Radioactivity 1

Safety Precautions in Handling Radioactive Materials

  • Display warning signs in the radiation laboratory; signs must be clearly visible to all readers.

  • Wear protective clothing: disposable latex gloves, full laboratory coat with sleeves buttoned, or a radiation suit, and closed shoes.

  • Keep extra clothing and shoes in the laboratory in case your clothes become contaminated.

  • Keep exposure time to radiation as short as possible.

  • Do not eat or drink in a room labeled with radioactive materials signs.

  • Do not leave radioactive materials unsecured in an unattended laboratory, even for a short time, unless the door is locked.

  • Supervise visitors to the laboratory; they must not be left unattended.

  • Use tongs, forceps, or robotic arms during practicals.

  • Never work alone in a radiation laboratory.

Safe Storage of Radioactive Materials

  • Store radioactive material or source in two strong lead-lined containers, one inside the other.

  • Storage container must have a labelled hazard symbol (warning sign) of radioactive materials.

  • Separate all waste by isotope and physical form.

  • Keep the container of radioactive material closed, except when materials are being added.

  • Request hazardous waste collection when you are ready for waste pick-up.

Background Radiation and Artificial Sources

  • What is background radiation? Low-level natural radiation that occurs on Earth’s surface.

  • Major causes of background radioactivity:

    • Cosmic rays (cosmogenic): radiation that reaches the Earth from space.

    • Rocks and soil (terrestrial): some rocks contain radioactive isotopes; radioactive radon gas comes from uranium in igneous rock.

    • Living things: plants absorb radioactive material from the soil, and it passes through the food chain.

  • Artificial sources of radiation:

    • Medical x-ray

    • Radioactive tracer

    • Radioactive waste from nuclear power stations

    • Nuclear missiles from atomic bomb explosions (nuclear weapon testing)

  • Radioactive emission occurs randomly in space at any time.

Instruments to Detect Radioactivity

  • Cloud chamber

  • Geiger–Müller (G–M) counter

  • Photographic film

Detection of Alpha (α), Beta (β), and Gamma (γ) Radiation

  • Detection methods: cloud chamber, Geiger–Müller counter, and photographic film.

  • Cloud chamber setup (components):

    • Perspex (window) chamber with detachable lid

    • Dry ice

    • Sponge (or pad or dark-coloured cloth) moistened with methyl alcohol

    • Radiation source or material

    • Bright light

    • Plastic or glass container

  • How detection works in a cloud chamber:

    • When radiation particles pass through, the alcohol evaporates and then condenses, leaving vapour tails (tracks) that reveal the path of the ray.

    • The path taken by ionising radiation is detected by the tracks left in the chamber.

  • Tracks in cloud chamber: various shapes depending on particle type (bold/straight tracks, much fainter tracks, very faint tracks) as shown in diagrams.

Geiger–Müller (G–M) Counter

  • Components:

    • Geiger–Müller counter tube

    • Thin mica window (small closed glass)

    • High-voltage supply

    • Anode (thin central wire)

    • Cathode (inside surface coated or metal wire)

    • Scaler or pulse counter or rate meter

    • Tube containing argon gas

  • How the G–M counter detects radiation:

    • Radiation enters the tube and ionises the gas (removes an electron).

    • Atoms in the gas become positively charged ions (ionised).

    • Negative ions (electrons) move to the anode, producing an electric current.

    • Positive ions move toward the cathode.

    • The current is read by the counting device (scaler or rate meter) and may be shown as an analogue reading, flashes of light, or a clicking sound.

    • More counts per second indicate higher radiation.

Photographic Film Detection

  • Photographic film darkens when it absorbs radiation; the more radiation absorbed, the darker the film.

  • Film badges are worn by people working in radioactive places.

  • Film badge composition (example): thin aluminium window, copper, lead–tin alloy window, and a plastic case with an open centre area.

Detection of Radiation in Cloud Chamber and Related Properties

  • The cloud chamber reveals tracks which provide qualitative information about the type of radiation.

  • Alpha particles typically produce dense, short tracks; beta rays produce longer, thinner tracks; gamma rays do not leave tracks in the same way but may interact indirectly.

Emission Characteristics and Randomness

  • Radioactive emission occurs randomly over space and time.

  • Radioactive isotopes spontaneously decay.

  • The decay of unstable isotopes is independent of environmental factors such as temperature, pressure, or state of the material.

  • Emissions have properties to consider:

    • Their nature (composition, relative charge, mass)

    • Their relative ionising effects

    • Their relative penetrating abilities

Penetrating Abilities and Deflection in Fields

  • In electric fields:

    • Alpha particles (positively charged) are deflected toward the negatively charged plate (negative electrode).

    • Beta particles (negatively charged) are deflected toward the positively charged plate (positive electrode).

    • Gamma rays have no charge and are not deflected in an electric field.

  • In magnetic fields:

    • Alpha particles are deflected by a magnetic field at a right angle to their velocity.

    • Beta particles are deflected more easily than alphas, at a right angle to their velocity.

    • Gamma rays are not deflected by magnetic fields.

  • Fleming’s left-hand rule can be used to determine the direction of deflection in a magnetic field for a conventional current (positive charge flow).

  • Note: For beta particles (negative charge), the direction indicated by the rule applies to the conventional current, so the actual force direction is opposite to the particle’s motion.

Ionising Effects and Relative Ionisation Power

  • Ionising power ranking (highest to lowest):

    • Alpha particles have the highest ionising effects.

    • Beta particles have moderate ionising effects.

    • Gamma rays have the lowest ionising effects due to their lack of charge.

Radioactive Decay and Nuclear Equations

  • Meaning of radioactive decay: The change of an unstable nuclide with the release of energy in the form of radiation (alpha particles, beta particles, and gamma rays).

  • General nuclide decay equations:

    • Alpha decay (emission of an alpha particle):
      ^{A}{Z}X ightarrow ^{A-4}{Z-2}Y + ^{4}_{2}\mathrm{He}

    • Example: Alpha decay always forms a helium nucleus ($^{4}_{2}\mathrm{He}$).

  • Examples of alpha decays:

    • Radium-226 decay:
      ^{226}{88}\mathrm{Ra} ightarrow ^{222}{86}\mathrm{Rn} + ^{4}_{2}\mathrm{He}

    • Uranium-234 decay (example provided in transcript):
      ^{234}{92}\mathrm{U} ightarrow ^{230}{90}\mathrm{Th} + ^{4}_{2}\mathrm{He}

  • Beta decay (emission of an electron):

    • General beta decay equation:
      ^{A}{Z}X ightarrow ^{A}{Z+1}Y + ^{0}_{-1}\beta^{-}

    • Examples from transcript:

    • Iodine-131 beta decays to Xenon-131:
      ^{131}{53}\mathrm{I} ightarrow ^{131}{54}\mathrm{Xe} + ^{0}_{-1}\beta^{-}

    • Thorium-234 beta decays to Protactinium-234:
      ^{234}{90}\mathrm{Th} ightarrow ^{234}{91}\mathrm{Pa} + ^{0}_{-1}\beta^{-}

  • Gamma emission:

    • Gamma emission causes no change in atomic (Z) or nucleon (A) numbers:
      ^{A}{Z}X^{*} ightarrow ^{A}{Z}X + \gamma

    • Emission of alpha and beta leaves the nucleus in an excited state, and protons and neutrons rearrange to form gamma radiation.

Half-life and Activity

  • Half-life (T1/2): The time taken for half of the radioactive nuclei to decay.

  • Activity (A): The average number of disintegrations per second.

  • SI unit for activity: Becquerel (Bq), where
    1\ \text{Bq} = 1\ \text{disintegration per second}

  • Decay relationships and simple calculations:

    • Number of undecayed nuclei as a function of time:
      N(t) = N0\left(\tfrac{1}{2}\right)^{\tfrac{t}{T{1/2}}}

    • Activity as a function of time:
      A(t) = A0\left(\tfrac{1}{2}\right)^{\tfrac{t}{T{1/2}}}

  • To measure half-life, use a GM tube and rate meter to monitor activity over time.

Decay Curves and Fractions after Half-lives

  • After four half-lives, the fraction of nuclei remaining is:
    \left(\tfrac{1}{2}\right)^4 = \tfrac{1}{16}

  • After five half-lives, the fraction remaining is:
    \left(\tfrac{1}{2}\right)^5 = \tfrac{1}{32}

  • These results illustrate exponential decay over successive half-lives.

Quick Reference: Notable Points

  • Emission is random in space and time; decay is spontaneous and largely independent of environment.

  • Background sources include cosmic rays, terrestrial radionuclides (and radon), and uptake through the food chain.

  • Artificial sources include medical X-rays, radiotracers, nuclear waste, and weapon testing.

  • Common instruments and their basic operation: cloud chamber (visual tracks), Geiger–Müller counter (count rates with audible/visual output), and photographic film (dose indication via darkness).

  • Safety protocols emphasize containment, PPE, time minimization, and supervision of non-staff in laboratories.