RADIATION PROTECTION

      A radionuclide is an isotope of one element which turns into another one during time with emissions of radiations: charges particles (α, β, e) and/or neutral particles (γ, n). Activity is the unit of this disintegration per time. Unit is Becquerel (1 Bq = 1 dis.s-1) or Curie (1 Ci = 3,7 1010 Bq, activity of 1 g of Ra, used till ≅ 1990). The interactions of particles with matter lead to ionizations, excitations, and thermal transfers. Absorbed dose is energy deposition to unit mass. Unit is Gray (1 Gy = 1 J.kg-1) or Rad (1 rad = 0,01 Gy, used till 1990)

      In radiological protection, the matter is human tissues or organs:

• For high absorbed doses: cells are destroyed.

• For low absorbed doses: cells are damaged.

      But the same energy deposition in a cell can:

• be due to different numbers of interactions, different sites of interactions, different natures of interaction,

• lead to different damages at the time of irradiation, and different biochemical and/or biological effects after the irradiation.

    The absorbed dose is an information about the damage. Absorbed dose does not make a complete report on the biological result and the biological future of the irradiated tissue.

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     The committed equivalent or effective dose is the time integral of the equivalent or effective dose rate to the organ. The integration is performed till 70 years lifetime. That is:

• for an adult (18 years old), the time integration is 52 years,

• for a teenager (15 years), the time integration is 55 years and so on.

     The committed equivalent or effective dose is expressed in Sievert. Committed equivalent or effective dose is the risk indicator for an individual in case of internal exposure.


Biological Effects of Ionizing Radiation

Cell

⦁ Charged particle: interactions along the trajectory, small penetration.

⦁ Neutral particle: interactions in local points, deep penetration.

⦁ Ionization: creation of secondary particles (e-, X, γ,…).

      Secondary particles will do interactions also. Each interaction: energy deposition, local damage: water radiolysis, free radicals,…


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Deterministic effects (high doses)

      Impairment of the integrity and function of organs and tissues. Acute and clinically observable effects: blotch (1 Gy), haematologic damage (1 Gy), cataract (3 Gy), burns (5 Gy), sterility (4 Gy), radiodermatitis (12 Gy), digestive system damage (12 Gy) …

Stochastic effects (low doses)

      Changes in cells which limit the ability to reproduce correctly. Delayed effects: radiation-induced cancer (thyroid, leukaemia, etc.), lower intelligence quotient (if antenatal exposure), hereditary effects (chromosomal abnormalities, in animals only). For one person: impossibility to predict if the effect will occur or not. No signature known till now for a radiation-induced cancer.


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Basic Principles of Radiation Protection

Justification

      Any decision that alters the radiation exposure situation should do more good than harm.


Optimization

      The likelihood of exposures, the number of people exposed, the magnitude of their individual doses should be As Low As Reasonably Achievable (ALARA), social and economic factors being taken into account.


Limitation

      The total dose to any individual from regulated sources in planned exposure situations other than medical exposure of patients should not exceed appropriate limits.


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Basic Principles of Regulations

⦁ To avoid any deterministic effects

⦁ To limit incurring stochastic effects at a socially acceptable level.

⦁ Respect of ICRP recommendations


Dose Limits

   The effective dose limits apply to external and internal exposures. For workers, limits used in world over 12 consecutive months: Effective dose limit: 20 mSv.

Equivalent dose limits:

⦁ hands, forearms, feet, ankles: 500 mSv

⦁ skin: 500 mSv (mean dose on any area of 1 cm2),

⦁ lens: 150 mSv,

Pregnant women: exposure of the unborn child < 1 mSv,

Lactating women: no internal exposure,

Persons: aged 16-18 years: effective dose < 6 mSv,

Exceptions: exceptional exposures (< twice the annual limit, must be planned and justified, special authorization for a specific area and duration), radiological emergency situation (100 mSv, 300 mSv to save persons).

3 essential rules:

⦁ Reduction of the time of exposure

⦁ Increase of the distance source - exposed person (use tongs, sticks, etc.),

⦁ Shieldings between the workers and the source, to reduce the particles fluence rates (wall and leaded windows, leaded aprons, etc.).

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