Radioactivity
Background radiation is the ionising radiation that is always present in our environment. It comes from natural and artificial sources. You must be able to identify the significant contributors:
- Radon gas: A radioactive gas released from rocks containing uranium; it contributes significantly to natural background radiation.
- Rocks and buildings: Materials like granite emit low levels of radiation.
- Food and drink: All food contains small amounts of radioactive isotopes (e.g., potassium-40).
- Cosmic rays: High-energy particles from space that strike the Earth's atmosphere.
- Artificial sources: Medical X-rays, nuclear fallout, and industrial waste.
Radioactive decay is the spontaneous and random process by which an unstable nucleus emits radiation to become more stable.
- Spontaneous: The decay happens on its own; it cannot be influenced by external factors like temperature, pressure, or chemical bonding.
- Random: It is impossible to predict exactly when a specific individual nucleus will decay. However, for a large sample, the rate of decay is predictable.
| Gamma ($\gamma$) |
|---|
| Electromagnetic wave (high energy photon) |
| 0 |
| Weakest (passes through matter easily) |
| Strongest (requires thick lead or concrete to reduce significantly) |
Why the differences?
- Ionising power: Alpha particles are heavy and have a +2 charge, so they interact strongly with atoms, stripping off electrons. Gamma rays have no charge and no mass, so they pass through without interacting much.
- Penetration: Because alpha particles lose energy quickly due to strong ionisation, they stop soon. Gamma rays do not lose energy easily, so they travel far.
When radiation passes through electric or magnetic fields, the charged particles are deflected. Gamma rays are not deflected because they have no charge.
- Alpha particles: Deflected slightly towards the negative plate (or opposite to the magnetic field direction). They have a large mass, so they are harder to deflect than beta particles.
- Beta particles: Deflected strongly towards the positive plate (or in the opposite direction to alpha particles). They have a very small mass, so they curve sharply.
- Gamma rays: Travel straight through without deflection.
Isotopes are atoms of the same element with the same number of protons but a different number of neutrons.
Some isotopes are stable, while others are radioactive. Radioactive isotopes are unstable due to:
- Excess neutrons: The nucleus has too many neutrons relative to protons (leads to beta decay).
- Nucleus is too heavy: The nucleus is very large (typically atomic number > 83), making the strong nuclear force unable to hold it together against electrostatic repulsion (leads to alpha decay).
Alpha Decay (\alpha):
The nucleus emits a helium nucleus (^{4}<em>{2}\text{He}). The mass number decreases by 4, and the proton number decreases by 2. The element changes.
^{A}</em>{Z}X \rightarrow ^{A-4}<em>{Z-2}Y + ^{4}</em>{2}\text{He}
Beta Decay (\beta^-):
A neutron in the nucleus turns into a proton and an electron. The electron is emitted as the beta particle. The mass number stays the same, but the proton number increases by 1. The element changes.
^{A}<em>{Z}X \rightarrow ^{A}</em>{Z+1}Y + ^{0}_{-1}\beta
Gamma Emission (\gamma):
The nucleus releases energy to become more stable. No protons or neutrons are lost. The element does not change.
^{A}<em>{Z}X^* \rightarrow ^{A}</em>{Z}X + \gamma
Alternatively, it is the time taken for the count rate (or activity) to fall to half its initial value.
Scenario: You have a table of count rates over time, but background radiation has NOT been subtracted. The count rate levels off at 20 counts/min because the background radiation is still being detected.
Step 1: Determine Background Radiation.
Look at the data as time approaches infinity. The count rate stops decreasing and becomes constant. This constant value is the background count rate (B).
Example: Final count rate = 20 counts/min. So, B = 20.
Step 2: Correct the Data.
Subtract the background from all measurements to get the corrected count rate (C_{corrected}).
C_{corrected} = C_{measured} - B
Example: If measured is 100, corrected is 100 - 20 = 80.
Step 3: Calculate Half-life.
Find the time it takes for the corrected count rate to halve.
- Initial corrected rate = 80 counts/min.
- Target half-rate = 40 counts/min.
- Find the time t where corrected rate is 40. This t is the half-life.
The choice of isotope depends on the type of radiation (penetration/ionisation) and the half-life.
Smoke Alarms: Use Alpha source (e.g., Americium-241).
- Why Alpha? Strongly ionising (creates a current in air) but low penetration (stopped by the casing, safe for home use).
- Why this half-life? Long half-life (~432 years) ensures the alarm works for many years without replacement.
Sterilisation of Equipment / Food Irradiation: Use Gamma source (e.g., Cobalt-60).
- Why Gamma? High penetration allows it to kill bacteria throughout the food or inside sealed medical equipment without damaging the packaging.
- Half-life: Moderate half-life ensures sufficient activity for a useful operational period.
Thickness Control (e.g., paper manufacturing): Use Beta source.
- Why Beta? Alpha is stopped by the paper; Gamma passes through completely without much change in count rate. Beta is absorbed partially, so changes in thickness cause measurable changes in count rate.
- Mechanism: A detector on the other side monitors count rate. If paper is too thick, count rate drops, and rollers adjust automatically.
Medical Diagnosis (Tracers): Use Gamma source with a short half-life (e.g., Technetium-99m).
- Why Gamma? It penetrates out of the body to be detected by an external camera.
- Why short half-life? The isotope decays quickly, minimizing the radiation dose to the patient. It disappears from the body before causing long-term damage.
Cancer Treatment (Radiotherapy): Use Gamma or Beta sources.
- Why? High energy radiation kills cancer cells by damaging their DNA. The beam is focused on the tumor to minimize damage to healthy tissue.
Effects on Living Things:
Ionising radiation can knock electrons off atoms in cells, damaging DNA. This can lead to:
- Cell death: Tissue damage.
- Mutations: Changes in genetic code, potentially leading to uncontrolled cell division (cancer).
Safety Precautions:
To reduce exposure to ionising radiation:
- Reduce Time: Spend less time near the source.
- Increase Distance: Radiation intensity decreases rapidly with distance (inverse square law). Use long tongs.
- Shielding: Use appropriate barriers.
- Alpha: Paper/air gap.
- Beta: Aluminium/plexiglass.
- Gamma: Thick lead/concrete.
- Storage: Store sources in lead-lined containers with interlocking doors to prevent accidental exposure.
Correct Understanding: Beta decay does change the element. The proton number (Z) increases by 1. Since the identity of an element is defined by its proton number, it becomes a different element (e.g., Carbon-14 becomes Nitrogen-14).
Correct Understanding: Always identify the background count rate (the value the graph levels off at) and subtract it from all data points. Calculate the half-life using the corrected values.
Examiner Accepts: 'Alpha particles have a larger mass and a greater charge (+2) compared to gamma rays (which have no charge). This allows them to interact more strongly with atoms, stripping off electrons more effectively.'
Why this works: It directly links the physical properties (mass/charge) to the mechanism of ionisation. Mentioning 'stripping electrons' is key.
Examiner Accepts: 'A gamma emitter is chosen because gamma rays have high penetration and can escape the body to be detected externally. A short half-life is chosen so that the radioactivity decays quickly, minimizing the radiation dose to the patient.'
Why this works: It addresses both criteria in the question: the type of radiation (penetration) and the half-life (safety/dose).
Correction: The question says 'deflected towards a negative plate'. Positive charges move towards negative plates. Alpha particles are positive (+2). Beta particles are negative (-1) and would move towards the positive plate. Gamma rays do not deflect.
Answer: Alpha (\alpha) particle.
30 \text{ min} / 10 \text{ min} = 3 half-lives.
Step 2: Halve the count rate 3 times.
Start: 800
After 1st half-life: 400
After 2nd half-life: 200
After 3rd half-life: 100 counts/min.
^{238}<em>{92}\text{U} \rightarrow ^{A}</em>{Z}\text{X} + ^{4}_{2}\text{He}
Proton number (Z): 92 - 2 = 90
Element with proton number 90 is Thorium (Th).
Answer: ^{234}_{90}\text{Th}
^{14}<em>{6}\text{C} \rightarrow ^{A}</em>{Z}\text{X} + ^{0}_{-1}\beta
Proton number (Z): 6 - (-1) = 7
Element with proton number 7 is Nitrogen (N).
Answer: ^{14}_{7}\text{N}