Radioactivity
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Notes
Background Radiation
- **Background radiation** is the radiation that exists around us all the time.
- Natural sources: **radon gas** (in air), **rocks and buildings**, **food and drink**, **cosmic rays**.
- Man-made sources: medical X-rays, nuclear waste, nuclear fallout, nuclear accidents.
- The **count rate** (decays per second) is measured using a **Geiger-Müller tube**.
- To correct for background radiation, measure count rate with no source and subtract from readings with source.
Types of Radiation
- **α (α)**: helium nucleus (2 protons, 2 neutrons), charge +2, range a few cm in air, stopped by paper, **highly ionising**.
- **β (β)**: high-speed electron, charge -1, range tens of cm in air, stopped by a few mm of aluminium, **moderately ionising**.
- **Gamma (γ)**: electromagnetic wave, no charge, infinite range in air, reduced by thick lead, **weakly ionising**.
- Radioactive decay is **spontaneous and random** – cannot predict which nucleus will decay next.
Ionising Power & Deflection
- Ionisation is the removal of an electron from an atom, creating an ion.
- **α** is most ionising (charge +2, large mass); **β** is moderate (charge -1); **gamma** is least (no charge).
- In an **electric field**: α deflects towards negative plate, β towards positive plate, γ undeflected.
- In a **magnetic field**: α and β deflect in opposite directions (due to opposite charges); γ undeflected.
- β deflects more than α because β has much smaller mass.
Radioactive Decay
- **α decay**: nucleus emits α-particle (₂⁴He). Mass number decreases by 4, atomic number decreases by 2.
- **β decay**: neutron changes into proton and electron; electron (β⁻) is emitted. Atomic number increases by 1, mass number unchanged.
- **Gamma decay**: nucleus emits γ-ray; no change in mass or atomic number, only energy reduces.
- Decay equations must balance mass and atomic numbers on both sides.
- Example: ₈₄²¹²Po → ₈₂²⁰⁸Pb + ₂⁴α
Half-Life
- **Half-life** is the time taken for half the nuclei in a sample to decay.
- It is constant for a given isotope and unaffected by conditions.
- Half-life can be found from an activity–time graph: read time for activity to halve.
- Correct for background radiation before calculating half-life.
- After n half-lives, fraction remaining = (½)ⁿ.
Uses of Radiation
- **α**: smoke detectors – α ionises air, smoke absorbs α, triggers alarm.
- **β**: measuring thickness of thin materials (e.g., paper, aluminium foil) – absorption varies with thickness.
- **Gamma**: sterilising medical equipment (penetrating, kills microbes), irradiating food, **radiotherapy** for cancer.
- **Tracers**: short half-life gamma emitters used in PET scans to diagnose cancer.
Dangers of Radiation
- Ionising radiation can damage DNA, causing **cell death**, **mutations**, or **cancer**.
- High doses cause skin burns and reduce white blood cells (weakened immune system).
- Safe handling: store in lead-lined boxes, use tongs, minimise time, maximise distance.
- Shielding (lead, concrete, water) absorbs radiation; radiologists stand behind barriers.
- Radioactive waste with long half-life is buried deep underground.
Alpha particle (helium nucleus) – 2 protons, 2 neutrons.
Deflection of alpha and beta particles in an electric field (analogous to ray deflection).
Penetrating power of alpha, beta, and gamma radiation.
Activity–time graph showing half-life determination.
Practice questions
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1.Which of the following is a natural source of background radiation?
Easy- ARadon gas
- BMedical X-rays
- CNuclear waste
- DNuclear fallout
2.What is the main reason radioactive sources are stored in lead-lined containers?
Easy- ATo reduce the amount of radiation emitted, protecting people nearby
- BTo increase the rate of radioactive decay
- CTo prevent the source from getting cold
- DTo stop the source from moving
3.Which type of radiation is the most ionising?
Easy- Aα particles
- Bβ particles
- CGamma rays
- DNeutrons
4.Which statement about radioactive decay is correct?
Easy- AIt is a random process and cannot be predicted when a particular nucleus will decay
- BThe rate of decay increases if the substance is heated
- CPlacing a radioactive substance in a lead box prevents it from decaying
- DThe decay always produces poisonous gases
5.A radioactive source has a half-life of 0.5 hours. A detector near the source shows a reading of 6000 counts per second. Background radiation can be ignored. What is the reading on the detector 1.5 hours later?
Medium- A750 counts per second
- B1500 counts per second
- C2000 counts per second
- D3000 counts per second
6.A student measures 210 counts in 3 minutes from a radioactive source. The background count rate is 20 counts per minute. What is the corrected count rate for the source?
Medium- A50 counts per minute
- B70 counts per minute
- C190 counts per minute
- D270 counts per minute
7.A nucleus of radium-226 (⁸⁸²²⁶Ra) undergoes α decay. What are the nucleon number and proton number of the new nucleus formed?
Medium- ANucleon number 222, proton number 86
- BNucleon number 222, proton number 88
- CNucleon number 224, proton number 87
- DNucleon number 230, proton number 90
8.Which type of radiation is most suitable for measuring the thickness of thin aluminium foil in a manufacturing process?
Medium- Aβ particles
- Bα particles
- CGamma rays
- DNeutrons
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