Explain safety precautions for all ionising radiation in terms of reducing exposure time, increasing distance between source and living tissue and using shielding to absorb radiation

5.2.5 Safety Precautions for Ionising Radiation

1. Why precautions are needed – core ideas

• Biological effects: Ionising radiation can strip tightly‑bound electrons from atoms in living cells. This can:

  • Kill cells (cell death)
  • Alter DNA – producing mutations that may develop into cancer
  • Damage germ‑cell DNA – leading to hereditary effects

• ALARA principle: The dose received must be kept as low as reasonably achievable. All safety measures aim to satisfy ALARA.

2. Radiation‑Protection Hierarchy (Time → Distance → Shielding)

The hierarchy is the order in which the three basic controls are applied. Each step reduces the dose and therefore helps to meet the ALARA principle.

1. Reduce exposure time
2. Increase distance from the source
3. Insert appropriate shielding between source and tissue

3. Types of ionising radiation encountered in school laboratories (core)

• Alpha (α) – e.g. ²⁴¹Am (smoke‑detector source)
• Beta (β) – e.g. ⁹⁰Sr (medical equipment source)
• Gamma (γ) / X‑ray – e.g. ⁶⁰Co (sterilisation source)
• Supplement (rarely used in schools): Neutrons – e.g. from a research reactor

4. Safe storage, labelling and authorised handling

• All sources are stored in a lead‑lined, clearly labelled container when not in use.
• Labels must show the radionuclide, activity, date of receipt and the radiation‑hazard symbol.
• Only staff who have been trained and authorised may move a source. They must use a shielded holder or trolley.
• Waste containing radioactive material is placed in a designated radioactive waste container and recorded in the laboratory log‑book.
• Every use of a source is entered in a radiation‑use log (date, source, activity, experiment, duration, personnel).

5. Reducing the dose – the three basic controls

5.1 Reduce the time of exposure (core)

• The absorbed dose D is directly proportional to the exposure time t:

  D = \dot{D}\;·\;t

  where \dot{D} is the dose‑rate (µSv h⁻¹).

• Plan the experiment so the source is uncovered for the shortest time possible.
• Use a timer or a log‑book to record the exact opening and closing times.
• Prepare the detector, shielding and data‑recording equipment in advance (“ready‑to‑use” set‑up).

5.2 Increase the distance from the source (core)

• Radiation intensity follows the inverse‑square law for a point source:

  I ∝ 1/r²

  Doubling the distance reduces the intensity to one‑quarter.

• Work from the greatest practical distance – use a viewing window, camera, or remote monitor.
• Mark a “safe distance” on the bench (e.g. 2 m for a strong γ source).
• Place shielding between the source and the operator, never behind the operator.

5.3 Shield the source (core)

Shielding works by absorbing or scattering the radiation. The material chosen depends on the type of radiation.

• Core tip: For mixed fields (e.g. a β source that also emits bremsstrahlung), place a low‑Z shield first, then add a thin lead layer.

6. Dose‑rate units and school‑lab limits (core)

• Units: 1 µSv h⁻¹ = 1 microsievert per hour; 1 mSv h⁻¹ = 1000 µSv h⁻¹.
• Typical regulatory limits for schools (public‑area exposure):

  • Background‑corrected dose‑rate < 0.1 µSv h⁻¹ in areas where students work.
  • Controlled‑area limit (e.g. behind shielding) ≤ 0.5 µSv h⁻¹.

• Always measure the ambient dose‑rate with a survey meter before starting work and compare with the limits above.

7. Practical activity – background‑rate correction (core)

1. Set the Geiger‑Müller counter (or scintillation detector) to “count‑rate” mode.
2. Record the background count‑rate for at least 60 s with no source present.
3. Place the sealed source in the holder, keep the same geometry, and record the total count‑rate for 60 s.
4. Calculate the corrected source count‑rate:

   Rcorrected = Rtotal – R_background

5. Use the corrected rate to estimate the dose‑rate (using the detector’s calibration factor) and then the dose for the planned exposure time.

8. Laboratory safety checklist (core)

• Store all sources in lead‑lined, clearly labelled containers when not in use.
• Wear appropriate PPE: lab coat, disposable gloves (if handling sources), and safety glasses.
• Check the area dose‑rate with a calibrated survey meter; ensure it is below the school limits.
• Post clear signage (radiation‑hazard symbol, required safe distance, activity level) around the work area.
• Ensure a spill‑kit / containment box is readily accessible.
• Know the emergency procedure:

  • Who to call (radiation safety officer or senior teacher)
  • Location of the emergency shower and eye‑wash
  • Evacuation route and assembly point

• Dispose of any contaminated waste in the designated radioactive‑waste container and record it in the log‑book.
• Complete a radiation‑use log for every experiment (date, source, activity, duration, personnel, any incidents).

9. Suggested diagram for a hand‑out (description)

• A sealed ⁶⁰Co capsule at the centre.
• Three concentric circles:

  1. Inner circle – a stopwatch symbol representing “time”.
  2. Middle circle – a ruler or arrow labelled with a distance (e.g., 2 m) representing “distance”.
  3. Outer circle – a block of lead between the source and a stylised worker representing “shielding”.

• The worker uses a remote manipulator or looks through a viewing window, wearing lab coat, gloves and goggles.
• Arrows show the direction of radiation and how each precaution reduces the dose.

10. Summary – core points

1. Ionising radiation can cause cell death, mutations and cancer; therefore doses must be kept as low as reasonably achievable (ALARA).
2. The radiation‑protection hierarchy – time, distance, shielding – is the logical order for reducing dose.
3. Use appropriate shielding for each radiation type; remember the inverse‑square law when increasing distance.
4. Know the dose‑rate units (µSv h⁻¹, mSv) and the school‑lab limits (< 0.1 µSv h⁻¹ for public areas).
5. Store sources safely, label them, keep a use log, and follow the practical checklist (PPE, survey meter, signage, emergency, waste disposal).
6. Always correct measured count‑rates for background before calculating dose.