Comprehensive Study Notes on Radioactivity

Radioactivity

Marie Curie's Life and Work

• Marie Curie and Pierre Curie's marriage in 1895:

  • Marked the beginning of a partnership that led to significant discoveries.

• Henri Becquerel's discovery in 1896:

  • Discovered radioactivity in uranium, which Marie Curie investigated further.

  • Marie Curie found radioactivity in thorium as well.

• Nobel Prizes:

  • 1903 Nobel Prize in Physics: Awarded to Marie Curie, Pierre Curie, and Henri Becquerel.

  • 1911 Nobel Prize in Chemistry: Awarded solely to Marie Curie.

  • Marie Curie was the first woman to win a Nobel Prize.

• Marie Curie's Early Life:

  • Exceptional memory from childhood.

  • Won a gold medal at the age of 16.

• Marie Curie's Achievements:

  • Understood the importance of accumulating intense radioactive sources.

  • For treating illnesses and maintaining research supplies.

  • The Radium Institute in Paris had a stock of 1.5.

Radioactive Decay

• Definition: Radioactive decay is the process where an unstable nucleus transforms into a more stable nucleus by emitting radiation (energy).

• Nature of the Process:

  • Random: It is impossible to predict which specific nucleus will decay next.

  • Spontaneous: The process is not influenced by external conditions like temperature and pressure.

• Radioisotopes:

  • Many elements have radioactive isotopes (radioisotopes) due to unstable nuclei.

  • Instability is often caused by a large number of nucleons, particularly an excess of neutrons.

  • Examples: cesium, cobalt, iodine, plutonium, radon, strontium, thorium, uranium.

Nuclear Stability

• Nuclear Instability:

  • In large nuclei, nucleons are not held together as tightly due to the short effective range of the strong nuclear force, leading to instability.

  • Adding or subtracting neutrons can influence nuclear stability.

Types of Nuclear Emission

• Emission Types:

  • Radioactive substances emit alpha particles, beta particles, or gamma rays.

  • Emission may also include energy in the form of rays.

• Transformation of Elements:

  • The atom transforms into an atom of another element due to changes in the number of protons in the nucleus.

• Outcomes of Emission:

  • Increased stability of the nucleus.

  • Reduction in excess neutrons.

Characteristics of Alpha, Beta, and Gamma Radiation

• Alpha Particles:

  • Symbol: α

  • Composition: 2 protons + 2 neutrons

  • Mass: Approximately 4 times the mass of a proton, $(\approx 4 \times \text{mass of proton})$

  • Speed: About $3 \times 10^7$ m/s

  • Charge: +2e

• Beta Particles:

  • Symbol:

  • Composition: An electron

  • Mass: Approximately $(\text{mass of proton} / 1840)$

  • Speed: About $2.9 \times 10^8$ m/s

  • Charge: -1e

• Gamma Rays:

  • Symbol: γ

  • Composition: Electromagnetic radiation

  • Mass: 0

  • Speed: $3 \times 10^8$ m/s

  • Charge: 0

• Origin of Beta Particles:

  • Beta particles originate from inside the nucleus, where a neutron decays into a proton and an electron.

  • Equation: $_{0}^{1}n \rightarrow _{1}^{1}p + _{-1}^{0}e$

Ionizing Power

• Depends on kinetic energy and charge.

• Alpha Particles:

  • Most ionizing due to large mass and kinetic energy.

  • Interact heavily with surrounding material, pulling electrons off atoms.

• Beta Particles:

  • Less ionizing than alpha particles.

• Gamma Radiation:

  • Least ionizing.

• Alpha particles are more harmful when inhaled or ingested.

  • Example: radon and thoron gases.

Penetrating Power

• Alpha Particles:

  • Easily absorbed.

  • Travel about 5 cm in air before being absorbed.

  • Cannot penetrate a thin sheet of paper or skin.

• Beta Particles:

  • Travel fairly easily through air or paper.

  • Absorbed by a few millimeters of metal, such as aluminum.

• Gamma Radiation:

  • Most penetrating.

  • Requires several centimeters of dense metal (e.g., lead) or several meters of concrete to absorb most of the radiation.

Deflecting Radiation

• Diagram illustrating the deflection of alpha, beta, and gamma radiation in a magnetic field.

• Magnetic field is directed into the page.

• Alpha particles are deflected in one direction, beta particles in the opposite direction, and gamma rays are not deflected.

Activity and Half-Life

• Geiger Counter:

  • Used to measure radiation.

  • Records the rate at which radiation is detected, known as the count rate.

  • Unit: counts per second (count/s) or counts per minute (count/min).

• Activity of a Radioactive Source:

  • The rate at which nuclei decay.

• Half-Life of a Radioactive Source:

  • The average time taken for half the atoms in a sample to decay.

Half-Life Equation

• Equation: $N = N<em>0 \times 2^{-t/T</em>{1/2}}$

  • $N_0$: Initial number of undecayed atoms (at time t = 0).

  • $N$: Number of undecayed atoms at time t.

  • $T_{1/2}$: Half-life of the substance.

• Applies to calculating count rate/mass.

Examples of Half-Life Calculations

• Example 1: Iodine-131

  • Initial atoms: 20,000,000

  • Half-life: 8 days

  • Time: 32 days

  • $N = 20,000,000 \times 2^{-32/8} = 625,000$

  • Alternatively, 32 days = 4 half-lives. $N = 10,000,000 / 2 / 2 / 2 / 2$

• Example 2: Radium-224

  • Half-life: 3.6 days

  • Original mass: 10 grams

  • Time: 7.2 days

  • $m = 10 \times 2^{-7.2/3.6} = 2.5$

  • Alternatively, 7.2 days = 2 half-lives. $m = 10 / 2 / 2 = 2.5 \text{ g}$

• Example 3:

  • Using a graph of activity vs. time to determine half-life.

• Example 4:

  • 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?

Background Radiation and Corrected Count Rate

• Background Radiation:

  • Radiation that is always present in the environment.

  • Mostly from natural sources.

• Corrected Count Rate:

  • $\text{Corrected Count Rate} = \text{Observed Count Rate} - \text{Background radiation}$

Background Radiation Example Calculation

• Example 5:

  • Observed count rate = 542 (sample + background)

  • Background count rate = 30

  • Initial count rate of sample = 542 – 30 = 512

  • Later count rate = 94 (sample + background)

  • Count rate of sample = 94 – 30 = 64

  • Decay: 512 -> 64 (512 / 64 = 8)

  • Number of half-lives: 3 ($2 \times 2 \times 2 = 8$)

  • Half-life = 12 minutes / 3 = 4 hours

• Another Example:

  • Initial corrected count rate = 180 – 20 = 160

  • After one half life 160/2 = 80

  • The count rate measured = 80 + 20 (background) = 100

• Using a graph of emission rate vs. time to estimate half-life.

  • The rate of emission of a radioactive source is measured until the reading reaches the background rate of 20 counts per minute.

Applications and Safety Precautions

• Safety Measures:

  • Reducing exposure time.

  • Increasing distance from the source.

  • Using shielding to absorb radiation.

• Effects of Radiation:

  • Cell death.

  • Mutations.

  • Cancer.

Effects of Radioisotopes on Cells

• High Dose of Radiation:

  • Causes significant ionization in cells, leading to cell death (radiation burns).

• Damage to DNA:

  • Can disrupt cell control mechanisms, leading to uncontrolled cell division and tumor formation (cancer).

• Effects on Gametes (Sperm or Egg Cells):

  • Damaged DNA can be passed to future generations, causing genetic mutations.

  • Mutations can be harmful, leading to developmental issues or genetic disorders.

Uses of Radioisotopes

• Based on:

  • Penetrating power.

  • Damage to living cells.

  • Detectability.

  • Radioactive decay and half-life.

• Fault Detection:

  • Gamma rays are used to detect faults in manufactured goods.

  • Rays escape through faults and are detected on photographic film.

• Thickness Measurements:

  • Beta radiation is used to measure thickness.

  • Radiation is directed through a sheet, and a detector measures the amount of radiation that passes through.

  • Thickness is adjusted based on the radiation detected.

• Smoke Detectors:

  • Americium-241 is used (long half-life, about 430 years).

  • Alpha radiation falls on a detector, creating a current that keeps the alarm silent.

  • Smoke entering the gap absorbs alpha radiation, stopping the current and triggering the alarm.

Sterilization and Food Irradiation

• Medical Products:

  • Sealed in plastic bags and exposed to gamma rays to kill microbes.

• Food Irradiation:

  • Preserves food by killing microbes with intense gamma rays.

  • Results in sterile food, often used in space or hospitals.

  • Single-celled organisms are killed due to cell damage.

Cancer Treatment

• Method:

  • A source of gamma rays (or X-rays) is directed at the tumor.

  • The source moves around the tumor to minimize radiation exposure to other tissues.

Radioactive Dating

• Principle:

  • Radioactive substances decay at a known rate, allowing the determination of the age of objects and materials.

• Radiocarbon Dating:

  • Measures the amount of carbon-14 in an object to determine when it was alive.

  • After death, carbon-14 decays.

  • Nuclear weapons testing in the 1950s and 1960s increased carbon-14 in the atmosphere, affecting the accuracy of dating living objects from that period.

• Dating Rocks:

  • Potassium-40 and argon are used.

Engineering and Radioactive Tracing

• Tracing Underground Water Flow:

  • Water containing a radioactive chemical is injected into the ground.

  • Detectors at ground level monitor its movement through underground cracks.

• Medical Tracing:

  • A radioactive chemical (tracer), such as technetium-99 (short half-life, about 6 hours), is injected into the patient.

  • A scanner is used to trace the path of the chemical.

5.2 Radioactivity (Summary)

5.2.1 Detection of Radioactivity

• Core:

  • Understand background radiation.

• Supplement:

  • Sources of background radiation:

    • Radon gas (in the air).

    • Rocks and buildings.

    • Food and drink.

    • Cosmic rays.

  • Ionizing nuclear radiation measured using a detector connected to a counter.

  • Use count rate (counts/s or counts/minute).

  • Use background radiation measurements to determine corrected count rate.

5.2.2 The Three Types of Nuclear Emission

• Core:

  • Emission of radiation from a nucleus is spontaneous and random.

  • Identify alpha (α), beta (β), and gamma (γ) emissions by:

    • Their nature.

    • Their relative ionizing effects.

    • Their relative penetrating abilities (ẞ* are not included).

• Supplement:

  • Deflection of α-particles, ß-particles, and γ-radiation in electric and magnetic fields.

  • Explain relative ionizing effects with reference to:

    • Kinetic energy.

    • Electric charge.

5.2.3 Radioactive Decay

• Core:

  • Radioactive decay is a change in an unstable nucleus that results in the emission of α-particles or ß-particles and/or γ-radiation.

  • These changes are spontaneous and random.

  • During α-decay or ß-decay, the nucleus changes to that of a different element.

• Supplement:

  • Isotopes may be radioactive due to an excess of neutrons and/or the nucleus being too heavy.

  • Describe the effect of α-decay, ß-decay, and γ-emissions on the nucleus, including increased stability and a reduction in excess neutrons.

    • The following change in the nucleus occurs during ß-emission: $neutron \rightarrow proton + electron$

  • Use decay equations, using nuclide notation, to show the emission of α-particles, ß-particles, and γ-radiation.

5.2.4 Half-Life

• Core:

  • Define half-life as the time taken for half the nuclei of an isotope in a sample to decay.

  • Recall and use this definition in simple calculations.

• Supplement:

  • Calculate half-life from data or decay curves from which background radiation has not been subtracted.

  • Explain how the type of radiation emitted and the half-life of an isotope determine its use in various applications, including:

    • Household fire (smoke) alarms.

    • Irradiating food to kill bacteria.

    • Sterilization of equipment using gamma rays.

    • Measuring and controlling thicknesses of materials.

    • Diagnosis and treatment of cancer using gamma rays.

5.2.5 Safety Precautions

• Core:

  • State the effects of ionizing nuclear radiations on living things, including cell death, mutations, and cancer.

• Supplement:

  • Describe how radioactive materials are moved, used, and stored safely.

  • Explain safety precautions for ionizing radiation in terms of reducing exposure time, increasing distance, and using shielding to absorb radiation.