Plasma Containment: Maintaining a plasma at millions of degrees Celsius means it cannot touch the walls of any conventional container. Magnetic confinement (using strong magnetic fields to trap the plasma) or inertial confinement (using lasers or particle beams to compress and heat fuel pellets) are the primary approaches. Both are technologically demanding and complex.
2. Plasma Stability: Plasmas at these temperatures are inherently unstable and prone to disruptions, making long-term, stable confinement difficult to achieve.
3. Energy Balance: The challenge is to produce more energy from the fusion reactions than is put in to heat and confine the plasma. This 'net energy gain' has been demonstrated for very short periods, but sustaining it for continuous power generation is a major hurdle.
4. Material Science: Reactor walls and components must withstand intense heat, high-energy neutron bombardment, and radiation over long periods without degrading. Developing suitable materials is an ongoing challenge.
5. Economic Viability: The immense research and development costs, coupled with the complexity of building and operating such reactors, make attaining an economically viable and competitive fusion power plant a long-term prospect.

Conservation Laws in Nuclear Reactions

When balancing nuclear equations, two fundamental quantities must be conserved: the mass number (A) and the atomic number (Z). The mass number, found at the top left of an element symbol, represents the total number of protons and neutrons. The atomic number, found at the bottom left, represents the number of protons, which also signifies the nuclear charge. The sum of these numbers on the reactant side must equal the sum on the product side.

Steps to Balance a Nuclear Equation

1. Identify all known particles and their mass and atomic numbers. 2. Sum the mass numbers on both sides of the equation. If they don't match, determine the missing mass number. 3. Sum the atomic numbers on both sides of the equation. If they don't match, determine the missing atomic number. 4. Use the calculated missing mass and atomic numbers to identify the unknown particle using a periodic table or common nuclear particle symbols (e.g., alpha particle: 2He or α, beta particle: ȡe or ȡβ, neutron: n, proton: p or H).

Common Particles in Nuclear Equations

Alpha particle (2He or 2α): Mass number 4, atomic number 2. Beta particle (-1e or -1β): Mass number 0, atomic number -1. Neutron (0n): Mass number 1, atomic number 0. Proton (1H or 1p): Mass number 1, atomic number 1. Gamma ray (0γ): Mass number 0, atomic number 0 (pure energy, no mass or charge). These are crucial for correctly balancing equations.

What is Radioactive Activity?

Radioactive activity is the rate at which unstable nuclei in a sample decay, measured in Becquerels (Bq). One Becquerel means one nuclear decay event per second. This activity is directly proportional to the number of unstable nuclei present in the source at any given time.

How Activity Changes Over Time

The activity of a radioactive source decreases over time due to the depletion of unstable nuclei. As nuclei decay, fewer are available to decay, leading to a reduction in the decay rate. This decrease is exponential: the activity halves over a constant period known as the half-life. After one half-life, activity is 50%; after two, 25%; after three, 12.5%, and so on. This shows a continuous, but slowing, reduction in activity.

Half-life and Total Decay

The half-life is a fundamental characteristic for each radioactive isotope. It's the time needed for half of the initial unstable nuclei to undergo decay, and concurrently, for the activity to halve. Since decay is a random process, it's impossible to predict when a single nucleus will decay, but for a large sample, the half-life provides a reliable measure of decay rate. Although activity continually decreases, theoretically, it never reaches exactly zero because there will always be a tiny fraction of undecayed nuclei remaining, no matter how many half-lives have passed.