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HKIE Professional Assessment - Nuclear Discipline study materials
HKIE Professional Assessment - Nuclear Discipline study resources

HKIE Professional Assessment - Nuclear Discipline: Radiation Protection, Shielding and Dosimetry

HKIE Professional Assessment - Nuclear Discipline · Work through your exam, one topic at a time.

All guide sections

Radiation Protection, Shielding and Dosimetry

This domain tests dose quantities, limits, ALARA, shielding calculations and practical dosimetry in nuclear facilities.

Core rules

  • Absorbed dose is measured in gray (Gy); equivalent dose uses radiation weighting factors and is measured in sievert (Sv); effective dose applies tissue weighting factors and is used for stochastic risk.
  • ICRP occupational limits are 20 mSv per year averaged over five years, with no more than 50 mSv in any single year. Public limit is 1 mSv per year. Lens limit is 20 mSv per year; extremities are 500 mSv per year.
  • ALARA requires dose reduction to as low as reasonably achievable, considering economic and social factors.
  • Shielding uses attenuation: N = N0 e^-μx. Half-value layer is HVL = ln2/μ; ten-value layer is TVL = ln10/μ. For a point source, dose rate also falls with inverse square of distance.
  • Personal dosimetry uses TLDs, OSLs or film badges; electronic dosimeters give real-time dose but are not a substitute for legal records. Contamination monitoring includes direct survey and smear tests.

Worked example

A Co-60 source produces 400 µSv/h at a point. Lead has an HVL of about 1.2 cm for Co-60 gamma rays. What thickness reduces the dose rate to 20 µSv/h, neglecting buildup?

The required reduction factor is 400/20 = 20. The number of half-value layers is n = ln20/ln2 = 4.32. Thickness = 4.32 × 1.2 cm = 5.2 cm. The reasoning is that each HVL halves the dose rate. In a broad beam, scattered photons add dose, so a buildup factor would require additional thickness; the calculated value is therefore a lower bound.

Traps

Do not add absorbed dose in Gy directly to effective dose in Sv without weighting. Do not ignore distance; inverse square can be more effective than thin shielding. Do not use the public limit for a radiation worker. Do not forget lens and extremity limits. Do not treat a smear test as a dose measurement; it measures removable contamination.

Key points

  • Occupational effective dose: 20 mSv/y averaged, 50 mSv maximum in a year.
  • Public effective dose: 1 mSv/y.
  • HVL = ln2/μ; TVL = ln10/μ.
  • Co-60 lead HVL is about 1.2 cm.
  • ALARA includes economic and social factors.
  • Distance, time and shielding are the basic dose controls.