Radioactivity and decay, forms of radiation, uses and dangers Notes

Why Do some Materials Give Out Radiation, And How Can Radiation Be Both Useful And Dangerous?

1. Radioactivity is a natural process in which an unstable nucleus changes into a more stable one by emitting radiation.
2. Although radiation can be harmful, it is also extremely useful in medicine, industry, and energy production.

Radioactivity Happens When Nuclei Are Unstable

1. Atoms consist of a small, dense nucleus (containing protons and neutrons) with electrons around it.
2. Some nuclei are unstable because the balance of forces in the nucleus does not produce a long-term stable arrangement.
3. When this happens, the nucleus undergoes radioactive decay.

Radioactive decay

1. A key feature of radioactive decay is that it is not affected by typical chemical or physical conditions (such as temperature or pressure).
2. Those conditions mainly affect electrons, not the nucleus.

The Random Nature of Radioactive Decay

1. It is impossible to predict when a single nucleus will decay.
2. Two identical nuclei can decay at very different times.
3. However, when many nuclei are observed together, clear patterns appear.
4. This randomness leads to the concept of half-life.

Background Radiation Is Always Present

1. Even without any "nuclear sources" nearby, you are exposed to radiation every day.
2. This is called background radiation, and most of it comes from natural sources.

1. Rocks and soil

1. Some rocks contain long-lived radioactive elements such as uranium and thorium.
2. Their decay products can also be radioactive.
3. Building materials made from these rocks can contribute to indoor background radiation.

2. Radon gas (radon-222)

1. Produced in the decay chain of uranium-238.
2. Radon is a noble gas, so it does not easily react and can seep out of rocks.
3. It can accumulate indoors if ventilation is poor.

3. Food and living things

1. Some foods contain potassium, and a small fraction of potassium atoms are radioactive potassium-40.
2. Foods with high potassium content can therefore have slightly higher natural radioactivity.

4. Cosmic rays

1. High-energy particles from the Sun and deep space strike the atmosphere, producing showers of particles.
2. Some reach the ground.

What Is Half-Life?

1. After one half-life, half of the original nuclei have decayed.
2. After two half-lives, only one quarter of the original nuclei remain undecayed.
3. Each radioactive isotope has its own characteristic half-life.

Alpha, Beta, And Gamma Radiation Behave Very Differently

1. Radioactive decay can produce three main types of nuclear radiation: alpha (α), beta (β), and gamma (γ).
2. All three (α, β, γ) are ionising, meaning they can damage cells by knocking electrons out of atoms and molecules.

1. Alpha Radiation: Very Ionising, Poor Penetration

1. Alpha radiation consists of particles made up of two protons and two neutrons.
2. Alpha particles carry a positive charge and have a relatively large mass.
3. Because of their large mass, alpha particles strongly ionize materials they pass through.
4. Alpha radiation has very low penetration and travels only a few centimetres in air.
5. A sheet of paper or the outer layer of human skin is sufficient to stop alpha radiation.

2. Beta Radiation: Medium Penetration, Medium Ionisation

1. Beta radiation consists of fast-moving electrons emitted from the nucleus.
2. Beta particles have a negative charge and a much smaller mass than alpha particles.
3. Beta radiation is moderately ionizing because the particles interact less strongly with matter than alpha particles.
4. Beta radiation has greater penetration than alpha radiation and can travel several metres through air.
5. A thin sheet of metal, such as aluminium, is sufficient to absorb beta radiation.

3. Gamma Radiation: Low Ionisation, Very High Penetration

1. Gamma radiation is a form of high-energy electromagnetic radiation.
2. Gamma rays have no mass and no electric charge.
3. Because gamma radiation does not consist of particles, it is weakly ionizing.
4. Gamma radiation is highly penetrating and can pass through large distances of air.
5. Thick layers of lead or concrete are required to reduce the intensity of gamma radiation.

Radiation Can Damage Cells By Causing Mutations

1. Ionising radiation can damage living tissue because ionisation can disrupt molecules inside cells, including DNA.
2. If DNA is altered, a mutation may occur.
3. Some mutations are repaired by the body, but others can lead to uncontrolled cell division, increasing the risk of cancer.
4. The source material highlights that mutation can be harmful, but it has also been deliberately used.
5. In the mid-20th century, radiation was used to create mutations in crops, and then plants were selected for useful characteristics.
6. Some crop varieties developed from such trials are still used today.

Managing Risk: Minimising Exposure Matters

1. Because radiation can cause biological damage, good practice is to minimise exposure to avoid unnecessary dose.
2. In practical school and laboratory settings, sensible precautions include:
   1. keeping a distance from the source,
   2. using appropriate shielding (paper, aluminium, lead, depending on the radiation type),
   3. limiting time of exposure,
   4. safe handling and storage.

Useful Applications Come From Controlled Radiation

Despite the dangers, radioactive sources are valuable because they can be detected easily and can deliver energy to matter.

1. Sterilising Equipment And Food With Gamma Rays

1. High doses of radiation can kill microorganisms, so gamma radiation is used to sterilise:
   1. medical tools (reducing infection risk),
   2. packaged food (extending shelf life).
2. A key idea is that the gamma rays do not remain in the items afterward.
3. The food and equipment are not made radioactive by this process (they are exposed to radiation, but they do not become a radioactive source).

2. Treating Cancer With Radiotherapy

1. Because radiation can kill cells, it can also be used to destroy cancer cells.
2. In radiotherapy, radiation is directed at the cancerous region to cause ionisation damage and ideally kill the tumour cells.
3. The source material notes that beta radiation is often used because it can penetrate to the affected area (more than alpha), causing ionisation within the tumour.

3. Monitoring And Controlling Industrial Processes

1. Radiation can be used as a non-contact measurement tool.
2. For example, the thickness of paper or plastic film can be monitored by placing a radioactive source on one side and a detector on the other:
   1. if the sheet becomes thicker, fewer particles/photons reach the detector, so the measured count rate decreases,
   2. if it becomes thinner, the detector count rate increases.
3. This allows continuous quality control during manufacturing.

Nuclear Fission

1. Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei.
2. This process releases:
   1. Energy
   2. Neutrons

Changes to Protons and Neutrons During Fission

1. When a uranium-235 nucleus undergoes fission:
   1. The nucleus splits into two smaller nuclei
   2. Extra neutrons are released
   3. A large amount of energy is released
2. Important conservation rule:
   1. The total number of protons and neutrons is conserved overall
   2. They are redistributed between the products

What Is a Chain Reaction?

1. A chain reaction occurs when:
   1. Neutrons released from one fission event
   2. Trigger fission in other nearby nuclei
2. This causes:
   1. Repeated fission events
   2. Continuous energy release

Why Control Is Essential

1. A reactor must keep the chain reaction steady:
   1. too few neutrons absorbed by fuel, and the reaction slows down,
   2. too many fissions too quickly, and energy release becomes dangerous.
2. This is why reactors use control rods that absorb neutrons, helping regulate the rate of fission.

Accounting For Background Radiation In Experiments

1. When measuring radiation in the lab (for example, how count rate changes with distance), you must separate the source's radiation from background radiation.
2. A practical method:
   1. Measure the detector count with no source present for a fixed time (for example 1 minute). This is the background count.
   2. Measure the count with the source present for the same time.
   3. Subtract background from each measurement to estimate the source-only count.

Nuclear Waste

1. Nuclear waste contains radioactive materials with long half-lives.
2. Some waste remains radioactive for thousands or millions of years.
3. Nuclear waste is produced by:
   1. Nuclear power stations
   2. Medical treatments
   3. Scientific research
4. Because of its long-lasting danger, nuclear waste must be carefully managed.

Storage and Disposal of Nuclear Waste

1. Nuclear waste is often stored underwater in deep pools.
2. Water acts as a shield against radiation and helps remove heat.
3. After some time, waste may be moved to deep underground storage sites.