RAD PROTECTION

Radiation

Energy that travels through space or matter as waves or particles from natural or artificial sources.

Ionizing Radiation

High-energy radiation able to remove orbital electrons from atoms, creating ions.

Non-Ionizing Radiation

Lower-energy radiation that excites, but does not ionize, atomic electrons.

Electromagnetic Radiation

Energy propagated as oscillating electric and magnetic fields; includes X-rays and gamma rays when ionizing.

Particulate Radiation

Ionizing radiation consisting of particles such as alpha, beta, protons, or neutrons.

Alpha Particle (α)

Helium nucleus (2p + 2n) emitted in radioactive decay; high ionization, low penetration—stopped by paper.

Beta Particle (β)

High-speed electron or positron emitted from a nucleus; moderate penetration—stopped by thin aluminum.

Gamma Ray (γ)

High-energy electromagnetic photon emitted from the nucleus; deeply penetrating—attenuated by lead.

X-Ray

Photon produced outside the nucleus when high-speed electrons brake or shift shells in an atom.

Neutron Radiation

Uncharged particles released from fission, reactors, or accelerators; best shielded by water or concrete.

Cosmic Rays

Ionizing particles and photons originating from the sun and stars (natural background source).

Terrestrial Radiation

Radiation from uranium, thorium, and other radionuclides in Earth’s crust.

Radon

Radioactive gas from uranium decay; largest natural source; emits alpha particles to the lung.

Internally Deposited Radionuclides

Natural radioisotopes (e.g., Potassium-40) metabolized within the body.

Diagnostic X-Rays

Largest man-made source of ionizing radiation to the population (~3.2 mSv/yr).

Somatic Effects

Radiation effects appearing in the exposed individual’s body cells or tissues (e.g., cancer).

Genetic Effects

Radiation-induced mutations in germ cells affecting future generations.

Deterministic Effect

Severity increases with dose; has a threshold (e.g., skin burns, cataracts).

Stochastic Effect

Probability increases with dose; no threshold (e.g., cancer, hereditary effects).

Fetal Irradiation Effects

Prenatal death, malformations, growth retardation, or childhood cancer depending on dose & stage.

Prevent deterministic effects and limit probability of stochastic effects.

Radiation Protection Aim

ICRP

International Commission on Radiological Protection—sets global dose guidance.

Justification

ICRP principle: perform a procedure only if expected benefit exceeds radiation risk.

Optimization (ALARA)

ICRP principle: keep doses As Low As Reasonably Achievable while achieving purpose.

Dose Limits

ICRP principle: legal cap on radiation received by workers or the public.

Gray (Gy)

SI unit of absorbed dose; 1 ___ = 1 J/kg.

Sievert (Sv)

SI unit of effective dose or equivalent dose accounting for radiation weighting; risk metric.

Roentgen (R)

Old unit of exposure in air; 1 __ = 2.58 × 10⁻⁴ C/kg.

Air Kerma (Gya)

SI measure of energy released in air; replaces roentgen for exposure.

Rad

Old unit of absorbed dose; 1 ___ = 0.01 Gy.

Rem

Old unit of effective/equivalent dose; 1 ___ = 0.01 Sv.

Becquerel (Bq)

SI unit of radioactivity; 1 ___ = 1 disintegration per second.

Curie (Ci)

Old unit of radioactivity; 1 ___ = 3.7 × 10¹⁰ Bq.

Exposure

Amount of ionization created by X- or γ-rays in air; measured in C/kg or Gya.

Absorbed Dose

Energy deposited per unit mass in tissue; measured in gray (Gyt).

Effective Dose

Dose weighted for tissue sensitivity; indicates whole-body risk; measured in sievert.

Radioactivity

Rate of nuclear transformations in a source; measured in Bq or Ci.

Health Physics

Science/practice of occupational and public radiation protection.

Health Physicist

Radiation scientist specializing in safety research, teaching, and operations.

Linear Nonthreshold (LNT) Model

Assumes any dose, no matter how small, carries some stochastic risk.

Time, Distance, Shielding—core methods to reduce radiation exposure.

3 Cardinal Principles

Minimize Time

Reduce beam-on duration to proportionally lower dose.

Maximize Distance

Increase separation from source; dose falls with inverse square law.

Shielding

Place attenuating material (lead, concrete) between source and person to block radiation.

Half-Value Layer (HVL)

Thickness that reduces beam intensity to 50 % of original.

Tenth-Value Layer (TVL)

Thickness that cuts beam intensity to 10 % of original.

Protective Apron

Personal shield containing 0.5 mm Pb; about two HVLs—reduces scatter to 10 %.

Positive Beam Limitation (PBL)

Automatic collimator that matches X-ray field to receptor size (±2 % SID).

Source-to-Image Receptor Distance (SID)

Distance from X-ray focal spot to detector; affects exposure & magnification.

Leakage Radiation Limit

Tube housing must emit < 1 mGya/hr (100 mR/hr) at 1 m during use.

Total Filtration (Diagnostic)

≥ 2.5 mm Al for tubes operated above 70 kVp (inherent + added).

Reproducibility

Variation in output for identical technique ≤ 5 % between exposures.

Linearity

Output proportional to mAs; adjacent mA stations vary ≤ 10 %.

Mobile X-Ray Distance Rule

Operator must stand ≥ 2 m from tube & patient during exposure.

Fluoroscopy Source-to-Skin Distance

≥ 38 cm (stationary) or 30 cm (mobile) to reduce skin dose.

Primary Protective Barrier

Shielded wall/floor targeted by primary beam; thicker (e.g., 1/16-in lead).

Secondary Protective Barrier

Shields against scatter & leakage; thinner; often gypsum or 0.4 mm Pb equiv.

Scattered Radiation

Photons deflected from patient or objects; principal source to staff.

Leakage Radiation

Photons escaping tube housing other than primary beam.

Use Factor (U)

Fraction of beam-on time a barrier receives primary radiation.

Workload (W)

Weekly operational output (mA-min/week) used in barrier design.

Occupancy Factor (T)

Fraction of time a protected area is occupied; higher T = thicker shielding.

Controlled Area

Location occupied by radiation workers; limit 1 mSv/week (100 mrem/wk).

Uncontrolled Area

Public space; design limit 0.02 mSv/week (2 mrem/wk).

Gas-Filled Detector

Radiation instrument using ionization of gas to create electrical signal.

Ionization Chamber

Gas detector used for precise exposure or area surveys; wide range.

Proportional Counter

Lab detector distinguishing α vs β particles; assays small activities.

Geiger-Müller Counter

Sensitive survey meter for contamination; audible clicks for single photons.

Scintillation Detector

Crystal produces light flashes when hit by radiation; basis of gamma cameras and survey meters.

Thermoluminescent Dosimeter (TLD)

LiF or CaF₂ crystal stores radiation energy; emits light when heated for dose readout.

Optically Stimulated Luminescence (OSL)

Al₂O₃ dosimeter read with laser light; more sensitive & re-readable than TLD.

Film Badge

Photographic film with filters; historical personnel dosimeter sensitive to x-rays.

Pocket Dosimeter

Pen-shaped ion chamber giving immediate dose reading to the wearer.

Dose Area Product (DAP)

Product of dose and beam area (cGy·cm²); reflects risk from both dose and field size.

Entrance Skin Dose (ESD)

Radiation dose to patient’s skin at beam entry; common patient dose metric.

Genetically Significant Dose (GSD)

Weighted average gonad dose producing same genetic impact on population as actual exposures.

Mean Marrow Dose (MMD)

Average radiation dose to active bone marrow from a procedure.

Contact Shield

Lead shield placed directly on patient (e.g., gonad cups, lens disks).

Shadow Shield

Lead device suspended in beam casting protective shadow over sensitive organs.

Elective Booking

Scheduling radiologic exams to avoid first 10-day window of potential conception.

Patient Questionnaire

Form asking women of childbearing age about pregnancy status before imaging.

Entire pregnancy limit 5 mSv; per month 0.5 mSv.

Embryo-Fetus Dose Limit

50 mSv (5 rem) for radiation workers per NCRP.

Occupational Annual Effective Dose Limit

150 mSv (15 rem) annually.

Equivalent Dose Limit – Lens of Eye (Workers)

Public Annual Effective Dose Limit

1 mSv (0.1 rem) for frequent exposure situations.

Negligible Individual Dose (NID)

0.01 mSv (1 mrem) or per NCRP 0.5 mSv/year threshold where regulation deemed unnecessary.